BOUGHT WITH THE INCOBIE FROM THE "^^ SAGE ENDOW-MENT FUND^ THE GJ^ OF iienrg Hit Sage iSqi /\.2L5k'&^ v:^\v<^\>^ Cornell University Library QE 365.R72 Introduction to the study of minerals; a 3 1924 014 810 448 Cornell University Library The original of tliis book is in tlie Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924014810448 INTRODUCTION TO THE STUDY OF MINERALS Publi-shed by the McGraw-Hill Boole Company Ne-w Voirlk Svtccc&sors fo iKc&OoU-Departmentjs of tKc McGraw Publishing Company Hill PubHshing Gjmpany Publishers of Dook.5 foi* ESectrical World TheEn^ineenng" and Mining Journal Engineering Record American MacKini^t Electric RaJKvay JoumQl Coal Age MeiaMurgical and CBiemical Engineering R>wer INTKODUCTION TO THE STUDY OF MINERALS A COMBINED TEXT-BOOK AND POCKET MANUAL BY AUSTIN FLINT ROGERS, Ph. D. ASSOCIATE PROFESSOR OF MINERALOGY AND PETROGRAPHY LELAND STANFORD JUNIOR UNIVERSITY McGRAW-HILL BOOK COMPANY 239 WEST 39TH STREET, NEW YORK 6 BOUVERIE STREET, LONDON, E. C. 1912 Copyright, 1912, by the McGraw-Hill Book Company PRINTED AND ELECTROTYPED BY THE MAPLE PRESS YORK, PA. XLo lib's parents THIS BOOK IS GRATEFULLY INSCRIBED PREFACE This work is intended primarily as a text-book for a one-year course in mineralogy. \\ hile full enough for class work it is condensed enough for field work. There is decided advantage in using the same book in the field as in the class room and laboratory. The author's aim has been to produce a well-balanced book covering the whole field of mineralogy as far as is practicable; and one in which the practical methods of identifying, minerals, the economic features, and the scientific aspects of mineralogy as a means of correlating hundreds of isolated facts, are all given their proper emphasis. The only way to bring out the important and essential points is to omit the great mass of details which are properly included in the reference books. Two hundred minerals have been included in the descriptions (Part VI). These have been selected with great care, although the list is necessarily an arbitrary one based upon the writer's experience and judgment. All the common minerals and the more important useful ones are included in the two hundred. Others have been selected for practice in testing and also to give a comprehensive view of the mineral kingdom. In most miner- alogical text-books brief descriptions of some of the rare minerals are given. In the writer's opinion this is not advisable, for one needs a full description of a rare mineral in order to identify it. Of the two hundred minerals given the one hundred most common, in the writer's opinion, are distinguished by larger type in the headings and determinative tables. This selection, like that of the two hundred, is somewhat arbitrary but it un- doubtedly includes all the common minerals of the world. In a short course in mineralogy attention may be confined exclusively VIU PREFACE to the hundred minerals given in large type. Certain other portions of the book would also be disregarded in a short course. On the other hand, Sections 16 to 19 of Part I (crystal measure- ment), Part III (crystal optics), and Part VII (occurrence, origin, and association of minerals) may each serve as the basis for an advanced course in the subject mentioned. The distinctive features of this book are: (1) The description of the optical and microscopic properties of crushed fragments, cleavage flakes, and recrystallized products. (2) Six new determinative tables. These are useful in getting the student familiar with the terms used as well as in the actual determination of minerals. (3) A num.bered list of the occurrences of each mineral, estab- lishing what the author calls paragenetic varieties. (4) A glossary of terms. Synonyms and varieties are usually given in the glossary rather than in the text. In a subject like mineralogy a glossary seems almost a necessity. In crystallography symmetry has been emphasized and the ideas of hemihedrism, etc., abandoned, though these terms are ex- plained. The thirty-two classes are included for the sake of completeness, but the ten important ones are given more space than the others. Groth's names of the thirty-two classes and von Fedorow's names of forms are used, but the other terms in common use are given in parentheses. The Miller indices have been employed exclusively. An attempt has been made to give a concise logical presenta- tion of the subject of crystal optics, beginning with the nature of light. Some of the figures may be colored by the student for future reference. This part of the book may serve as an intro- duction to the study of petrography. The blowpipe part has been modelled somewhat after Brush- Penfield's excellent manual. A qualitative scheme, especially applicable to minerals in that calcium phosphates, borates, and fluorid are provided for, has been included. PREFACE IX In Part VI the order of the minerals is like that of Dana's System of Mineralogy except where there is some special reason for change. The silicutes, following Klockmann, Groth, Nau- mann-Zirkel, and Renard and Stciber, are placed last. The results of analyses of the oxy-salts (excepting silicates) are stated in terms of metals and acid radicals instead of in the usual form of basic and acid arih\'drids. This method shows the variation, isomorphic replacement, and molecular ratios as well as the ordinary method, and is not based upon antiquated notions. In stating the results of analyses of acid, basic, and hydrous salts the water percentage is given, as its determination is often of value in the identification of minerals. The analyses are taken mainly from Hintze's Handbuch and Dana's System (5th and 6th editionsj. In the description of minerals (Part VI) a standard form of headings has been used for convenience of reference. Any partic- ular point may easily be found. Free use has been made of other books, especially those men- tioned in the Select Bibliography. I am greatly indetjted to Professoi- James Perrin Smith for as- sistance in reading and correcting proof. Austin F Rogers. STANFoitij University, April, 1912. CONTENTS Select Bibliography Introduction Page xvii 1 Section 1.- Section 2.- Section Section Section Section Section Section 8.- Section 9." Section 10.- Section 11.- Section 12.- Section 13.- Section 14.- Sbction 1.5.- Section 16.- Section 17.- Section 18.- Section 19.- PART I The Form op Minerals -The General Properties of Crystals -The Symmetry of Crystals -The Forms of Crystals -The Symbols of Crystal Faces -The Classification of Crystals -The Orthorhombic System -The Monoclinic System -The Triclinic System -The Tetragonal System -The Hexagonal System -The Isometric System -Twin-crystals . . -Crystalline Aggregates -The Internal Structure of Crystals . -The Growth and Habit of Crystals -The Measurement of Crystals -The Stereographic and other Projections of Crystals -The Determination of the Geometrical Constants Crystals ... -The Representation of Crystals . . PART II The Physical Properties of Minerals of Section 1. — Cleavage Section 2. — Parting . Section 3. — Tenacity Section 4. — Hardness 3 5 10 15 20 27 31 37 39 44 55 63 67 68 74 78 82 88 97 103 105 106 107 XI xu CONTENTS Section 5.- Section 6.- Section 7.- Section 8.- Section 9.- -Etch-figures -Specific Gravity -Luster -Color -Streak PART III The Optical Properties of Minerals Section 1. — The Nature of Light Section 2. — Refraction of Light Section 3. — ^Polarized Light Section 4. — Double Refraction . , Section 5. — The Niool Prism Section 6. — The Polarizing Microscope Section 7. — Interference Colors . Section 8. — Vibration or Extinction Directions Section 9. — Direction of the Faster and Slower Rays Section 10. — Classification of Crystals from an Optical Standpoint Section 11. — Interference Figures Section 12. — Optical Properties of Twin-crystals Section 13. — Absorption and Pleochroism Section 14. — Dispersion Section 15. — Rotary Polarization Section 16. — Optical Anomalies Section 17. — Optical Orientation ... Section 18. — The Optical Constants and Their Determination PART IV The -Chemical Properties of Minerals Introductory Chemical Types . ■Derivation of Chemical Formuto •Polymorphism . . . ■Isomorphism . . . . ... Blowpipe Analysis . . ... Apparatus used in Blowpipe Analysis Reagents used in Blowpipe Analysis ■The Operations of Blowpipe Analysis Section 10. — Select Blowpipe and Wet Tests . . . Section Section Section Section Section Section Section Section Section 9.—' Page 108 109 111 112 112 113 115 119 120 122 123 125 133 134 136 139 145 146 149 151 154 156 158 163 166 169 172 173 175 176 178 181 195 CONTENTS PART V The Determination of Minerals Section Section Section Section Sect lux -Introduction -Crystal Form -Physical Tests -Optical Tests 5. — Blowpipe and Chemical Tests Page 209 209 210 211 214 Table 1. — Minerals arranged according to Crystal System and Habit 215 Table 2. — Minerals arranged according to Structure and Cleavage 216 Table 3. — Minerals arranged according to Color . . 217 Table 4. — Minerals arranged according to Specific Gravity 219 Table 5. — Minerals arranged according to Optical Tests 220 Table fi. — Minerals arranged according to Blowpipe and Chemical Tests . . 222 PART VI The Description of Important Minerals Introductory Section 1. — Elements . Section 2. — Sulfids Section 3. — Sulfo-salts Section 4. — Haloids Section 5. — Oxids Section 6. — Aluminates, Ferrites, etc. Section 7. — Hydroxids Section 8. — Carbonates Section 9. — Phosphates, Arsenates, etc. Section 10. — Nitrates, Borates, and Uranates. Section 11. — Sulfates . . . Section 12. — Tungstates and Molybdates Section 13. — Silicates PART VII The Occurrence, Association, and Origin of Minerals Section 1. — Distribution and Occurrence Section 2. — Association and Order of Succession Section 3. — Origin of Minerals . Section 4. — Paragenetic Varieties 253 257 269 288 297 305 318 324 332 352 364 369 381 384 447 448 449 449 XIV CONTENTS Page Section 5. — Synthesis of Minerals . 450 Section 6. — Alterations, Pseudomorphs, and Replacements 451 Section 7. — Igneous Rooks . . 453 Section 8. — Volcanic Exhalations . . 455 Section 9. — Pegmatites 455 Section 10. — Clastic Rocks 456 Section 11. — Organic Deposits 457 Section 12. — Veins 457 Section 13. — Spring, Lake, and Sea Deposits 459 Section 14. — Metamorphic Rocks 459 PART VIII The Uses op Minerals Table. — Mineral Products of the United States 462 A. Minerals used in the Natural State 463 466 467 468 469 470 470 471 472 ., 472 472 473 473 B. Minerals used for the Extraction of Rfetals and Alloys Section 14. — Aluminum 473 Section 15. — Antimony 474 Section 16. — Bismuth 474 Section 17. — Copper 475 Section 18. — Gold . 476 Section 19. — Iron . 476 Section 20. — Metals used as Alloys with Steel 478 Section 21 . — Lead . 479 Section 22. — Mercury 480 Section 1.- — Precious Stones Section 2.- — Ornamental Stones Section 3.- —Abrasives . . Section 4.- —Refractory Materials . Sbction 5.- —Glass, Porcelain, and Pottery Sbction 6.- — Paints and Graphic Materials Section 7.- —Fertilizers Section 8.- —Fluxes Sbction 9.- —Lubricants Section 10.- -Paper-making Materials Section 11.- -Plaster . Section 12.- —Mica . Section 13.- -Sulfur CONTENTS XV Page Seitkj.v 23. — Platinum . . . , . . . 480 Sectkjn 24.— Silver. . , 481 Section 25. — Tin . 481 Section 20. — Zinc 482 C. Miiicrnh used in the Chemical Industries Section 27. — Aluminum Compounds . 482 Section 28. — Arsenous (J.xid 483 Section 29. — Borax and Boracic Acid 483 Skition 30. — Bromin ... 484 Section 31. — Cobalt Compounds . ... - - 484 Section 32. — Chromium Compounds ... . 484 Section 33.— lodin . , . .484 Section 34.— Lithium Salts ...... 485 Section 35. — Magnesium Compounds ..... 485 Sectio.n 30. — Nitrates - . . 485 Section 37. —I^otassium Salts , , 486 Section 38. — Sodium Salts 486 Sectio.n 39. — Strontium Compounds 487 Section 40. — Sulfuric Acid 487 Section 41. — Thoria 487 Section 42.— Zinc Oxid. 488 Glossary ... 489 Index . ... 513 SELECT BIBLIOCJRAPHY PART I The Form of Minerals Bauermann: Text-book of Systematic Mineralogy. Longmans, Green and Co., London, 1889. Guoth: Physikalische Kryslallographii. Wilhelm Engelmann, Leipzig, 1905 C4th edition). ^ Hilton: Mathematical Crystallography and the Theory of Groups of Movements. Clarendon PresH, Oxford, 1903. Lewis: A Treatise on Crystallography. Cambridge University Press, 1899. Liedisch: Grundriss der Physikalischen Krystallographie. Veit u. Co., Leipzig, 1896. Moses: Characters of Crystals. Van Nostrand, New York, 1899. Reeks: Hints for Crystal Drawing. Longmans, Green and Co., London, 1908. Stoby-Maskelyne: Crystallography. Clarendon Press, Oxford, 1895. Tutton: Crystallography and Practical Crystal Measurement. Macmillan, London, 1911. Williams: Elements of Crystallography. Henry Holt and Co., New York, 1902 (3rd edition). PART II The Physical Properties of Minerals Groth: Physikalische Krystallographie (see above). Liebisch: Grundriss der Physikalischen Krystallographie (see above). Moses: Characters of Crystals (see above). PART III The Optical Properties of Minerals DuPARC ET Pearce; Traits de Technique MinSralogique et Petrographique. (Premiire Partie.) Veit u. Co., Leipzig, 1907. xvii xviii SELECT BIBLIOGRAPHY Groth: Physikalische Krystallographie (see above). Iddings: Rock Minerals. Wiley, New York, 1911 (2d edition). Liebisch: Grundriss der Physikalischen Krystallographie (see above). Luqubr: Minerals in Rock Sections. Van Nostrand, New York, 1908 (3rd edition). Moses: Characters of Crystals (see above). Weinschenk-Clark : Petrographic Methods. McGraw-Hill Book Co., New York, 1912. Winchell AND Winchell: Elements.of Optical Mineralogy. Van Nostrand, New York, 1909. PART IV The Chemical Properties op Minerals Arzeuni: Physikalische Chemie der Krystalle. Vieweg u. Sohn, Braun- schweig, 1893. Brauns: Chemische Mineralogie. Tauchnitz, Leipzig, 1896. Brtjsh-Penfield : Manual of Determinative Mineralogy. Wiley, New York, 1906 (16th edition). Doelter: Physikalisch-Chemische Mineralogie. Joh. Barth, Leipzig, 1905. Groth: Chemische Krystallographie. Engelmann, Leipzig, 1906 (4 vols.). Groth-Marshall : Introduction to Chemical Crystallography. Wiley, New York, 1906. Plattner-Kolbeck: Probierkunst mit dem Lotrohre. Johann Barth, Leipzig, 1907 (7th edition). Prescott and Johnson: Qualitative Chemical Analysis. Van Nostrand, New York, 1908 (6th edition). PART V The Determination of Minerals Brush-Penfield : Manual of Determinative Mineralogy (see above). Kraus and Hunt: Tables for the Determination of Minerals. McGraw-Hill Book Co., New York, 1911. PART VI The Description of Important Minerals Brauns: Das Mineral Reich. Fritz Lehmann, Stuttgart, 1903. Dana: System of Mineralogy. Wiley, New York, 1892 (6th edition). 1st appendix, 1899. 2nd appendix, 1909. SELECT BIBLIOGRAPHY xix fiiioTH: Tabdtdrischi Uebersichl der Mineralien. Vicwcg u. Sohn, Braun- schweig, 18!J8 (4tli edition). Foiite: Cinnplete Mineral Catalog. I'oote Mineral Co., Philadelphia, 1909. Hintze: Ilandbuch dcr Mhuralogie. Veit u. Co., Leipzig, 1897. II Band. (I Band, not complete). Miehs: Mineralogy. Macmillan, London, 1902. Nau.\ian.n'-Ziukel:' Elemerde der Mincraloijie. Engelmann, Leipzig, 1901 (14th edition). RosE.vDOsiH-WiJLPiNo: Mikroskopische Physiographic der pelrographisch wiclitigen M incralina. E. Schweizerbartscho Verlagshandlung, Stuttgart, 1905. PART VII TilE OfTUKRENCE, ASSOCIATION, AND ORIGIN OF MINERALS Heck- Weed: T/ie Nature of Ore Deponits. Mc(iraw-Hill Book Co., New York, Pni (2nd edition). Braunn: Chemische Mineralogie (see above). Cl.\rke: The Data of Geochemistry. U. S. (Geological Survey, Washington, 1908 (Bulletin .330). CIeikie, .Ja.meh: Structural and Field Geology. Oliver and Boyd, Edin- burgh, 1908 (2nd edition). Ke.mp: Handbook of Hocks. Van Nostrand, New York, 1911 (5th edition). Klockmann: Lehrbuch der Mineralogie. Ferd. Enke, Stuttgart, 1907 (4th edition). L.iCKOix: Mineralogie de la France it de ses Colonies. Librairie Poly- technique, Paris, 1893-1910 (4 vols). Meunier: Les Methodes de Synthase en Mineralogie. Librairie Poly- technique, Paris, 1891. Pirsson: Rocks and Rock-Minerals. Wiley, New York, 1908. PART VIII The Uses of Minerals Bauer: Edelsleinkunde, Tauchnitz Leipzig, 1896. BEyscHLAG-KRUSCH-VooT: Die Lagerstdtten der nutzbaren Mineralien und Gesteine. Ferd. Enke, Stuttgart, 1909. Bbannbr: Syllabus of Economic Geology. J. C. Branner, Stanford Univer- sity, Cal., 1911 (3rd edition). Farrington: Gems and Gem Minerals. A. W. Mumford, Chicago, 1903. XX SELECT BIBLIOGRAPHY KuNz: Gems and Precious Stones of North America. Scientific Publishing Co., New York, 1892 (2nd edition). Mbrkill: The Non-Metallic Minerals. Wiley, New York, 1905. Moses and Parsons: Elements of Mineralogy, Crystallography, and Blow- pipe Analysis. Van Nostrand, New York, 1909 (4th edition). Pbnfield: Tables of Minerals. Wiley, New York, 1907 (2nd edition). RiES: Economic Geology. Macmillan, New York, 1910 (3rd edition). The Mineral Industry. Published annually since 1892 by the McGraw-Hill Book Co., New York. The Mineral Resources of the United States. Published annually since 1883 by the U. S. Geological Survey, Washington. INTRODTCTION Minerals are those parts of the earth's crust which have a definite chcinical composition. Such substances as oil, coal, clay, and soil are not considered as minerals, for they do not have a definite chemical composition. Rocks such as granite, lirae.stone, sandstone, etc., are mixtures of minerals though, in a few cu.scH, they are composed of a single mineral. Materials of organic origin such as amber, coal, and pearls are by common (imsent excluded, but petrifactions formed by the replacement of organisms by minerals are included. Aljout a thousand definite kinds of minerals have been dis- covered, though many of them are very rare and are confined to one or two localities. It is with the two hundred more common and important minerals that we are especially concerned. The purpose of this book is to explain and record the promi- nent characters of minerals so that one may learn to recog- nize them by sight or by simple physical and chemical tests and also to discuss their uses and their manner of occurrence. Minerals, like plants and animals, have chaiacteristic forms, though they are not the result of growth in the ordinary sense of the term. Nearly all minerals under favorable conditions are found in characteristic geometric forms called crystals, which are often very regular and beautiful. They have very appro- priately boon called " flownrs of the earth. " Though often seem- ingly unlimited in vaiicty, the crystals of a given mineral are all derivatives of a simple type. Minerals do not, however, always occur in distinct isolated 1 2 INTRODUCTION crystals. They usually consist of a confused aggregate of grains, fibers, or plates. Such an aggregate is said to be crys- talline for the separate particles are crystals in one sense of the word, that is, they have the internal structure of crystals. After all the regular arrangement of its molecules is the characteristic feature of a crystal, the external form being simply an outward expression of the internal structure. In crystals the cohesion between particles varies with the direction and hence minerals usually break more easily in cer- tain directions than in others. This property is known as cleavage and is useful in determining minerals. The subject treating of action of light, especially polarized light, upon transparent crystals constitutes crystal optics, or if limited to minerals, optical mineralogy. The secrets of the rocks revealed by the polarizing microscope are not only wonderful, but are also useful in the determination of minerals. One of the most fundamental properties of minerals is the chemical composition and hence a knowledge of chemistry is in- dispensable in the study of minerals. For the sight recognition of minerals various physical characters are important, but for identification chemical and blowpipe tests are often necessary. The blowpipe, especially, is a great aid in rapid testing. It can be used to advantage because one usually knows what to test for and can readily confirm his sight determinations. The economic side of mineralogy will appeal to many on account of its practical application. The history of mineralogy began with the discovery of metallic ores and other useful minerals and the study of mineralogy as a pure science has been kept alive principally because of the aid it has given in the development of mineral resources. Perhaps the most fascinating branch of mineralogy is that dealing with the occurrence, association, and .origin of minerals. While necessarily a thoroughly scientific subject pursued for its own sake by the specialist, its use in determining the origin of ores will give it a standing which it otherwise might not have. PART I thp: form of minerals 1. THE GENERAL PROPERTIES OF CRYSTALS In beginning the study of crystallography the student's atten- tion may be directed to crystals of the common minerals such as calcite (Figs. 151-162), quartz (Figs. 435-438), pyrite (Figs. 198- 205), gypsum (Figs. 102-105), and orthoclase (Figs. 90-93). Then for the time neglecting how they were formed and what they are composed of, their form or geometrical properties may be considered. Crystals are solids bounded by flat, more or less smooth, sur- faces called faces. Intersections of two faces are called edges, and intersections of three or more faces are called vertices. The number of faces plus the number of vertices is equal to the num- ber of edges plus two. (F-|- V = E-|-2). The faces of crystals vary greatly in number, in shape, and in position. On many crystals it will be noticed that there are several kinds of faces. All the faces of one kind on a crystal constitute a form. For example in Fig. 5, the top and bottom six-sided faces constitute one form, and the six rectangular faces another form. Some crystals have only a single form, but most of them are combinations of two or more forms. It is the great possible number of combinations that gives the variety to crystals, for there is no limit to the number of forms on a crystal. The angles on any crystal are the plane angles of the faces, the interfacial or dihedral angles over the edges, and the solid or poly- hedral angles at the vertices. On account of the diflaculty of ac- curate measurement the plane angles, though characteristic, are 3 INTRODUCTION TO THE STUDY OF MINERALS little used. The measurement of interfacial angles is the start- ing-point in the description and determination of crystals. An interfacial angle (dihedral angle of geometry) is defined by the plane angle formed by cutting a plane normal to the intersection edge of the two faces. It will be noticed that there are two possi- ble angles to measure, an internal and an external or supplement angle. For various reasons the supplement angle is the one used Fig. 1. — Contact goniometer. in crystallography. In a hexagonal prism, for example, the interfacial angle is read 60° instead of 120°. The interfacial angle may be measured approximately by means of a contact goniometer, which, in the simplest type, consists of a semicircular cardboard protractor provided with a celluloid arm (Fig. 1). The plane of the protractor is placed perpendicular to the inter- section edge. One face of the crystal is brought in contact with the arm and the protractor is revolved until the other face is parallel to, but not quite in contact with, it. THE FORM OF MINERALS 5 One of the most striking and important properties of crystals is the recurrence of the faces, edges, and vertices. This property of recurrence is known as symmetry. It varies for different kinds of crystals, and is the basis for the classification of crystals. The arrangement of faces in belts of planes with parallel inter- section edges called zones is another notable feature of crystals. On looking over a number of crystals one might fail to see any order, system, or regularity so great is their variety, and might decide that crystals are fortuitous solids. But such is not the case, for between the faces, angles, and zones of crystals there exi.st exact mathematical relations. Given the angles between a few faces of a crystal, the angles between any two of the many crystal faces possible may be calculated. Crystal faces intersect only at certain definite angles. A facet cut at random on a crys- tal is not a crystal face, for the faces are the result of a definite internal structure. The practical importance of crystallography lies in the fact that a given mineral or artificially prepared compound occurs in crystals characteristic of that substance, and hence the crystal form may often be used in the determination of the substance. 2. THE SYMMETRY OF CRYSTALS The repetition of the faces, interfacial angles, and vertices of crystals in accordance with some fixed law is called symmetry. Symmetry is perhaps the most important property of crystals, for among natural objects it is a property peculiar to crystals, and besides furnishes the basis for the classification of crystals. The symmetry of a crystal may be defined by the operations necessary to bring it into coincidence with its original position. The symmetry operations are rotation about an axis, reflection in a plane, and a combination of rotation with reflection (rota- tory-reflection). If a solid can be revolved about some line through its center so that similar faces recur a certain number of times in a complete 6 INTRODUCTION TO THE STUDY OF MINERALS revolution, that line is called an axis of symmetry (denoted by A). In crystals the period of the axis or the number of times of recur- rence is either two (2), three (3), four (4), or six (6). An axis about which similar faces recur every 180° is said to be a two-fold axis (Aj); every 120°, a three-fold axis (A3); every 90°, a four- fold axis (A^); and every 60°, a six-fold axis (A^). Figs. 2, 3, 4, and 5 illustrate these various axes of symmetry. The vertical FiQ. 3. Fio. 4. Representing axes of symmetry. FiQ. 5. lines through the center are the axes of symmetry. The plane figures above are plans showing the amount of rotation necessary to bring the figures into self-coincidence. A plane that divides a solid into two parts so that similar faces occur on opposite sides of the plane is called a plane 'of symmetry (denoted by P) . Fig. 6 represents an orthoclase crystal in which the shaded area is a plane of symmetry. The faces are either perpendicular to this plane, or occur in pairs, one on each side of it. A crystal may have several planes of symmetry. A cube, for example, has nine, three perpendicular to the faces and six through the edges. THE FORM OF MINERALS 7 The recurrence of .similar faces hy rotation about an axis com- bined with ref]ection in a plane is called composite symmetry (denoted by 19). The period of the axis may be 2, 4, or 6, but never 3. The particular operation denoted by IP^ is called inversion. As may be seen from Fig. 7 the front part of the crys- tal if revolved 180° becomes the reflection of the rear part (dotted lines). A solid which is brought into self-coincidence by inver- sion is said to have a center of symmetry (denoted by C) , for a line drawn from any point through the center encounters an exactly similar point on the opposite side. Fig. 7 represents a crystal Fig. 6. — Plane of symmetry. FiQ. 7. — Center of symmetry. of axinite with a center of symmetry. Every face has a similar parallel face on the opposite side of the crystal. This is the easiest test for a center of symmetry. In Fig. 8 the vertical line is an axis of four-fold composite symmetry, for the upper part of the crystal revolved 90° is a reflection of the lower part. Simi- larly the vertical line in Fig. 9 is an axis of six-fold composite symmetry. Axis, plane, and composite axis with plane are collectively known as elements of symmetry. In crystals the elements of symmetry are combined in various ways. With the limitation imposed by the rationality of the indices, or with the assumption of a crystal structure made up of particles at small, finite dis- INTRODUCTION TO THE STUDY OF MINERALS -tances apart only axes of 2-, 3-, 4-, and 6-fold symmetry are pos- sible, and in fact no other axes of symmetry have ever been found in crystals. Various methods of combining the axes of symmetry with each other, and with planes of symmetry, lead to the result that only thirty-two kinds of symmetry are possible among crystals. These thirty-two combinations of symmetry elements constitute the thirty-two crystal classes. In the above discussion the term similar faces has been used so often that an explanation is necessary. By similar faces are Fig. 8 — J?^. Fig. 9.— ^„. meant faces which are more or less alike in shape, size, and ap- pearance. On crystals which have been formed quietly without disturbing influences similar faces have the same size and shape. But on account of various external influences similar faces are rarely exactly of the same size and shape. The effect of external influences may be illustrated by alum which crystallizes in octa- hedrons. Alum crystallizing on the bottom of a beaker will form in more or less flattened octahedrons like Fig. 11, while if it crystallizes about a weighted string suspended in the solution, the crystals will be more or less perfect octahedrons like Fig. 10. THE FORM OF MINERALS The irregularity in tlic size and shape of similar faces is one difficulty in the study of crystals. While the faces may vary the angles are quite constant, as is expressed in the law of constancy Fio. 10. Fio.— 11. Alum crystals. of interfacial angles: "In all crystals of the same substance the angles between corresponding faces are constant." So that for many crystals the interfacial angles must be measured in order to determine the symmetry. Thus the crystals represented in cross-section by Figs. 12, 1.3, 14 are bounded by hexagonal prisms £0/ v^O' 60, — ^60 60/ \6U and have an axis of six-fold symmetry if the interfacial angles are all WP On the other hand, the crystal represented in cross- section by Fig. 15, though apparently a hexagonal prism, is a combination of two forms (a rhombic prism and a pinacoid) , and 10 INTRODUCTION TO THE STUDY OF MINERALS has an axis of two-fold symmetry because the interfacial angles are 62° and 56°. Another property of crystals used in determining symmetry is the physical character of the faces. So that crystallography is by no means merely a branch of geometry. Similar faces are those with the same luster, the same kind of striations, pits, or other markings. Geometrically a cube has nine planes of sym- metry, three parallel to the cube faces, and six through the cube edges, but a cube of pyrite with striations like those of Fig. 16 has only three planes (those parallel to the cube faces). A crys- FiG. 17. Sphalerite. FiQ. 18. Apophyllite. tal of sphalerite represented by Fig. 17 is geometrically an octa- hedron, but crystallographically it is a combination of two tetra- hedrons, one with smooth faces, the other with striated faces. Fig. 18 illustrates another good example of this kind. Apophyl- lite occurs in crystals which are apparently cubes modified by the octahedron. Close examination, however, shows that the side faces are striated and have a vitreous luster, while the top and bottom faces are smooth and have a pearly luster. The forms, then, are a pinacoid and square prism instead of a cube. The apparent octahedron is in reality a tetragonal bipyramid or a double-ended square pyramid. 3. THE FORMS OF CRYSTALS The similar faces of a crystal constitute a form, form being used here in a special technical sense. Similarity of faces on many crystals may be observed at a glance, but for others not only THE FORM OF MINERALS 11 careful examination and measurement, but also etching with some solvent is necessary. Similar faces will have the same kind of etch-figure.s (see page 108). There are many kinds of forms. The most logical method is to name the forms according to their shape, regardless of their position on the crystal. A single face is a pedion (Fig. 19). Two Fro 19.— Pedion. Fio 20. — Pinaooid. Via. 21.— Dome. FiQ 22.— Sphenoid. Fio. 23. Ditrigonal prism. Fig. 24. Di tetragonal prism. Fio. 25. Dihexagonal prism. parallel faces constitute a pinacoid (from the Greek word for a board). Fig. 20. Two non-parallel faces are called a dome (Latin word for house) Fig. 21, if astride a plane of symmetry, but a sphenoid (Greek word for wedge). Fig. 22, if not astride a plane of symmetry. Ditrigonal. r.n. Pigs. 26-39.— THE FORM OF MINERALS 13 Next we have three, four, six, eight, or twelve similar faces in one zone. These are called prisms and are distinguished according to their cross-section as trigonal (Fig. 2), rhombic (Fig. 3), tetrag- onal (Fig. 4), hexagonal (Fig. 5), ditrigonal (Fig. 23), ditetrag- onal (Fig. 24), and dihexagonal (Fig. 25). Pyramids are similar Fig. 40. Fig. 41. Fig. 42, faces intersecting in a point. They are defined by the shape of the cross-section just as the prisms are. See Figs. 26 to 29, and 34 to 36. Bipyramids, geometrically considered, are two pyramids placed end to end. They are defined by cross-section just as prisms and pyramids are. See Figs. 30 to 33 and 37 to 39. Trapezohedrons are double-ended forms with the symmetry A^-nA^. They are distinguished as trigonal (Fig. 40), tetragonal (Fig. 41), or hexagonal (Fig. 42), according to the cross-section. Bisphenoids are forms consisting apparently of two sphenoids placed together sym- metrically. They are called rhom- bic (Fig. 48), or tetragonal (Fig. 8), according to cross-section. A rhombohedron is a form consisting of six rhombic faces, three at each end of a six-fold axis of composite symmetry (Fig. 9). It is like a cube distorted along one of its diagonals. Scalenohed- rons are double-ended forms consisting of scalene triangular Fig. 43. Fig. 44. 14 INTRODUCTION TO THE STUDY OF MINERALS faces meeting in zigzag edges- They are distinguished by their cross-section as ditrigonal (fig. 43), and ditetragonal (Fig. 44). There are fifteen more kinds of forms which are ;-estricted to the isometric system. Some of these such as cube, octahedron, and tetrahedron are simple, but as most of them are complicated, their description is deferred until the isometric system is taken up- Of the above mentioned forms, the pyramids, prisms, pina- coid, dome, sphenoid, and pedion cannot occur by themselves; and for that reason are called open forms. All the others are called closed forms for they enclose space of themselves. Fig. 45. Fig. 46. Fig. 47. Fig. 48. Congruent tetrahedra. Enantiomorphoug rhombic bisphenoids. Two forms are said to be congruent if one of them may be made coincident with the other by rotation. For example, the tetrahedra of Figs. 45, and 46 are congruent. Two forms are said to be enantiomorphous if they are non-superposable and the mirror-image of each other. The rhombic bisphenoids of Figs. 47 and 48 are enantiomorphous. Two forms are said to be com- plementary when their combination is geometrically indistin- guishable from another kind of form. For example, the two tetrahedra of Fig. 17 are complementary, for geometrically they form an octahedron. Another method of naming forms that is in general use is based upon the position of faces with respect to the axes of ref- erence iq-v.). Thus a pinacoid is defined as a form that cuts one THE FORM OF MINERALS 15 axis and is parallel to the other two. A prism is defined as a form that is parallel to the vertical axis and cuts the other two. A form that is parallel to one of the lateral axes and cuts the other two is a dome. A form that cuts all three axes is in general called a pyramid. A pyramid developed at only one end of the vertical axes is known as a hemimorphic pyramid. In the mono- clinic and triclinia systems the names of forms are based upon the analogy of these systems with that of the orthorhombic. For example, in the monoclinic system {hkl\ is called a hemi-pyramid because there are one-half as many faces as in the corresponding form of the orthorhombic system while [hkl] in the triclinic system is a tetarto-pyramid, there being but two faces instead of eight. In the monoclinic system {hOl} is a hemi-dome because {hOl} in the orthorhomljic system is a dome. In the triclinic system {hkO} is a hemi-prism consisting of two opposite parallel faces instead of the four faces of the prism [hkO] of the orthorhombic system. But as said before the logical names of forms are based upon their shape and not upon their position on the crystal. 4. THE SYMBOLS OF CRYSTAL FACES Crystal measurement proves that exact mathematical relations exist between crystal faces. To make use of this fact the posi- tion of crystal faces is defined by the method of analytic geome- try, which consists in referring them to three (in one case, four) suitably chosen coordinate axes passing through the center of the crystal. These axes are called axes of reference to distinguish them from axes of symmetry. The selection of these axes is morfe or less arbitrary, but they are chosen so as to yield the simplest relations possible. They are therefore lines parallel to prominent edges, which are usually either axes of symmetry or normals to planes of symmetry. Any face may be defined by its intercepts on the axes of refer- ence. In Fig. 49 the axes are the dot-and-dash lines OX, OY, and OZ intersecting at the origin, 0. The intercepts of the plane 16 INTRODUCTION TO THE STUDY OP MINERALS ABC (extension of the face m) are OA, OB, and OC. The inter- cepts of the plane HKL (extension of the face n) are OH,' OK, and OL. Now it has been found that the ratios OA: OH, OB: OK, and OC : OL on any one crystal are usually simple rational numbers such as 1:3, 1:2, 1:1, 2:1, 4:1, etc., while the ratios OA:OB:OC and OH: OK:OL are, in general, irrational. InFig. 49, the •-Y ratios OA:OH = 1:2, OB: OK = l:l,OC:OL = 3:2. In the case of the mineral barite the rela- tive intercepts of some of the faces are as follows (the letters referring to Fig. 50) : Fia 49. Intercepts Weiss symbols Miller indices m 0.815: l:oo a: 6:aoc 110 a 0.815: 00:00 a: c/j6: ooc 100 u 0.815: 00:1.313 a: oo6: c 101 d 0.815: oo: 0.656 o: oo6: J^c 102 I 0.816: 00:0.328 a: oob: \c 104 c 00 : 00:1.313 coa: oo6:c 001 00 : 1:1.313 ooa: h:c Oil y 0.815:0.5:0.656 a:\ h:\c 122 z 0.815: 1:1.313 a: b:c 111 The expressions for the intercepts are cumbersome. A simple method of notation is suggested by the fact that these values for different faces are in the ratio of simple rational numbers. We may select the expression for one of these faces THE FORM OF MINERALS 17 Fig. 50. — Barite crystal. as a standard and represent the other faces by the simple num- bers. .In barite the face z with the intercepts 0.815 : 1 : 1.313 has been taken as the unit face. This establishes the axial ratio as a:6:c = 0.815:l:1.313. The symbols for the other faces may be written as in the second column. These are called the Weiss symbols from the name of the German crystallographer who devised them (1818). As the order a, b, c is always un- derstood these letters may be omitted and as infinity gives trouble in mathematical calculations, the reciprocal values of the ratios may be used. We then have the symbols of the third column. If OA : OB : OC are the intercepts of a unit face the sym- bol of another face with the intercepts OH:OK:OL is hkl in the . OA OB OC expression qjj:-^j^:q^ = h:k:l. The three simplest whole numbers that ex- press this ratio are called the Miller indices, as Miller, formerly professor of mineralogy at the University of Cambridge, was the first to make extensive use of this method. The symbol hkl is an algebraic expression standing for certain num- bers and so is called a type symbol. Besides a face that cuts all three axes of reference we have the faces hkO, hOl, and Okl each of which cuts two axes and is parallel to the third and hOO (100), 0/eO(010), and OOZ(OOl) each of which cuts one axis and is 2 Fio. 51. — The seven type faces. 18 INTRODUCTION TO THE STUDY OF MINERALS parallel to the other two. These constitute the seven so-called type symbols and are represented in Fig. 51. (Fig. 557, page 413, represents an olivine crystal with seven actual type forms.) As in analytic geometry the front, right, and top ends of the axes are considered as positive, whUe the back, left, and bottom ends are considered as negative. A negative index is indicated by a line over the letter. There are eight planes which cut the axes at the same relative distances, but in different octants. They are hkl, hkl, hkl, hkl, hkl, hkl, hkl, and hkl. These symbols as just written are face-symbols. The symbol of one face hkl may be taken to repre- sent the form. The form- symbol is often written with brackets {hkl} to distinguish it from a face- symbol hkl or (hkl). In order to determine the type symbol it is only necessary to write the indices h, k, I, in the order of the axes a, b, c, substi- tuting when the face is parallel to an axis. In writing type symbols the reciprocal idea may be disregarded. The determination of the symbol involves calculation by means of trigonometry, or the corresponding graphical solutions. A simple case is illustrated by Fig. 52. Here we have a rectangular zone of (hkO) faces, a, m, r, b, where a is (100) and b is (010), the axes of reference being parallel to these two faces. Assuming m to be (110) the problem is to determine the symbol of r. Move r parallel to itself until r and m intersect the a-axis at a common point. Then the intercept of r on the 6-axis, it may be seen, is one-third of that of m. The intercepts of the r-face are la:^b:aoc or ^a-.^b:^. The Miller indices are (130) (read one, three, zero). Fig. 52. — Giaphic determination of indices. THE FORM OF MINERALS 19 The law of the lutionality of the indices, which has been cstal)li,shc'(l by the nicasurcincnt of thousands of crystals, is the foundation of geometrical crystallography. The axial ratio for a given substance having once been established by a unit face, all the other possible faces may be predicted, for their interfacial angles can be calculated by the formula of plane and spherical trigonometry. ^^'l)y the symbol (111), in tlie case of barite for example, does not represent a face that cuts the three axes at equal lengths is one of the most difficult points for the student of crystallography to comprehend. Several illustrations may clear up this point. Take two cities laid out according to different plans. Say in one, the blocks are 47.j feet long and 325 feet wide, and in the other 650 feet long and 300 feet wide. A pedestrian on inquiring about a certain building in each place might be directed to go two blocks north and three blocks east. Yet the actual distance for him to walk in the two cities would be different for the lengths of the blocks are different. The lengths of the blocks are on record in the city engineer's office, but the pedestrian is not directly con- cerned with them, but with the directions given him. The axial ratios for crystals are established and on record in reference books, but in the description of the various crystal faces and forms use is always made of Miller indices or other symbols rather than of the actual intercepts of the faces. Another analogous case that will appeal to the student of chem- istry is the law of definite proportions and the law of multiple proportions. In the chemical formulae, CuO and CujO, CuO means that there are 63.6 parts (by weight) of copper and 16 parts of oxygen, while Cu^O means that there are 127.2 (2x63.6) parts of copper and 16 parts of oxygen. The atomic weights have been determined and are given in tables, but they are not expressed in chemical formulae. In order that the symbols may be as simple as possible it has been found convenient to have six kinds of axes of reference, to some one of which the crystals of every known substance are 20 INTRODUCTION TO THE STUDY OF MINERALS referred. The axes of reference differ in relative length and also in their inclinations to each other. In one case four axes are used instead of three. The kinds of axes of reference serve as a basis for the crystal systems, which are defined in the next section. 5. THE CLASSIFICATION OF CRYSTALS The modern classification of crystals is based upon symmetry. Only axes of 2-fold, 3-fold, 4-fold, and 6-fold symmetry have ever been found on crystals. With this limitation it may be demon- strated that only thirty-two combinations of symmetry elements are possible. A rigid mathematical proof will not be given here, but the following table shows the relation between the various combinations. A single axis alone (singular axis) gives the five classes of row 1. For the sake of completeness the axis of one- fold symmetry (AJ is included. 1 A^ A, A3 A. Aa 2 (A,-1A,) A^-SAj A,-4A2 A,-6A, 3 A,P A,P A3P A.P A,-P 4 (A,- IP) A2-2E Aj-SP Aj-4P A,-6P 5 (A,-A,-2P) A2-2A,-3P Aj-SAjtf A,-4A,-5P A;-6A,-7P 6 JP, ^?4 ^,. 7 .•P,-2A,-2P JP^-3A^-3P If this axis has combined with it an axis of 2-fold symmetry, there must be as many of these axes as the period of the singular axis indicates. We then have the five classes of row 2, but Aj- 1 Aj is the same as Aj. The singular axis of row 1 may have a THE FORM OF MINERALS 21 plane of symmetry perpendicular to it wliicli gives the five classes of r(jw 3. If tlie planes of symmetry are through the singular axis tlieie must be n of them for A^. Thus we have row -1, but AflP is the same as Aj-P. Combining the first four rows in various ways we have row 5. Rows 6 and 7 represent axes of comiKisitc syinmctr)- combined in various ways. It will be noted that there is no composite axis of 3-foId (or one-fold) symmetry. A; of low 1 is really no symmetry at all while Ai'l' is simply a jjlane of symmetry by itself. By combining several axes with ]jeriods fi;i-eater than 2 we have five more classes: 3A2'4A3; :5A,-4A3-3A2; ^A^^Aj-.SP; 3A2-4A3-6P and eA^^Aj-SA.-QP. These constitute the thirty-two crystal classes. Examples of all of these but one (Aj'F) have been found either among minerals, or compounds made in the laboratory. It is interesting to note that just as Mendel6ef, the Russian chemist, predicted the existence and even the properties of several chemical elements by the discovery of the periodic law, so Hessel, a German mathe- matician, in 1830 predicted the thirty-two crystal classes when representatives of only seventeen of them were known. The table on pages 22-3 gives the name of the class, the number of faces in the general form, the symmetry, and typical examples of mincials arjd ))i-ci)ar(;d compounds. The name of the form with the symliol \hld\ or [hkil], or the general form as it is called, gives the name to the class. In contradistinction, the other forms are called limit forms. This term may be explained by considering a pyramidal face {hid). By increasing its inter- cej)t on the vertical axes it becomes steeper and steeper, its limit in this case being the prism (MO). By decreasing its intercept on the vertical axis it becomes less steep, its limit in this case bcinfr the pinacoid (001). The forms corresponding to the type symbols in any class may be found from the symmetry by a graphical method. Indicate the a- and /;-axcs by two dotted lines at right angles, their inter- section being the projection of the c-axis. Then indicate the symmetry elements in their proper positions by the following INTRODUCTION TO THE STUDY OF MINERALS £ ft £-« Si o ■* o O f W'o' w o CD owl o9>o q_ td t-'^ o td c3 ■a fi 2 M ■a .2 ^ g S c3 ■a 1^ ^6 1 03 1 a >> o 12; g CO "', o !2; y. y-> O M) CI « /3 rfj « 0? e^ C3 ^ O ^ .1l- "tr! ^ a ii 'S ^ e ?, -t^ &, s o d '^ a; 0) -11 ^ o -< p^ ^„ > ■; ^. „ -i1 Ph P^ 't^ CO CO ^ ^. -51 ^; to 9, •" trl '~^ ^ m o 1^ c ® -<^ -* -^ '^ -^ CO 3 «l M •„ rt C-) N C-l 1< c! a o 11 K ^ ■^ J3 ■3 '■^ N '-1 a) f- n. f^ &- PH iJ '^ -a -S "3 g a o =3 _^ a a O G SB tfj .. -C , -*j -fj -t- * - - o o §) gi fc/j bC bC S S O I- Vj Oi O "-< CI CO -t »0 CO t^ g o -S! o H o o w w 'o 03 00 05 O l-H (N N (N CO CO M ou^aniosj 24 INTRODUCTION TO THE STUDY OF MINERALS conventions. A full line represents a plane of symmetry. If the plane of the paper is a plane of symmetry draw a heavy circle of convenient diameter. Denote axes of 2-, 3-, 4-, and 6-fold symmetry by small ellipses, triangles, squares, and hexagons. As an example let it be required to find the forms represented by the type symbols of the rhombic pyramidal class with the sym- metry, A2'2P. In Fig. 53 the two planes of symmetry are repre- sented by two full lines which coincide with the projection of the axes, a and h. Their \ T ^ intersection is the axis X of two-fold symmetry. Projections of the faces in the upper octants are -^ ^"ooi ^l ^^^^ small crosses. Faces parallel to the vertical axis may be indicated 5J« ^0? by arrows. The general T ^hhd form of this class is a rhombic pyramid, for riG.53.-GrapMcai^methodrf^deterHuningthe ^^^ symmetry requires {hkl), (hkl), and (hkl) to accompany (hkl). For a face (Okl), the symmetry requires (Okl); the form, then, is a dome. Similarly {hOl] is a dome; {M-0} is a rhombic prism; {100} and {010} are each pinacoids, while {001} is a pedion consisting of a single face. In the lower half of the crystal {hkl\ is a rhombic pyramid; {Okl} and [hOl] , domes; while {001} is a pedion. The forms on the lower half of the crystal in this case are independent of those on the upper half. Certain classes have properties in common and may be assembled in larger groups called systems. The first two classes have no directions fixed by symmetry, class 1 having no sym- metry at all, while in class 2 any direction is an axis of 2-fold composite symmetry. These two classes constitute the tri- clinic system. Classes 3, 4, and 5 each have one direction fixed by symmetry, either an A^ or a line normal to P. These three ./ THE FORM OF MINERALS 25 classcH fcinKtitutc the monoclinic system. Classes 6, 7, and 8 have tliiee directions at lisht angles which are fixed by sym- metry. They constitute the orthorhombic system. Classes 9 to 15 have one direction of 4-fold symmetry (including two classes with composite symmetry) and constitute the tetragonal system. Classes 16 to 27 have one direction of either 3-fold or 6-fold symmetry and constitute the hexagonal system. The isometric system consists of five classes each with four directions of 3-fold symnietrj-. Except for the isometric system these fixed directions are axes of reference. The axes of reference for the various systems are as follows: Triclinic. Three non-interchangeable axes at oblique angles. FIk- 108, page 38. Monoclinic. Three non-interchangeable axes, two at oblique angles, the third at right angles to the plane of these two. Fig. S2, page 32. Orthorhombic. Three non-interchangeable axes at right angles. Fig. 62, page 27. Tetragonal. Three axes at right angles, two of which are interchangeable. Fig. Ill, page 39. Isometric. Three interchangeable axes at right angles. Fig. 17."j, page 55. Hexagonal. Four axes, one at right angles to three inter- changeable ones which are in one plane and intersect each other at angles of 120°. Fi^. V.',2, page 44. The forms of the crystal class with the highest grade of symmetry in each system are sometimes called holohedral or whole forms, while many of the forms of the classes of lower grade of symmetry are called hemi- hedral or half forms, because they have half the number of faces of the holohedral forms. ^^^ ssa.— Tetrahe- There is a geometrical resc;mblance between dron derived from the these two kinds of forms. A tetrahedron is the hemihedral form of an octahedron, for it may be 26 INTRODUCTION TO THE STUDY OF MINERALS derived by extending alternate faces and suppressing the others as in Fig. 53a. A cube has no hemihedral form, or rather the hemihedral and holohedral cubes are geometrically identical, for the suppression of alternate octants still leaves the cube (see Figs. 58-61). The symmetrical suppression of the faces of the general forms of the six holohedral classes gives rise to twenty-six divisions. These together with the six holohedral divisions lead to the thirty-two classes before mentioned. The general forms of the five isometric classes may be derived from the hexoctahedron 1 w 55 / B 58 59 60 Pigs. 5t-61. — ^To illustrate hemihedrlsm. 61 thus: The suppression of faces of alternate octants (Fig. 56) gives the hextetrahedron, the suppression of alternate faces (Fig. 56) gives the gyroid, the suppression of faces in pairs astride the planes of symmetry (Fig. 54) gives the diploid, while the com- bination of any two of these methods (Fig. 57) gives a twelve- sided figure called the tetartoid. As this form has one-fourth the number of faces of the hexoctahedron it is called a tetartohedral or quarter form. Figs. 58 to 61 show that the cube is common to all five isometric classes. As hemihedrism is due to the symmetrical suppression of faces it is mainly a question of terms. In most of the modern works on crystallography hemihedrism is not considered, for there is no THE FORM OF MINERALS 27 structural connection between the holohedral and hemihedral forms. The holohedral classes are sometimes called normal groujjs, but they are really no more normal than the classes with lower symmetry. 6. THE ORTHORHOMBIC SYSTEM The orthorhombic system includes all crystals that can be referred to three non-interchangeable axes of reference at right angles to each other. The axial elements are a:b:c. Conven- tionally b is unity and a is always less than unity. These values -^-^- Fio. 62. Axes for topaz. Fio. 63. Axes for barite. Fio. 64. Axes for cerussite. differ for every orthorhombic substance. Figs. 62, 63, and 64 represent the unit lengths of the axes for topaz, barite, and cerussite respectively. Rhombic Bipyramidal Class. sAj-aP (C) {Holohedral) The three axes of 2-fold symmetry are the axes of reference. The selection of the c-axis is arbitrary, but of the other two a is always shorter than h. 28 INTRODUCTION TO THE STUDY OF MINERALS Rhombic bipyramid 8 faces jfaklj (Rhombic pyramid)' Rhombic prism 4 faces jOkl) (Brachydome) Rhombic prism 4 faces IhOU (Macrodome) Rhombic prism 4 faces |hkO! (Rhombic prism) Pinacoid 2 faces ilOOi (Macropinacoid) Pinacoid 2 faces 10101 ( Brachypinacoid) Pinacoid 2 faces |001| (Basal pinacoid) 65 hkl Figs. 65-71. — The seven orthortiombic type forms. Rhombic Pyramid {hkl} (Rhombic pyramid). The general form of this class consists of eight faces. In the ideal form the faces are scalene triangles (Fig. 65). For any one substance there is a great variety of forms possible depending upon various rational values of h, k, and I. If h and k are equal we have {hhl} of which there is a series with varying values of I. As these forms are in a vertical zone with the unit prism {110}, they are called bipyramids of the unit-series. {111} is the unit bipyramid. Rhombic Prism {Okl} (Brachydome). A horizontal open form composed of four faces each parallel to the a-axis (Fig. 66). There is a series of all possible rational values of k and I. Rhombic Prism {hOl} (Macrodome). A horizontal open form composed of four faces each parallel to the 6-axis (Fig. 67). There is also a series varying from {001 } to {010} for all possible values of h and I. Rhombic Prism {hkO} (Rhombic prism). An open form consisting of four vertical faces (Fig. 68). For each orthorhom- 1 These names are used by some authors. THE FOliM OF MINFUAL.S 29 hie iiiiiicial a wlicile scries of prisms is possible ranging from 1010) to {100|. The unit prism is jllOj. Pinacoid {100| (Miu/rojjinacoid). This form may be called {\\c front pinacoid as it consists of two opposite parallel faces, one in front and one behind (Fig. 69). As there is only one pinacoid of this kind possible, the symbol {100} is used instead of j/iOO}. Pinacoid {010} (Brachypinacoid). This form, consisting of two parallel faces, one on the right and one on the left, may be calle:l:1.3\S. Usual forms: c{001), m{110}, o{011}, d{102j, Z{104}. (IleuvuKO parallel to c and m. Inter- facial angles: mm(110: iTO) =78° 22^'; cto(001 : 110) =90°; co(001 :011) =52-^ 43'; cd(00 1:102) =38° 51'; cZ(001 : 104) =21° 56'. -axi8 (often called the ortho-axis), are independent. Fig. 85 rejjresents [hQl]. Pinacoid {lOOj (Orthopinacoid). This form may be called the front pinacoid, but is also known as the orthopinacoid. Fig. 87. S3{hkl\. 84|0/c;|. S5{IMI\ 8a{hlc0\ 87il00|. 88J010J. SOlOOli. Figs, 83-89. — The seven monoclinic type forms. Pinacoid jOlOj (Clinopinacoid). This* may be called the side-pinacoid, but is also known as the clinopinacoid. Fig. 88. Pinacoid {001} (Basal pinacoid). This form is usually known as the basal pinacoid, but its faces are inclined and not perpen- dicular to the c-axis. Fig. 89. Combinations. All monoclinic crystals are necessarily com- binations of two 01- more forms, as all the forms are open ones. As in the orthorhombic system the habits are quite diversified. If the angle /? is close to 90° there is often close resemblance to orthorhombic crystals, but this result may also be due to equal development of front and back faces. Prismatic crystals are usually elongated in the direction of the c-axis, but occasionally in the direction of the fc-axis, as in the case of epidote, and in the direction of the a-axis, as in orthoclase. .3 34 INTRODUCTION TO THE STUDY OF MINERALS Examples Many minerals and also artificially prepared compounds crystal- lize in this class. Ortlioclase (KAlSijOg), diopside (CaMgSijOj), augite (R"Si03), and gypsum (CaS04.2H20) are given as good examples for study and practice. Orthoclase. a:6:c = 0.658:l:0.555; ^ = 63° 57'. _ Usual forms: c{001}, 6{010}, m{110}, 2{130}, a;{T01}, t/{201}, n{021}, o{lll}. Cleavage parallel to c and b. Interfacial angles: Fig. 90. Fig. 91. Fig. 92. Figs. 90-93. — Orthoclase crystals. Fig. 93. mm(110:lT0)=61° 13'; 6z(010: 130) =29° 24'; ca;(001 :102) = 50° 16'; c!/(001 :201) =*:80° 18'; cn(001 :021) =44° 56'; 6o(010: 111) =63° 8'; 6c(010:001)=90° 0'; c?w(001:110) =67° 47'. Figs. 90-93 represent usual types of crystals. Diopside. a:&:c = 1.092:l:0.589; /3 = 74° 10'. Usual forms: c{001}, 6{010], aflOO}, m{110}, p{lll), oj221}, rf{T01), //{311}, s{lll}. Interfacial angles: mm(110: 110) =92° 50'; ab(100:010)=90° 0'; oc (00 1 : 001) =74° 10'; 6c(010:00_l) =90° 0'; pp(lll:lll)=48^ 29'; cp(001 : Ul) =33° 50'; ss(lll:lll) = 59° 11'; 00(221:221) =84° 11'; JJ(311:311) =37° 50'; cd(001: 101) = 31° 20'. Figs. 94—97 represent typical crystals of diopside. The striations on Fig. 94 are due to polysynthetic twinning with (001} as twin-plane. Augite. Axial elements, usual forms, and interfacial angles practically as in diopside. Figs. 98 to 101 represent the common THE FORM OF MINERALS 35 Fig. 94. P P IP V Fio. 95. Fig. 96. Figs. 94-97. — Diopsidc crystals. m Fig. 97. Fig. 98, a m.. Fig. 99. Fio. 100. Figs. 98-101. — Augito crystals. ,.i' •ii Fic. 101. Fio. 102. Fig. 103. Fio. 104. FiGB. 102-105. — Gypsum crystals. Fig' 105 36 INTRODUCTION TO THE STUDY OF MINERALS types of augite crystals. Twins with { 100} as twin-plane are common as shown in Fig. 101. Gypsum. a:6:c = 0.690;_l:0.412; ^9 = 80° 42'. Usual forms: mfllO}, Zjlll}, 6{010|, e{103}. Cleavage paralleUo &. Inter- facial angles: mm(110: iTO) =68°_30^; ZZ(lll:lTl) =36° 12'; &e(010:T03) =90° 0'; ae(edge iTO, 110:103) =87° 49'. The usual combination is hml, but with varying habit as represented in Figs. 102 and 103. Fig. 105 represents a twin crystal with {100} as twin-plane. DoMATic Class. P. {HemihedraT) The 6-axis is normal to the plane of symmetry, the a and c axes being in the plane of symmetry. Domes {hkl\, {hkl\, [hkl\, {hkl\ Domes {Qkl\, jOM} Pinacoid _ ._ {010} Pedions {100}, (TOO); {0011, {OOl}; \mi\, [hJOl]. A c / j \ 1 C' \m nix . )) f\ • / \ r \^« a v^ \y Fig. 106.— KaSiOe. Fig. 107. — Tartaric add. Clinohedrite, CaZn(OH)2Si03, is the only mineral known to belong to this class. There are .several representatives among artificially prepared compounds, among them potassium tetra- thionate, KjS^Oj, a crystal of which is represented by Fig. 106. rilE FORM OF MINERALS 37 Hl'ilKNOIDAL ("J>\SS. Aj {Hem iinorphic) The uxis of 2-fold symmetry is the b-a,xiB, the a and c axes being in a plane normal to this axis. Sphenoids {hkl\, \hkl\, \hTu\, \hTd\; \Okl\, \Okl\; \hkQ\, \hTcQ\ Pinacoids \mi\, \Kol\; jlOO), lOOlj Pedions jOlOj, iOTOi Fichtelite, CigH32i belongs to this class. I'ig. 107 represents a I lystal of tartaric acid, which also belongs here. Cane sugar is anotlier (■xamijlc. 8. THE TRICLINIC SYSTEM 'i'he triclinic .system includes all crystals that can be referred to three non-inlcrchangeable axes at oblique angles. The axial clc-rncnts are a:h:c, {h being unity, and a always less than unity) and the angles a, [i, and y between the axes h and c, a and c, a and h respectively. Fig. 108 repicsents a triclinic axial cross. Pinacoidal Class. IP^iC) (Holohe/lral) The choice of axes is arbitrary, but they are usually taken parallel to the intersection edges of the three most prominent faces. Pinacoids 2 faces |hkl!, IhklJ, IhElj, lEElj Pinacoids 2 faces |h01|, ihOl} Pinacoids 2 faces lOkli, {Okl{ Pinacoids 2 faces {bkOj,{bEOi Pinacoid 2 faces jlOOi Pinacoid 2 faces lOlOJ Pinacoid 2 faces !001 1 (Tetarto-pyramids) (Hemi-macrodomes) ( Hemi-brachydomes) (Hemi-prisms) (Macropinacoid) ( Brachypinacoid) (Basal pinacoid) All forms are jnnacoids consisting of two opposite parallel faces. 38 INTRODUCTION TO THE STUDY OF MINERALS Combinations. The appearance of triclinic crystals depends largely upon the obliquity of the axes. Many of them closely approach monoclinic crystals in angles. (See Fig. 534, page 389.) This is especially the case with the plagioclase feldspars. In albite, for example, a = 94° and ;- = 88°. '7- 1 ^ /^^ a a Fig. 108. — Triclinic axes. Fig. 109. — Axinite. Examples Comparatively few minerals crystallize in this class. Axinite, though without much variation in habit, is the most available triclinic mineral for study. Axinite. a :b:c = 0.492: 1:0.479; a = 82° 54', _^ = 91° 52', ;-= 131° 32'. Usual forms: a{100}, m{110}, M{_110}, s{201), p{lll}, r{lll}. Interfacial angles: mM(110: 110) =44°_29'; ms(110:201)=27° 57'; sr(201:lll) =36° 25'; Mr(lTO:lTl) = 45° 15'; sp(201 : 111) = 16° 7'. Fig. 109 is a drawing of a typical crystal of axinite. Pedial Class. (No symmetry) (Hemihedral) Although there are no symmetry elements whatever, this con- stitutes a crystal class, for crystals without symmetry may have faces with rational indices. The rationality of indices is entirely independent of symmetry. THE FORM OF MINERALS 39 In this class all forms are pedions consisting of one face only. Two parallel faces, even if they have about the same size and shape, are separate forms. There are twenty-six separate kinds of forms possible, using the sign permutations with the seven type symbols. No mineral representative of this class has yet been discovered, but several artificially prepared compounds belong here. Fig. 110 represents a crystal of hydrous acid strontium tartrate with the forms: c{001J, c'{OOT}, 6{010), 6'joTo}, a{100}, a'jTOO}, u\122], and/{101J. 9. THE TETRAGONAL SYSTEM The tetragonal system includes ; all crystals that can be referred to ; ^ three axes at right angles, two of . j which are interchangeable. The t^'^ ^^ axes are designated Oi : Oj : c, ttj and ,'i^'' i a^ each equal to unity and c either j greater or less than unity. Fig. ) 1 1 1 represents the axes for zircon. p^^ m. -Tetragonal axes, Ditetragonal Bipyramidal Class. A^^AjSP (C) (Holohedral) The axis of 4-fold symmetry is the c-axis. As there are four axes of 2-fold symmetry at 45° to each other, either pair at right angles to each other may be selected as the lateral axes. Ditetragonal bipyramid 16 faces {hklj (Ditetragonal pyramid) (Pyramid of first order) (Pjrramid of second order) (Ditetragonal prism) (Prism of first order) (Prism of second order) (Basal pinacoid) Tetragonal bipyramid 8 faces Ihhll Tetragonal bipyramid 8 faces ihoi! Ditetragonal prism 8 faces IhkOl Tetragonal prism 4 faces 1110) Tetragonal prism 4 faces iioot Pinacoid 2 faces 1001! 40 INTRODUCTION TO THE STUDY OF MINERALS Ditetragonal Bipyramid {hkl} (Ditetragonal pyramid). The general form consists of sixteen faces, the faces in the ideal form being scalene triangles (Fig. 112). The angles over alternate polar edges are equal. Tetragonal Bipyramid {hhl} (Pyramid of the first order). This form cuts the lateral axes at equal distances (Fig. 113). Tetragonal Bipyramid {hOl} (Pyramid of the second order). A form consisting of eight faces each parallel to one lateral axis (Fig. 114). This form and {hhl} are identical except in position. Ditetragonal Prism {hko } (Ditetragonal prism) . An open form consisting of eight faces, each parallel to the vertical axis (Fig. 115). The faces meet in angles which are alternately equal. f=4=^ =^^^ -~T'— 113{hhl] ni\hOl\. 115\hk0}. 116J110}. 117J100J. llSjOOlj Figs. 112-118. — The seven tetragonal forms. Tetragonal Prism {110} (Prism of the first order). This is an open form with four faces each parallel to the vertical axis (Fig. 116). Tetragonal Prism {lOOj (Prism of the second order). This is an open form similar to {110} except in position (Fig. 117). Pinacoid {001} (Basal pinacoid). This form possesses two parallel faces, an upper one and a lower one. (Fig. 118). Combinations. The bipyramids are closed forms, but the prisms and pinacoids are open forms, and hence must occur in combination. In habit tetragonal crystals are usually prismatic, pyramidal, or tabular. Pseudo-octahedral and pseudo-cubic crystals are not uncommon. THE FOliM OF MINERALS 41 Examples Zircon. c = 0.040. Usual foirn.s: m[\\0\, ajlOOJ, pflllj, u\X',]\, ijSll}. Interfacial angles: mp(110: 111) =47"^ 50'; ?AnA/(l]0:110)=90° 0'; 7na(l]0: 100) =4.7^ 0'; r«7;(110: 111) = Fig. IHJ. P / ;> Fig. 120. Fig. 121. I'jf.s 119-122.— Zircon. Fig. 122. 47'^ 50'; ap(]00:lll)=6P 40'; mM(110:331) =20° 12'; a;a;(311: 311) =32° 57'; 7>p(lll:lll) =.JGM0' Figs. 119 to 122 represent the usual combinations and habits. Apophyllite. c = 1.2."jl. Usual forms: ajlOOj, c'OOlj, pjlll}, y\Z\0\. Cleavage parallel to c. Interfacial angles: cp(001:lll) = 00"^ 32'; a7>(100:lll)=.j2° 0'; ppflll : iTl) =76° 0'; 02/(100: 310) = 18''-' 2(i'. r==^=^ V. JJ '^ 2/-. Fig. 123. Fig. 124. Fio. 12.5. FioH. 123-126. — Apophyllite Fig. 126. 42 INTRODUCTION TO THE STUDY OF MINERALS DiTETRAGONAL PYRAMIDAL ClASS. A^^P {Hemimor-phic hemihedral) Ditetragonal pyramids [hkl], [hkl] Tetragonal pyramids {hhl\, {hhl} Tetragonal pyramids \hJOl], lhOl\ Ditetragonal prism { hkO } Tetragonal prism lllOj Tetragonal prism j 100 1 Pedions jOOl}, lOWJ No known minerals belong here. Hydrous silver fluorid, AgF-HjO (Fig. 127), is a representative of this class. FiQ. 127. Fig. 128. Fig. 129. Tetragonal Bipyramidal Class. A4-P-(C) (Pyramidal hemihedral) Tetragonal bipyiamids \hkl], {khl\, {hhl], \hOl\ Tetragonal prisms {hkO], \khO}, {llOj, jlOO} Pinacoid 1001} Scheelite, CaWO^, is the best example of this class. (Fig. 128). Tetragonal Trapezohedral Class. A^-4A2 (Trapezohedral hemihedral) Tetragonal trapezohedrons \hkl], [khl] Tetragonal bipyramids \hhl}, \hOl\ Ditetragonal prism j hkO } Tetragonal prisms i 100 [ , i 1 10 j Pinacoid 1 001} THE FORM OF MINERALS 43 No known minerals belong here. But by means of etch- figures it has been shown that the artificial salt, NiS04-6H20, has the symmetry of this class. Fig. 129 shows the etch-figures on {1111. Tetragonal Scalenohedral Class. {Sphenoidal hemihedral) Tetragonal scalenohedrons Tetragonal bisphenoids Tetragonal bipyramid Ditetragonal prism Tetragonal prisms Pinaooid i?',-2A2-2P \hkl\, \hhl\, 1101 {hkl\ [hhl] \hOl\ \hkQ] ilOO! 1001! Chalcopyrite, CuFeSj, is the best known representative of this class. Fig. 130 represents a combination of the tetragonal bisphenoid ^{772j and the tetragonal scalenohedron ;(;{122}. FiQ. 130. Fig, 131. Tetragonal Pyramidal Class. {Hemimorphic tetartohedral) Tetragonal pyramids \hkl\, j khl ] , Tetragonal pyramids i hhl \ , { hhl j , Tetragonal prisms {hkO\, \khO]; \ Pedions- j Wulfenite, PbMoO^, may perhaps belong to this class. Crystals like Fig. 131 have been found but etch-figures indicate the tetrag- \hkl] \khl\ \hOl] \hOl\ 1101, 1100! 0011, 100] 1 44 INTRODUCTION TO THE STUDY OF MINERALS onal bipyramidal class. The artificial compound, Ba(SbO)2- (C4H^Og)2"H20, certainly belongs here. Tetragonal Bisphenoidal Class, ^i (Sphenoidal tetartohedral) The crystals of this class have composite symmetry with respect to a four-fold axis and a plane normal to it. Tetragonal bisphenoids \hhl\, \khl\; \hhl], hhl]; \hOl\, {Okl\ Tetragonal prisms {hkO}, {khO]; lllOj, {100} Pinacoid {001 } An artificial compound, CajAljSiOj, has been assigned to this class by Weyberg (1906). 10. THE HEXAGONAL SYSTEM i The hexagonal system includes all crystals which can be referred to four axes, three interchange- I able ones in a plane at right angles to the fourth. The posi- tive ends of the three lateral axes make angles of 120° with each other as shown in Fig. 132. a> i;:r::^^]<^=^^::^^^IIL_ The index on the third axis is ' I ~^ ' designated by i, so that the Miller I symbol for the general form is {hkil}, in which h + k=i. The I axes may be designated Uj^-.a^: I 0,3'- c, in which a^, a^, and a^ are Fig. 132.— Hexagonal axes. Unity and c either greater or less than unity. The axis of 3-fold or 6-fold symmetry is always taken as the c-axis. 'Twelve crystal classes are assigned to this system. They are sometimes treated under two systems, the trigonal and the hexagonal. As there are three different methods of subdivision, upon which there is no general agreement, it is preferable to con- sider them all under one system. THE FORM OF MINERALS 45 Dihexagonal Bipyramidal Class. A„6A,7P(Cj (Holohedral) Dihexagonal bipyramid Hexagonal bip3n'ainid Hexagonal bipyramid Dihexagonal prism Hexagonal prism Hexagonal prism Pinacoid 24 faces |hkll| 12 faces |hh-2El| 12 faces ihOhll 12 faces IbkiOj 6 faces |ll20i 6 faces |10lO! 2 faces |0001| (Dihexagonal pyramid) (Pyramid of 2d order) (Pyramid of 1st order) (Dihexagonal prism) (Prism of 2d order) (Prism of 1st order) (Basal pinacoid) Dihexagonal Bipyramid jhkllj (Dihexagonal pyramid). This form consists of 24 faces (scalene triangles in the ideal form) cut- ting the four axes at unequal distances. The angles over alter- nate polar edges are equal, dig. 133.) Fig. i:y.i\hkll]. Fig. 13.5 1 A0« Hexagonal Bipyramid }hh-2hl| (Pyramid of the second order) . The faces cut two of the lateral axes at equal but greater dis- tances than the third lateral axis. (Fig. 134.) Hexagonal Bipyramid {hOhlJ (Pyramid of the first order). The faces cut two of the lateral axes, but are parallel to the third. (Fig. 135.) This form differs from {h-h-2h-l] only in position. Dihexagonal Prism {hkiOj. All the faces are in a vertical zone, each being parallel to the vertical axis. Alternate angles are equal. (Fig. 136.) Hexagonal Prism {1120} (Prism of the second order). This form consists of six faces making angles of 60'' livith each other. (Fig. 137.) 46 INTRODUCTION TO THE STUDY OF MINERALS Hexagonal Prism {1010} (Prism of the first order). This is similar to {1120) except in position. (Fig. 138.) Pinacoid {0001} (Basal pinacoid). This form consists of two opposite parallel faces. (Fig. 139.) FlQ. 136. j hkiO 1 Fig. 137. j 1120}. Fig. 138. { 1010 } . Fig. 139 { 0001 [ . Combinations. The habit is prismatic, pyramidal, or tabular. Simple combinations are tl(e rule in this class. As beryl is the only common mineral belonging to this class, it is the only ex- ample given for practice. Example Beryl. c = 0.498. Usual forms: c{0001}, m{10T0}, p{10Tl}, s{1121}, •y{_2131}. Interfacial angles: mm(1010:0110) =60° 0'; cs(0001:1121)=44° 56'; cp(0001 :1011) =29° 57'; m!;(1010:2131) Fig. 140. Fig. 141. Fig. 142. Fig. 143. = 37° 49'; ms(1010:1121)=52° 17'. Figs. 140 and 141 are the ordinary combinations. Fig. 142 has in addition the general form v{2131 }. Fig. 143 represents beryl of tabular habit, which is rare as compared with the prismatic habit. THE FORM OF MINERALS 47 DiHEXA(;oNAL Pyuamidal Class. A„-6P {Hemiiiiorphic hemihedral) Dihexagonal pyramids \hkil\, \hkil\ Hexagonal pyramids \h-lr2kl\, \h-h-2h-l\; \hO>il\, \hJ0Til\ Dihexagonal prism I hkiO | Hexagonal prisms jlOTO!, j 11201 Pedioas lOOOlj, (000T| The forms in the upper half of the crystal are different from those in the lower half. This is apparent from Fig. 144, which represents iodyrite (Agl)_. The form8_are cjOOOl j, w|4041}, aill20j, and ;r|4045j. Fio. 144. Fio. 145. IlK.XAfJO.VAL BiPYKAMIDAL ClasS. AjPC (Pyramidal hemihedral) Hexagonal bi pyramids Hexagonal prisms Pinacoid \hkil\, \khil\; \hOhl\, \h-h-2K-l] ■\hkiO\, \khlO\; ill2oi, |10T0! 10001 i Apatite, Ca5F(POj3, is the best example of this class. Fig. 14.J represents a crystal with c 1 000 1 ), m{10T0j, s{ 1121 j xjlOTlj and the general form /^{2131j. Hexacjonal TaAPEZOHEDRAL Class. A„-6A2 (Trapezohedral hemihedral) Hexagonal trapezohedrons \hkil\, \khil\ Hexagonal bipyramids \h-h-2h-l\, \hQhl\ Dihexagonal prism ! hkiO \ Hexagonal prism |1120i, jlOTOj Pinacoid 10001 1 48 INTRODUCTION TO THE STUDY OF MINERALS No mineral is known to belong to this class, but a complex double salt Ba(SbO)2(C^H^Oe)2'KN03 is assigned to this class on the basis of the etch-figures (Fig. 146). Here the forms are c{0001}, ?3{10Tl}, and mflOTO). Fig. 146. Fig. 147. Hexagonal Pyramidal Class. Ag {Hemimorphic tetartohedral) Hexagonal pyramids [hkil], [khil], {hkil}, \khil\: Hexagonal pyramids [hOhl], {h-h-2h-l}, [hOhl], {h-h-2h-l\ Hexagonal prisms {hkiO\, \khiO]; {llloj, {lOTOj Pedions (0001 j, 10001} Nepheline (NaAlSiOJ is assigned to this class because of the etch-figures represented in Fig. 147. Ditrigonal Scalenohedral Class. iPsSAj-SPCC) (Rhombohedral hemihedral) The lateral axes are the axes of 2-fold symmetfy and the c-axis, the axis of 3-fold symmetry. The crystals of this and some other classes of the hexagonal system are sometimes referred to three interchangeable axes at oblique angles as represented in Fig. 148. In this case the Miller symbol has three indices hkl and the axial element is a, the oblique angle between the axes. Scalenohedrons 12 faces ihkii!, jkhll}* (Scalenohedrons) Hexagonal bipyramid 12 faces |hh-2Eli (Pyramid of 2d order) Rhombohedrons 6 faces (hOhl!, iOhhlj ( Rhombohedrons) Dihexagonal prism 12 faces {hkiOl (Dihexagonal prism) Hexagonal prism 6 faces jlOlO) (Prism of 1st order) Hexagonal prism 6 faces ill20S (Prism of 2d order) Pinacoid 2 faces ioooi! (Basal pinacoid) * In these symbolfl h >k; for example, {2131} is [hk'l] and {12J1} is [mi]. THE FOKM OF MINERALS 49 Scalenohedrons {hkJJ|,|khilj. The general form of this class is a 12-si(l(Ml figure, each face of which is a scalene triangle. There are three kinds of edges, short polar, long polar, and middle edges, eacli with their characteristic interfacial angles. The \hLll\ form is called jjositive and the {Lh~il\ form, negative, l-'ig. 149 is a positive scalenohedron. Fio. 148. Fia. 149. 1^5;!. Fia. l.W, \hJOhl\ Hexagonal Bipyramid {h-h-2hl} (Pyramid of the second order). This form consists of twelve faces, each an isosceles triangle (Fig. 134). Rhombohedrons {h0hlj,|01ilil{. A rhombohedron consists of six rhombic faces. It is like a cube distorted in the direction of one of its diagonals. A rhombohedron is distinguished as acute or obtuse according to whether the angle over the polar edges is greater or less than 90°. The rhombohedron with faces in the middle front, right rear, and left rear dodecants is called positive and has the symbol IhOhl], while the rhombohedron with faces in the right front, left front, and middle rear dodecants is called negative and has the symbol \OhTd]. Fig. 1.50 is an obtuse posi- tive rhombohedron. Dihexagonal Prism |hkiO(. There are twelve faces in a ver- tical zone. (Kig. \'',2) =21° 18'; xx(12:52::i212) = 13° 22J'. Figb. 167 to 170 represent typical crystals of tourmaline. Thioonal Trapezohkdhal Class. Aj-SAj ( Trapf'zohedral tetartohedral) The axes of symmetry arc tlic axes of reference. Trigonal trapezohedrons [hlnl], \khU\ Trigonal bipyramitl \h-h-2h-l\ Rhombohodrons \hOhl\, [Qhfil] Trigonal prisnis 1 1 120 j , IT2T0 [ Ditrigonal jiriKins \hkiO\, \khiO\ Hexagonal prism jlOlOi Pinacoid joOOlj Quartz, SiOj, and cinnabar, HgS, are the only known minerals belonging to this class. The general form is comparatively rare and always occurs as a subordinate form. The forms {hkil} and {khil\ are distinguished as right-handed and left-handed forms respectively. Fig. 171 represents a right-handed quartz cryst_al with the forms mfloTO), rjlOlT}, zfOlTlj, s{1121j, and XJ5161}. Figs. 375 and 376, page 152, show a right-handed crys- tal and a left-handed crystal side by side. For angles of quartz see page 305. Tjiigo.ntal Rhombohedral Class. .^o(C) (Rhomhohedral tetartohedral) Rhombohedrons \hkil], [khil], \ikhl], \kihl] Rhombohedrons \kh-2Ji-l],_\ 2h-h-hJ \ Rhombohedrons [hOhl], {Ohhl\ Hexagonal prisms \hkiO], \khiO] Hexagonal prism |1120) Hexagonal ijrixm 11010} Pinacoid 1 0001} rhenacite, Ba^SiO^, is a representative of this class. Fig. 172 is a simple combination with a{1120} and a;{2132}. 54 INTRODUCTION ^TO THE STUDY OF MINERALS Trigonal Bipyramidal Class. Ag-P {Sphenoidal tetartohedral) Trigonal bipyramids Trigonal prisms Hexagonal prism Trigonal prisms Trigonal bipyramids Trigonal bipyramids Pinacoid {hkil], \khil\ \hkiQ\, [khiO] 111201 {1010}, {0110} \h-h-2M\, {2h-h-h-l\ [mU], \Qhhl\ {0001} This is |he only class for which no example has yet been found. The forms given above may be predicted from the symmetry. Fig. 171, FiQ. 172. Fig. 173. Fig. 174. Trigonal Pyramidal Class. (Ogdohedral) Trigonal pyramids Trigonal pyramids Trigonal pyramids Trigonal pyramids Trigonal prisms Trigonal prisms Pedions [hkil], [khil], {ikhl\, {kihl}; _ \hMl}, {khTl},JiWl\, {^W} \h-h-2h-l], \h-h-2h-l], \2h-h-h-l] ,_\2h-h-h'l\ \mhi\, \hm\; \Qm\, \QhTi\ \hkiQ\\ \khiQ]; {1120}, {1210} {1010}, {0110} {0001}, {0001} Hydrous sodium periodate, NalO^'SHjO, belongs to this THE FORM OF MINERALS 55 cl;is.s_ Fig. 173^ represcntfi a crystal with the forms r{10Tl}, r J0221 }, f, {0001 j, antU|"'-4-9-10}. (After eakle.) DrruicoNAL Bipyramidal Class. A3-3A2-4P(C) {Sphenoidal hemihedral) Ditrigonal bipyramids [hkll], {khil] Hexagonal bipyramid ^ j h-h-2K-l \ Trigonal bipyramids \hOhl\, \Ohhl\ Ditrigonal prisms \hklO\ {khlO\ Hexagonal prism j 1 120 } Trigonal prisms ! 1 OTO ) , 1 iTO } Pinacoid | 00011 Acid silver phosphate, AgJiPO^, has been assigned to this class Jjy Dufet. _Fig. 174 represents a crystal with the forms m{1010}, mjono], r{1011}, pl202l}, and g(0221}. Benitoite, BaTiSijOo, a mineral from San Benito County, California, de- scribed by Louderback was also assigned to this class, but accord- ing to Jezek it belongs to the ditrigonal pyramidal class. ! 11. ISOMETRIC SYSTEM i,^" This system includes all crys- tals that can be referred to three interchangeable axes at right angles. The axes may be desig- nated a I, a^, and a, as in Fig. 175. The five classes of the fi°- i75.-i,omotrio axes. isometric system have four axes of 3-fold symmetry in common. The cube and the dodecahedron occur in all five classes. The angles of the corresponding forms in all crystals aie equal. 56 INTRODUCTION TO THE STUDY OF MINERALS Isometric Hexoctahedral Class. GAj^Aj SA^-QPCC) (Holohedral) The crystals of this class have the maximum degree of sym- metry possible in crystals, the axes of reference. The four-fold axes ni symmetry are Hexoctahedron 48 faces \hU} Trapezohedron 24 faces {hkk\ Trisoctahedron 24 faces \hhl\ Tetrahexahedron 24 faces \hkO\ Dodecahedron 12 faces 1110! Octahedron 8 faces illll Cube 6 faces liooi Hexoctahedron {hkl}. The general form consists of forty- eight faces, the symbols of which may be derived from the form symbol by taking six permutations of letters and eight permu- tations of signs. Fig. 176 represents the hexoctahedron {321}. Fig. 176. Fig. 177. \hkk\. Fig. 178. [hhl]. Trapezohedron {hkk}. Each face is a trapezoid. This form is sometimes called the tetragonal trisoctahedron to distinguish it from the next mentioned form, the trigonal trisoctahedron. Fig. 177 represents the form {211} which is common on garnet, leucite, and analcite. Trisoctahedron {hhl } . Each face is an isosceles triangle. With this form the intercept on the third axis is greater than the inter- cepts upon the other two which are equal, while with {hkk} the intercept on the third axis is less than the intercepts upon the other two. Fig. 178 represents the trisoctahedron {221}. THE FORM OF MINERALS 57 Tetrahexahedron jhkOj. This form is so called because it apparently consists of a four-fticcd pyramid on each cube face. Fig. IT'.I roprcsfiils the form j210j. Dodecahedron |110j. This form (Fig. 180) consists of twelve faces each riionibic in shajjc. The interfacial angles are 60° and 90°. Octahedron |lllj. As its name implies, this is an eight-faced form. Ivicli face is an equilateral triangle. The interfacial angles are 70° ?,2' . (Fig. 181.) Cube {lOOj. The cube or hexahedron is a six-faced form with interfacial angles of 90°. (Fig. 182.) Fio. 179. \hkO\ Fio. 180 110 Fro. 181. Ill Fia. 182. jlOOJ. Combinations. The cube, octahedron, and dodecahedron are much more conimon than the other forms. They occur alone and in combination with each other. See Figs. 441-4 page 309. The hexoctahedron, trisoctahedron, and tetrahexahedron usually occur as small faces modifying simple forms. Although quite varied in aspect, isometric crystals, unless much distorted, are of about equal dimensions in all directions and this fact aids in their identification. Galena, garnet, and fluorite are given as typical examples for study and practice. Examples Galena. Usual forms: ajlOOj, o{lll) aa(100:010)=90° 0'; oo(lll : iTl) =70° Interfacial angles: 32'; ao(100:lll) = 58 INTRODUCTION TO THE STUDY OF MINERALS 54° 44'. Figs. 183 to 187 represent usual combinations varying from the cube alone to the octahedron alone. FiQ. 183. Fig, 184. Fig, 185. Fig. 186. Fig. 187. Garnet. Usual forms: d{110}, n{211}. Interfacial angles: rfd{110:101)=60° 0'; n?i(211:121)=33° 33^; nn(211:2Tl) = 48° IH'; dn(110:211) =30° 0'. Figs. 188 to 191 represent usual combinations varying from the dodecahedron alone to the trape- zohedron alone. Fig. 188. Fig. 189. Fig. 190. Fig. 191. Fluorite. Usual forms: a {100}, /{SlOj, <{421}. Cleavage parallel to {HI}. Interfacial angles: aa(100:010) =90°; 0/(100:310) =18° 26'; ai(100:421) =29° 12'. Figs. 192 to 195 Fig. 192. Fig. 193. Fig. 194. Fig. 195. represent frequent combinations. The plane formed by the dotted lines in Fig. 192 represents octahedral cleavage. Fig. Diploids 24 faces |hkl( Trapezohedron 24 faces |hkk Trisoctahedron 24 faces jhhlt Pyritohedrons 12 faces jhkOI Dodecahedron 12 faces 1110! Octahedron 8 faces {1111 Cube 6 faces llOOj THE FORM OF MINERALS 59 195 represents a twin crystal in which two cubes are twinned about a cube diagonal. Isometric Diploidal Class. 3A2 4A3 6P(C) {Pentagonal hemihedral) The axes of 2-fold symmetry are the axes of reference. Ikhl! ikhOl Of these forms, all but the diploid and pyritohedron are geo- metrically similar to those of the preceding class. Diploids {bkl),{khl}. The general form is a 24-faced form, the faces of which lie in pairs astride the planes of symmetry hence the name, which means double. The two forms {hkl] and \khl], which are congruent, are distinguished as positive and nega- tive. Fig. 196 represents the posi- tive diploid {321}. Pyritohedrons {hkO}, {khO}. fig. 196. The pyritohedron is so named because it is common on the mineral pyrite. The two forms given are distinguished as positive and negative. On pyrite the most common form is the positive pyritohedron {210}, repre- sented by Fig. 197. Examples Pyrite. Usual forms: a{100}, e{210}, o{ HI}, s[321}, n{211}. Interfacial angles: ae(100:210) =26° 34'; ee(210:210) =53° 8'; ee (210:102) =66° 25'; eo(210:lll) =39° 14'; ao(100:lll) =54° 44'; se(321:210) = 17° H'; sa(321:100) =36° 42'; so(321:lll) =22° 60 INTRODUCTION TO THE STUDY OF MINERALS 12i'; a?i(100:211)=35° 16'; on(lll:211) = 19° 28'; Figs. 198 to 205 represent common types of pyrite crystals. The cube faces are usually striated as in Fig. 198. Fig. 198. Fig. 199. Fig. 200. ^^ Fig. 201. Fig. 202. Fig. 203. Fig. 204. Figs. 198-205.— Pyrite Crystals. Fig. 205. Isometric Hextetrahedral Class, SAj 4A3-6P {Tetrahedral hemihedral) The axes of 2-fold symmetry are the axes of reference. Hextetrahedrons 24 faces Ihkli, jhEli Tris tetrahedrons 12 faces IhkkS Ihkk Deltohedrons 12 faces Ihhll, Ihhll Tetrahedrons 4 faces llUi, 1111 Tetrahexahedron 24 faces IhkO) Dodecahedron 12 faces 1110} Cube 6 faces 1100) The first four forms are geometrically different from the corre- sponding forms in the hexoctahedral class and are described below. Hextetrahedrons {hkl), {hkl). This 24-faced form is called the hextetrahedron as it apparently consists of a 6-faced pyra- mid built upon each face of a tetrahedron. (Fig. 206.) The two forms given are distinguished as positive and negative. They occur in alternate octants. THE FORM OF MINERALS 61 Tristetrahedrons |hkk}, {hkkj. These forms resemble three- faced pyramids built upon each tctrahedral face, hence the name. The two forms, wliich occur in alternate octants, are distinguished as positive and negative. Fig. 207 is a positive form. Deltohedrons {hhl}, (hEl). These two forms are also positive and no};ativc according to the octant in which they occur. Fig. 208 represents a positive form. The name refers to the shape of the faces. 206. \hkl\. Tetrahedrons {lllj, {lllj. This is the rci^ular tetrahedron of geometry, the interfacial angles being 109° 28' (Fig. 209). Like the previously mentioned forms, the positive and negative forms are exactly alike except in i)osition. The two forms {lllj and {111) in equal combination constitute an octahedron and there- fore are called complementary. Combinations. Crystals of this class usually have a tetrahe- dral aspect. Fia. 210. Fia. 211. Pio. 212. Figs. 210-213.— Tetrahedrite. Fia. 213. Example Tetrahedrite. Usual formj^ o { 1 1 1 j , Oj { 11 1 j , n { 2 1 1 ) , c/ { 1 10 j . Interfacial angles: oo(lll : 111) = 109° 28'; /t/i(211 : 121) =33° 62 INTRODUCTION TO THE STUDY OF MINERALS 33i'; no(211:lll) = 19° 28'; rfo(110:lll) =35° 16'. Figs. 210 to 213 represent usual types of tetrahedrite crystals. Boracite, Fig. 508, page 366 is also a good example of this class. Isometric Gyroidal Class. 6A2'4A3-3A4 (Gyroidal hemihedral) The axes of 4-fold symmetry are the axes of reference. The only form geometrically different from those of the hexoc- tahedral class is the general form, which is called a gyroid. The two forms [hhl] and [hlk] are enantiomorphous and are distin- guished as right and left. Sylvite, KCl, and sal-ammoniac, NH^Cl, crystallize in this class. Fig. 214 represents a crystal of NH4CI with the form {943}. Fig. 214.— Gyroid. Fia. 215. — Tetartoid. Fig. 216. oA2'4A3 Isometric Tetartoidal Class. (Tetartohedral) The forms in this class are the tetartoid, the tristetrahedron, the deltohedron, the pyritohedron, the dodecahedron, the tetra- hedron, and the cube. The only geometrically new form, the tetartoid, has twelve faces. There are four kinds of tetartoids possible: {hkl}, [khl], {hkl}, {khl}. Fig. 215 represents the form {321}. Ullmannite, NiSbS, has been placed in this class because for this mineral { 210} THE FOHM OF MINERALS 63 is a pyritohedron and {111} a tetrahedron. Fig. 216 represents a crystal of BaCNOg), with a{ 100 j , o { 1 1 1 ) , Oj { iTl } , and the general form, >i{4'21). 12. TWIN -CRYSTALS Loose isolated crystals are comparatively rare in nature. Crystals are usually grouped in parallel position or in the most irregular manner. A discitssion of these cases will occupy the next section. A jieculiar sort of grouping is linown us twinning and crystals so grouped arc called twin -crystals. Many crystals are found to be composed of two parts, one half of which has apparently been Fio. 217. Contact twin. Fig. 218. Penetration twii: Fia. 220. Polysynthotic twin. revolved 180° about a line called the twin-axis. The plane normal to tliis axis is called the twin-plane. The face of union of the two individuals is called the composition-face. It may or may not be the twin-plane. The twin-plane is always a crystal face or a possible crystal face, hut never a plane of symmetry. The twin-axis is always a jiossible crystal edge or is normal to a possible crystal face, but it is never an axis of even symmetry. Twin crystals are distinguished as (1) contact twins in which case the composition face is definite or as (2) penetration twins in which the composition face ie irregular. Fig. 217 represents a contact twin and Fig. 218, a penetration twin. In the case of contact twins the twin-law is defined with respect to a twin- plane, while in penetration twins it is defined with respect to a twin-axis. 64 INTRODUCTION TO THE STUDY OF MINERALS Fig. 221.— Plagioclase. Twins are usually recognized by the presence of reentrant angles, but there are exceptions to this general rule. In addition to twins composed of two individuals, there are also multiple twins made up of three or more parts. If the same face serves as twin-plane for a series of individuals we have a poly- synthetic twin (Fig. 220). But if different faces (of the same form) are twin-planes we have a cyclic twin (Fig. 219). A polysynthetic twin may consist of a large number of individuals and some of these may be so narrow that they appear as striations. Cleavages of calcite and of plagioclase often show twinning striations. In calcite the rhombohedron {0112} is the twin-plane and so on the cleavage face {1011} the striations are parallel to the long diagonal as represented in Fig. 235. In plagioclase the twin-plane is usually &{010}, and so the twin striations appear on the c{001} cleavage face as narrow bands parallel to the (001 : 010) edge as shown in Fig. 221. Examples Fig. 222 represents a twin of gypsum with {100} as twinning plane. In Figs. 223 (augite) and 224 (hornblende) {100} is also twin-plane. Fig. 225 represents a Carlsbad twin of ortho- clase. This is a penetration twin with the c-axis as twin-axis. Fig. 226 represents a cruciform penetration twin of staurolite. In Fig. 227 a contact twin of aragonite with m{ 110} as twin-plane is shown. A penetration trilling of cerussite is illustrated by Fig. 228. Fig. 229, a twin of marcasite, apparently has an axis of 5-fold symmetry. The angles in this case would be exactly 72° (-^ of 360°), but accurate measurement proves them to be 74° 55' instead.. Figs. 230 to 233, inclusive, represent various kinds of rutile twins, but in all cases (101) is the twin-plane. Fig. 230 is a THli FORM OF Ml\EliALS G5 simple contact twin; V'l^. '2'.i\ shows twin sti'iatioiis. A sinjilc Ijarid inserted in twinning position like Fi;;. 'I'.i'I is called a twin- X^ Fig. Fio. L'L'O. Fiu. 2Zi. Fig. l'L'I. Fig. 227. Fig. 228. Fig. 22y. Fig. 230. Fig. 231. ^ Fio. 232 Fig. 233. seam. Fig. 2:y,) is a cyclic twin. These four figures are ortho- graphic projections. 66 INTRODUCTION TO THE STUDY OF MINERALS The next row of figures illustrates the four twin-laws known for calcite. Fig. 234 is the scalenohedron {2131} twinned on {0001} . Fig. 235 represents a calcite cleavage with twin lamellae inserted parallel to {0112}, which is the most common twin-law Fig. 234. Fig. 235. Fig. 236. Fig. 237. for calcite. Fig. 236 is a calcite twin with {1011} as twin-law, while Fig. 237 is a scalenohedron twinned on {0221}, the rarest of the twin-laws for calcite. The next four figures represent twins of the isometric system. Fig. 238 (with 111 as twin-plane) is called the spinel twin because it is so common for the mineral spinel. Fig. 239 represents twin Fig. 238. Fig. 239. Fig. 240. Fig. 241. striations observed on cubic cleavages of galena. Here the twin- plane is {441}. A penetration twin of fluorite with the cube diagonal as twin-axis is represented in Fig. 240, while Fig. 241 is a twin of pyrite with the a-axis as twin-axis. This is known as the " eiseneres Kreuz." THE FORM OF MINERALS 67 The tendency of twinning is to apparently raise the grade of symmetry. This is especially the case with pseudo-hexagonal orthorhom})ic minerals such as chiysoberyl, aragonite, witherite, and cerussite. ^^ee Figs. 460, 492, 493, and 497. 13. CRYSTALLINE AGGREGATES Large distinct crystals are comparatively rare among minerals. Most minerals consist of crystal aggregates which do not possess definite crystal faces. Such minerals are, however, crystalline, as may lie determined by certain physical properties, particularly the optical properties. The kinds of crystalline aggregates are distinguished by certain terms which are constantly used in the descriptido of minerals. A mineral made up of plates is called lamellar (example, barite). If the layers are readily separated the term micaceous is used. An aggregate of more or less parallel imperfect crystals is called columnar (example, aragonite) and the same on a smaller scale is fibrous (example, gypsum). A flat columnar aggre- gate is said to be bladed (example, cyanite.) The term granular needs no explanation (example, magnetite). The forms assumed by many aggregates derive their names from some natural object. Nodular is the term used for irregular rounded lumps (example, pyrite). Mammillary refers to low rounded prominences (example, smithsonite) . Botryoidal is from a Greek word meaning a bunch of grapes (example, chal- cedony). Reniform means kidney-shaped (example, hematite). Pisolitic is the term used for an aggregate of shot-like masses (example, bauxite), while oolitic is similar except in size, being like fish-roe (example, calcite). Stalactitic indicates that the mineral is found in icicle-like forms (example, calcite). Den- dritic means branching like a tree (example, copper). Concre- tions are more or less spherical masses formed by the tendency of matter to gather around a center (example, siderite). A geode is a hollow concretion usually lined with crystals (example, quartz.) A vug is a cavity in a rock or vein lined with crystals. 68 INTRODUCTION TO THE STUDY OF MINERALS Other terms such as mossy, wire-like, radiating, rosette-shaped, leafy, globular, etc., are self-explanatory. Very few minerals are strictly amorphous. Chalcedony, though never occurring in distinct crystals, always shows a crys- talline structure when examined between the crossed nicols of a polarizing microscope. Opal, on the other hand, is amorphous, for it remains dark between crossed nicols. Both cryptocrystal- line minerals like chalcedony and amorphous minerals like opal often occur in botryoidal and mammillary forms. 14. THE INTERNAL STRUCTURE OF CRYSTALS It is the general belief that matter is coarse-grained, being made up of discrete particles (or groups of particles) at small, but not infinitesimal, distances apart. Many physical phenom- ena such as expansion on heating, transmission of light, and the polarity of tourmaline can only be satisfactorily explained by means of this hypothesis. In crystals it is only fair to assume that the particles have a regular arrangement, while in amorphous substances the ar- rangement is haphazard. The law of rational indices is one of the best proofs of some kind of molecular structure. A crystal is the outward expression of an internal structure. This internal structure is the essential character of a crystal. A fragment of quartz, though without crystal faces, may easily be distin- guished from a fragment of glass by polarized light. A study of the physical properties (especially the optical properties) of crystals reveals the fact that the properties are the same for all parallel directions, but in general are different for directions not parallel. From these and other c(jnsiderations we are forced to the conclusion that the arrangement of molecules about any one molecule is the same as that about any other. Haiiy, the founder of crystallography, made the first contribu- tion to the theory of crystal structure. A crystal of calcite. THE FOh'M OF MlNI<:i{.\LS (i'.t aci'idciitally ilr()p])C(l, broke into clcaN'af^o fraf;ineiils. From tills and otlif'i- ohscrvalioiis ilaiiy was led lo su])])os(' that rrystals arc made up of minute cleavage ])articles. H\- Iniilding up these j)aiticles any of the known foi-nis of a minei-al can be imitatctl. l''or exarniile, in galena the cleu\'age particles are cubes. Omitting a cei'taiu number of these cubes various secondary faces would result . Thus in figs. 242, 243, and 244 the dotted lines rejjresent (110), (210), and (310) faces resijectively. If th(> particles were very minute the faces would ap))eai- smooth. In this wa^' IIaii\' discoveicd the law of I'ational indices. - / *■■' .' J.-' -J--'" Imi:. LM2. ]'"i<;. 2'l:i Fia. 2.|4. It is pi-obable that the units of ciystal structure are not in con- tact , also it is not necessai'y that 1 hey be cleavage jjarticles, for soiue minerals ha\'e no cleavage. The ceul(>rs of the molecules or their centers of influence, as they aic ])robably in constant motion, luay be icpresented l)y points. Then we need not as- suine any parlicuhu' sliapt^ for the paiticles. Bravais est,ablished tlie fact that only foui'teen kinds of s]jace- lattiees or kinds of arrangement of the molecules are possible in ciystals. They aic as follows: cube (Fig. 24.1), centei'cd cube (Fig. 24(i), cube with centered faces (Fig. 247), square prism (Mfi. 24S), centered square j^rism (l''ig. 2I'.I), hexagonal prism (Fiji. 2.')0), rhombolie(bon (Fig. 2.')1), rhondnc prism (Fig. 252), centered rhouibic prism (Fig. 2.j3), rei't angular prism (I'ig. 2.")4), centered rectangular prism (Fig. 2.1.")), clinorhombic prism (Fig. 2.")()), rncmoclinic parallelepiped (Fif;. 2.17) , and triclini(' prism or general jjarallelepiped (Fig. 2')H) . In order to account for the thirty-two classes Bravais assumed that the molecules them- 70 INTRODUCTION TO THE STUDY OF MINERALS Fig. 245. Fig. 246. Fig. 247. Fig. 248. Fig. 249. Fig. 250. Fig. 251. Fig. 256. Fig- 257. Fig- 258. Figs. 245-258.— The fourteen space-lattices of Bravais. THE FORM OF MINERAL.^ 71 sel\'<>s i^isscss symmetry, hut this is not iicccssary for even if they iirc without symmetry, their ori(Mit;itioii in the unit cell of tlic spacc-hittict' will iiccdunt for all ohscrvcil tyj)es of symmetry. Hy considei'int!; infinite groups of movements, which include transhitions as well as symmetry operations, Fedorow, Schoen- flies and Barlow have proved that two hundred and thirty types of ei-ystal struetui'e are possilih'. Tliere is an intimate connection hetweeii the axes of symmetry, regular arranjiement of crystal molecules, and the rationality of iiidiees. Only axes of 2-, 3-, 4-, and 6-fold symmetry have ever been found in ciN'stals. Though this rests upon an empirical basis it would seem to be a law of nature for, of the thirty- two crystal classes deduced mathematically by assuming only these axes of symmetry, representatives have been ^^"- ^^^- ^'°- ^^'*- found of all but one. More- over, no crystal has ever been found which does not belong to one of these thirty-one crystal classes. Following Lewis, it can he shown by a consideration of I'igs. 2.59 and 260 that if we assume a system of particles arranged at small, finite distances apart we can prove that only axes of 2-, 3-, 4-, and 6-fold symmetry are possible. Let Aj, Aj, etc., repre- sent centers of the particles and let AjA^ be the smallest possible distance between any two of them. A^ and A^ also represent axes of symmetry. In Fig. 2')'.) a revolution around Ai brings Aj to A3 and a revolution around A^ brings Aj to A^. If the angle of revolution is 60°, A3 = A4 and AiA3 = A2A3 = AiAj. In this case we have an axis of 6-fold symmetry. If the angle of rotation is less than 60°, AjAj is less than AjAj, which is contrary to hy- pothesis. Therefore no axis of symmetry of degree greater than six is possible. For an axis of 5-fold symmetry we have an angle of rotation of 72° (J of 360°). Now a rotation of 72° around A, 72 INTRODUCTION TO THE STUDY OP MINERALS (Fig. 260) brings A^ to A3 and a rotation around A2 brings A; to A4. Then we have A3A4 less than AjAj, which is contrary to hypothesis. Therefore only axes of 2-, 3-, 4-, and 6-fold sym- metry are possible if crystals have a regular internal structure. As only a.xes of 2-, 3-, 4-, and 6-fold sj'mmetry have ever been found on crystals, in order to account for the observed facts we must assume a regular internal structure. A regular internal structure necessitates the rationality of the indices, for the various crj'stal faces are planes formed by the omission of a certain number of the particles in certain directions. It is often stated that the indices are simple whole numbers. This does not follow from the above argument nor does it fit the ob- served facts, for the indices of crystal faces are often large numbers. For example, on calcite crystals from Seguache County, Colorado, the author found as the dominant form a scalenohedron with the symbol {49-41-90-8!. The faces were perfectly smooth and gave sharp images affording measurements which checked almost exactly with the calculated values. There are many such cases on record. According to the structure theory of Bravais, the faces nifist apt to occur are those in which the centers of the particles are closest together. These are, of course, faces with simple indices such as {110}, {120), {130}, {210), {310), {410}, etc. But there is no reason why faces with large indices cannot occur, and in fact they do occur and are not to be explained as accidental. Out of 400 combinations of cerussite crystals Hubrecht men- tions {110) as occurring 374 times; {111}, 370 times; {010}, 357 times; {021), 279 times; {Oil}, 201 times; {130}, 192 times, while such forms as {013-1), {14141), {11131J and {4-86-4.i| occur only once or twice in the 400 combinations. We cannot prove by direct measurement that the indices are rational for all measurements are suljject to certain errors. That is, we cannot distinguish rational numbers from irrational num- bers by any measurements wc can make. But in an indirect way there is proof of the rationality of the indices as we have seen THE FORM OF MISEHM.S 73 alioNc. 'J'lie lat iiiiialit\- (if the iiiciiccs and I'f'Kulur iiitci'iia] struc- ture of crystals rest upon tlic criiijirical liasis of the syiniiifl ry of crystals. If we assuiiic crystal sti'ucturc it is easy to explain the relati\'e al/uiiclancc of crystal faces, the constancy of intcrfacial anglcK, varying crystal hal)it,and cleavage, as well as other jjhysical properties. iMg. 21)1 repicsenls in two diniensioiis the ])Ossil)le >tructure of an orthorhoinhic mitiei-al. It is easy to account for * / * T 7 m / ■ ' 1 • / • / / / / m / • ■ / ' / ' / • X. 010 ' h:^ 1'' i/' / • ^ /'i'-^> / • y^^ ^^ , / AlWy^ ^ y ^orrietric crystal. The faces most likely to occur are those in which centers of molecules are chtsest together. The probability of the occurretice of faces is, theti, in the following order: (1) (lOOj, (010); (2) (110); iW) CilOj, (120); (4) (310), (130); and {:>) (320j, (230). This agrees with the observed facts. Figs. 203a and 203b lejnesent respectively the plan and side elevation of an orthoclase ciystal in outline. In the monoclinic 74 INTRODUCTION TO THE STUDY OP MINERALS system there are two space-lattices possible, the clinorhombic prism and the monoclinic parallelepiped. As we shall see, the former suits orthoclase better. The small circles in the side elevation represent centers of molecules above and below the plane of_the drawing. The drawings show why (010), (110), 130, (001), (101), and (201) are the faces of common occurrence on FiQ. 263a. FiQ. 263b. The probable structure of orthoclaae. orthoclase and why such faces as (120), (210), and (101) are of rare occurrence. According to Bravais cleavage is more prominent the greater the distance between adjacent rows of molecules. Thus in orthoclase we have cleavage parallel to (001), (010), and (110) as shown by the dotted lines. 16. THE GROWTH AND HABIT OF CRYSTALS Crystals grow by accretion. They possess no organs as do plants and animals. Except for external faces a crystal is alike THE FORM OF MINERALS 75 in all its parts. They go through no growth stages, such as those observed in organisms. In the natural glass, obsidian, minute forms known as crystallites are very common. They consist of hair-like, fern-like, and feather-like forms as represented in Fig. 264. These are supposed to be incipient crystals and in some cases have been referred to the mineral augite. Crystallites have no action on polarized light. Larger forms (the swallow- tail forms of Fig. 264) , which become dark and light with crossed nicols, are called microlites. In the case of sulfur crystallizing from carbon bisulfid solutions some observers have found that small spheres were formed first. These became arranged in rows like strings of beads and then united in acute forms and finally minute pyramidal crystals were the result (Fig. 397) . Crystals usually grow more rapidly in certain directions than in others. The general appear- ance of the crystal due to this difference of growth in different directions is called the crystal habit. Such crystals as the micas usually have a limited growth in one direction and are flat. This is known as tabular habit. It may be described as thin tabular or thick tabular. In other cases the growth is largely in one direction, and we have the prismatic habit. In extreme cases we have acicular or needle- like crystals, or capillary crystals with the relative proportions of fine hairs. Other habits are defined as pyramidal, cubic, octahe- dral, etc. The habit of crystals is probably partly inherent in the sub- stance and partly due to external conditions such as temperature, pressure, and the presence of foreign substances. While little is known as to the cause of the habit of crystallized minerals, Fig. 264. — Crystallites and microlites. 76 INTRODUCTION TO THE STUDY OF MINERALS Fig. 265. Fig. 266. Fig. 267. some important experimental observations have been made in connection with this subject. For e.\ample, calcite may be obtained in small crystals from a solution of CaCOg in carbonated water. Vater, a German investigator, found that if no foreign substance was present in the solution, the crystals were unit rhombohedra, but on the addition of sodium sulfate, calcium sul- fate, etc, the habit of the crystals was changed. As more and more of these substances was added, the crystals became steeper rhombohedra and finally passed into prismatic crystals. Figs. 265, 266, and 267 illustrate the change in habit on adding sodium sulfate. Alum, which usually crystallizes in octahedrons, sepa- rates from alkaline solutions in cubes. The habit of a mineral is often characteristic. Barite is usually tabular in habit. Crystals of barite prismatic in the direction of the c-axis, when first found were described as a new mineral so unusual is this habit. Other minerals are marked by a great divei'sity of habits. The mineral calcite, for example, occurs in endless variety of combinations and the habit may be tabular, pyramidal, rhombohedral, scalenohedral, prismatic, as well as pseudo-cubic. (See Figs. 151 to 162, page 51.) If the habit of a crystal changes during its growth this fact is often made apparent in transparent crystals by a so-called phantom-crystal which is different in color or marked by a row of inclusions. Fig. 268 represents a phantom crystal of barite from England. At an earlier stage the face (010) was present. Striations on crystals may often be explained by the oscillatory combination of two forms. Thus while the prism of a quartz crystal is being formed there is a tendency all the while to form the rhombohedron, and this tendency is manifest in the striations. A magnified cross-section of a quartz crystal would appear as in Fig. 269. THE FORM OF MLS Ell A LS 11 ('urvcil ii\ ^tuls may be the result of oscillatoiv combination of several forms or of small individual crystals in not quite parallel position. Sk'|(ir,ii crystals are formed during the rapid growth of some crystals. Their formation may be illustrated by Fig. 27(J. Taking a rhombic crystal as a nucleus it will be seen that the amount of material available for a unit of surface is greater at the ■AniL\t'>. Hence the crj'stals under certain conditions will jrrow more rapidly in ihe.se directions, finally producing the hollow forms known as skeleton crystals. The snow crystal of Fig. 439, page '.W.i. is a skeleton crystal and not a twin crystal. i']<. l-ir,. L'oy. \\'.. .'70. On some cr3'slals there occur faces, with very high indices, the distribution of which agrees with the syninietr}' of the crystal. These are known as vicinal faces because they are ii(;(ir common faces with simjjle indices. 'J'he best e.xarnple is the case of fiuor- ite in wiiich the cube faces, especially in twin cr\'stals, are replaced by a veiy flat four-sided pyramid (Fig. 195). This is a tetrahexahedron with the indices 13210|. It often happens during the growtli of a crystal, es];ecially if rapid, that foreign minerals are included within the ciystal. These inclusions may be irregularly arranged in various positions and for these the term poikilitic is used. In othei cases there is a regular arrangement of inclusions as, for example, rutile 78 INTRODUCTION TO THE STUDY OF MINERALS Fig. 271. needles in phlogopite mica from Canada. Here the rutile needles are arranged in three directions at angles of about 60°, which are crystallographic directions of the mica (Fig. 271). This mica shows the phenomenon of asterism. When a hole in cardboard is viewed through a thin sheet of the mica held up to the light, a six-rayed star appears. Inclusions are sometimes arranged zonally in bands parallel to the outline of the crystal. In case of rock-salt or halite the mother liquor often occupies a cubic cavity known as a negative crystal and frequently there is a bubble which changes position on moving the specimen. Quartz and topaz also sometimes show a moving bubble. 16. THE MEASUREMENT OF CRYSTALS The starting point for all exact work in the description or determination of crystals is the measurement of their interfacial angles. It is the external or supplement angle that is used rather than the internal angle, the reasons being: (1) the sum of the sup- plement angles of a zone is equal to 360°, (2) it is easier to estimate the supplement angle with the eye, and (3) the angle read off on the reflection goniometer is the sup- plement angle. Interfa- cial angles are measured either by a contact goniometer, which is a protractor made of cardboard or metal, or by an instrument called the reflection goniometer. Fig. 272. — Contact goniometer (2/3 size). IIIK 1-llliM OF MI\h:H.\LS 79 A \<-vy s.'il isfaclnr\- (■(Hiiurt L^diiioniclcr i-, tli;il i|c\'isf'(| li\- I'l'lilii'M wliii-li (-(pii-i.-,!- Ill' a •^\■■Al\\\:^\(■t\ ,~,ciiii-circlc ))rii]tci| ud rai-'lliiianl willi a |ii\c,tci| -lri|) ol cell iilnn I (sec l-'i^ |, jiajic 4). V\'^. Ill is a liK-lal ^zuiiKiiJicI IT liiaili- l)\' I'licss nf licilili. With cai'i' faiily ;:ii(i(| M'-.iih- ai-iaiial c tu ahoiit V' can Ijc uIiI ailicil wilh liii- conlacl ;:nniiiiiii'tc|-, Inil ihr faia^s of l)ic crN'sIa] luiKst Ijf.' siiiDiitli ami III a |i| iri'i'ialili- si/.c. fi L"'i'i'inH'1ir I'll- iiuiic ai-ciiralc woi'k. csf^cciallN' on iiiinulo crystals, llic fofloct ioi] ;/oi]ioi(ii'l IT I-; iiscil. The pfiiiciplo of nii'tisiiifinonl is as follows: ff a l>fi;.'ht faci- of a ciS'stal is lichi close to the eye a I'c fleet lorj of a ilist a nl ohject siief] as a. wimlow liai' can lie old airieil. Or] t.urniijf.' the^ Cf\'s1al aUoiit, ledections afiMilit allied fi'oni othei' fa(;(;s. 'J'Ik; arif^lo tlifouf^h wliicfi the (T'ystal is fi'volveil to olitain th(j images from two adjoining faccH is the supplement angle. so isTitoDi'criox TO riiK stidy of mixeiials The reflection goniometer, i In' iii\ I'lH imi of WOllaston in ISO!), originally consistcil of a \-ci'tical ;;i'a(luatcil ciiclc with a lioi-izoii- tal axis Ijearing the ci'vstal cai'i'icr. In the iiioilei-ii tA'iie of gon- iometer the graduated circh' is horizontal and the axis of revolu- tion is vei'tieal. Model I\', made \>\ Fuess of Bei'lin, Fig. 273, is undouhtedh- the hest lioinomeler for sludent use. .\ eentral Fig. 27-4, — Two-cirr-le goniometer (1/4 size.) a.\is .■< liears a rr\stal caiTici' (axis of t ho .uraduated cirele) with adjustments which are two slidin;^- motions 0/ and r) at right angles and two tipping motions on cireidar ares at riglit angles (fi and //). A collimator .1 with a l>ieonca\'e slit at the end and a telescope /) wdiicdi may he set at any angle to tlie collimator complete the e([uipnient. A source of light, such as a Wclsbach THE FORM OF MINERALS 81 burner, placed at the end of the collimator furnishes a beam of light which is reflected from the crystal, when in a certain position, into the telescope. Looking into the telescope one sees a bicon- cave-shaped imago which may be bisected by the cross-wires in the telescope and then the reading on the vernier of the graduated circle is taken. The crystal carrier with the mounted crystal is revolved until an image is reflected from another face and so on for all faces of a zone. A separate set-up, which consists of making edges parallel to the axis of rotation by means of the adjustments on the crystal carrier, must be made for each zone. Another type of goniometer is the two-circle or theodolite goniometer which consists of two graduated circles at right angles. Fig. 274 shows the two-circle goniometer of C'zapski, made by Fuess of Berlin. Two angles corresponding to the longitude and to the co-latitude of a place on the earth's surface are obtained for each face. The advantage of the two-circle goniometer lies in the fact that only one set-up is required for all the faces on one-half of a crystal. In the case of small crystals with dull faces the polarizing microscope with rotating stage may be used to advantage in measuring angles (see Fig. 333, page 124). A simple reflection goniometer, may be made from the Penfield contact goniometer by fitting a wooden axis through the eyelet, the axis being provided with a wire pointer. Fig. 275 illustrates this device. By holding the goniometer, with the crystal mounted on the end of the axis with wax, so that the Fig. 275. — Simple reflection goniometer. 82 INTRODUCTION TO THE STUDY OF MINERALS intersection edge of the faces is in line with the axis, an image of a distant object, such as a window-bar, on a crystal face is made to coincide with the edge of a table or similar line of reference. The reading of the pointer is taken. Then carefully getting the same image again, the goniometer is held firmly and the axis carrying the crystal is rotated until a similar image is obtained from an adjacent face. The supplement angle is the difference between the two readings. And so on for other faces of the zone. As the protractor includes but 180°, only part of the zone can be measured at one time. 17. THE STEREOGRAPHIC AND OTHER PROJECTIONS OF CRYSTALS In order to study the symmetry and the angular relations of crystals use is often made of a projection of a crystal. By representing each face by a point, their shape and size are elimi- nated and the symmetry made apparent. There are various methods of projection, but the principal one used in crystallography is the stereographic projection. Imagine a crystal within a sphere, their centers coinciding. Radii are drawn normal to the faces of the crystal, intersecting the sphere in points. Each face then is replaced by a point called the pole of the face (Fig. 276) . As it is dif&cult to make measurements on a spherical surface, a projection of the poles is made on a plane. The plane of projection is a plane through the center of the sphere. The projection of any face on this plane is the inter- section of a line drawn from the south pole of the sphere to the pole of the face with this plane. In Fig. 276 the dots are poles of the faces and their projections are the small crosses. The most important property of the stereographic projec- tion is that all circles on the sphere are projected as circles or as straight lines. All vertical great circles appear as diameters of the primitive circle, as the projection of the horizontal great THE FORM OP MINERALS 83 circle is called. All oblique great circles appear as arcs of great circles passing through the extremities of diameters. All faces of a zone appear on the arc of a great circle. The method of projecting poles depends upon their position. Poles on the horizontal great circle are laid off directly on the - s. - -V^A \ A J ^ mh^ ) ~~~ xC J. je' V' \~^ / / Fig. 276. — Illustrating the stereographio projection. primitive circle. Poles on vertical great circles are projected by first laying off angles on the primitive. Lines are then drawn from these points to the south pole on the opposite side (Fig. 277). The projections are the intersections of these lines with 84 INTRODUCTION TO THE STUDY OF MINERALS the trace of the vertical great circle. The poles on an oblique great circle are projected by first finding what is called the pole of the great circle. This is a point on the diameter normal to the great circle and 90° from its intersection with the great circle. This pole is found as indicated in Fig. 278, p being the pole. Angles on oblique great circles are laid off on the primitive. The intersection of lines drawn from these points to the pole of the great circle with the great circle itself is the required projection (Fig. 279). _ A great circle may be drawn when three points are known. The center of the circle is the intersection of the perpendicular bisec- tors of the chords of any two arcs of the great circle. A great circle of long radius may be drawn by a beam compass or by a piece of thin flexible steel bent so as to fit the known points. Fig. 277. Fig. 278. Fig. 279. Fig. 280. An important property of the stereographic projection is that the angles between great circles are projected in their true value. To determine the angle between two great circles draw the great circle (dotted arc of Fig. 280) of which their intersection is the pole. The angle between the two great circles is the same as the interfacial angle on this great circle. This may be determined by the method illustrated in Fig. 279, for the intersection of the two great circles corresponds to the point p. The work of plotting and measuring the angles of a stereo- graphic projection has been much simplified by Penfield. He used large sheets with a graduated circle of 7 cm. radius on which are printed several scales, one for laying off stereographic THE FORM OF MINERALS 85 degrees on a diameter, another for giving radii of various great circles, and another for giving radii of small vertical circles. Besides these sheets he devised several protractors printed on transparent celluloid to fit a circle of 7 cm. radius. One is a great circle protractor giving arcs of great circles for every other degree. This may be pivoted at the center and swung around to Fig. 281. — Penfield's stereographio protractors. pick out all the faces of a zone. Another protractor is of small vertical circles. This laid down on a projection will give the angular distance between any two points on the arc of a great circle. The right half of Fig. 281 shows the great circles for every ten degrees and the left half, the small vertical circles for every ten degrees. The stereographic projection affords a ready means of solving a 86 INTRODUCTION TO THE STUDY OF MINERALS spherical triangle graphically. For example, in Fig. 282 given the angle A and the sides b and c of a spherical triangle, to find the angles B and C and the side a. From any point A, the side b is laid off on the primitive, then a great circle is drawn making the angle A with the primitive by laying off the stereographic degrees on the diameter 90° from A. The side c is laid off on this great Fig 282. — Graphic solution of a spherical triangle. circle and this determines the point B. A great circle BC is drawn. The angle C is measured on a diameter drawn 90° to C and the angle B on a great circle of which B is the pole. The work is very easy if the Penfield sheets and protractors are used. In the example: given A = 49, 6 = 73°, c = 50°; found graphically a = 48°, B = 103°, C = 51°. THE FORM OF MINER AL^^ 87 Another method of projection is the gnomonic projection in which the plane of projection is a plane perpendicular to the c-axis of the crystal and at a unit's distance from the center. This plane is parallel to the 001 face in all except the mono- clinic and triclinic systems. From the center of the crystal lines are drawn normal to the crystal faces and extended to cut the plane of projection. The principal advantages of the gnomonic projection are: (1) that zones are represented by straight lines and (2) that the Miller indices may be obtained directly from the 1 I "x m \ u u 1 '/ i \ /'"' /^ ^ \ \ < / \ \ 2 ,'-' V V"~~~--^^^ / /i \ / V- 4'- ^^ i/J / »J Fig, 283. — Gnomonic projection. projection. Fig. 283 is a gnomonic projection of the topaz crystal represented in Fig. 77. The circle has a radius of 45° (gnomonic degrees) . The prism faces m and I are represented at the edge of the drawing by arrows as they are projected at infinity. The linear projection which is used in connection with crystal drawing, is made up of intersecting lines which repre- sent faces. The lines are the intersections of faces, shifted parallel to themselves so that they cut the vertical axi^, c, at 88 INTRODUCTION TO THE STUDY OP MINERALS unity, with the plane of projection which is a plane through the center of the crystal perpendicular to the c-axis. Fig. 284 is the linear projection of the topaz crystal represented by Fig. 77. Zones are points at the intersection of lines. Fig. 284. — Linear projection. 18. THE DETERMINATION OF THE GEOMETRICAL CONSTANTS OF CRYSTALS In the case of new minerals or new forms of described minerals, it is necessary to determine the geometrical constants of crystals. Although work of this kind may never engage the student in after- life it will be of great advantage to him in the proper understand- ing of crystals. There are three distinct steps in this work, namely : (1) The graphic determination of Miller indices from the measured angles. (2) The calculation of the axial elements from selected measure- ments. (3) The calculation of the theoretical interfacial angles from the selected measurements and the graphically determined indices. For the determination of the Miller indices a graphic method is preferable to calculation, for it furnishes a check on the other work which consists of the solution of plane and spherical triangles. Besides it presents a picture of the mathematical operations THE FORM OF MINERALS 89 involved. 'I'lic graphic inctliod in based upon a stereographic projection. Tlie dctcrininatidii of tlic indices is mucii simplified by making use of zonal rclatioHK. Fiiccs of crystals very commonly occur in several prominent zones. By inspection, the parallelism of edges is the test of a zone. A more accurate method is to test the reflections of various faces in the reflection goniometer. The indices of a zone aic three numbers obtained in the following ■;. 28.'>. — To fxpliiin meaning of a zone syinl>ol. manner from any two faces of the zone, (hid) and {h'k'V) . {kl'-lk') , {Ih'-h'V), (hk'-lch'). These three values, usually written [uvw] in brackets to distinguish them from face or form indices, are the coordinates of a point which determines a parallelepiped puch that its diagonal from the origin to the point mentioned is parallel to the intersection edge of the two faces (Fig. 285) . In the figure the faces are (122) and (212) ; the indices of the zone, [223]. For 90 INTRODUCTION TO THE STUDY OF MINERALS any face (pqr) that belongs to the zone [uvw] the following must be true up + vq + wr = 0. This is known as the equation of zone con- trol. If two indices of a face in a known zone are given, we may then find the third index in this way. If a face (hkl) is common to two zones [uvw] and [u'v'w'] the following relation is true: h = vw'-v'w; k = uw'-u'w; l = uv'-u'v. In all of these careful atten- tion must be given to signs. The method of determinants may be used to ascertain whether any three given faces are in a zone or not. The indices are writ- ten in the form: 1 1 1 "■3 "'3 '3 The sum of the left to right products Qi^ Xk^Xh) +{kiXl2X h^) + {l^Xh^Xk^ must equal the sum of the right to left products {l^Xk^Xh^ -\- {kj^Xh^Xl^ -\- {\Xl2Xk^ if the three faces are in a zone. This fact may usually be told by inspection if the three faces are written in the determinant form as above. A graphic method of obtaining the Miller indices of an ortho- rhombic crystal is indicated in Fig. 286, which represents the cerussite crystal sketched in the lower corner of the drawing. The known faces' areo{100}, m{110}, 6{010}, and/b{011}. The problem is to determine the indices of the faces r, x, i, y, and p. Let us suppose that measurements have been made in the follow- ing zones: [amrb], [xkib], [apk], and [ay]. A stereographic pro- jection is first made from the measurements of the interfacial angles with [amrb] on the primitive circle. The other faces are projected as indicated on page 83. First take the zone of hkO's. A prolonged radius is drawn through m(llO). From the intersection of a tangent at a with this radius a line RT perpendicular to Ob is drawn. This consti- tutes the unit scale line. Intercepts of the radii of hkO and hkl faces on this line are in the ratio oi h:k in terms of the radius ^ Unit facea must be known or assumed. THE FORM OF MINERALS 91 taken as unity. Thus for the radius through r the intercept RT' is JRT. .So the symbol of r is (130). For the zone of Old's a similar method is employed, but the angles must be laid off in another quadrant. By folding this over 90° it takes the position Mb with x at x', k a.t k' , and i at i'. Fig. 286. — Graphic determination of Miller indices. As k is the unit face (Oil), a radius OS is drawn through k'. A tangent from b parallel to OM determines another unit scale line NS. The radius through x intercepts this scale line at S and as NS' = iKH, the indices of x are (012). Similarly the indices of i are (021) for NS" = 2NS. For hOl faces a similar method is employed, but as there is only 92 INTRODUCTION TO THE STUDY OF MINERALS one QiQl) face the position of Q, a possible unit face, must be determined. This is on the great circle through h and p. To determine the indices of the hkl faces use is made of zonal relations. For zones through (100) the ratio k:l is constant for all faces and similarly for zones through (010) the ratio h:l'm con- stant and for zones through (001) the ratio h:k\s constant. So that for all hkl faces the corresponding Qkl, hOl or hkO faces are found by constructing the appropriate arcs of great circles. In the figure the zone circles akp, bpQ, and Opm (a straight line) are drawn. So that the symbol for p is (411) as it satisfies both k:l = 1:1 and h:k = 1:1. The zone circle through b and p deter- mines Q, the pole of a possible face (101). To determine the symbol of y the zone aO is folded over to oU, the pole y and the pole of a possible face Q taking the posi- tions y' and Q'. Radii through y' and Q' are drawn and another unit scale line VW is established. As the distance VW = ^VW the symbol of y is (102). Fig. 287 is a diagram' used to determine the symbol of faces in a given rectangular zone if the symbol of one of them is known. Its construction is simple. In a square of convenient size radii are drawn from one corner for every other degree. From the top line perpendiculars are dropped for each of the radii. Hori- zontal lines divide the sides of the square into fractional parts corresponding to the more common indices, namely: 1:6, 1:5, 1:4, 1:3, 2:5, 1:2, 3:5, 2:3, 3:4, 4:5, and 5:6. The method of using the diagram may be illustrated by an example. In cerussite the angle (001 :011) is 35° 52'. Along the radius for 36°, the horizontal line reading 1 : 2 (on the left) cuts the vertical line for 20°. The calculated angle for (001:012) is 19° 52^. The angle for (001 : 021) is found at the intersection of this same horizontal line (it reads 2 : 1 on the right) with the vertical for 36°. This intersection is on the diagonal for 55° 30' ' G. H. F. Smith describes a similar device called the moriogram. Mineralogical Maga- zine, Vol, 14, p. 49, 1904, THE FORM OF MINERALS 93 (caliulatcd :..j° 2(J'). The value of the c-axis is given at the bottom of the vertical through 36° as 0.725 (calculated 0.723). a\ga\ iiun Unit Angle 45 y1 Fig. 287. — Diagram for determining indices of crystal faces. In the preceding example the unit angle was less than 45°. If the unit angle is greater than 4.5° the diagram is turned upside down. The readings for angles less than the unit are made on the 94 INTRODUCTION TO THE STUDY OF MINERALS verticals, the indices being found on the left while the readings for angles greater than the unit are given on the radii with the indices on the right. For example, in barite (001 : Oil) =52° 43', (001:012) as found = 33° 15' (calc. 33° 18') and (001:021) as found = 69° (calc. 69° 9J'). In this case c from the scale at the bottom (now the top) is about 1.32 (calc. 1.313). For establishing the axial elements, which consist in the most general case of the axial ratios a:b:c and the axial angles a, ^, and ]', it is necessary either to take the measurement of selected faces (usually unit faces) and use these for the calculation or to combine the measurements of all the faces, weighting those from the best signals. The formulae used in the simpler cases are given below. After the axial elements have been established, these values and the graphically determined indices for the various faces are sub- stituted in the formulae and the theoretical angles obtained become part of the record for the mineral. The same formulae may be used throughout. Thus the formula a = t; tan {100:hkO) may be solved for the axis a, for the angle (100 -.hkO) and even for the ratio h : k, though a graphic determination of the indices is preferable to calculation. Formulas used in Simple Cases Orthorhombic System : a = tan(100:110)=r^ tan(100:M0) a = cot(010:110) =^ tan(010:M0) From Fig. 288 it may be seen that UhkO-hkO) = (lOOihkO) and i{hkO:hkO) = {OlO:hkO). c = tan(001:011)=^ tan(001:OA;0 c = cot(010:011)=|-cot(010:0H) THE FORM OF MINERALS 95 From Fig. 289, ^(()/,/:0A;0 = (001 :0/cO and KOH:0/cl) = (010:0W). '' =tan(001:101)=^ tan(001:M)0 a n ^-=cot(010:101)=^ cot(010:A00 a n From Fig. 290, \{mi -Ml) = {001 -Ml) and \{mi:hOl) = {om-.mi). Fig. 288. Fio. 289. Fig. 290. Monoclinic System : /3= 100 A 001 _ cot(010:110) _ A cot(01Q:/t/tO ) sin ;9 /c sin /? tan(100:110)_/i tan(100:fe/cO) sin p k sin ;9 tan(00 1:011 ) _ I tan(001: OfcO sin^ A; sin^ a = c = _cot(010:011)_i cot(010 :0/cO ~ sin/? /c sin/3 c _ sin(0 01:101)_ I tan(0 01:AOO a ~sin(100ri01) ~^ sin(100TA00" Triclinic System. The mathematical relations are very com- plex here. Five independent angles are necessary to establish the five axial elements a, c, a, /3, and /- (6= 1). 96 INTRODUCTION TO THE STUDY OF MINERALS Tetragonal System : c= tan(001 :011) =|- tan(001 :OA;0 c = cot(010:011)=^cot(010:0/cO c = \/2'tan(001:lll)=^ \/2 ta.r\.{mi -.hhl) c = \/2coi{nO:lll) = ^^\/2cot{nQ:hhl) The angles in the prism zone are constant for all tetragonal crystals. Some of the prominent angles are: (100: 110) =45° 0' (100:430) =36 52 (100:320) =33 41 (100:210) =26 34 (100:310) = 18 26 Hexagonal System : c = cos 30° tan(0001 : lOTl) =-^ cos 30° tan(0001 :^0^0 c = tan(0001 : 1 122) =^ tan(0001 : h-h-2h-l) The angles in the prism zone are constant for all hexagonal crystals. The prominent angles are: (1010:1120) =30° 0' (1010:2130) = 19 6 A (1010:3140) = 13 54. Isometric System. There are no axial elements to be deter- mined. All isometric crystals have the same angles for corre- sponding forms. The more prominent angles are: (100:111) =54° 44'; (100_: 110) =45° 0'; (111 : 110) =35° 16'; (110:101) =60° 0'; (111 : 111) =70° 32'; (210: 100) =26° 34'; (211: 121) =48° Hi'. For other isometric angles see Dana, System of Mineralogy, Qth edition, pp. xx-xxvi. THE FORM OF MIAEHALS 97 19. THE REPRESENTATION OF CRYSTALS llepix'seiitations of crystals may be made by means of models or drawings. On account of their size and ideal symmetry crystal models are extensive]}' used in the elementary study of crystallography. Although seemingly njore perfect than crystals, yet in reality they are crude in comparison. The angles are never as exact as in crystals and they fail to give the clue to symmetry by differences in luster, striations, and other markings on various Fig. 2!) 1. -Crystal net. faces that crystals do. frystal models are njade of wood, glass, pasteboard, or plaster of Paris, Those most used are the mod- els made from pear-tree wood by German dealeis. A drawing like Fig. 291, which cut out and pasted together, forms a hollow pasteboard model is called a crystal net. The student will find it very instructive to make crystal nets for the simple crystals of various systems. These can be filled with plaster of Paris and the paper removed when the plaster has solidified. 7 98 INTRODUCTION TO THE STUDY OF MINERALS The principal purpose of a crystal drawing is to give an idea of the crystal habit. While the various forms are represented in their relative size and shape the drawing represents, as a rule, a crystal in ideal symmetry free from distortion. The reason for this is that the ideal drawing stands for crystals with all possible distortions. In some cases where the crystal has a peculiar habit or where a single crystal is at hand, the faces may be shown in their actual develop- ment. Fig. 292 illustrates a topaz crystal drawn in this way. Fig. 292. — Clinographic drawing. Fig. 293. — Orthographic drawing There are two kinds of crystal drawings; the orthographic in which the projectors are normal to the plane of projection which is usually a horizontal plane, and the clinographic in which the projectors are inclined to a plane of projection which is usually a vertical plane. An orthographic drawing is made by dropping perpendiculars from the vertices to the plane of projection. All faces perpen- dicular to the plane of the drawing appear as lines inclined to THE FORM OF MINERALS 99 each other at their true angles. Horizontal edges of the crystal appear in their true length, but oblique edges are foreshortened. An orthographic projection gives a good idea of one end of a crystal, but it does not show how steep the oblique faces are or how prominent are the faces perpendicular to the plane of the drawing. But b}' combining two orthographic projections made on planes at right angles to each other, a plan and an elevation are obtained, which though lacking in the appearance of solidity, together give a good idea of the habit. Fig. 293 is a plan and Fig 294. Fig. 'Mr,. Fig. 296. front elevation of a topaz crystal with the forms cjOOl), )/(041}, wIllOj, Z{120), i(223), and m(111}. It will be noticed that corresponding points in the plan and elevation are all connected by parallel vertical lines, which fact facilitates the construction. The usual method of representing crystals is by means of a clinographic drawing made in parallel perspective instead of ordinary perspective as the parallelism of edges is one of the prominent features of crystals. The projection is made on a vertical plane by inclined projectors taken so that one sees both the top and the right side of the crystal. The clinographic drawing may be made either from an axial cross or directly from a stereographic projection. An isometric axial cross (Fig. 294) is first constructed so that the angle between OA and OC is. 116° 17', and that between 100 INTRODUCTION TO THE STUDY OF MINERALS OB and OC is 93° 8', the proportional lengths of the axes being 0.1 :OS:OC = 37:100:104. This is equivalent to a rotation of 18° 26' to the left and to a tipping forward of 9° 28'. This axial cross is modified for other systems. For example, the axial cross for an orthorhombic crystal with the axial ratio G-8:l:l-3 is 08X37 : 100 : 1-3 X 104. In the hexagonal system there is a third lateral axis. In Fig. 29-5 make OS equal to 1-732XOA. Join S with B and B'. Bisect B»S and B5' at the points R and Q. ROR', QOQ', and BOB' are the three lateral axes. In the monoclinic and triclinic systems the angles between the Fig. 297. — Showing method of clinographic drawing. axes are also modified. For the angle ^ between a and c the position of the a-axis is changed as follows: On the axis OC (Fig. 296) the distance OM = cos^xOC is laid off and on the axis OA the distance 0A^ = sin/3 XOA is laid off. Then the 6-axis is the line QOQ' , QO being the diagonal of a parallelogram MQNO. The lengths of the axes are modified as in the other systems. After the axes are projected in their proper positions the next step is to plot a linear projection of the crystal on these axes by taking the reciprocal of the Miller indices and then n:aking the third term equal to unity. The desired direction of the intersection edge of the two faces is a line joining the intersection of the linear THE FORM OF MINERALS no-.iu no-.iii 00 /OO.-JII 100.110 Fig. 298. Ill 110 100 111 110 010 Fia. 299. 102 INTRODUCTION TO THE STUDY OF MINERALS projection of the two faces with the extremity of the vertical axis. Fig. 297 shows the method of construction of the clino- graphic drawing of the wernerite crystal of Fig. 299. The dot- and-dash lines are the axes of reference; the heavy lines, the linear projection constructed on the axial cross; while the dotted lines are the directions of the intersection edges. The direction intersection of faces like (111) and (110), which do not intersect, is simply the direction of the lines. In Fig. 298, representing the construction of the wernerite crystal of Fig. 299, use is made of a stereographic projection. The plane of projection is an oblique plane in which the angle of rotation is .18° 26' (tan"' J) and the tipping angle, 9° 28' (tan"' J). These angles determine the point m. A great circle SmS' is drawn. N is the pole of this arc, wN equaling 90° (stereo- graphic degrees) . The intersection of a zone with the faces (1 1 1) and (111) with this great circle is the point r. A line is drawn from N through r intersecting the primitive circle at t. A line per- pendicular to the radius through t is the desired intersection edge of the two faces (111) and (111) and all other faces of the zone. The advantage of this method is that nothing need be known of the indices or axial elements. PART II THE PHYSICAL PROPERTIES OF MINERALS The crystal form is an outward expression of an internal molec- ular structure, anil even when the external form is wanting the crystalline nature may usually be determined by some physical property. Some of the physical properties such as specific gravity are independent of the direction and are called scalar properties, while others such as cohesion and effect of light, heat, and electricity can be represented by a line of given length and direction, hence the term vectorial properties. A crystal may be defined as a homogeneous solid body the properties of which are the same in parallel directions, but in general are different in non- parallel directions. In this part of the book the simpler physical properties such as cleavage, hardness, and specific gravity are considered, while the optical properties are left for another part. The thermal and electrical properties of minerals are of very little practical impor- tance and are therefore omitted. 1. Cleavage Many crystals have the property of breaking with smooth .surfaces in certain directions which are parallel either to crystal faces or to possible crystal faces. This important property is called cleavage. Galena, which usually crystallizes in cubes, has a cubic cleavage (Fig. 300) while fluorite, also crystallizing in cubes, has an octahedral cleavage (Fig. 301), but the octahedron is a form sometimes found on fluorite. Cleavage is defined accord- ing to the direction as cubic, rhombohedral, prismatic, etc., and according to the character of the surface, such terms as imperfect, ins 104 INTRODUCTION TO THE STUDY OF MINERALS good, perfect, and very perfect being used. Thus the micas have a very perfect cleavage parallel to (001) (Fig. 302), while the feldspars have a perfect cleavage parallel to (001) and good cleav- age parallel to (010) (Fig. 303). Fig. 300. — Cubic cleavage. Fig. 301. — Octahedral cleavage. Fig. 302. — Mica cleavage. Fig. 304. — Barite cleavage. Fig. 303. — Feldspar cleavage. Fig. 305. — Calcite cleavage. Cleavage conforms to the symmetry of the crystal. In barite, an orthorhombic mineral, the cleavage is perfect in one direction parallel to (001), and less perfect intwo directions parallel to (110) THE PHYSICAL PROPERTIES OF MINERALS lOo (Fig. 304). In gypsum there is a very perfect cleavage parallel to (010), an imperfect cleavage with conchoidal surface parallel to (j^OO) and an imperfect cleavage with fibrous surfacf parallel to (111). The relation of a cleavage fragment to a crystal is shown in Fig. 30(5, the inner rhombic figure being the result of cleavage. .\ line normal to tlie paper is an axis of two-fold symmetry for the cleavagf- fragment as well as for the crystal. In fluorite the octahedral cleavage conforms to the s\ninietry of the hexoc- tahedral class of the isometric S3'stem. In calcite, whatever the shape of the crystal, the cleavage is perfect rhombohedral in three directions at angles of 74° .j.j' to each other. Fig. 30.5 represents a cleavage of calcite with three surfaces and intersecting cleavage traces on each. Cleavage is a fairly constant property of minerals and is invaluable in the rapid recog- nition of minerals. Such minerals as calcite, fluorite, feldspars, amphiboles, and gypsum are distinguished principally by their cleavage. On the other hand, such minerals as quartz and garnet, possess no cleavage. They break with an irregular fracture. In chalcedony the fracture is con- choidal (curved like the interior of a shell). Other terms ap- plied to fracture are splintery, hackly, even, and uneven which are self-explanatory. A table giving cleavage directions of important minerals will be found on pages 219-222. 2. Parting This term is applied to the separation due to some molecular disturbance such as twinning. Cleavage may be obtained in any part of a crystal in the given direction, the size and the number of the cleavage particles being limited only by mechanical appli- FiG, 306.— Relation of f-'leavage to crystal (gypsum). 106 INTRODUCTION TO THE STUDY OF MINERALS ances. Parting, on the other hand, only takes place in certain definite planes, those of the molecular disturbance. In a cubic cleavage of rock-salt, if pressure is applied in a direction normal to a vertical diagonal plane, a surface normal to the direction of pressure is developed, the shaded plane in Fig. 307. In this case we have an example of dodecahedral parting. If pressure is applied by a dull knife edge normal to the obtuse edge of a cleav- age rhombohedron of calcite a small portion of the calcite will be reversed in position, forming a twin with (0112) as twinning Fig. 307. — Halite parting. Fig. 308. — Calcite parting. plane. This small portion of calcite may be easily removed, the parting being parallel to (0112) and produced in this case by twinning. This is illustrated in Fig. 308. Prominent examples of parting are the following: basal part- ing (001) in diopside (Fig. 94, page 35), basal parting (001) in stibnite, octahedral parting (111) in magnetite, and rhombohe- dral parting (0112) in calcite. 3. Tenacity The terms brittle, tough, and malleable are used in describing minerals. A malleable substance is one that can be cut by a knife and also flattened out with a hammer. Gold is so malleable a sub- stance that it may be beaten into sheets about one-three hundred thousandth of an inch thick. A sectile substance is one that can be THE PHYSICAL PROPERTIES OF MINERALS 107 cut by a knife, but not flutteiicil out with a hammer. The term flexible refers to the fact that the mineral bends easily and re- mains bent, while in contrast elastic means that the mineral after being bent returns to its original position. The micas are elastic while chlorite is flexible. 4. Hardness The resistance that a substance offers to abrasion is called hardness. It is not a property that is capable of exact definition or measurement, but comparative tests are expressed in terms of a so-called scale of hardness. The scale of hardness consists of ten minerals ranging from talc, a mineral which has a soapy feel and is very easily scratched by the finger nail, up to diamond, the hardest known suljstance. The minerals are numbered as follows; 1 Talc Knife Blade 2 (iypKum 6 Orthoclase I inger Nail 7 Quartz 3 falcite 8 Topaz 4 Fluorite 9 Corundum 5 Apatite 10 Diamond The finger nail is about 2 J, for it scratches gypsum, but is scratched by calcite. A knife blade is about 5 J for it scratches apatite, but is scratched by orthoclase. The hardness of a mineral is judged both by its effect on the minerals of the scale and their effect upon it. If a mineral scratches fluorite but is scratched by apatite, it has a hardness of 4 j. Two minerals of the same hardness will scratch each other. Great care should be used in determining the hardness. A foreign substance embedded in the. mineral will often give too high a value. A soft mineral leaves a chalk-mark on a harder one, so the mark left by a mineral should be a distinct groove. Minerals made up of grains or fibers often appear too low simply because the particles are forced apart. Thus a sandstone made 108 INTRODUCTION TO THE STUDY OF MINERALS up of sand grains with hardness of 7 may appear to have a hard- ness of about 3 because the grains are rather loosely cemented. The value recorded in the description of minerals is the maximum value for crystallized varieties. For example, hematite in crystals has a hardness of 6, but some massive varieties have a hardness of 3 or even less. In some minerals the hardness varies with the direction, as it does for all theoretically. Cyanite in a direction parallel to the c-axis has a hardness of 4J, while at right angles to this the hard- ness is about 7. For most faces calcite has a hardness of 3, but on the basal pinacoid (0001) the hardness is about 2, being scratched by the finger nail. 5. Etch-figures An important method of determining the symmetry of a crystal is by means of etch-figures. When a crystal is acted upon by a solvent, the action is not uniform, but begins at certain points and proceeds more rapidly in some directions than in others. If the action is stopped at the right time the faces of the crystal are usually found to be covered with little angular figures of definite shape and orientation called etch-figures. The etch-figures are usually shallow depressions bounded by minute faces. The fact that these faces are often general forms enables one in many cases to determine the crystal class. Examples are given in Figs. 129, 146, and 147. Without etching it would have been impossible to assign these substances to their proper crystal class. The shape of the etch-figures varies with the solvent, time, and temperature, but whatever their shape they always conform in symmetry to the class to which the crystal belongs. On simi- lar faces the etch-figures are alike and on dissimilar faces they are unlike. The faces of etch-figures lie in well developed zones, but they have high indices. There is no rule to follow in obtain- ing etch-figures as it is simply a question of ease of solution. Crystals soluble in water may give them by passing a moistened THE PljysiCAL PHOPEHTIES OF MISEUALH 109 cloth over tlie surface while soiiio refiactory minerals, such as topaz, require fused caustic potash. 6. Specific Gravity The dcns.ty of a substance compared witii llic density of water under standard conditions (4'=' C.) is called the specific gravity. A specific gravity of 3 means that the substance weighs three times as much as an equal volume of water. The specific gravity is the weight of a substance divided by the weight of an equal volume of water. Fise methods of finding the specific gravity are described. (a). A rough, but rapid, method is to weigh out a gram of the mineral and then to find its volume with a burette, care being taken to eliminate air-bubbles. If one gram is used the specific gravity is the reciprocal of the volume. (6). A more accurate melliod is based on the fact that a body immersed in watei' loses in weight an amount equal to the weight • -'^"^^ , _ n^^ Fig. 309. — Specific gravity tjalance. of the water displaced. The substance has the wejirlit .1 in air, say. Suspended by a fine thread in a \esHel of water, it has the weight W. Then (1= <1 being the specific gravity. Nu- merous precautions must be taken to insure accuracy. (c) The most convenient specific gravity balance for the prac- tical identification of minerals is that represented in Fig. 309 which was designed by the author and is used in his laboratory. It con- 110 INTRODUCTION TO THE STUDY OF MINERALS sists essentially of a brass or wooden beam supported near one end by a knife edge. The short arm carries two pans, the lower one to be immersed in water. The end of the long arm rests within a guard, which limits the motion of the balance. The long arm of the beam is graduated so that the specific gravity may be read off directly. This may be done by always placing the counterpoise in the notch near the end of the long arm when weighing in air. Whatever the weight of the counterpoise its distance (x) from the fulcrum, and the distance (y) of the counterpoise from the fulcrum when weighing in water are connected by the equation, G = — , G being the specific gravity. The distance y for various x—y values of G is marked on the beam. Thus in the balance figured, X is 15 inches. Then if Gis2, y is 7.5. So 2 is marked at a point 7.5 inches from the fulcrum. If G is3, y is 10. So 3 is marked at a point 10 inches from the fulcrum. The balance is adjusted by a device just above the fulcrum. When in adjustment the balance will look like the figure, the lower pan being immersed in water and the long arm of the balance free. The mineral is placed on the upper pan and the counterpoise in the notch near the end of the long arm. Wire loops are added to the hook of the counterpoise until the mineral is balanced. Then the mineral is transferred to the lower pan. It is well to moisten the mineral before immersion so as to rid it of air bublsles. The mineral will lose weight so the counterpoise is moved toward the fulcrum until balance is restored. The specific gravity is indicated directly on the beam. This method is rapid and the results are accurate enough for the practical purposes of determination. (d) For accurate work a pycnometer or specific gravity flask may be used. The pycnometer itself is first weighed (A). The coarsely powdered mineral is introduced into the pycnometer and another weighing (B) made. The flask is then filled with distilled water, air bubbles being eliminated by boiling. After cooling, the weight (C) is taken. Then the flask is emptied and filled with distilled water, weight' (D). Then G= ',~ — 77. With THE PHYSICAL PROPERTIES OF MINERALS 111 proper precautions this method is very accurate. Fibrous or porous minerals should be finely powdered, otherwise the value is too low. (e) A number of heavy liquids are useful in determining the specific gravity. Methylene iodid, CHjIj, has a specific gravity of 3.3 and may be diluted with benzol (sp. gr. =0.98), forming a liquid with any desired intermediate specific gravity. A water solution of potassium mercuric iodid, KI-Hglj, also called Thou- let solution, has a specific gravity of 3.19 and may be mixed with water in any proportion. The specific gravity of a mineral may be determined by diluting these liquids until fragments of the mineral neither sink nor float, but remain suspended. A West- phal balance is used to determine the specific gravity of the liquid. The heavy liquids are especially useful in separating mixtures of minerals for the purpose of analysis. A table showing the specific gravities of the common and important minerals is given on pages 229-232. 7. Luster Luster is the term applied to the quality of light reflected from a substance. Metallic luster is the brilliant luster of metals possessed by most sulfid minerals such as galena, pyrite, etc.,' as well as some oxids such as hematite and magnetite. Minerals with metallic luster are opaque even on the thinnest edges. Adamantine is the brilliant luster of transparent or translucent minerals with high index of refraction. Examples are diamond (n = 2.41) and cerussite (n= 1.80-2.07). Vitreous is the luster of broken glass possessed by most transparent or translucent minerals such as quartz, calcite, etc. Pearly luster is due to continued reflection from a series of parallel plates and is pos- sessed by minerals with eminent cleavage such as gypsum and talc. Silky luster is due to fibrous structure and is illustrated by fibrous serpentine and fibrous gypsum. Waxy, greasy, pitchy, and dull are self-explanatory terms used to describe luster. 112 INTRODUCTION TO THE STUDY OF MINERALS 8. Color In some minerals such as cinnabar, orpiment, malachite, and azurite the color is a property of the substance and hence is constant. The color of a metallic mineral is always quite con- stant, but as these minerals are susceptible to tarnish a fresh fracture should always be observed. But in the majority of non-metallic minerals the color is due to some impurity which usually exists in very small amounts and varies in different localities. Thus quartz, calcite, and fluorite, if pure are colorless, but they are found in practically all colors and the color may even vary in the same specimen. A table for determining minerals by color will be found on pages 223-227. 9. Streak The streak of a mineral refers to the color of its powder. It may be determined by rubbing a corner of the mineral on a piece of unglazed porcelain called a streak-plate. In the absence of a streak-plate a smooth piece of light colored flint or chert may be used. A thin slab of novaculite also makes an excellent streak- plate. The streak, though colorless for most non-metallic minerals and dark-gray or black for many metallic ones, is especially valuable in the determination of a few common minerals such as hematite (streak, red-brown), and limonite (streak, yellow brown) . PART III THE OPTICAL PROPERTIES OF MINERALS Among llip phy.sical pro])eitic« the optical properties take first rank in the accurate description and determination of minerals. Most of the ojjtical determinations can be made by means of a special form of microscope known as the polarizing microscope, but for the more accurate determination of the optical constants the refractomcter, the goniometci-, and the axial angle apparatus must be used. Minerals for optical determinations may be prepared in three different form.s: (1) oriented sections or sections cut in definite crystallographic directions, (2) thin rock sections by means of which minerals in fine grained rocks may be determined, and (3) fragments, cleavages, and minute crystals. As fragments are easily prepared simply by crushing the mineral, the method is a general one for the examination of all but opaque minerals. The polarizing microscope should have a place in the mineralogical laboraUinj and should supplement the blowpipe in the determination of ininends. 1. THE NATURE OF LIGHT It is now generally believed that light consists of a vibratory motion or some kind of disturbance in the ether, a hypothetical medium which is supposed to pervade all space and even material bodies. The wave-motion, as it is called, is regarded as the resultant of simple harmonic motion and a uniform motion at right angles to this. Fig. 310 explains the wave-motion. Sim- ple harmonic motion is uniform motion in a circular path as it would appear on a diameter of the circle. A point moving from 8 113 114 INTRODUCTION TO THE STUDY OF MINERALS a to d appear§jto move from a' to d. This constitutes a periodic vibration with varying velocity which may be represented by the swinging of a pendulum. If this is compounded with linear motion from A to A' we have the harmonic curve ADGJ A', which is a sine curve represented by the equation y = a sin ~jfX, in which a is 2n OD, called the amplitude, and -=r, the angular velocity in the circle. Thus light consists, to the best of our knowledge, of a periodic vibration (or some kind of disturbance in the ether) transverse to the direction of transmission, though we know nothing of its physical nature. It may be illustrated by the waves observed along the sea-shore. A floating object, in general, simply moves up and down, while the wave as a whole advances toward the shore. d Fig. 310. — Wave motion. The maximum displacement, OD (Fig. 310), is called the amplitude of the vibration. The point with the maximum upward displacement, D, is called the crest of the wave and the point with maximum downward displacement, J, the trough. The distance be- tween two successive crests or troughs, or a corresponding distance such as AA', is called the wave-length (denoted by the Greek letter, ^). By phase is meant the position of any point and the direction in which it is moving. Two points are in the same phase when THE OPTICAL PROPERTIES OF MINERALS 115 they are in the same relative position and moving in the same direction. The phase difference of two points is the distance apart of their ordinates in terms of the wave-length. . A ray of light is a line to indicate the direction of transmission of the wave motion. A wave-front is the surface determined by all the parts of a system of waves which are in the same phase. A ray is perpendicular to its wave-front. The intensity of light depends upon the amplitude of the vibra- tions, and the color of the light depends upon the wave-length of the vibrations. The wave-length for the violet end of the spec- trum is 380/z// (millionths of a millimeter) and for the red end of the spectrum, 760/*^ (millionths of a millimeter). White light is the sum of light of various wave-lengths. For this reason monochromatic light (light of approximately one wave-length) must be employed in all accurate optical work. The simplest method of obtaining monochromatic light is to ignite a sodium, lithium, or thallium salt on platinum wire by which is pro- duced yellow {589 /i/i), red {670fi/i), or green (535///i) light respectively. The sodium salt is the one most frequently used. 2. REFRACTION OF LIGHT When light passes from one medium into another, in general there is a change of direction. This is known as refraction. A familiar illustration is the apparent bending of a stick in water. In Fig. 311 the angle i is called the angle of incidence and the angle r, the angle of refraction. There is found to be a constant relation between the sines of these angles and whatever the direction of sin i/ transmission, —. — = n (a constant) . The constant, n, which is ' smr- . called the index of refraction depends upon the substance and upon the kind of monochromatic light used. The index of refraction for the violet end of the spectrum is greater than for the red end of the spectrum as shown in Fig. 312. The following 116 INTRODUCTION TO THE STUDY OF MINERALS table gives the indices of refraction (yellow light) for some of the common minerals: Fluorite Quartz Apatite Garnet Sphalerite 1.43 1.55 1.63 1.76 2.37 Substances with a high index of refraction (1.9 or over) have the brilliant appearance called adamantine luster, while minerals with lower index of refraction have ordinary vitreous luster. Several particular directions of transmission should be men- tioned. In the formula -. — =-n, if i=0°, r = 0°; so for normal SIU T incidence there is no refraction. If i = 90°, the equation becomes sinr = -; r for this particular value is called the critical angle. The critical angle is a constant for the substance. A graphic determination of this angle is shown in Fig. 313. The indices / / v \ \ Fig. 311. FiQ. 312. Fio. 313. of refraction of the two substances are the radii of two concentric circles, AB being the boundary between the two substances. A tangent is dropped from the intersection of the inner circle with the boundary line. A radius is then drawn through the point where this tangent intersects the outer circle. The angle r is the critical angle. Rays of light passing from the denser (lower) medium to the THE OPTICAL PROPERTIES OF MINERALS 117 rarer (upper) medium along the line PO will graze the surface OA. Rays of light passing from the denser into the rarer medium at angles greater than the critical angle cannot enter the rarer medium, but are reflected back into the denser medium as illus- trated by the dotted line of Fig. '-iVi. This phenomenon is called total reflection. An empty test-tuVje immersed in a beaker of water has a peculiar silvery appearance because of total reflection. This silvery reflection disappears when the test-tube is filled with water. The great brilliancy of the cut diamond, as compared with natural crystals of the uncut diamond, is due principally to the fact that the facets are arranged so that most of the light is totally reflected, the critical angle for diamond being very small (24° as compared with 48° for ordinary glass) . The simplest means of determining the index of refraction of a mineral is hj means of the Becke test. Fragments of the min- eral are embedded in a liquid of known index and examined on the stage of a microscope with the substage lowered. On focus- ing sharply on the edge of the mineral and then throwing it slightly out of focus by raising the microscope tube, a bluired white line will appear on the side of the substance having the greater index of refraction. On lowering the microscope tube the white line appears on the side of the substance having the smaller index of refraction. By using a numl)er of liquids the index may be obtained within certain limits. The most useful liquids are as follows: Water 1.33 Oil of cloves 1.53 Bromoform 1..59 «-.Vronobromnaphthalin 1.66 Methylene iodid 1.74 The Becke test may be explained by referring to Fig. .314. S and S' are two substances in contact along a vertical boundary, S' having the greater index. A cone of light rays from the 118 INTRODUCTION TO THE STUDY OF MINERALS 789654 32 1 mirror passes upward through the two substances. Light will go from each substance into the other. The rays 1 to 6 will all go into the denser medium S.' Of the rays 7 to 12, 10 to 12 will go from the denser substance S' into the less dense substance S, while the rays 7 to 9 will be totally reflected, provided the critical angle is such that a ray between 9 and 10 grazes the boundary. Then on changing the focus slightly to the dotted line shown in the figure, there is a concentration of light on the side of the denser substance S', which constitutes the white line. The index of refraction may also be judged by the appearance of the fragment in the liquid. If the fragment and the liquid have about the same index of refraction, the fragment will appear smooth and will scarcely be visible. This is called low relief (Fig. 316). If, on the other hand, the indices of refraction of the two substances are quite different the surface of the fragment 1 2345J 7891011 Fig. 314. — Explanation of Becke test. Fig. 315. Fig. 316. Fig. 317 will appear rough and the borders dark. This is called high relief. It should be noted that the fragment will have high re- lief whether its index is less (Fig. 315) or greater (Fig. 317) than that of the liquid. THE OPTICAL FROPEHTIES OF MINERALS 119 3. POLARIZED LIGHt The light transmitted through a slice of a colored tourmaline crystal cut parallel to the c-axis has acquired peculiar properties as may be seen \>y allowing the light from one tourmaline to fall upon another similar tourmaline. When similar dircftions of the two tourmalines are parallel, a maximum amount of light is transmitted, while if similar directions of the tourmalines are perpendicular no light at all is transmitted. The simplest expla- nation is that the light is vibrating in one plane only. Whether the vibration plane is the plane of the c-axis or a plane perpen- dicular to this we do not know, but we may assume it to be the plane of the c-axis.' ' '. .1 ' ' ' '' ■'t T Fig. 31 S. — Polarization by absorption. Fig. 319. — Polarization by refraction. In ordinary light the vibrations are in all planes, while the light transmitted by the tourmaline slice is in but one plane. This light is known as polarized light. Fig. 318 is a diagrammatic representation of ordinary light and polarized light as trans- mitted Vjy tourmaline. The light reflected from non-metallic surfaces such as glass or polished wood is more or less polarized. If the light reflected • Homn physiciHts assume that the other plane is the vibration-plane. According to the electro-magnetic theory oi light thiro are disturbances in both planes. In the plane of the c-axis there is an electrical disturbance, while in the other pkne there is a magnetic diHf urbanee 120 INTRODUCTION TO THE STUDY OF MINERALS from a sheet of glass is examined with the tourmaline, it will be found that for a certain angle of incidence light is almost extin- guished when the c-axis of the tourmaline is parallel to the plane of incidence, while light is fully transmitted when the direction is perpendicular to the plane of incidence. This means that the reflected light is partially polarized, the plane of vibration being perpendicular to the plane of incidence as shown in Fig. 320. Fig. 320. — Polarization by reflection. Another method of producing polarized light is by continued refraction through a series of parallel glass plates. The emerging light is partially polarized and vibrates in the plane of incidence. Fig. 319 shows this as well as the fact that the reflected light is polarized and vibrates in a plane normal to the plane of incidence. The most practical method of obtaining polarized light is by means of a Nicol prism, but this involves a discussion of double refraction which is the next topic. 4. DOUBLE REFRACTION If a dot is viewed through a clear cleavage rhombohedron of calcite (Iceland spar) two images of the dot are seen and on revolution of the calcite one dot remains stationarj-, while the other dot appears to revolve around the fixed one. A ray of light gives rise to two rays, the ordinary ray (the fixed one) and the extraordinary ray (the one that revolves). In Fig. 321 the image of the ordinary ray is marked o, and that of the extraordi- THE OPTICAL PROPERTIES OF MIXERALS 121 nary e. This phenomenon is known as double refraction and though possessed by most minerals, only in a comparatively few is it marked enough to be seen witii the naked eye. Fio, a24a. Fig. .324b. Figs. 321-.324b. — Double refraction in calcite. Light that emerges from a doubly refracting substance such as calcite has acquired peculiai- properties as may be demonstrated by examining this double image with a tourmaline section cut parallel to the c-a.\i.s. \\'hon the c-axis of the tourmaline is Fig. 32.5. Fig. .326. Fig. .327. Fig. 328. Fig. 329. parallel to the long diagonal of the cahitf rhomb (Fig. 322) only the image due to the ordinary ray, o, is seen, but when tho c-axis of the tourmaline is parallel to the short diagonal of the cal- cite rhomb (Fig. 323) only the image due to the extraordinary 122 INTRODUCTION TO THE STUDY OP MINERALS ray, e, is seen. Hence for the ordinary ray the vibrations are in the plane of the long diagonal and for the extraordinary ray the vibrations are in the plane of the short diagonal. See Figs. 324a and 324b. If the double image formed by a piece of Iceland spar be viewed through another piece of Iceland spar, in general four images are visible, pairs of which wax and wane in turn as one of the Iceland spars is revolved. In certain positions 90° apart, only two images appear. Figs. 325-329 show diagrammatic ally the changes that take place. The symbols o and e refer to images produced by the first rhomb, while with the second rhomb o gives rise to Og and o^ and e to Bg and e^. This behavior is good evidence, if not proof, that in doubly refracting crystals light is polar- ized and is vibrating in two planes at right angles to each other. In Figs. 325-329 the short lines through the circles indicate the vibration planes. 5. THE NICOL PRISM The principal device for producing polarized light is a Nicol prism, so-called from the name of its inventor, Nicol. It is a piece of apparatus to which we are indebted for much of our knowledge of crystal optics. A clear piece of calcite or Iceland spar (this variety is obtained almost exclusively from cavities in the basalt at a certain locality in Iceland) of suitable di- mensions is cut through in a plane at right angles to the principal section and 93° to the terminal faces. After polishing, the two halves are cemented by Canada balsam and mounted. Fig. 330 represents a vertical cross-section of a Nicol prism together with a horizontal plan. Now as the refractive index of the ordinary ray of calcite THE OPTICAL I'UOI'KHTIKS OF MINERALS 123 is l.(ir) and tha(, of the lialsani ahoul. ] SA, it will lie seen frdin the figuic that tlu' ordiuai'v ray o, in passiiiy the balsam, as for this ])ai-licular direction of transmission they lia\'e almost the s:ime index of refraction. So that thci'e emerges fi'om the upper suiface of the uicol, plane ]>olai'izeil light with ^•ibra.tions jiarallel to the short diagonal fif the caicite rhondi as indicated in Fig. i?3(). If a Xicol prism is examined with a tourmaline, darkness i-e- sults when the e-axis of the tourmaline is parallel to the long diagonal of the calcit<' I'hond). If two iiicols ha^■e theii' slioit diagonals pai'allel, light goes through unaffected, save the intensity is diminished, but on revolving one of the nicols, the - ^^ 1 light giadually fades until their _. _^_ ^_ _2 __ _, _; — -I shol't diagf)nals are at right Fic;.:S.31. — Polarizing microscope (l/4 size). angles, \\-hen dai'kness results. |WI|1I«*I_^!SP^ 6. THE POLARIZING MICROSCOPE The polarizing or pet iT)gra])hic microscojje (Fig. 331) differs frfiin an oi'dinary micioscope in th(^ addition of a I'otating stage 124 INTRODUCTION TO THE STUDY OF MINERALS for measuring angles and of two Nicol prisms one above, the other below, the stage. Two nicols are necessary, for the effects due to polarized light cannot usually be distinguished except by another nicol. The lower nicol is called the polarizer and the upper one, the analyzer. The lower one fits into a socket so it may be rotated, but ordinarily it is set so that its vibration plane is at right angles to that of the upper nicol. Then the field should be dark and the nicols are said to be crossed. In some cases we use the lower nicol alone and then its vibra- tion plane must be known. A cleavage fragment of calcite has high relief when its long diagonal is parallel to the plane of vibra- tion of the nicol (see Fig. 364, p. 147). A Fig. 332. — Centering the stage. FiQ. 333. — Measuring an angle. In order to use the rotating stage, its center must coincide with the optical center of the microscope tube. The method of centering the stage may be explained by referring to Fig. 332. A and B are centering screws 90° apart, located either on the stage or on the microscope tube, preferably on the latter. The two perpendicular lines across the field represent the cross-hairs. An object, o, on a glass slide is placed at the intersection of the cross-hairs. Suppose on revolution of the stage it appears to revolve in the dotted circle until it resumes the position o. Then revolve the stage 180°, correct for one-half the error by the THE OPTICAL PROPERTIES OF MINERALS 125 centering screws, the other half by moving the slide on the stage. In the figure the error is oo' , the components of which in the di- rection of the cross-hairs are o'e and o'd. Then by the centering screw A, o' is first moved the distance We and by the centering screw B, o' is moved the distance Wd. The object, o, then takes the position c and the slide itself must be moved the distance co, when the stage should be centered. To measure a plane angle in a thin section or the interfacial angle of a small, flat crystal the stage is centered with the inter- section of the two edges at the center of the cross-hairs. A reading is made when one edge of the crystal is parallel to the east-west cross-hair, say, then the stage is revolved until the other edge (dotted line in Fig. 333) is parallel to the same cross- hair, but on the opposite side of the center, when another reading is taken. The difference between the two readings is the supple- ment or external angle (a in the figure). The microscope is often used for measuring very small distances such as the dimensions of minute crystals. For this purpose a special eye-piece (micrometer eye-piece) containing a scale etched on glass is used. On the stage of the microscope a scale reading hundredths of a millimeter is placed. It is then simply necessary to see how many hundredths of a millimeter each division of the eye-piece is equivalent to. 7. INTERFERENCE COLORS If thin plates of singly refracting (isometric) crystals are examined between crossed nicols there is no result, for the original field remains dark. But if thin plates of doubly refracting crystals are examined between crossed nicols there results the beautiful color effects known as interference colors. In order to explain these colors it is necessary to consider the re- sults obtained in examining the doubly refracting plates in monochromatic light. If two light waves or train of waves of the same wave-length travel along the same path, after they 126 INTRODUCTION TO THE STUDY OP MINERALS have travelled different distances in another medium, they combine in general to produce a new wave, the abscissa at any point of which is equal to the sum of the abscissae of the two original waves. This is called interference. There are two special cases of importance. (1) The two waves have the same amplitude and a phase difference or path differ- ence of iX. As can be seen from Fig. 334 they neutralize each other and darkness is the result. That is, two light waves can Phase differences-^ Figs. 334-336. combine so as to produce darkness. (2) The two waves have the same amplitude and a phase difference of ^. In this case the new wave will have the same phase, but the amplitude will be doubled (Fig. 335). For intermediate cases such as phase dif- ference of l^, the resultant wave will have an intermediate ampli- tude (Fig. 336) . Interference results when light waves from the same source go over the same path one in advance of the other. There are two methods of obtaining interference, one by the use of thin films, the other by the use of doubly refracting crystals. THE OPTICAL PROPERTIES OF MINERALS 127 Let Fig. 337 represent a thin film of air in a selenite cleavage. AB and A'B' are oblique incident rays. On reaching the surface of the film they are partly reflected and partly refracted. So that for a point B' there will be two rays travelling along B'D'; one of them is the reflected ray of A'B', the other the reflected ray of the refracted ray EC. These two rays are in a position to interfere for one of them has travelled a greater distance than the other. For while one has travelled A'B' the other has travelled ABE. Hence the ray from AB is the distance ECB' behind the ray from A'B'. Fio. 337. Fig. 338. If we use monochromatic light and adjust the thickness of the film so that the retardation, or lagging of one ray behind the other, is equal to iX, we have, for that particular thickness, dark- ness, while for a retardation of X we have light of maximum inten- sity. So that with a wedge-shaped film in monochromatic light we have a series of dark bands, for retardations of ^y and nX produce the same effect as -^ and }. respectively. Now let us consider the case of doubly refracting crystals. A series of parallel light waves from the polarizer or lower nicol enter the crystal and are broken up into two sets of waves one vibrating in the plane of the paper, say, and the other in the 128 INTRODUCTION TO THE STUDY OF MINERALS plane normal to the paper. See Fig. 338. At certain points on the upper surface there will emerge two sets of waves travelling in the same path but vibrating in planes at right angles to each other, and oblique to the planes of vibration of the nicols. In order that they may interfere it is necessary to reduce the vibrations to one plane and for this pur- pose an analyzer or upper nicol is necessary. The effects produced depend upon the rela- tive positions of the nicols, upon the position of the mineral with reference to the nicols, and upon the phase difference of the two sets of polarized light waves. Fig. 339 explains diagrammatically what happens when a doubly refracting crystal is examined between the crossed nicols of a polarizing microscope. A ray of light enter- ing the lower nicol is broken into two rays, e and o. One of these (o) is totally reflected and disappears. The remaining ray (e) is broken into two rays (e' and o') by the mineral plate. These two rays, which are vibrating at right angles to each other, are each broken into two rays (e", o" , and e'", o'"), by the upper nicol. One from each of these (o" and o'") is totally reflected and finally there emerges from the top of the upper nicol two sets of waves (e" and e'") vibrating in the same plane and these inter- fere with each other. With crossed nicols darkness results when the phase difference is X. In Fig. 340 PP' is the vibration plane of the lower nicol and AA' of the upper nicol, RR' and SS', the Fio. 339. THE OPTICAL PROPERTIES OF MINERALS 129 vibration planes of the crystal plates, r and s represent vibra- tions in the same phase. The components of these in the plane of the upper nicol are p and a, which are opposite and equal. Hence they annul each other. With crossed nicols there is maximum light when the phase difference is \X. In Fig. 342 r' and s represent vibrations of ^X phase difference. Their com- ponents in the plane of the upper nicol are p' and a, equal but on the same side of the origin. Hence the intensity is doubled. ^^— — Y / " \ ' \ / tN^ /V ■ ^ \ / \' 1 V >r' a' A A-i-iX ^ — i;;— — -..,,,^^ ^/^ / \ / {' \ [ \ / \ f" 1 \ / 1 r 1 \ / ' \/ / \/ t s'V /R Fig. 340. FiQ. 341. Fia. 342. For a phase difference of \X, we have the intensity shown in Fig. 341. Thus the intensity varies between for a retardation of I, and a maximum for retardation of \'k. As can be seen from Fig. 343, the retardation produced by a wedge-shaped section of a crystal will vary from point to point. fX A retardation of nl will give darkness and a retardation of ^A will give a maximum intensity. Hence a wedge examined in mono- chromatic light between crossed nicols will appear as a series of 9 130 INTRODUCTION TO THE STUDY OF MINERALS parallel dark bands interspersed with colored spaces, the color depending upon the monochromatic light used. For white light we have the combined effect of all the colors of the spectrum. Some idea of the interference colors seen in white light may be gained by a study of Fig. 344. (This diagram may be colored by the student.) The top of the figure represents a wedge-shaped section as viewed in various kinds of monochro- matic light, there being a dark band at positions which give retardations of nX. For each of these colors a medium value of A 2 4" 3X 2X 5_\ 2 3X OM mwmm^^ Fig. 343. the wave length is chosen as follows: red, 700/i/i; orange, 620fi/i; yellow, dGOfi/i; green, BlBfiji; blue, 460///j; violet, ilOfi/i. The top row of figures gives the value of retardations in /ufi. The lower part of the figure indicates the interference colors as seen in white light. Let us consider the colors in succession. The intensity of different parts of the spectrum varies and has an influence in determining the color. Yellow is the most intense, and violet, the least intense part of the spectrum. The color chart, as it is called, begins with darkness, succeeded by dark gray which gradually becomes lighter. At about 250fifi all the colors combine to form white light. At 280/i/t(^ for yellow) yellow is at a maximum, but mixed with white gives straw yellow. At 310/i/x and at SdOftfi orange and red respectively, are at a maximum, but THE OPTICAL PROPERTIES OF MINERALS 131 the great intensity of the yellow modifies these colors and places them further to the right, for at SiOftp. the color is bright yellow. At 575fifi violet is the color. Though of weak intensity, violet is produced here because the other colors are practically extin- A lOO 200 800-400 500 000 700 BOO 900 1000 1100 12001800 140O 1600 1000 1700 1800 1 9C0 20O0 2100 1 ! ; i ! 1 i : : ! 1 1 1 ' 1 ■ 1 Eed 1 ' 1 ! 1 ' M ( ' ' ' 1 ' li 1 i \\ 1! 1 1 : 1 i 1 X ; 1 ■ 1 1 1 2A i ■ 1 3A Orange Aim i i M 1 ll ill i i i 1 ■ : • ! A 2AI 1 1 3A Yellow 1 M M 1 M M II J i j ! -A ; 1 ,„ % I ' 1 3A Green Mill : 1 1 ; : i A 1 1 1 i !2A 1 : 1 M 1 3Ai ' 14X1 Blue i i i 1 1 1 1 1 : 1 i Ai ' 1 :_ i.\ 1 1 3A : i •JA Violet M III Mi 1 I 1 1 1 1 1 '■ 1 i 1 1 .X ; ; 1 2A 1 1 : 3A ; ' 1 Ha ; _ ^5x; 1 5 1 |g §§ 1 ,s sl 1 1 1 1 , , . 1 1 1 l| , 1 1 1 1 1 2 n,-n .002 .004 .006 .008 .010 .012 .014 .016 .018 .020 .0^2 .024 .020 .02H Sm .CB2 .034 .080 .088 .040 .042 | 0.M 0.08 • / /; A/'Xl-^.'^'Or^'^^^k^J^^^i^^ ^i i^ ,^ r-^ ''l \\iy /\//yc^>AA i i 1 1 1^^^^ '!'''!' 1 1 trder 1 1 1 a 100 300 aOO 400 [iOO 600 7U0 800 UOO lOOij 1100 1200 1 luoo 1700 1800 11)00 aooo aioo First Oraer | Second Order 1 'lUud< 1 Fourth Order Fia. 344. — The color chart and its derivation. guished. As can be seen from the diagram the colors follow in order: blue, green, yellow, and red. At 1125 fi/x violet appears again. At this point only red, blue, and violet are near a maxi- mum. But red and blue together produce violet. Then in order 132 INTRODUCTION TO THE STUDY OF MINERALS we have blue, green, yellow, and red again. After this the colors are paler and pass into neutral tints and finally into white which resembles ordinary white light. There is a repetition in the colors, but they gradually become fainter. The colors from black up to the first violet (J = 57 5 fifi) are called first order colors, from this violet up to the second violet (J = 1125 fifi), second order colors and so on. In white light six or seven orders may be distinguished, but in monochromatic light there is no limit to the number of orders as determined by the dark bands. By trial it may be found that the interference color depends upon three factors: (1) the double refraction which is a constant for the crystal ; (2) the orientation or direction in -which the crystal is cut (in a thin section of sandstone the quartz grains, cut in various directions, have a great variety of interference colors) ; (3) the thickness as may be seen in a quartz or selenite wedge. The formula J = t{n^ — n) gives the relation between J, the retardation in fifi, t, the thickness of the plate and (n^ — n), the double refraction, w^ and n being the two values of the indices of refraction for a particular section. For a given substance with known indices of refraction the thickness may be measured and the interference colors predicted. Or the thickness may be measured, the retardation determined from the color chart, and the double refraction calculated. Or the thickness may be cal- culated that will give a certain interference color for a crystal with known double refraction. In the color chart at the bottom of Fig. 344, the horizontal lines represent the thickness from 0.00 to 0.05 mm., ordinary rock slides and fragments being from 0.03 to 0.05 mm. in thickness. The vertical lines give retardations in /ijj. while the diagonal lines represent the amount of the double refraction. A crystal of 0.03 mm. thickness and double refraction of 0.02 has a retardation of 600/ifi (0.000600 = 0.03X0.02), and gives a violet interference color. THE OPTICAL PROPERTIES OF MINERALS 133 8. VIBRATION OR EXTINCTION DIRECTIONS If a section of a doubly refracting crystal is revolved between crossed nicols there is darkness four times in a complete revolu- tion. This is called extinction and is simply due to the fact that for these particular positions the crystal has no effect upon the dark field produced by the crossed nicols. The two directions in the crystal parallel to the vibration planes of the two nicols are called extinction directions or vibration directions. These two directions are directions of the two plane polarized waves pro- duced in doubly refracting crystals by the plane-polarized light of the lower nicol. Fig. 345. Parallel extinction. Fig. 346. Oblique extinction. Fig. 347. Symmetrical extinction. Now according to the position of these directions with refer- ence to the crystal outlines we have various kinds of extinction which are characteristic of crystals of the various systems. In case the directions are parallel to the outline we have parallel ex- tinction. This is represented by the convention of Fig. 345, the cross-hairs of the microscope being parallel to the vibration- planes of the nicols, and the crystal placed in the position of darkness. If the directions are not parallel to the outline we have oblique extinction (Fig. 346). The particular case in which these directions make equal angles with the edges of the crystal is called symmetrical extinction (Fig. 347) . The angle between an extinction direction and the prominent 134 INTRODUCTION TO THE STUDY OP MINERALS edge of a crystal is called the extinction angle, and is characteris- tic of certain crystals in certain directions. The extinction angle is determined by taking a reading when the outline is parallel to one of the cross-hairs (the stage being centered) and then revolv- ing the stage until maximum darkness results, when another read- ing is taken. The difference between the two readings is the extinction angle. In Fig. 346 the extinction angle indicated by the arrow is about 15°. It may be noticed that there are two possible extinction angles which are complementary. The smaller angle (<45°) is usually taken. Accurate determinations of the extinction angle are made in monochromatic light. A convenient determination in white light may be made by using a selenite plate which gives a field showing red of the first order. This is called the sensitive tint, the least change giving either orange-red or violet-blue. When inserted in the slot provided for test-plates a doubly refracting mineral appears the same tint of red as the red field only when in the extinction position. 9. DIRECTION OF THE FASTER AND SLOWER RAYS For a section of a doubly refracting crystal cut in any direction there are in general two values of the index of refraction corre- sponding to the two vibration direc- tions at right angles to each other. One of the values is greater than the other, otherwise there would be no double refraction. Interference is caused by one ray getting behind the other. This is the slower ray, the other being the faster ray. The ray with the greater index of refraction is the slower ray and the one with the smaller index of refraction is the faster ray, as may be proved by means of Fig. 348. Parallel rays p-p' on passing from air into a section FiQ. 348. — The relation of velocity to index of refraction. THE OPTICAL PROPERTIES OF MINERALS 135 arrive at the points rr' together. But while the ray p' is going the distance r's' in air, the ray p has gone the distance rs, the velocity in the section being less than in air. The wave-front in the section is then ss' constructed l>y drawing a line from s' tangent to an arc with radius rs (rs being the velocity as com- pared with the velocity r's' in air). Thus there is a change in direction of the light rays. The I'ays are bent toward the per- pendicular. Hence the section with velocity less than in air has an index of refraction greater than air. It is often necessary to know which of the two directions is the slower ray. The determination of the extinction fixes the posi- tion of the two rays, but in order to determine which is which use is made of a section in which these directions are known. The superposition of one doubly-refracting section on another has the same effect as thickening or thinning the section by causing the interference colors to go up or down on the color chart, "up" being toward the thick end of a wedge and "down" toward the thin end of a wedge. Several test -plates are used for this purpose. The one most frequently used is the mica plate, a cleavage of muscovite of a thickness sufficient to produce a retardation of 140/i/z {{X for medium yellow and hence called the ^-undulation mica plate). The direction of the slower ray of this plate is marked by an arrow. This direction is the line joining the branches of the hyperbola of the interference figure seen in con- vergent light. The mica plate itself gives a pale bluish-gray interference color of the first order and when placed in the slot provided for the purpose in the microscope tube causes the inter- ference color of a crystal section placed in the position of maxi- mum illumination to be lowered or raised by liO/ifi (see color chart). The color goes up when similar directions are parallel (when slower ray of crystal coincides with slower ray of the mica plate) and down when dissimilar directions are parallel (when slower ray of crystal coincides with faster ray of the mica plate or vice versa). This test is usually employed to determine the elongation of 136 INTRODUCTION TO THE STUDY OF MINERALS the crystal, that is, to test whether the long direction of the crystal is the slower ray or the faster ray. Examples of the two cases are given in Figs. 349 and 350, the dotted rectangle repre- senting the mica plate with arrow indicating the slower ray. In the first case, the crystal originally with red interference color is changed to blue, when its length is parallel to the arrow of the mica plate. Hence, the elongation is parallel to the slower ray. Fig. 349. — Positive elongation. Fia. 350. — Negative elongation. This is called positive elongation. In the other case, the red crys- tal is changed to blue, when its length is parallel to the faster ray of the mica plate, the faster ray being always perpendicular to the slower ray. This is called negative elongation. 10. CLASSIFICATION OF CRYSTALS FROM AN OPTICAL STANDPOINT From an optical standpoint there are three divisions of crystals: isotropic (isometric), uniaxial (tetragonal and hexagonal) and biaxial (orthorhombic, monoclinic and triclinic). In discussing the optical properties it is convenient to employ a geometrical representation of the optical structure. The figure formed by taking as radius vector the index of refraction in various direc- tions is called the optical indicatrix. The indicatrix is, in general, an ellipsoid, sections of which are ellipses. The important prop- THE OPTICAL PROPERTIES OF MINERALS 137 erty of the indicatrix is that the major and minor axes of any elliptic section are the extinction directions for that section and, moreover, the lengths of these axes are proportional to the indices of refraction for that particular section. The shape of the indicatrix varies for the three divisions of crystals mentioned. In isometric crystals the index of refraction is the same for all directions. The optical indicatrix is therefore a sphere. All sections of a sphere are circular, so there is no double refraction and, hence, no interference colors. A section cut in any direction will remain dark between crossed nicols. Isometric crystals and amorphous substances such as glass are said to be isotropic. Crystals of the remaining systems (those except the isometric) have double refraction so that the term anisotropic is used for them in contrast with the term isotropic. Tetragonal and hexagonal (also the trigonal if that is consid- ered as a separate system) crystals constitute the uniaxial divi- sion. If sections of these crystals, cut in various directions, are examined between crossed nicols, it is found that basal sections (sections perpendicular to the c-axis) remain dark between crossed nicols and that all sections parallel to the c-axis give some inter- ference color which is a maximum for a given thickness, while sections oblique to the c-axis give interference colors varying from a maximum to darkness, the color depending upon the ob- liquity, but those of equal obliquity around the c-axis give the same interference color. From these tests it will be seen that the indicatrix is an ellip- soid of revolution. The axes of an elliptical section through the c-axis represent the indices of refraction, one of which designated n^., is the maximum of all possible values in the crystal, while the other, designated n^, is the minimum of all possible values. In some cases 7-,' or the slowest ray, is the c-axis, while in other cases, a, or the fastest ray, is the c-axis. This divides the uniaxial crystals into two divisions, the optically positive (c = y) and the ' a, 0, and j- are directions in a crystal and Ua, np, and riy are indices of refraction for these directions. 138 INTRODUCTION TO THE STUDY OF MINERALS optically negative {c = a), the terms positive and negative being purely arbitrary. The indicatrix of positive crystals is an oblate spheroid and that of negative crystals, a prolate spheroid. The determination of the optical character, that is, whether positive or negative, is made by ascertaining the faster and slower rayin a section parallel to the c-axis, or in a basal section by testing the interference figure, obtained by convergent light, with a mica plate. Crystals of the remaining systems, orthorhombic, monoclinic, and triclinic, constitute the biaxial division. Of all the possible values of the indices of refraction in a biaxial crystal it is found that one section contains the direction ;- corresponding to the maximum index of refraction n^ and also the direction a corre- sponding to the minimum index of refraction n^. For other sec- tions the indices of refraction are intermediate between the maximum and minimum. The direction perpendicular to the plane of y and a is called /? or sometimes the optic normal. The index of refraction n^ is intermediate between n^ and n^, but is not a mean value. The indicatrix for biaxial crystals made by laying off on three rectangular axes the values n„, n^, and n^ or indices of refraction for the three directions mentioned is a triaxial ellipsoid. The maximum interference color for a given thickness is given by the section which includes jr and a. The interference colors vary from this maximum to a minimum, but there is no section that remains dark between crossed nicols as in uniaxial crystals. Fig. 351 is the section of a triaxial ellipsoid which contains y and a. There are two circular sections of this ellipsoid. Lines normal to these circular sections are peculiar directions corre- sponding, somewhat to the single direction or c-axis of uniaxial crystals. These directions are called optic axes, hence the term biaxial. A plate normal to an optic axis appears uniformly bright between crossed nicols for all positions of rotation. The optic axes always lie in the plane of y and a, which is called the plane of the optic axes or the axial plane, and the acute angle THE OPTICAL PROPERTIES OF MINERALS 139 between the optic axes is called the axial angle, denoted by the term 2V. In Fig. 351 OA and OA' are the optic axes and AOA' the axial angle. The optic axes are always symmetrically placed with respect to y and a. The line bisecting the axial angle is called the acute bisectrix, denoted by Bxa- The line bisecting the obtuse angle be- tween the optic axis is called the obtuse bisectrix, denoted by Bxo. There are two divisions of biaxial | crystals according to whether the acute bisectrix is y or a. The former are called positive {Bxa = Y) and the latter, negative (Bxa = a), a purely arbitrary designation. Fig. 35 1 repre- sents a positive crystal. The determination of the optical character may be made by testing for the faster and slower ray in a section known to be perpendicular to the acute bisectrix. It may also be determined in this kind of section by obtaining an interference figure and testing it with a mica plate. 11. INTERFERENCE FIGURES The tests mentioned up to this have been made by using ordi- nary parallel light or polarized parallel light. A unique series of effects, important in the identification and description of minerals, may be obtained by examining suitable sections in convergent polarized light. For this purpose either a polariscope or a polarizing microscope may be used. A polariscope, Fig. 352, is an instrument consisting essentially of an analyzer and polarizer with slight magnifying power and strongly convergent lenses both above and below the stage. If the polarizing microscope is used, a high power objective (No. 7 Fuess, No.V Seibert, or J Bausch and Lomb), and also a Fio. 3S1. ^ 140 INTRODUCTION TO THE STUDY OF MINERALS cnmlcnsing lens i)lacod just Ijelow tlic stago must Ijo suljstituted for the ordiiuiry set-up. Either the eyc-piccc must ]')C removed or a special lens, called the ]'ei'ti-and lens, must Ijc inserted in the microscoiie tube l)et\veen the upjjer nicol and the eye-piece. The color effects seen when hasal sections of uniaxial crj's- tals and sections of biaxial crystals cut normal to the acute bisectrix are examined in convei'gent light between crf)ssed riicols are known as interference figures. With isometric crystals no interference figures arc 0I3- tained for there is no double refraction. Uoubile refraction is necessar}' for the produc- tion of interference figures. In fact, interference figures are simpljr the result of inter- ference colors, due to varying doul^Ie refraction in different directions, combined and modified liy the darkness due to crossed nicols. Basal sections of uniaxial ci'ystals e.xamined in mono- (diromatic convergent light lietween crossed nicols give a dark cross with dark con- centric rings (Fig. 353). The explanation is sim]:>le (see Fig, 3")G). Strongly convergent I'ays of ligjit ti'aA'(>rse (lie crj'stal in various ol)lique directions and the effect is the same as if rays of parallel light traverse a wedge of the crystal. So that along any radius we get a dark band where \ THE OPTICAL PROPERTIES OF MINERALS 141 the retardation is nX for the particular light used. Midway be- tween the bands we get the maximum color. As the structure is the same all around the c-axis in uniaxial crystals the dark bands are circular. It remains to explain the dark cross which it should be noticed is stationary on rotation of the section on the stage. The crj^stal, it may be imagined, is made up of innumerable parts each with extinction directions at right angles. These parts are arranged radially around a center and on rotation of the stage as vibration directions of successive parts become parallel to the vibration directions of the nicols, Fig. 353. Fig. 354. Fig. 3.5.5. darkness results for that particular part. As new radii are always coming into extinction there is always a dark cros?, the arms of which are parallel to the vibration directions of the two nicols and so remain fixed. For sections not quite parallel to the basal plane the dark cross on rotation of the stage is eccentric, but the arms always remain parallel to the vibration planes of the nicols and revolve in the same direction in which the stage is rotated (see Fig. 355). With ordinary white light we still have the black cross, but the dark rings become colored rings, the colors of which vary from black at the center through gray, yellow, red, and so on until after six or seven orders there is practically while light. The number of rings depends upon the thickness and also upon the double refraction. In very thin sections there may be no rings visible. Very thick sections show the full number of rings. 142 INTRODUCTION TO THE STUDY OF MINERALS Quartz with weak double refraction shows for ordinary thickness no rings at all, while calcite with very strong double refraction shows a large number of rings. The optical character of a uniaxial crystal may be determined from the interference figure by inserting in the slot a mica plate with the slower ray, ;-, in the 45° position. The interference figure Fig. 356. is changed, the daric cross disappearing and two dots appearing in two opposite quadrants as represented diagrammatically in Fig. 354. If the imaginary line Joining the two dots is perpendicular to f of the mica plate, the crystal is positive (Fig. 354), while if parallel to )■ of the mica plate, it is negative. This is due to the fact that the interference colors " go up " in two opposite quad- THE OPTICAL PROPERTIES OP MINERALS 143 rants and "go down" in the other two quadrants. The rings then are not continuous, but broken and the two rings nciirest the center are the two dots. Sections of biaxial crystals cut normal to the acute bisectrix show an interference figure something like Fig. 357, with a black cross and two sets of concentric ellipses passing into 8-shaped curves or leminscatcs. In monochromatic light the rings are dark and in white light colored. On revolving the section on the stage the dark cross opens up and passes into an hyperbola as Fig. 357. Fia. 358. FiGa. 357 and 358. — Biaxial interference figure. shown in Fig. 358, which represents the 45° position. The line joining the centers of the ellipses is the trace of the axial plane, the center of the ellipses representing the emergence of the optic axes. Hence the distance apart of the branches of the hyperbola is a measure of the axial angle. If these are close together the axial angle is small, if far apart the axial angle is large. If the axial angle is too large, the hyperbola does not appear at all using No. 7 Fuess objective. The biaxial interference figure may be explained by means of the diagrammatic Fig. 359. The optical structure of biaxial crystals is symmetrical to three planes at right angles to each other. One of these is the plane of the paper, while the other two are represented by their traces, the vertical and horizontal lines of the figure, which are also vibration planes of the two 144 INTRODUCTION TO THE STUDY OF MINERALS nicols. The two small circles are traces of the optic axes. The extinction directions for various parts of a crystal may be obtained by bisecting, internally and externally, the angles formed by joining any point with the traces of the optic axes. The dotted lines represent this procedure for one point. In similar manner the small crosses were obtained for different parts of the crystal. In the normal position a black cross will be formed along the vertical and horizontal lines. On revolving the section the black cross disappears. In the 45° position an hyper- FiG. 359. — Explanation of a biaxial interference figure. bola is formed by the darkness of different parts along the hyper- bola as can be seen from the figure. In the 90° position the cross is restored. The number of rings depends upon the strength of the double refraction and upon the thickness, but the distance between the branches of the hyperbola remains constant whatever the thickness. The optical character is determined by means of a quartz wedge on which is marked the slower ray y. The quartz wedge is inserted in the slot thin end first when the interference figure shows hyperbolae. Then when ;- is parallel to the trace of the THE OPTICAL PROPERTIES OF MINERALS 145 axial plane the ellipses appear to expand for positive crystals and to contract for negative crystals. A section normal to an optic axes shows a series of concentric rings crossed by a dark bar which revolves in an opposite direc- tion from the rotation of the stage. 12. OPTICAL PROPERTIES OF TWIN -CRYSTALS One-half of a twin crystal has the same position with respect to the other half that it would have if rotated 180° about an axis from its original position. From this it can be seen that the extinction angles in two halves of a twin crystal are equal, but opposite in sign, if the section is cut normal to the twin-plane. Thus in a cleavage of a selenite twin the extinction directions are inclined to each other. Consequently if examined between Fig. 360. Fig. 361. Fig. 362. crossed nicols one-half of such a crystal is dark, while the other half is light (see Fig. 360). On rotation the light and dark parts interchange. A section of a polysynthetic twin such as plagioclase shows a series of dark and light bands, the extinction directions in alter- nate bands being parallel (see Fig. 361). Many orthorhombic crystals such as aragonite and KjSO^ are pseudo-hexagonal by twinning, and basal sections between crossed nicols are divided into six sectors like Fig. 362, opposite pairs of which extinguish together. Basal sections of aragonite, like Fig. 362, examined in convergent polarized light show a biaxial 10 146 INTRODUCTION TO THE STUDY OF MINERALS interference figure in each sector but arranged so that the axial planes are parallel to the outline. An optical examination reveals the composite nature of many apparently simple crystals. 13. ABSORPTION AND PLEOCHROISM The color of a transparent substaijce is due to the residual color of the spectrum left after the substance has absorbed a certain part of it. Many colored anisotropic crystals have the property of absorbing different amounts or kinds of light in different directions. Absorption has reference to the amount or intensity of light absorbed and hence may be tested in mono- chromatic light while pleochroism refers to the kind of light absorbed and so must necessarily be tested in white light. A prismatic crystal of epidote from the Sulzbachthal in Tyrol, if not too thick, will appear green in a certain position, while on revolving it 90° about its long axis it will appear brown. Few Fig. 363. — Dichroscope. substances show such a marked change as this, and at any rate it is not always possible to turn a crystal about and look through it in various directions. The determination of pleochroism and absorption is made either by means of one. nicol of a polarizing microscope in which case minute crystals may be examined, or by means of the dichro- scope when large crystals are available. A dichroscope is simply a piece of Iceland spar set in a cylin- drical frame that is provided with a small aperture at one end and either open or provided with a lens at the other end. Fig. 363 is a diagrammatic representation of the dichroscope. When held THE OPTICAL PROPERTIES OF MINERALS 147 up to the light the dicliroscope shows two images of the aperture side by side, the diameter of the aperture being made such that the images do not overlap. If a pleochroic crystal is viewed through the dichroscope two colored images are seen simultaneously. The reason for this is that in a doubly refracting calcite crys- tal we have two sets of light waves vibrating in planes at right angles to each other. The color of some crystals, as we have seen, varies with the direction, but by using a Nicol prism we may observe the two colors successively, one when the vibration plane of the nicol is parallel to the length of the crystal, and one when perpendicular to the length of the crystal. These- colors are called axial colors. Fig. 364.— Calcite. Fig. 365.— Biotite. In uniaxial crystals there are two axial colors, hence the term dichroic. In biaxial crystals there are three axial colors, hence the term trichroic. In order to determine the axial colors for particular directions it is necessary to ascertain the viljration plane of the lower nicol. For this purpose a rock-section containing biotite may be used. Biotite sections showing cleavage have very strong absorption. On revolving the section the biotite becomes very dark every 180°. The cross-wire which is parallel to the length of the biotite when it is darkest represents the vibration plane of the lower nicol as illustrated in Fig. 365, the arrow indicating this direction. 148 INTRODUCTION TO THE STUDY OP MINERALS If a rock-section containing biotite is not available the test may be made by examining minute cleavage fragments of calcite obtained by pounding to a coarse powder any kind of calcite. If mounted in oil of cloves or Canada balsam, the calcite rhombs have a marked relief when their long diagonals are parallel to the vibration plane of the lower nicol, but slight relief when their short diagonals are parallel to this direction as represented in Fig. 364. The vibration direction of the lower nicol is indicated by the arrow. The trichroism of a biaxial crystal is beautifully illustrated by the soda amphibole, glaucophane, as seen in thin rock-sections Fig. 366. Fig. 367. Fig. 368. Figs. 366-368. — ^Pleochroism of glaucophane. under the microscope. Three kinds of cross-sections may be distinguished as follows: pseudo-hexagonal (Fig. 366), stout rectangular (Fig. 367), and thin with oblique ends (Fig. 368). These sections are respectively almost perpendicular to the c, b, and a axes. These sections when rotated on the stage of the microscope show respectively the following pairs of colors: neu- tral and violet, blue and violet, blue and neutral. It will be seen that the color for the a-axis is the neutral tint for the mineral has this color when the a-axis is parallel to the vibration plane of the nicol (represented by the arrow) . Similarly the violet is the color for the 6-axis and blue for the c-axis. The three axial colors may be combined in an axial cross (Fig. 369). THE OPTICAL PROPERTIES OF MINERALS 149 In glaucophane it has been found that b = ^ and a (almost) = a and c (almost) = y. The absorption may be indicated by the following: ;'>/?> a. This is called the absorption scheme and means that more light is absorbed in the y or c-direction than in the /? or 6-direction and more in the p or 6-direction than in the a or a-direction. Sometimes Gothic letters are used instead of a, /?, and 7-. 13 Fig. 369. — Axial colors of glaucophane. 14. DISPERSION By dispersion is meant the divergence in value or in position of the optical constants for different colors of the spectrum. For isometric crystals we have simply dispersion for the indices of refraction. The index of re- fraction for the violet end of the spectrum is greater than that for the red end of the spectrum as indicated in Fig. 312, page 116. For diamond the dispersion is very large n^ — nr = 0.063, hence the "fire" of the diamond. For fiuorite the dispersion is very small n^ — 71^ = 0.006 hence fluorite is sometimes employed in making achromatic microscope lenses. For uniaxial crystals we have different values of the indices of refraction n^ and n^ for different parts of the spectrum. For some crystals one part of the spectrum will give a positive interference- figure and another part a negative figure, while for an inter- mediate color of the spectrum the mineral will be isotropic. In biaxial crystals there is more variety in the dispersion, not only varying values for n^, rip, and n^, but also variation in the position of the optic axes and axial plane and in the size of the axial angle. In the orthorhombic system there can be no dispersion of the axial plane or of the bisectrices, as a, /?, and ;- always coincide with 150 INTRODUCTION TO THE STUDY OP MINERALS the crystallographic axes. The color distribution is then sym- metrical with respect to the axial plane and also a line normal to it. This is shown diagrammatically in Fig. 370, the dotted circles and hyperbolae representing the blue part of the spectrum, say, while the full circles and hyperbolae represent the red part of the spectrum. This is known as symmetrical dispersion. In the monoclinic system the only fixed relation is that the h- axis is always either a, p, or y. Hence the axial plane is either parallel or perpendicular to the (010) face. If the axial plane is perpendicular to (010) , the position of the axial plane will vary for the different colors. If the &-axis is the acute bisectrix we have Fig. 370. Fig. 371. Fig. 372. Fig. 373. Fig. 374. Symmetrical Crossed dis- Horizontal Inclined Asymmetric dispersion. persion. dispersion. dispersion. dispersion. the color of the interference figure symmetrical to the center of the figure as in Fig. 371. This is called crossed dispersion. If the 6-axis is the obtuse bisectrix, then the color distribution of the interference figure of a section perpendicular to the acute bisectrix will be symmetrical to a line perpendicular to the axial plane as shown in Fig. 372. This is called horizontal dispersion. There is a third case in which the axial plane is parallel to the 010 face and the 6-axis coincides with /?. A section normal to the acute bisectrix will give an interference figure with color sym- metrical to the trace of the axial plane as in Fig. 373. This is called inclined dispersion. In the triclinic system there is both dispersion of the axial THE OPTICAL PROPERTIES OF MINERALS 151 plane, optic axes, and axial angles and no necessary connection between a, b, c and a, /?, y. The interference figure lias no color symmetry and for this reason it is called asymmetric dispersion (Fig. 374). To make these observations it is necessary to use rather thick sections in a polariscope. Although these various kinds of dis- persions are theoretically possible in the orthorhombic, mono- clinic, and triclinic systems, they are not marked enough in most crystals to be observed. For good, though perhaps exaggerated, illustrations of these various kinds of dispersion see plate II in Tschermak's Lehrbuch der Mineralogic, 6th edition (1905). 15. ROTARY POLARIZATION If a thick (1 mm. or more) basal section of a quartz crystal is examined in monochromatic light with crossed nicols it is found that, instead of being dark as most uniaxial crystals are, some light will be transmitted, but on rotating one nicol through a certain angle, depending upon the thickness and the kind of light, the field will become dark. Thus quartz has the curious property of rotating the plane of polarization. For a section of quartz 1 mm. thick, using red light the angle of rotation is 13°. For cinnabar, the only other mineral known to possess this prop- erty, the angle of rotation is 325° under similar conditions. As the angle of rotation depends upon the wave length of light used, in white light the section is never dark, but on rotation succes- sive colors appear. In convergent light the interference figure is like the normal uniaxial figure except there is a light circular area at the center. A substance that rotates the plane of polarization is said to be optically active. Besides the minerals quartz and cinnabar, this property is possessed by many artificially prepared crystals such as sodium chlorate, sodium periodate, strychnine sulfate, etc., and also by solutions of cane-sugar, tartaric acid, and many other organic compounds with asymmetric carbon atoms. In solutions 152 INTRODUCTION TO THE STUDY OF MINERALS the angle of rotation depends upon the amount of substance in solution. Sugar analysis is based upon this fact. The sugar solution, the strength of which is to be determined, is placed in a tube between the crossed nicols of an instrument called the polarimeter, the nicols being horizontal and one of them capable of being rotated. The amount of rotation necessary to produce darkness determines the strength of the solution. Both in crystals and in solutions the rotation may be right- handed (clock-wise) or left-handed (counter-clock- wise) . There Fig. 375. Fig. 376. Enantiomorphous quartz crystals. seems to be a curious connection between the crystal form and the optical activity. Quartz crystallizes in the trigonal-trapezohedral class of the hexagonal system. Occasional crystals are found with small faces of the trigonal trapezohedron a;{5161} to the right and above mfloTO). These crystals are called right- handed and one is illustrated by Fig. 376. Other crystals have faces of the complementary form, a;j{6151} , a face to the left and above ?ra{1010}. These, in contrast, are known as left-handed THE OPTICAL PROPERTIES OF MINERALS 153 (see Fig. 375). The two crystals figured are exactly alike in shape, yet one cannot be derived from the other by any possible rotation. They bear the same relation to each other that the right and left hands do, one being the mirror-image of the other. Such crystals are said to be enantiomorphous.' Basal sections cut from a right-handed quartz crystal rotate the plane of polari- zation to the right and similar sections from a left-handed crystal rotate the plane of polarization to the left. r\ 001 no lie 001 /\ oil y 101 101 X"'' \ 110 100 100 110 xl f N^ ■ vi_ . v^ 101 101 Fig. 377. Fiq. 378. Enantiomorphous tartaric acid crystals. The two modifications of tartaric acid, C^HjO^, (dextro- and laevo-) each crystallize in the monoclinic sphenoidal class in exactly similar but enantiomorphous forms. While d-tartaric acid possesses the sphenoid {011} at the positive end of the b-axis (Fig. 377), Z-tartaric acid possesses a corresponding sphe- noid {011} at the negative end of the &-axis (Fig. 378). Now solutions of d-tartaric acid rotate the plane of polarization to the right and solutions of Z-tartaric acid to the left. This important relation was discovered by Pasteur. The difference between these two compounds is believed to be due to the space arrangement of the atoms, assuming an asymmetric carbon atom. From such considerations is built up the science of stereo- chemistry. ' In Figs. 37S and 376, also 377 and 378, the left-handed crystals are drawn as if the axes had been rotated to the right instead of to the left, as ordinarily. This brings out the enantiomorphous relation better. The left-handed crystal may be drawn by tracing the drawing of the right-handed crystal through the back of the drawing, thus reversing it. 154 INTRODUCTION TO THE STUDY OF MINERALS Racemic acid is identical in composition and chemical proper- ties with the other two forms of tartaric acid, but in solution is optically inactive. From a solution of racemic acid, equal quantities of d-tartaric and Z-tartaric acid crystallize out. There are a number of such racemic compounds, as they are called. All of these facts in regard to optical activity have an important bearing upon the theory of crystal structure. 16. OPTICAL ANOMALIES Among the optical properties of crystals there are many anomalies. Certain isometric crystals are doubly refracting and some uniaxial crystals give a biaxial interference figure with a small axial angle. There are also anomalies in extinction, in interference colors, and in interference figures. Among isometric minerals only a comparatively few, such as spinel and cuprite, are always normal. Sections of many iso- metric crystals show double refraction either in the section as a whole or in cer- tain parts. Fig. 379 represents an octa- hedral section of garnet, a mineral which often exhibits optical anomalies in thick sections. This section in polarized light consists of a triangular isotropic area „„„ „ bounded by three doubly refractive sec- FiG. 379.— Garnet. , \. , • i • • tors, each of which extmguishes parallel to and normal to the triangular outline. Sections of leucite, an isometric rock-forming mineral common in the lavas of central Italy, often show very weak double refrac- tion and fine polysynthetic twinning lamellae. Little spherical portions of the hyalite variety of opal, one of the very few amorphous minerals, often show a dark cross be- tween crossed nicols which is due to the extinction of a doubly refracting substance. Basal sections of some crystals of apatite, beryl, and vesuvia- THE OPTICAL PROPERTIES OF MINERALS 155 nite, all uniaxial minerals, exhibit in convergent polarized light biaxial interference figures with small axial angles. Most of these anomalies are clue to molecular strain developed by sudden cooling, rapid growth, pressure of surrounding rocks, etc. There are experimental proofs for all these cases. Thus, double refraction can be produced in glass, an amorphous sub- stance, bj' subjecting it to pressure in a vise. In the case of leucite the anomaly may be explained by dimor- phism. On heating a leucite crystal to about 433° C. it becomes isotropic. Hence the compound, KAl(Si03)2, is dimorphous. Above 433° C. it is isometric, while below that temperature it is biaxial with weak double refraction. On crystallizing from fusion in an igneous rock it assumed an isometric form, the trape- zohedron, which it retained on cooling to the ordinary tempera- ture, while it changed from an isotropic substance to an aniso- tropic substance. The Berlin blue interference color is the best example of an anomalous interference color. This is a deep blue of the first order which cannot be found on the color chart, for it is between the black and the white of the first order. It is characteristic of chlorite and zoisite and is due to the fact that the axial angle for these minerals varies considerably for different colors of the spectrum. As the axial angle for yellow light is nearly 0°, the resultant color, leaving out the yellow, is blue. In some uniaxial minerals, such as vesuvianite, the crystal may be optically positive for a certain color and optically negative for , another color, while for some intermediate color it is isotropic and hence the interference color in white light is the result of all the colors of the spectrum except that color. Crystals with strong dispersion of the optic axes such as titanite (2Er = 54°; 2E^ = 33°) will not give complete extinction in white light. The interference figure will be abnormal in that the hyperbolae are colored instead of black. 156 INTRODUCTION TO THE STUDY OF MINERALS 17. OPTICAL ORIENTATION / (c) By this term is meant the position of a, /?, -jr, the acute bisec- trix, and the axial plane of the optical indicatrix with reference to the crystallographic axes. In uniaxial crystals there are but two possible orientations: (1) c coincides with y (positive crystals), (2) c coincides with a (negative crystals). The tests for optical character have been mentioned in Section 11. In orthorhombic crystals the three directions a, /?, and j- must coincide with the crystallographic axes as there is parallel extinc- tion on (100), (010), and (001). The axial plane is always aplane of symmetry. There are the following six possibilities in the orientation a^j-, ay^, ^ay, Pya, ya^, y^a, the order being a, b, c in each case. There are two ways of de- termining the orientation, one by testing for the faster and slower rays in various sections, the other by obtaining the inter- ference figure. The first case is illustrated by Fig. 380 (a, b, c) , which represents sec- tions parallel to (100), (010) and (001). The faster and slower ray is determined for each of the section and marked by the letters / and s. As the c-axis is the slower in two sections it must be ;-. The (001) section containing a and b must contain a and ^. Of these a is the slower, hence it is p. So we have the orientation a = ^,, b = a, c = y or ^ay. Fig. 381 represents a basal section of topaz (topaz has a good cleavage parallel to (001), so chips are easily prepared) bounded by faces of (110) and (120). The interference figure lies across the short diagonal or in the direction of the a-axis as represented conventionally in the figure. Now the plane of the optic axes contains a and y, so that in this case b = p. By determining the ,001 / - / 100 010 (a) (h) Fig. 380. Optical orientation. THE OPTICAL PROPERTIES OF MINERALS 157 faster and slower ray in this section we find that the short diag- onal is the faster ray. We know that a, and not y, is faster than /?, so that we have a=a, b = j^, c = y. Or if the optical character be determined it is found to be positive so that ;-, the acute bisectrix, coincides with c. Another example is Fig. 382, representing a basal cleavage of muscovite. The interference figure lies along the 6-axis so that the a-axis is /?. The 6-axis is then either a or j-. By testing it is found to be the slower ray. So we have the orientation a = ^, b = f, c — a. The optical character is negative so that c, the Fig. 381.— Topaz. Fig. 382. — Muscovite. acute bisectrix, is a. In reality muscovite is monoclinic and a makes a small angle (1° 30') with the c-axis, but this may be disregarded. In monoclinic crystals the crystallographic axis b always coin- cides with either a, /3, or 7-. The other two are in the plane of the a and c axes, but make variable angles with these axes. These angles are the extinction angles on (010), the extinction on (100) and (001) being either parallel or symmetrical. The axial plane in the monoclinic system is either parallel to, or normal to (010), the plane of symmetry. In triclinic crystals none of the indicatrix axes coincide with the crystallographic axes and consequently the extinction on all faces is oblique. It is difficult to determine the optical orienta- tion of a triclinic crystal. 158 INTRODUCTION TO THE STUDY OF MINERALS 18. THE OPTICAL CONSTANTS AND THEIR DETERMINATION The optical constants differ for the various crystal systems. The only optical constant for isometric crystals is the index of refraction, which varies with the kind of light used. Thus for diamond nLi = 2.408; n^^ = 2An] nTi = 2.425, the subscripts referring to the fact that lithium, sodium, and thallium salts were used for producing monochromatic light. In uniaxial crystals there is the optical character (positive or negative), two indices of refraction n^ and n^ for various kinds of monochromatic light, the double refraction, n^-n^, and if the crystal is colored the axial colors for a and y, and the absorption ay-j- OT f> a. It is to be noted that the optical constants for tetragonal and hexagonal crystals are exactly the same. Crystals of these two systems can only be distinguished by outline, by cleavage, or by etching-tests.. For orthorhombic crystals we have the orientation, the optical character (positive or negative), three indices of refraction, n„, n^, and n^, the double refraction n—n^, and the axial angle 2V , all of these for various kinds of monochromatic light. We also have, if the mineral is colored, the axial colors for a, p, and y and the ab- sorption scheme a^/3^;-. For monoclinic crystals we have all the optical constants for the orthorhombic system in addition to the extinction angles ^ A c, ^ A c, or a A c, and the position of the axial plane. For triclinic crystals we have, in addition to the constants for the orthorhombic system, the position of the principal optical sections, which planes contain a and j9, ^ and ;-, and a and j. It will be seen that the principal quantitative optical determi- nations are index of refraction and axial angle. Accurate deter- minations of these as well as of extinction angles must be made in monochromatic light. One method for determining the index of refraction is called the prism method. The crystal cut in the form of a prism of about 60° is mounted on a reflection goniometer or spectrometer. Light THE OPTICAL PROPERTIES OF MINERALS 159 from the collimator is refracted both on entering and emerging from the prism as shown in Fig. 384. The angle of deviation, d, is a minimum for a particular position of the prism which is found by trial. Then n = ".--, — — where d is the angle of •' smAp ^ deviation and p, the internal angle of the prism. In isometric crystals a prism cut in any direction gives but one value of the index of refraction, while in uniaxial crystals a prism with refracting edge parallel to the c-axis gives two values n„ and n^. In biaxial crystals it is necessary to have two prisms to ob- tain the three values n^, ?i^, and n^. Another method for determining the index of refraction de- pends upon total reflection and may be illustrated by the dia- FiG. 383. — The Smith refractometer. grammatic cross-section of the Smith refractometer of Fig. 383. It consists of a metal frame holding a hemisphere, h, of highly refracting glass (n= 1.79), a totally-reflecting prism, P, and lenses at Z', P, and l^. The substance, the index of refraction of which is sought, is placed over the glass-hemisphere at c and in close contact with it by means of a drop of a-monobromnaphthalene or methylene iodid. As the glass hemisphere has a greater index of refraction than the substance (this is a necessary condition) , rays of light entering at l^ are in part totally reflected back into the hemisphere, the rays represented by the dotted lines entering the substance. The totally reflected rays fall on a scale S, engraved on glass and are reflected by the prism P into the eye-piece at l^. 160 INTRODUCTION TO THE STUDY OF MINERALS The field of vision appears as in the circle above the eye-piece, one-half being light and the other half dark. After the instru- ment has been calibrated the index of refraction may be read off directly on the scale. The reading in the figure indicates a doubly refracting substance with indices of refraction of 1.583 and 1.607. Accurate observations should be made in mono- chromatic light, but examination in white light will indicate the amount of dispersion. d ^ Fig. 384. The axial angle of a biaxial crystal may be measured by means of an axial angle apparatus which is practically a reflection gonio- meter plus Nicol prisms. If a suitable crystal is mounted, so that it can be rotated around its /? direction as an axis, between horizontal crossed nicols, so arranged that the interference figure shows an hyperbola, the apparent axial angle can be determined by reading the circle when the vertices of the hyperbola are tangent to the cross-wires. Fig. 385 represents a section of a biaxial parallel to the axial plane. It will be seen that the angle measured is not 2V, but another angle which is called 2E and related to it by the following equation, sin E = |9 sin V, j3 being the index of refraction in the direction of the optic axis. The value 2E is very often recorded as it is obtained directly. Another less accurate method of measuring the axial angle is based upon the fact that the distance apart of the vertices (d in Fig. 385) . of the hyperbola of an interference figure is propor- tional to the value of 2E. This determination may be made by using a micrometer eye-piece. The distance apart of the branches of the hyperbola of a substance with previously determined value of 2E is measured. This determines the constant C in the THE OPTICAL PROPERTIES OF MINERALS 161 equation sin E = d-^C. So for other crystals using the same microscope and combination of lenses the value E may be calcu- lated from the measurement of d and substitution of C. Synopsis of the Optical Properties and Constants for the Crystal Systems o Isometric All sections n No inter- ti ^ J -a remain dark ference o CO B I figure 1— 1 t-i " d Basal sec- Interference o •43 Tetragonal tion dark. Tla. nj- figure with 3 Parallel ex- dark cross 1 tinction in and colored u most sec- circles. ^ ° tions. s-a Basal sec- Interference Hexagonal tions dark. figure with M Parallel ex- Ua, Ur dark cross ■| tinction in and colored 2 most sec- circles. tions t-H Parallel ex- Interference Symmetrical s Orthorhom- tinction in ria, np, tij figure with dispersion. 2V o BIC. most sec- dark hyper- o tions. bola and 2 colored 2 < t3 'o 0. ellipses and lemniscates. Parallel ex- Interference Horizontal, 3 0) MONOCLINIC tinction in ria, np, Uj. figure with inclined, or 2V J "^ 100.001, dark hyper- crossed i'i and hOl bola and . dispersion. t^l sections. colored a« Oblique in ellipses and 1 others. lemniscates. TrI CLINIC Oblique ex- Ua, npj Ur Interference Asymmetric 2V d tinction in all sections. figure with dark hyper- bola and colored ellipses and lemniscates. dispersion. 11 PART IV THE CHEMICAL PROPERTIES OF MINERALS 1. INTRODUCTORY As minerals are natural substances of definite chemical compo- sition it is necessary for the student to have a knowledge of chemistry before he can intelligently study mineralogy, much less determine minerals. The commonly accepted classification of minerals is primarily a chemical one and while a few simple physical tests often serve to identify a mineral, a more or less complete chemical analysis is frequently necessary. On the other hand, the student may derive much chemical knowledge from a study of minerals. The compounds of such elements as titanium, molybdenum, tungsten, and vanadium are perhaps best studied in minerals. With the exception of some inert gase^ of the atmosphere (argon, krypton, neon, and xenon) all of the eighty-two known elements occur in minerals. Some are exceedingly rare and con- fined to one or two minerals, while others are very common and widely distributed in minerals. The following table of the ele- ments with their symbols and atomic weights shows to some ex- tent their occurrence in minerals. 163 164 INTRODUCTION TO THE STUDY OF MINERALS Table of Elements with their Atomic Weights (International Committee, 1912) Element Symbol Atomic weight Occurrence Aluminum . . Antimony . . , Argon Arsenic Barium Beryllium . . , Bismuth .... Boron Bromin Cadmium. . . Caesium .... Calcium Carbon Cerium Chlorin Chromium . . Cobalt Copper Dysprosium . Erbium Europium. . . Fluorin ...... Gadolinium . Gallium Germanium . Gold Helium Hydrogen . . . Indium lodin Iridium Iron Krypton .... Lanthanum . Lead Lithium Lutecium . . . , Magnesium. . Manganese . . Mercury Molybdenum Neodymium . Al Sb A As Ba Be Bi B Br Cd Cs Ca C Ce CI • Cr Co Cu Dy Er Eu F Gd Ga Ge Au He H In I Ir Fe Kr La Pb Li Lu Mg Mn Hg Mo Nd 27.1 120.2 39.9 74.9 137.4 9.1 208.0 11.0 79.9 112.4 132.8 40.1 12.0 140.2 35.5 52.0 59.0 63.6 162.5 167.7 152.0 19.0 157.3 69.9 72.5 197.2 4.0 1.0 114.8 126.9 193.1 55.8 82.9 139.0 207.1 6.9 174.0 24.3 54.9 200.8 96.0 144.3 Corundum, AI2O3 Stibnite, Sb20a In the atmosphere Arsenopyrite, FeAsS Barite, BaS04 Beryl, Be3Al2(Si03)6 Bismuthinite, Bi2S3 Colemanite, Ca2B60ii.5H20 Bromyrite, AgBr Greenockite, CdS PoUucite, H2Cs2Al2(Si03)5 Calcite, CaCOa Graphite, C Monazite, (CeLa)P04 Halite, NaCl Chromite, FeCr204 Smaltite, (Co,Ni)As2 Chalcopyrite, CuFeS2 With the rare earths Sipylile, ErNb04 With the rare earths Fluorite, CaF2 In gadolinite In sphalerite Argyrodite, AgaGeSe Gold Au In uraninite Water, H2O In sphalerite lodyrite, Agl Iridosmine, (Ir.Os) Hematite, Fe203 In the atmosphere Lanthanite, La2(C03)3.9H20 Galena, PbS Lepidolite, K,Li,Al silicate With the rare earths Magnesite, MgC03 Pyrolusite, Mn02 Cinnabar, HgS Molybdenite, M0S2 In monazite THE CHEMICAL PROPERTIES OF MINERALS Table of Elements with their Atomic Weights — Continued Element Symbol Atomic weight Occurrence Neon Nickel Niobium Niton Nitrogen Osmium Oxygen Palladium .... Phosphorus . . . Platinum Potassium . . . Praseodymium Radium Rhodium Rubidium. . . . Ruthenium. . . Samarium .... Scandium Selenium Silicon Silver Sodium Strontium .... Sulfur Tantalum Tellurium Terbium ThaUium Thorium Thulium Tin Titanium Tungsten Uranium Vanadium .... Xenon Ytterbium .... Yttrium Zinc Zirconium .... Ne Ni Nb Nt N Os O Pd P Pt K Pr R Rh Rb Ru Sa Sc Se Si Ag Na Sr S Ta Te Tb Tl Th Tm Sn Tl W U V Xe Yb Y Zn Zr 20.2 58.7 93.5 222.4 14.0 190.9 16.0 106.7 31.0 195.2 39.1 1 10.6 226.4 102.9 85.4 101.7 150. 44. 79. 28. 107. 23.0 87.6 32.1 181.5 127.5 159.2 204.0 232.4 168.5 119.0 48.1 184.0 238.5 51.0 130.2 172.0 89.0 65.4 90.6 In the atmosphere MUlerite, NiS Columbite, (FeMn) (NbOs): Radium emanation Soda Niter, NaNOs Iridosmine, (Ir.Os) Water, H2O Palladium, Pd Apatite, Ca5F(POi)3 Platinum, Pt Sylvite, KCl In monazite In uraninitc In platinum In rhodizite Laurite, RuSa In samarskite In euxenite Clausthalite, PbSe Quartz, SiOs Argentite, Ag2S Halite, NaCl Celestite, SrS04 Sulfur, S Tantalite, Fe{Ta03)2 Calverite, AuTe2 In gadolinite Lorandite, TIASS2 Thorite, ThSiOi In gadolinite Cassiterite, Sn02 Rutile, Ti02 Wolframite, (Fe,Mn)W04 Uraninite, UaOs Vanadinite, PbiiCl(V04)3 In the atmosphere In gadolinite Xenotime, YPO< Sphalerite, ZnS Zircon, ZrSiO^ 166 INTRODUCTION TO THE STUDY OF MINERALS Of the eighty-two elements enumerated only about eighteen occur as minerals. They are carbon, sulfur, selenium, tellurium, arsenic, antimony, bismuth, mercury, copper, silver, gold, lead, iron, platinum, palladium, iridium, osmium, tantalum, and tin. This leaves out of consideration the free gases of the atmosphere. From a chemical standpoint the elements may be divided into two classes, the metals and the non-metals. The metals include such elements as copper, silver, gold, lead, iron, and platinum. Some of these occur as alloys, such as electrum (Au,Ag), amalgam (Ag,Hg), nickel-iron (Fe,Ni), and iridosmine (Ir,Os). The non-metals include such elements as oxygen, hydrogen, nitrogen, phosphorus, and sulfur. Arsenic, antimony, and bismuth are intermediate in their properties between metals and non-metals and are sometimes called semi-metals or metalloids. 2. CHEMICAL TYPES Most minerals are, of course, chemical compounds, or combi- nations of two or more elements. These compounds are the chemical types recognized by chemists, namely, oxids, acids, bases, and salts with their various subdivisions. Acids are compounds, the dilute water solutions of which contain hydrogen ions. According to the theory of ions it is the hydrogen ions that give the acid properties such as sour taste and the change of blue litmus to red. The strength of the acid depends upon the proportion of hydrogen ions present or upon the degree of dissociation. Hydrochloric and sulfuric acids are strong acids, while carbonic and silicic acids are weak acids. Acids are compounds of hydrogen with the halogens (CI, Br, I, or F), with sulfur, or with certain radicals, such as CO3, SO^, PO4, ASO4, AsSj, AsS^, SbSg, SbS^. These are called acid radicals. The most common acids are those containing oxygen and are known as oxygen acids or oxy-acids. The oxy-acids are mono- basic (HNO3), dibasic (HjSO^), tribasic (HgPO^) or tetrabasic (H^SiO^), according as- they have one, two, three, or four replace- THE CHEMICAL PROPERTIES OF MINERALS 167 able H atoms. The polybasic acids, as they are called, are capable of forming condensed acids by subtracting water. This is especially prominent with the silicic acids. Orthosilicic acid is H.SiOj; H,SiO,-H20 = H2Si03, metasilicic acid; 2H,SiO,- H20 = H„Si20j, diorthosilicic acid; 2H,SiO,-3H20 = H2SiA, dimetasilicic acid. There are also H^SigOg {ZB.^^\0 ^- 4:B.^0) and H,Si30,„ (3H,SiO,-2H,0). The replacement in the oxy-acids of O by S gives compounds called sulfo-acids. Thus HjAsO^ is arsenic acid or oxy-arsenic acid, while HgAsS^ is sulfarsenic acid. H3ASO3 is arsenious acid, while HgAsSg is sulfarsenious acid. Various condensed acids may be derived from the above by the subtraction of HjS, en- tirely analogous to the condensed oxy-acids. Thus we have HAsS2(H3AsS3 — HjS), metasulfarsenious acid and H^AsjS^ (2H3- AsS^— HjS), pyrosulfarsenic acid. Very few of these acids exist either as minerals or prepared compounds, but salts of all of them are known as minerals. Bases are compounds the dilute water solutions of which con- tain hydroxyl (OH) ions. It is the hydroxyl ions that give the basic properties such as soapy feel, and the changes of red litmus to blue. The strength of the base is proportional to the number of hydroxyl ions present. The strong bases such as KOH and NaOH are called alkalies. Weak bases are represented by re(0H)3 and A1(0H)3. Among the bases represented by min- erals are MgCOH)^, MnCOH)^, A1(0H)3, A102H(A1(0H)3-H30), Fe02H(Fe(OH)3-H20), and Fe.OeH, (4Fe(OH)3-3H20). Oxids are compounds of the elements with oxygen. Elements, the oxids of which form bases with water, are called metals. These oxids are called basic anhydrids for this reason. Elements, the oxids of which form acids with water, are called non-metals. These oxids are called acid anhydrids. Salts are compounds formed by the union of bases with acids, the metal of the base uniting with the non-metal or acid radical of the acid to form the salt, while the hydroxyl of the base unites with the hydrogen of the acid to form water thus, NaOH-|-HCl 168 INTRODUCTION TO THE STUDY OF MINERALS = NaCl + H20. In dilute solutions salts are dissociated into two parts or ions as they are called. The metal is one ion, and is called the cation, while the non-metal or acid radical is the other ion, and is called the anion. Among salts we may distinguish halogen salts, oxy-salts, and sulfo-salts corresponding to the acids of which they are the derivatives. The following represent salts found as minerals : Sulfid, PbS; selenid, PbSe; tellurid, PbTe; arsenid, FeAsj; antimonid, AggSb; sulfarsenite, AgjAsSg; sulfarsenate, CujAsS^; sulfantimonite, AgjSbSg; sulfantimonate, CujSbS^; sulfoferrite, CuFeSj; sulfochromite, FeCrjS^; sulfovanadate, CugVS^; sulfo- germanate, AggGeS^; sulfostannite, PbSnSj; chlorid, AgCl; bromid, AgBrjj iodid, Agl; fluorid, CaFj,' fluosilicate, KjSiFg,- carbonate, CaCOg," meta-aluminate, MgAljO^; metaferrite, FeFejO^; meta- chromite, FeCr204; chromate, PbCr04; metaniobate, Fe(Nb03)2; metatantalate, Fe(Ta03)2; phosphate, LiFePO^; arsenate, FeAs- O4.H2O; vanadate, BiVO^; antimonate, CajSbjOj; nitrate, NaNOg,- borate, AIBO3; sulfate, BaSO^; chromate, PbCrO^; selenite, CuSe03-2H20; tellurite, Fe(Te03)3-4H20; tungstate, CaWO^; molybdate, PbMoO^; metatitanate, CaTiOgj orthosili- eate, (Mg,Fe)2Si04; metasilicate, CaSiOg; trisilicate, KAlSijOj; dimetasilicate, LiAl (81305)2; diorthosilicate, Pb3Si207. All the above are normal salts, that is, all the hydrogen of the acid or hydroxyl of the base has been replaced by metals or by acid radicals respectively. A compound in which only part of the hydrogen of the acid has been replaced by a metal is called an acid salt. Among minerals we have KHSO^, and H2CuSi04, which are called acid potassium sulfate, and acid copper silicate respectively. A compound in which only part of the hydroxyl of the base is replaced by an acid radical is called a basic salt. Among minerals we have Cu2(OH)2C03, Cu4(OH)|;Cl2, CujCOH)- AsO^, and many others. These three minerals are called basic copper carbonate, chlorid, and arsenate respectively. The formulae of some minerals are written as if they consist of two or more separate molecules. These are called molecular THE CHEMICAL PROPERTIES OF MINERALS 169 compounds for want of a better name. Among molecular com- pounds are double salts and hydrates or hydrous salts. Double salts are (1) salts composed of two metals with a common acid radical (example, dolomite CaC03-MgC0g), (2) salts of a single metal with two distinct acid radicals (example, arsenopyrite FeSj'FeAsj) or (3) salts in which both the metal and acid radical are different (example, kainite MgSO^-KCl-SHjO). Acid and basic salts may also be written in the form of double salts. KHS0, = K2S0,-H,S0„ Cu2(OH)2C03 = CuC03-Cu(OH)2. Another kind of compound is SbjSjO, antimony oxy-sulfid, which may be written 2Sb,S3-Sb,03(3Sb2S20). Similarly Pb^OCl^ (or PbCl^-PbO) is lead oxy-chlorid and Pb^CljCOj (or PbCl/Pb- CO3) is lead chloro-carbonate. Acid and basic salts when heated in the closed tube at a rela- tively high temperature (usually above 200° C.) give off water and this water is called water of constitution. In other compounds water is more loosely held, and is given off on heating at a temperature varying from about 100° C. to 200° C. This is the so-called water of crystallization, but as Alexander Smith has pointed out this term is a misnomer, water not being necessary for crystallization. Many anhydrous com- pounds and minerals occur in well formed crystals. Salts which give off water at low temperatures are called hydrates or hydrous salts. The formula is written as if they contain water as such. Examples, hydrous calcium sulfate, CaS04-2H20; hydrous sodium borate, NajB^O,- IOH2O. There may be various hydrates, for example, MgSO.-Hp, MgSO,.6H20, and MgSO^-VH^O. The following are examples of complicated salts which occur as minerals. HNa3(C03)2-2H20, hydrous acid sodium carbonate; Fe^(OJi)2{SOJ^-l7'iifi, hydrous basic ferric sulfate. 3. DERIVATION OF CHEMICAL FORMUL.^ A mineral has a definite chemical composition and hence may be represented by a formula. The formula is obtained by dividing the percentage composition of the various elements or 170 INTRODUCTION TO THE STUDY OF MINERALS radicals by the corresponding combining weights as found in a table of atomic weights (pages 164-5). The ratio of these ex- pressed in the simplest whole numbers possible gives the empirical formula. Example: An analysis of chalcopyrite from Phoenix- ville, Pennsylvania, gave Smith the following: I II III IV Cu 32.85 H- 63.6 =0.516 34.57 Fe 29.93 -H 55.8 =0.536 30.54 S 36.10-^32.1 =1.121 34.89 Pb 0.35 Dividing by the combining weights given in the second column we have the figures of the third column, lead being omitted. These numbers are nearly in the ratio 1:1:2, hence the formula CuFeSj. The theoretical percentages for chalcopyrite are given in the fourth column. Discrepancies in analyses may be explained in two ways. (1) Impurities may be present and should be eliminated before analysis if possible. If not, the analysis may be recalculated. (2) Similar metals or acid radicals may replace each other in various proportions. In this case the molecular ratios of replac- ing metals or acid radicals are added together. An analysis of brown sphalerite from Roxbury, Connecticut, gave Caldwell the percentage composition "of column I. Dividing by the I II III IV Zn 63.36-=-65.5 =0.967 1 Fe 3.60-^55.8 =0.064 / S 33.36 H- 32.1 =1.039 1.039 combining weights of column II we have the molecular ratios of III. The sum of the molecular ratios for Zn and Fe (1.031) is to the molecular ratio for S (1.039) practically as 1:1. Hence the formula, (Zn,Fe)S, which means that iron replaces zinc in varying amounts. Analyses of sphalerite show an iron content varying from nil up to 18 per cent. Analyses of oxids, haloids, sulfids, and sulfo-salts are given as THE CHEMICAL PROPERTIES OF MINERALS 171 •percentages of the elements. This cannot be done with the oxy- gen salts as there is no way of determining oxygen directly. So that the percentage composition of the oxygen salts must be expressed either as oxids or as metals and acid radicals. At present it is customary to use the oxids. This is in accordance with the electro-chemical theory of Berzelius in which dualistic formulae were used. Thus FeSO^ was considered as FeO-SOg, FeO being the base or electropositive radical and SO3, the acid or electronegative radical. As these views are considered anti- quated by modern chemists, it is preferable to employ the metals and acid radicals in stating the results of analysis. Thus FeSO^ may be given as Fe" and SO^, indicating the valence of iron. The present author has given the results of analyses of oxygen salts (except the silicates) in this form, though he has had to recalcu- late the analyses. This method is especially applicable if haloids or sulfids are present. In the ordinary method there is an excess of oxygen equivalent to the amount of halogen or sulfur present which must be deducted. For example, apatite is Ca5F(P04)3. The calculated percentage compositions are: CaO = 55.5; P205 = 42.3; F = 3.8; Total = 101.6. The excess over 100 per cent, is due to the fact that only part of the calcium is combined with the oxygen as can be seen by expressing the formula in another way. QCaO.SPjOjCaFj. The oxygen equivalent of F is ^0 with atomic weight of 8. The percentage compositions given above have been figured on the basis of formula weight of 512.5 (504.5 + 8). 512.5:504.5: : 101.6: 100. A much better way is to express the percentage composition thus: Ca = 39.7; F = 3.8; P04 = 56.5; total = 100.0. But in many silicates it is not possible to determine the silicic acid of which they are salts and in these cases nothing can be done except to use the oxids, antiquated though the method may be. In this book analyses of silicates are all given in the form of oxids. In the case of hydrous, acid, or basic salts of any kind, the water percentage is given as the determination of water is often a practical means of identifying a mineral. 172 INTRODUCTION TO THE STUDY OF MINERALS Some facts regarding the relation between chemical composi- tion and crystal form are important in the study of minerals, for upon these facts the commonly accepted classification of minerals largely rests. 4. POLYMORPHISM Many mineral substances exist in two or more distinct forms, that is, in crystals belonging to different crystal systems (not simply different crystal habits) and having dissimilar physical properties. Such compounds are called pol3miorphs. A famil- iar example is carbon which occurs as graphite in soft, opaque hexagonal crystals, and as diamond in very hard, transparent isometric crystals. Polymorphous elements like carbon are called allotropic. Among polymorphous minerals may be men- tioned the following: „ Q / Pyrite — Isometric ^ \ Marcasite — Orthorhombic p_ / Calcite — Hexagonal ' \ Aragonite — Orthorhombic T^Aici- r\ f Orthoclase — Monoclinic 1^ Microchne — Tnchmc The above minerals are dimorphous. The following minerals are trimorphous : r Rutile —Tetragonal c = 0.64 TiO^ I Octahedrite —Tetragonal c = 1.77 [ Brookite — Orthorhombic [ Cyanite — Triclinic AljSiOj •! Andalusite —Orthorhombic a = 0.986, c = 0.702 [ Sillimanite — Orthorhombic a = 0.970, c = 7 The three last mentioned minerals have the same empirical formula, but Groth has suggested that cyanite is (A10)2Si03, and the other two, Al(A10)Si04. Numerous examples of polymorphism occur in prepared com- THE CHEMICAL PROPERTIES OF MINERALS 173 pounds. Sulfur may be prepared in at least four modifications: a-sulfur is ortliorhombic; /3-sulfur, monoclinic; ^-sulfur, also monoclinic, but with different axial ratio from /?-sulfur; 5-sulfur, rhombohedral. Mercuric iodid, Hglj, exists in a red tetragonal modification (from solutions) and also in a yellow orthorhombic modification (from fusion or sublimation). In many cases it is the temperature wliich determines the modification formed. Thus calcite forms from aqueous solutions below 30° C, while aragonite forms above 30° C. For example, the crust formed in a tea-kettle is aragonite. Quartz (SiOj) forms below 800° C, while tridymite (a modification of SiOj) forms above 800° C. 5. ISOMORPHISM Many compounds of similar chemical composition, especially salts with the same acid radicals and related metals, have almost identical crystal forms. Such compounds are said to be isomor- phous. Isomorphous substances have similar form, but except in the isometric system it does not mean that the form is identical. For example, the angle (110:lT0) for barite, BaSO^, is 78° 22J', while for celestite, SrSO^, it is 75° 50' and for anglesite, PbSO^, it is 76° 16^'. Barite, celestite, and anglesite form an isomorphous group. Among prominent isomorphous groups of minerals are the following: ' Pyrite FeS^ Smaltite (Co,Ni)As2 Cobaltite CoAsS GersdoriEte NiAsS Marcasite FeSj Arsenopyrite FeAsS Lollingite FeAsj Glaucodote (Co,Fe)AsS Safflorite CoAs^ Rammelsbergite NiAsj Tetrahedrite GujSbSs Tennantite CujAsS, 174 INTRODUCTION TO THE STUDY OF MINERALS f Proustite AgjAsSs Y Silvers < _ \ Pyrargyrite AgjSbS, / Corundum Al,03 \ Hematite Fe,03 Cassiterite SnOj Rutile TiO, Diaspore A1(0H)3-A1,03 Goethite Manganite Fe(OH)3Fe,03 Mn(OH)3-Mn20 Calcite CaC03 Magnesite MgC03 Ankerite Ca(Mg,Fe)C03 Siderite FeC03 Rhoohrosite MnCO, Smithsonite ZnCOj Aragonite CaC03 Strontianite SrCOj Witherite BaC03 Cerussite PbCO, Fluor-apatite Ca3F(POj3 Chlor-apatite Ca,Cl(P0,)3 Dahllite Ca5(C03)j(POJ Voelokerite Ca,04(PO,)3 Svabite Ca,F(AsOj3 Pyromorphite Pb,Cl(P0j3 Mimetite Pb^CUAsOJ, ^ Vanadinite Pb5Cl(VOj3 ' Barite BaSO, Celestite SrSO, ^ Anglesite PbSO, Ilmenite FeTi03 Geikielite MgTiO, Pyrophanite MnTiO, ^ Senaite (Fe,Mn,Pb)Ti03 Grossularite Ca3Al,(SiO,)3 Pyrope Mg3Al,(SiO,)3 llaTTiota Almandite Fe3Al,(SiO,)3 VJIdllitl/Ub ' Spessartite Mn3AU(SiO„)3 Andradite Ca3Fe,(SiO,)3 Uvaroyite Ca3Cr,(SiO,)3 THE CHEMICAL PROPERTIES. OF MINERALS 175 Many isomorphous compounds are capable of crystallizing out together in various proportions forming what are known as isomorphous mixtures. There are many such cases among min- erals and this fact is very useful in interpreting mineral analyses. Ankerite is an isomorphous mixture of CaCOj with MgCOg and FeCOg and is represented by the formula Ca(MgFe)C03, which means that the proportions of Ca, Mg, and Fe vary in different specimens and that the molecular ratios of Mg and Fe together equal that of Ca. Among prominent isomorphous mixtures are: Sphalerite (Zn,Fe)S, smaltite (Co,Ni)As2, tetrahedrite (Cu,Fe,Zn,Hg,Ag)3(Sb,As)S3, columbite (Fe,Mn)(Nb,Ta)20e, campylite Pb5Cl(As,P)30i2, endlichite Pb5Cl(As,V)30,2, pi- sanite (Fe,Cu)SO/7H30, wolframite (Fe,Mn)W04, actinolite Cu(Mg,Fe) 3(8103),, epidote Ca2(Al,Fe)3(OH)(SiOJg. The gar- nets are isomorphous mixtures of minerals given on page 174. It is rare to find an analysis of garnet that will correspond ex- actly to any one of these formulae. The physical properties of isomorphous mixtures vary con- tinuously and for this reason the term solid solution is sometimes used. Some authors restrict the term isomorphous to compounds which are capable of forming mixed crystals. But this hardly seems feasible with minerals, for most of them are intractable compounds. 7. BLOWPIPE ANALYSIS The advantage of blowpipe analysis lies in the fact that the tests are simple, the apparatus portable, and the reagents few in number. By means of the blowpipe an intense heat (about 1500° C.) can be obtained on a small scale, and a variety of chem- ical effects can be brought about. At the same time it is the author's opinion that with a few exceptions, blowpipe analysis is of value only in the determination of minerals. Blowpipe analysis will be discussed under four headings: (8) apparatus, (9) reagents, (10) operations, and (11) select tests. Section 10 may be used in preliminary tests and also as determi- 176 INTRODUCTION TO THE STUDY OF MINERALS native tables, while tests for the metals and acid radicals may be found in section 11, arranged alphabetically by elements. 8. APPARATUS USED IN BLOWPIPE ANALYSIS Blowpipe. The blowpipe is made in a variety of forms. The simplest blowpipe is a brass tube about 10 inches long bent at one end. A bulb is sometimes added to condense moisture. A more elaborate form is a nickel-plated tube with a moisture chamber at one end and a smaller tube at right angles which is provided with either a brass or a platinum tip. Where gas is available, the gas blowpipe is undoubtedly the most convenient form on account of the perfect control of the flame. The gas blowpipe (Fig. 386, page 181) is similar to the nickel-plated form just described, but the smaller right-angled tube is a double one, the inner one for air, the outer one for gas. Fuel. Gas is the most convenient and commonly used fuel. If a Bunsen burner is used, it is well to use a small tube which fits the top of the Bunsen burner, and is provided with a flange in which the tip of the blowpipe rests. The luminous flame of the Bunsen burner should be used with the blowpipe. Where gas is not available, alcohol, lard oil, or olive oil may be burned in a lamp with a wick. Candles are even more conven- ient. For field use a good combination is alcohol for heating and candles for use with the blowpipe. Charcoal. Slabs of charcoal about 4 inches long, 1 inch wide, and f inch thick are used. They may be purchased from dealers in chemical apparatus. Plaster. A paste of plaster of Paris with water is poured out on oiled glass in sheets about J inch thick. Before hardening, it is marked off in rectangles about 4 inches long and 1 inch wide. Platinum Tipped Forceps. These forceps are essential for testing the fusibility of minerals, and are useful for other pur- poses. Arsenic, antimony, lead, and copper minerals should be fused on charcoal, for these metals alloy with platinum. THE CHEMICAL PROPERTIES OF MINERALS 177 Hammer. A small square-faced hammer of about one-fourth pound weight is indispensable. Anvil. A small block of steel, square or rectangular in cross- section, and about J inch thick is convenient for powdering minerals. Platinum Wire. No. 27 platinum wire is the best size for general use. The wire may be fused into a piece of glass tubing, or held in a special holder made for the purpose. Test Tubes. The most convenient size is 4 inches long and ^ inch in diameter. Glass Tubing. Soft glass tubing of 7 mm. outside diameter is best for most purposes, but it is well to have a variety of sizes. For some tests hard glass tubing is preferable. Watch Glasses. These are needed especially for solubility tests. The best size is 2 inches in diameter. Magnet. A magnetized knife-blade answers the same purpose and is more convenient. Lens. A Coddington or aplanatic triplet of f inch focus is recommended. A triangular file, blue glass, funnels, and filter- paper are also essential. The following pieces of apparatus are not essential, but will be found very useful. Diamond Mortar. A mortar made of a piece of cylindrical tool-steel about 1 inch long and about 1 inch in diameter, with a convenient size cylindrical cavity and pestle to fit, is very conven- ient for reducing a mineral to a coarse powder. Agate Mortar. A small agate mortar, IJ inches in diameter, is used for fine grinding of minerals. Steel Pliers are used for breaking oil fragments of minerals. Platinum Foil. A thin sheet of platinum about J by | inch may be used for soda fusions. Dropping Bulbs are useful for reagents that are needed in small amounts, such as Co(N03)2. Small beakers, crucibles, wash-bottles, etc., may often be used to advantage. 12 178 INTRODUCTION TO THE STUDY OF MINERALS 9. REAGENTS USED IN BLOWPIPE ANALYSIS A. Dry Reagents Dry reagents should be kept in wide mouthed glass bottles. It is convenient to have a set of four or six of these bottles in a wooden stand. Soda or Sodium Carbonate, NajCOj. Baking soda (NaHCOj) may be used instead. Soda is used principally for fusions. Borax, NajB^Oy-lOHjO is used principally for bead tests. The ordinary commercial salt can be used. Borax glass is simply fused borax, used in silver cupellation. Sodium Metaphosphate, NaPOg. This is used for the bead tests, in which salt of phosphorus, HNaNH4P04-4H20, is usually employed. It can be made by fusing salt of phosphorus, and is much more convenient, as a salt of phosphorus bead usually drops off the loop of platinum wire when heated. Potassium Acid Sulfate, KHSO^. This is used in bismuth flux, in boracic acid flux, and also independently. Bismuth Flux is made by grinding together 1 part KI, 1 part KHSO4, and 2 parts S. It is used on plaster tablets and also on charcoa;l. Boracic Acid Flux is a mixture* of 1 part of finely powdered fiuorite (CaF^) with 3 parts of KHSO,. Cupric Oxid, CuO. Powdered malachite may be used instead. Tin. Ordinary tin-foil (sheet lead with a thin coating of tin) is used as a reducing agent. Test Lead. Lead in a granulated form such as is used in assay- ing. Bone-Ash, such as is used in assaying, is moulded into cupels on charcoal. B. Wet Reagents The following are the more important wet reagents used in the determination of minerals, though occasionally' any of the reagents of the chemical laboratory may be found useful. Hydrochloric Acid, HCl. Two parts concentrated acid (sp. gr.. THE CHEMICAL PROPERTIES OF MINERALS 179 1.20) with 3 parts distilled water is the acid used for general purposes. (.IN.)* Nitric Acid, HNOg. One part concentrated acid (sp. gr. 1.42) with 2 parts water. (")N.) Sulfuric Acid, H,SOj. One part concentrated acid (sp. gr. 1.S4) with 4 parts of water (5N). It should be diluted with great care by pouring acid into the water rather than the reverse. Citric Acid. As this is a solid it may be used in the field for testing carbonates. It is only necessary to make a water solution. Aqua Regia is a mixture of 3 parts of cone. HCl and 1 part of cone. HNO3. It is made up when needed. Ammonium Hydroxid, NHjOH. One part of concentrated NH^OH (sp. gr. 0.96) to 4 parts of solution. Ammonium Oxalate, (NHJ^CI^O^^HjO. 40 grams of salt to a liter of solution. (iN.) Sodium Acid Phosphate, Na2HP04-12H20. 60 grams to a liter of solution. (iN.) Ammonium Molybdate, (NHJ2M0O4. It is difficult to pre- pare this reagent. Dissolve 100 grams of M0O3 in 250 com. NH^OH (sp. gr. 0.96) with 2.10 ccm. of water. After cooling, pour this solution into 750 ccm. HNO3 (sp. gr. 1.2) with 750 ccm. water while stirring. Silver Nitrate, AgNOj. 43 grams of the salt to a liter of solu- tion. (iN.) Barium Chlorid, BaCl2'2H20. 61 grams of salt to a liter of solution. (^N.) Cobalt Nitrate, Co(N03)2-6H20. 73 grams of the salt to a liter of solution. (AN.) C. Additional Reagents used in Qualitative Analysis Acetic Acid, HC2H3O2. 30 per cent. acid. (5N.) Alcohol. 95 per cent, ethyl alcohol. *N means a normal solution, i. e., one that contains one gram-equivalent of the sub- stance in one liter, a gram atom of hydrogen being the unit. _, 180 INTRODUCTION TO THE STUDY OF MINERALS Ammonium Carbonate, (NHJ2CO3. 192 grams to a liter of solution, including 100 ccm. of NH.OH. (4N). Ammonium Chlorid, NH^Cl. 267 grams to a liter of solution. (5N). The solid reagent is also used for some purposes. Ammonium Sulfid, (NHJjS. Saturate cone. NH^OH with HjS, and add an equal volume of NH^OH. Dilute with three volumes of water. (4N.) Ammonium Sulfid, Yellow, (NH4)3Sa;. Made by adding flowers of sulfur to (NHJjS. Barium Hydroxid, Ba(OH)2-8H20. Calcium Carbonate, CaCOj. The solid reagent. Chloroplatinic Acid, HaPtClj. This is made by dissolving scrap platinum (after cleaning) in aqua regia. Ether -Alcohol. Equal volumes of ether and absolute alcohol. Ferrous Sulfate, FeSO^-THjO. Concentrated solution. Lead Acetate, Pb(C2H302)2'3H20. 95 grams to a liter of solu- tion. (iN.) Potassium chromate, K2CrO^. 49 grams to a liter of solu- tion. (JN.) Potassium Cyanid, KCN. 33 grams to a liter of solution. (^N.) Potassium Ferricyanid, KgFeCCN),. 55 grams to a liter of solution. (^N.) Potassium Ferrocyanid, K4Fe(CN)8-3H20. 53 grams to a liter of solution. (iN.) Potassium Hydroxid, KOH. Solid reagent. Sodium Acetate, NaC2H302. The solid dissolved in ten parts of water. Sodium Carbonate, NajCOg. Solid reagent. Sodium Cobaltic Nitrite, Na3Co(N02)e- This is made by add- ing 1 part of Co(N03)2 solution to 3 parts of acetic acid and 5 parts of a 10 per cent, solution of NaNOj. Sodium Hydroxid, NaOH. Solid reagent. Sodium Nitrate, NaN03. Solid reagent. Stannous Chlorid, SnCl2-2H20. 56 grams to a liter of solu- tion. (JN.) Tartaric Acid, C Ji^Oe. THE CHEMICAL PROPERTIES OF MINERALS 181 10. THE OPERATIONS OF BLOWPIPE ANALYSIS The list of tests given here serves both as an outline to follow with known substances, and also as determinative tables for unknown minerals. Only important tests are included so that to be of value decided results must be obtained. I. Use of the Blowpipe. To produce a steady flame, maintain a reservoir of air by keeping the checks slightly distended, and by breathing through the nose. Oxidizing Flame (O.F.). The extreme outer tip (Fig. 386) of a small flame produced by a rather strong blast of air is most favorable for oxidation. If a candle, lamp, or Bunsen burner is O.F. FiQ. 386. used, the tip of the blowpipe is held just within the flame. One's ability to produce a good oxidizing flame may be judged by fusing borax on a | inch loop of platinum wire and then adding a little M0O3. The bead should be colorless. Reducing Flame (R.F.) . The tip of the inner luminous cone (Fig. 386) of a large flame produced by a gentle blast of air is most favorable for reduction. If a candle, lamp, or Bunsen burner is used, the blowpipe tip is held just outside the flame and the whole flame is directed toward the assay. A borax bead made amethyst colored with a little MnOj in O.F. should become colorless when heated in a good reducing flame. The reducing flame should be luminous, and just hot enough to pre- vent the deposition of soot. 182 INTRODUCTION TO THE STUDY OF MINERALS II. Flame Tests. In the high temperature of the blowpipe flame many com- pounds are volatilized. The colors produced are often charac- teristic, and should be viewed against a black background such as a piece of charcoal. The chlorids are, as a rule, the most vola- tile compounds of the metals, so HCI should be used, but in some cases H2SO4 is better. Platinum wire is used except for com- pounds of As, Sb, Pb, and Cu, which may be heated on char- coal. The wire should be cleaned with HCI after each test, but it should never be placed in a reagent bottle on account of danger of contaminating the reagent. Red Flames. Purplish red — lithium compounds. Crimson — strontium compounds. Orange red — calcium compounds. Yellow Flames. Intense yellow (masked by blue glass) — sodium compounds. Green Flames. Yellowish green — barium compounds. Yellowish green — molybdenum compounds. Emerald green — copper compounds (without HCI). Bright green (use HjSO^) — boron compounds. Pale bluish green (use HjSOJ — phosphates. Pale bluish green — tellurium compounds. Pale bluish green — antimony compounds. Bluish green — zinc compounds. Blue Flames. Azure blue — copper compounds (with HCI). . Pale blue — arsenic compounds. Pale blue — lead compounds. Violet Flames. Pale reddish violet (use blue glass) — potassium compounds. The spectroscope must be used to detect such elements as rubidium, calcium, thallium, indium, etc., and also to detect very small amounts of the above mentioned elements. THE CHEMICAL PROPERTIES OF MINERALS 183 III. Open Tube Tests. Glass tubes about 4 inches long and open at both ends are used. The substance is placed about 1 inch from one end of tube. The tube is heated gently in a horizontal position at first and then is gradually inclined while still heating, thus producing a current of air. Odor of burning matches (SO.) — sulfids and sulfo-salts. Sublimate of minute brilliant crystals (AsjOg) — arsenids and sulfarsenites. Non-volatile amorphous sublimate (Sl)^04) on under side of tube — antimony sulfid and sulfantimonites. Gray metallic globules (Hg) — mercury sulfid. IV. Closed Tube Tests. Glass tubes closed at one end are used. Two closed tubes may be made at the same time by fusing a piece of tubing about 5 inches long at its middle point and pulling it apart when hot. Tubes should be clean and dry before using. 1. Change in Appearance. Decrepitates (flies to pieces) — characteristic of many minerals. Turns black — copper minerals. Turns dark red — iron minerals. Turns yellow — lead minerals. Turns yellow (white on cooling) — zinc minerals. 2. Formation of Sublimates. Yellow sublimate (S) — some sulfids. Black metaUic mirror (As) — arsenids. Reddish yellow (AsS) — arsenic sulfids and sulfarsenites. Reddish brown (SbjSjO) — antimony sulfids and sulfanti- monites. White volatile sublimate — ammonium salts. Water (HjO) — hydroxids, hydrous, basic, and acid salts. 3. Formation of Gases. Colorless and odorless (COj) — carbonates (detected by Ba(0H)2. 184 INTRODUCTION TO THE STUDY OF MINERALS Colorless and odorless (O) — manganese dioxids (detected by glowing charcoal). Brownish-red and pungent odor (NOj) — nitrates. V. Treatment on Charcoal. The substance, either alone or intimately mixed with some reagent, is heated in a shallow circular cavity, at one end of the charcoal, made by revolving a coin or end of a knife handle. O.F. or R.F. is used according to the desired effect. 1. Evolution of Gas. Odor of burning matches (SOj) — sulfids and sulfo -salts (use O.F.). Arsin odor (AsHg) — arsenids and sulfarsenites (use R. F.). 2. Formation of Sublimates. (Use 0. F.). White sublimate near assay (SbjOg) — antimony compounds. White sublimate far from assay (AsjOj) — arsenic compounds. White sublimate, yellow when hot (ZnO) — zinc compounds. White sublimate, yellow near assay (PbSOJ — ^lead sulfid. Yellow sublimate (PbO) — ^lead compounds. Yellow sublimate BijOj — ^bismuth compounds. 3. Reduction with Soda. Mix intimately 1 part of the finely powdered substance with 3. parts of soda (NajCOg) and fuse in R. F. on charcoal. Magnetic particles (Fe304,Ni,Co) — iron, nickel, and cobalt compounds. Metallic button, gray and malleable (Pb) — ^lead compounds. Metallic button, malleable but brittle on edges (Bi) — bis- muth compounds. Metallic button, malleable white (Ag) — silver compounds. Metallic button, malleable yellow (Au) — gold compounds. Metallic button, malleable red (Cu) — copper compounds. Metallic button, malleable white (Sn) — tin compounds. 4. Soda Fusion Test for Sulfur. An intimate mixture of a finely powdered sulfid or sulfo-salt with about three parts of soda is heated in O.F. on a thin sheet of THE CHEMICAL PROPERTIES OF MINERALS 185 mica (or platinum, if absence of As, Sb, Pb, and Cu is assured). The fused mass placed on a bright silver coin with several drops of water and crushed will give a black stain (AgjS). The reac- tions are : R"S + Na^COg = Na^S + R"C03. Na^S + 2 Ag + H^O + = Ag2S + 2NaOH. Tellurids give the same test. Try a blank test to see if gas or soda contains sulfur. Sulfates also give this test if heated in a strong R.F. on char- coal instead of on mica. Powdered charcoal helps the reaction. The sulfur compounds sink into the charcoal, hence the same spot cannot be used more than once. 5. Treatment with Cobalt Nitrate. The substance is heated intensely on charcoal before and after adding a dilute solution of cobalt nitrate. In this way cobalt aluminate, cobalt zincate, etc., are formed. Deep blue coloration — ^infusible aluminum compounds and zinc silicates. (Almost any fusible substance will give a blue color for a cobalt glass is formed.) Bright green coloration — zinc compounds, except the sili- cates which give a blue coloration. Bluish-green coloration — tin compounds. Pale pink coloration — magnesium compounds (not very satisfactory) . VI. Treatment on Plaster with Bismuth Flux. An intimate mixture of the substance with an equal quantity of bismuth flux (2 parts S, 1 part KI, and 1 part KHSOJ is heated gently at one end of a plaster tablet. In this way iodids of the metals are obtained. Yellow sublimate (Pbl2) — ^lead compounds. Orange sublimate stippled with peach-red (Sblj) — antimony compounds. Purplish-chocolate sublimate (Bilg) — bismuth compounds. Scarlet sublimate (dark greenish-yellow if overheated) (Hglj) — mercury compounds. Deep blue sublimate (Molg?) — molybdenum compoun(^s. 186 INTRODUCTION TO THE STUDY OF MINERALS VII. Treatment on Charcoal with Bismuth Flux. As above but with charcoal instead of plaster as a support. Greenish-yellow sublimate (Pblj) — lead compounds. Scarlet sublimate (Bilj) — bismuth compounds. Faint yellow sublimates (Hglj, etc.) — mercury, arsenic, and antimony compounds. VIII. Borax Bead Tests. Borax beads are made by fusing borax in an J-inch loop of platinum wire. Great care should be used in O.F. and R.F. Sulfids should be first roasted by gently heating the powdered substance spread out on charcoal. It is well to preserve the beads in a little frame or glass tube for future reference. Many elements giving colorless or pale yellow beads are not mentioned. The colors refer to cold beads, except when otherwise mentioned. Violet Blue Green Red Brown Yellow Colorless Co O.F.,R.F. Cr O.F.,R.F. Cu O.F. R.F. (opaque) Fe R.F. O.F. Mu O.F. R.F. Mo R.F. O.F. Ni O.F. (hot) O.F. (cold) R.F. (turbid gray) Ti R.F. O.F. U R.F. O.F. V R.F. O.F. W R.F. O.F. THE CHEMICAL PROPERTIES OF MINERALS 187 IX. Sodium Metaphosphate Bead Tests. Beads of sodium metaphosphate, NaPOg, are made just as with borax. Salt of phosphorus or microcosmic salt, HNaNH^- P04-4H20 may also be used, for on heating it loses NH3 and HjO, and is converted into NaPO,,. The colors refer to cold beads. Violet Blue 1 Green Red Yellow Colorless Co O.F.,R.F. Cr O.F.,R.F. Cu O.F.- R.F. (opaque) Fe O.F. (pale) O.F.,R.F. Mn O.F. R.F. Mo R.F. O.F. Ni O.F.,R.F. Ti R.F. O.F. U O.F.,R.F. V R.F. O.F. W R.F. O.F. Silica is insoluble in a NaPOj bead, while with silicates, the bases dissolve (sometimes coloring the bead) while the silica remains as a translucent mass, often the shape of the original fragment, which floats around in the bead. A few other com- pounds, such as AI2O3, are also insoluble in a NaPOj bead or are very slowly soluble. 188 INTRODUCTION TO THE STUDY OF MINERALS X. Reduction Color Tests. Saturate several NaPOg beads with the substance, and heat. on charcoal with metallic tin in R.F. Dissolve in dilute HCI, add tin, and then boil. Violet solution — titanium compounds. Deep blue solution — tungsten compounds. Brown solution — molybdenum compounds. Green solution — chromium, uranium, and vanadium com- pounds. XI. Soda Bead Tests. Beads of soda are made as with borax and sodium metaphos- phate. Use O.F. Bluish-green opaque bead (Na2Mn04) — manganese com- pounds (a very delicate test). Yellow opaque bead (NajCrOj) — chromium compounds. Effervescence— silica. Na2C03-|-Si02 = Na2Si03 + C02. (The bead will be clear if equal molecular quantities are used.) XII. Treatment with Acid Potassium Sulfate. The substance is mixed with KHSO4 and heated in a test-tube or closed tube. Red-brown fumes with pungent odor (NOj) — nitrates. Colorless gas with HCI odor (HCI) — chlorids. Colorless gas which etches glass (HF) — fluorids. Colorless gas with disagreeable odor (HjS) — sulfids. Colorless, odorless gas (CO2) — carbonates. XIII. Fusibility Tests. Long thin splinters of the mineral about 1 mm. in diameter held with platinum-tipped forceps or wrapped with a coil of platinum wire are heated in the hottest part of the flame which is THE CHEMICAL PROPERTIES OF MINERALS 189 just beyond the tip of the inner cone (see Fig. 386, page 181) of a small sharp O.F. fiame (rather strong blast). Metallic sub- stances should be heated on charcoal as they may contain As, Sb, or Pb which will alloy with the platinum. Powders or substances which fiy to pieces when heated may be ground into a paste with a little water, which after careful drying can be heated in the forceps or on charcoal. Scale of Fusibilty 1. Fuses easily in luminous flames (gas or candle) — stibnite. 2. Fuses with difficulty in luminous flame — chalcopyrite. 3. Fuses easily in blowpipe flame — almandite garnet. 4. Fuses on edges easily in blowpipe flame — actinolite. 5. Fuses on edges with difficulty — orthoclase. 6. Fuses only on thinnest edges — enstatite. 7. Infusible, even on thinnest edges — quartz. Not only the degree, but also the manner of fusion ehould be noted. The substance may fuse either to a clear, opaque, or colored glass, quietly, with intumescence (bubbling), or with exfoliation (spreading out like leaves of a book). XIV. Silver Cupellation. A qualitative test for silver in ores may easily be carried out by means of the blowpipe. The method is similar to that used in assaying except that it is on a smaller scale. By using an assay centner (100 mg.) of ore and measuring the silver button obtained on an ivory scale made for the purpose, one may obtain quantitative results which, after some practice, are very satisfactory. (1) Mix finely powdered ore intimately with one volume of borax glass (made by fusing borax), and one volume of test lead. If ore is galena, it is not necessary to add test lead. (2) Fuse mixture in a deep cavity in charcoal with a strong R.F. for several 190 INTRODUCTION TO THE STUDY OF MINERALS minutes. (3) When cool remove lead from charcoal and hammer off the slag. (4) Add fresh borax glass and heat in O.F. until the quantity of lead is considerably diminished. Again hammer off every particle of the slag. (5) Prepare a cupel by filling a large cavity in charcoal with very slightly moistened bone-ash and making a smooth concave depression with a mold (the end of a large test-tube will do). Heat cupel gently and remove all loose particles. (6) Place the cube of lead on the cupel and fuse in O.F. by blowing across the top of it, using a small flame and strong blast. The oxidation produces a thin film of lead oxid showing interference colors, but when the lead 'is all absorbed, the film suddenly disappears or "blicks," and a minute sphere of silver, which may also contain gold, remains. If the button "freezes" or is not spherical, it is better to begin again. XV. Solubility Tests. In the absence of any special phenomena the only accurate way of testing solubility is to boil the solvent with the substance for some time and then to filter or decant the clear liquid and evaporate to dryness. A residue indicates that the substance is soluble. If in doubt as to the solubility run a blank test with an equal quantity of solveiit alone. A water solution of the residue gives a precipitate with NajCOg, except in the case of alkali compounds, but among minerals these are all readily soluble in water. Soluble in water — nitrates, some chlorids, some sulfates, some borates, some carbonates. Soluble in HCl — all carbonates, some sulfids, some sulfates, borates, some phosphates, some silicates (see p. 241-244), iron oxids, and iron hydroxids. Soluble in HNO3, but insoluble in HCl— -most sulfids and sulfo- salts. Soluble in aqua regia — gold and platinum. Soluble in HF — silica and nearly all the silicates. THE CHEMICAL PROPERTIES OF MINERALS 191 Insoluble in acids but soluble in other liquids — cerargyrite, soluble in NH^OH; anglesite, soluble in NH^CjHgOj; sulfur, soluble in CS,. Insoluble, but decomposed by fusion with NajCOg — most sili- cates, chromite, wolframite, barite, and celestite. For method of treatment see note 4, for silicates, p. 203. Insoluble, not completely decomposed by soda fusion, but de- composed by fusion with KOH — cassiterite, corundum, and rutile. Evolution of Gas. Colorless, odorless gas (CO.) — carbonates. Colorless gas with disagreeable odor (HjS) — some sulfids. Colorless, pungent gas (CI) — manganese dioxid. Brown red, pungent gas (NO,) — sulfids. Color of Solution. Amber solution — iron compounds. Green solution — copper and nickel compounds. Blue solution — copper compounds. Pale red solution — cobalt compounds. Insoluble Residue. Gelatinous residue — some silicates. White residue (PbS0,),(HSb03),(AgCl)— lead, antimony, and silver minerals. Yellow residue (WO3) — calcium tungstate. XVI. Wet Tests and Group Reagents. A . Wet Tests for Bases HCl precipitates AgCl, HgCl, and PbClj. HjS in acid solutions precipitates AgjS, PbS, HgS + Hg, BijSj, CuS, HgS, AS2S3, AsjSg + S, Sb^Sj, SnS, and SnS^. 192 INTRODUCTION TO THE STUDY OF MINERALS NH4OH in the presence of HCl (or NH^Cl) precipitates Pb (OH)^, Hg^NH^Cl, HgNHjCl, BiO(OH), SbO(OH), SnCOH)^, Sn(OH)„ Al(OH)g, CrCOH),, Fe(0H)3, FeCOH)^, and also CagCPOJj, CaFj, and CaCBOa)^. (NH4)2S in neutral solutions precipitates AgjS, PbS, HgS, CuS, Bi^Sg, Sb^Sg, SnS, A1(0H)3, CrCOHj), FeS, FeS+S, ZnS, MnS, CoS, and MS. (11114)2003 precipitates from alkaline solutions carbonates of all the non-alkali metals except Mg. With Ag, Cu, Co, Ni, and Zn the precipitate is soluble in excess. NajHPO^ precipitates all the metals except the alkalies as phosphates, Hg as basic chlorid, and Sb as oxid. Na2C03 precipitates all the metals except the alkalies as fol- lows: AgjCOg, Hg^CO,, CdCOg, FeC03, MnC03, BaCOg, SrCOg, CaCOg, Fe(0H)3, A1(0H)3, Cr(0H)3, Sn(0H)2, H2Sn03, Sb203, HgaOClj, HjSbO^, and basic carbonates of Pb, Cu, Zn, Co, and Ni. H2SO4 precipitates PbSO^, BaSO^jSrSO^, CaSO^ (incompletely), and HgSO^ (incompletely). NaOH precipitates AgaO, Hg20, HgO, Cu(0H)2, Cd(0H)2, BiO(OH), SbO(OH)3, Sn(0H)2, SnO(OH)2, Fe(0H)3, Fe(0H)2, M(0H)2, Co(OH)2, Mn(0H)2, Ba(0H)2 (incompletely), Sr(0H)2 (incompletely), Ca(0H)2 (incompletely), Mg(0H)2, and the fol- lowing which are soluble in excess: Pb(0H)2, Sb203, SbO(OH)3, Sn(0H)2, SnO(OH)2, A1(0H)3, Cr(0H)3, Zn(0H)2. NH4C2O4 precipitates oxalates of all the metals except the alkalies from alkaline solutions. B. Wet Tests for Acids With BaCl2 as a reagent. A white ppt. insoluble in HCl indicates SO^. A white ppt. soluble in HCl, but insoluble in acetic acid indi- cates F. THE CHEMICAL PROPERTIES OF MINERALS 193 A yellow ppt. soluble in HCl but insoluble in acetic acid indicates CrO^. A white ppt. soluble in HCl and in acetic acid indicates BOj or B.O,, P0„ CO3, or AsO,. With AgNOj as a reagent. A yellow ppt. soluble in HNO3 indicates PO^. A red or red-brown ppt. soluble in HNO3 indicates AsO^ or CrO,. A white ppt. soluble in HNO3 indicates BOj or B^O^. A white ppt. insoluble in HNO3 indicates CI. A black ppt. soluble in HNO3 indicates S. XVII. Preparation of Solution. Water is the first solvent used, and after that either hydrochloric or nitric acids. For some minerals HCl is the best solvent and for some HNO3 is the best so that it is well to try a small quantity of the mineral with each of these solvents to determine which is the better. For sulfids HNO3 is the best solvent, but if Pb, Sb, and Sn are present white residues are formed. HCl will precipi- tate chlorids of Ag, Pb, and Hg. If the substance is insoluble in both HNO3 and HCl it may be soluble in aqua regia (1 part HN03-F3 parts HCl). Many minerals, especially silicates, are insoluble in aqua regia, and require fusion with NajCOj on platinum foil or in a porcelain crucible. A water solution of the fusion will generally contain sodium salts of various acids while an acid solution of the residue will generally contain the metals. The following minerals are not decomposed by NajCOg and require fusion with KOH in a nickel or silver crucible; corundum, AI2O3; cassiterite, SnOj,' and rutile, TiOj. 13 194 INTRODUCTION TO THE STUDY OF MINERALS XVIII. Qualitative Scheme (for the more common elements). 1. Add cold dilute HCl in excess. Ppt. 2. Filtrate 6. 2. Wash ppt. with hot water on filter-paper. Residue 3. Filtrate 5. 3. Add NHjOH to residue drop by drop. A blackening indicates Hg. Divide filtrate into two portions 4 and 5. 4. Acidify filtrate with HNO3. A white ppt. indicates Ag. 5. Test filtrate with K^CrO,. A yellow ppt. indicates Pb. 6. Pass HjS into warm, slightly acid solution. Ppt. 7. Filtrate 16. 7. Digest ppt. with (NH4)2S. Filter. Residue 8. Filtrate 13. 8. Digest residue with hot dilute HNO3. Filter. Residue 9. Filtrate 10. 9. Dissolve residue in aqua regia. Boil off CI. A ppt. with SnClj indicates Hg. 10. Add a little cone. H^SOj and drive off excess. A white ppt. indicates Pb. Filtrate 11. 11. Add NHjOH in excess to filtrate. A white ppt. indicates Bi. Filtrate 12. 12. A blue filtrate indicates Cu. Add KCN until blue color disap- pears. Then pass HjS. A yellow ppt. indicates Cd. 13. Add dilute HCl to filtrate. Heat ppt. formed with cone. HCl. A residue indicates As. Filtrate 14. 14. Into the dilute solution, heated to almost boiUng, pass HjS. An orange red ppt. indicates Sb. Filtrate 15. 15. Into the cool diluted filtrate pass HjS. A yellow ppt. indicates Sn. 16. Boil off HjS, add a few drops of HNO3. Add NH^Cl and NH^OH. Ppt. 17. Filtrate 22. 17. Dissolve ppt. in least possible amount of HCl. Add 50% alco- hol and dilute H2SO4. A crystalline ppt. indicates Ca. Filtrate 18. 18. Boil off the alcohol, make filtrate alkaline with NHjOH. Ppt. 19. Reject filtrate. 19. Fuse ppt. with NajCOj and NaNOj on platinum foil. A bluish- green mass indicates Mn. Digest fused mass in hot water and filter. Residue indicates Fe. Divide filtrate into two portions, 20 and 21. 20. A yellow filtrate giving red ppt. with AgNOj indicates Cr. 21. Acidify with HCl. Add soUd NH^Cl and boil. A ppt. indi- cates Al. o THE CHEMICAL PROPERTIES OF MINERALS 195 22. Into the warm alkaline filtrate pass HjS. Ppt. 23. Filtrate 29. 23. Wash ppt. on filter with cold dilute (1: 10) HCl. Residue 24. Filtrate 26. 24. Dissolve residue in aqua regia. Evaporate to dryness, add a little water and make strongly basic with NaOH. Add tartaric acid but not enough to make the solution acid. Heat sHghtly and pass H^S. A ppt. indicates Co. Filtrate 25. 25. Acidify filtrate with HCl. A ppt. indicates Ni. 26. Boil filtrate to remove HjS. Add KOH in excess. Ppt. 27. Filtrate 2S. 27. Fuse ppt. with NajCO,. A bluish-green mass indicates Mn. 28. Add H^S to the filtrate and heat. A white ppt. indicates Zn. 29. Evaporate filtrate to rather small volume. Add (NHJjCO, and alcohol. After standing a half -hour, filter. Ppt. 30. Filtrate 34. 30. Dissolve ppt. in hot dilute acetic acid and add KjCrOj. A yellow ppt. indicates Ba. Filtrate 31. 31. Add NH,OH and alcohol. A yellow ppt. indicates Sr. Fil- trate 32. 32. Dilute and add (NHJjCjOj. A white ppt. indicates Ca. Fil- trate 33. 33. Add NH.OH and Na^HPO,. A white ppt. indicates Mg. 34. Evaporate filtrate to dryness. Ignite to drive off ammonium salts. Add NaOH and NajHPO^. Heat and add alcohol. A white ppt. indicates Li. Filtrate 35. 35. To the filtrate add NajCoCNOj)^. A yellow ppt. indicates K. Note. — The original substance must be tested for Na and NH,. 11. SELECT BLOWPIPE AND WET TESTS Aluminum, Al. 1. Infusible aluminum minerals (also zinc silicates) ignited before and after adding cobalt nitrate solution give an intense blue color. Fusible minerals may give a blue cobalt glass whether aluminum is present or not. 2. Ammonia gives a white gelatinous precipitate, A1(0H)3, in solutions containing aluminum. Iron hydroxid, chromium hydroxid, calcium phosphate, calcium borate, and calcium fluorid are also precipitated by NH^OH along with A1(0H)3. o 3 O o 196 INTRODUCTION TO THE STUDY OP MINERALS Ammonium, NH^. 1. Ammonium salts heated in the closed tube with KOH, NaOH, or CaO (made by heating calcite, CaCOg) give the characteristic ammonia odor. Antimony, Sb. 1. Antimony minerals heated on the charcoal in O.F. give a white coating (SbjOj) near the assay and dense white fumes without odor. 2. With bismuth flux on plaster antimony compounds give a peach-red coating or an orange coating stippled with peach-red. 3. In the open tube antimony minerals give a non-volatile, amorphous, white sublimate (Sb204) on the under side of the tube. 4. Concentrated HNO3 oxidizes antimony sulfids and sulfo- salts to HSbOg, a white precipitate soluble in KOH. Arsenic, As. A. Compounds without Oxygen 1. On charcoal most arsenic minerals give a white volatile coating (AsjOg) far from the assay and fumes with characteristic odor of arsine (AsHj). 2. In open tube minute, brilliant, colorless crystals (AsjOj). 3. In closed tube a black mirror of arsenic. 4. HjS precipitates yellow AsjSg, which is soluble in (NH^jS^, but insoluble in concentrated HCl. B. Arsenates 5. Arsenates heated intensely in closed tube with charcoal give a black metallic mirror. 6. Nitric acid solutions of arsenates give a yellow precipitate with (NIl4)2Mo04 when heated to boiling. Barium, Ba. 1. Yellowi.sh-green flame (not made blue by HCl). 2. Dilute H2SO4 precipitates white BaSO^ from dilute solutions. THE CHEMICAL PROPERTIES OF MINERALS 197 3. KjCrOj or KjCrjO^ gives a yellow precipitate. 4. (NHJ2CO3 or (NHJjCjO^ gives a white precipitate soluble in acids. (Sr and Ca also). Beryllium, Be. 1. Be (OH) 2 is precipitated along with A1(0H)3 by NH4OH. The precipitate is dissolved in dilute HCl and the solution evaporated nearly to dryness. A little water is added, and also KOH in amount sufficient to dissolve the precipitate which forms at first. The solution is diluted and boiled when Be(0H)2 separates out. (Brush-Penfield). Bismuth, Bi. 1. With bismuth flux on plaster a purplish-chocolate coating with underlying scarlet. 2. With soda on charcoal R.F., a metallic button brittle on the edges and also a yellow sublimate. 3. To a nitric acid solution from which the excess of acid has been evaporated HCl is added. On dilution with water, a white precipitate, BiOCl, is formed. Boron, B. 1. Borates give a green flame especially if moistened with H2SO4. Silicates containing boron give a green flame when heated with boracic acid flux (3 parts KHSO^ to 1 part powdered fluorite, CaFj). BF3 is formed. 2. Alcohol added to a solution of a borate will burn with a green flame. 3. Turmeric paper moistened with a HCl solution of a borate, and dried carefully on the outside of the test-tube containing the boiling solution becomes reddish-brown. This color is changed to black by NH^OH. It is well to run a blank test at the same time. Calcium, Ca. 1. The microchemical gypsum test is the most satisfactory test for calcium. A drop of solution containing calcium is placed 198 INTRODUCTION TO THE STUDY OP MINERALS on a glass slip and alongside of it a drop of dilute H2SO4. The two drops are brought into contact at some point. In a few minutes time small crystals of CaS04-2H20 (gypsum) make their appearance. (See Fig. 387.) 2. In a rather concentrated solution diluteHjSO^ precipitates crystalline CaS04-2Il20. The addition of alcohol makes a more complete precipi- tation. 3. Yellowish-red flame with HCl. 4. (NHJ2C2O, or (NH,)2C03 gives a white precipitate soluble in acids, as do also Ba and Sr. Ba gives a yellow pre- cipitate with KaCrO^. Ca(N03)2 is solu- Fio. 387. bie in ether-alcohol, while Sr(N03)2 is Microcnemical gypsum. . _ , ^ msoluble. 5. Calcium borates, fluorids, and phosphates are all precipi- tated from acid solutions on the addition of NH4OII. Carbon, C. 1. Carbonates effervesce in dilute acids (some in the cold, others only upon heating) with the evolution of a colorless, odor- less gas which gives a white precipitate with Ba(0H)2 or lime- water. 2. Citric acid, a solid, serves as a convenient field reagent. Carbonates effervesce in a water solution of citric (or tartaric) acid. 3. Hydrocarbons, such as asphaltum, albertite, coal, etc., which are not minerals, properly speaking, heated in the closed tube give oils and tar-like substances. Chlorin, CI. 1. To a NaPOg bead saturated with CuO (or malachite) a little of the powdered substance is added. On heating, an intense azure-blue flame is obtained. 2. In chlorid solutions AgNOg gives a white curdy precipitate which is soluble in NH^OH. THE CHEMICAL PROPERTIES OF MINERALS 199 Chromium, Cr. 1. The borax and sodium metaphosphate beads are emerald green in both O.F. and R.F. 2. The sodium carbonate bead is yellow in O.F. KNO3 or NaNOj helps the reaction. 3. Chromate solutions give a dark red precipitate with AgNOj. Cobalt, Co. 1. The borax and sodium metaphosphate beads are deep blue in both O.F. and R.F. This is a very delicate test. 2. Heated on charcoal in R.F., cobalt compounds become magnetic. Copper, Cu. 1. Green flame made azure-blue with HCl. 2. Borax and sodium metaphosphate beads are blue in O.F. and opaque red in R.F. In the presence of iron, the O.F. bead is green or bluish-green. 3. On charcoal with soda in R.F. and also with NaPOg and metallic tin on charcoal, metallic copper (malleable) is obtained. 4. Solutions of copper minerals are blue (green in the presence of iron). NH^OH produces a deep blue coloration. 5. Copper solutions touched to a bright surface of iron, such as knife-blade or hammer, give a coating of metallic copper. Fluorin, F. 1. Fluorids are soluble in concentrated HjSO^ with evolution of HF which etches glass. A lead dish, or watch-glass coated with paraffin, should be used. 2. Fluorin compounds heated in a closed tube with 4 parts of NaPOj will etch glass and deposit a ring of SiOj, which cannot be washed off with water. 3. Fluorin compounds heated with concentrated HjSO^ and powdered silica give fumes which condense on moistened black paper. (Browning.) 200 INTRODUCTION TO THE STUDY OF MINERALS 4. Fluorids give a momentary green flame when heated with borax and KHSO4. This flame is due to the formation of vola- tile BF3. Gold, Au. 1. With soda on charcoal gold compounds give a malleable yellow button. 2. Gold may be identified in some of its rich ores by panning and washing away light quartz, rock, etc. Mercury is added to the concentrates. By grinding in a mortar an amalgam of gold is obtained. This may be heated on charcoal or in a closed tube and the mercury driven off. The residue is heated with a little borax on charcoal and a globule of gold obtained. Hydrogen, H. 1. Minerals with so-called water of crystallization give off water when heated in a closed tube at a comparatively low tem- perature (100-150° C). With hydrous sulphates of iron, copper, and aluminum the water has an acid reaction which is due to the SO3 given off. 2. Acid salts and basic salts give off water at comparatively high temperatures (usually above 150° C). Iron, Fe. 1. On charcoal R.F., especially with soda, iron minerals become magnetic. (Also Co and Ni.) 2. In O.F. the borax bead is amber colored and in R.F., bottle green. 3. NH4OH precipitates brownish-red Fe(0H)3. A few drops of HNO3 should always be added to the solution to insure oxida- tion of the iron, 4. To detect state of iron, a borax bead made blue with CuO (or malachite) is changed to opaque red by a ferrous compound and to green by a ferric compound. (Use a neutral flame.) 5. To detect the state of iron in insoluble silicates, fuse pow- THE CHEMICAL PROPERTIES OF MINERALS 201 dered mineral with a large excess of borax in a test-tube. Break tube and dissolve contents in HCl. Test the solution with K^Fe- (CN)e (ferric compounds give a blue precipitate) and with KgFe- (CN)g (ferrous compounds give a blue precipitate). Lead, Pb. 1. On charcoal with soda in R.F. a malleable button of lead and a yellow coating of PbO. PbS also gives a white coating of PbSO,. 2. On plaster with bismuth flux, a lemon-yellow coating. 3. From solutions containing lead, HCl precipitates PbClj, which is soluble in hot water, but recrystallizes on cooling the solution as white acicular crystals with adamantine luster. Lithium, Li. 1. A purplish-red flame, most intense at first. 2. For separation from the other alkalies, see item 34, page 195. Magnesium, Mg. 1. In the presence of NH^OH and NH^Cl, NajHPO^ precipi- tates NH^MgPO^-eHjO, which forms slowly. Other metals (ex- cept alkalies) must be absent as they also give precipitates. 2. White magnesium compounds give a pink color when ignited with cobalt nitrate solution. (This test is not very satisfactory) . Manganese, Mn. 1. Bluish-green soda bead (a very delicate test). 2. The borax or NaPOg bead is amethyst colored in O.F. and colorless in R.F. 3. With HCl manganese dioxids give off chlorin, a gas recog- nized by its penetrating odor. Mercury, Hg. 1. In closed tube with dry soda gives globules of mercury. 2. On plaster with bismuth flux a scarlet sublimate when 202 INTRODUCTION TO THE STUDY OP MINERALS gently heated. If overheated, the sublimate is dark greenish- yellow. 3. Most mercury compounds rubbed on a copper coin with HCl give a white amalgam. Molybdenum, Mo. 1. NaPOj bead is green in R.F., but colorless in O.F. The R.F. beads dissolved in HCl with tin give a brown solution. 2. Na2HP04 gives a yellow precipitate with nitric acid solu- tions of molybdenum compounds. Nickel, Ni. 1. The borax bead in O.F. is violet when hot, reddish-brown cold, while in R.F. the bead is turbid gray. 2. With nickel solutions NaOH gives a pale green precipitate which is insoluble in excess. With NH^OH a precipitate is formed which is soluble in excess to a pale blue solution (fainter than copper). 3. The separation of nickel and cobalt may be effected thus: To a solution free from NH^OH add NaOH until strongly basic. Then add tartaric acid, but not enough to make the solution acid. Heat slightly and pass HjS. The cobalt will be precipi- tated, while the nickel remains in solution. After filtering, the nickel may be precipitated from the filtrate by acidifying with HCl. Niobium, Nb. 1. When fused with borax and then dissolved in HCl, the addition of metallic tin gives a deep blue solution similar to that obtained for tungsten. Nitrogen, N. 1. In closed tube with KHSO4 nitrates give brown-red fumes of NO2. 2. A concentrated solution of FeS04 added to a solution of a nitrate in concentrated HjSO^ gives a brown ring. THE CHEMICAL PROPERTIES OF MINERALS 203 Oxygen, 0. No direct tests for oxygen are easily made. The higher oxids of manganese dissolve in HCl with the evolution of CI. Phosphorus, P. 1. An excess of (NH4)2Mo04 added to a nitric acid solution of a phosphate gives a yellow precipitate which is soluble in NH^OH. The solution should be only slightly heated, for arsenates give a similar precipitate on boiling. 2. Bluish-green flame when moistened with HjSO^. Platinum, Pt. 1. Insoluble in any single acid, but soluble in aqua regia. In rather concentrated, slightly acid solutions KCl gives a yellow precipitate, KjPtClg, insoluble in alcohol. Potassium, K. 1. Violet flame, masked by sodium, but visible through a blue glass. 2. Sodium cobaltic nitrite, Na3Co(N02)e, (see p. 180), gives a yellow precipitate insoluble in alcohol. 3. With HjPtClg solution gives a yellow crystalline precipitate insoluble in alcohol. Silicon, Si. 1. In the NaPOj bead silica and the silicates are only partially dissolved, leaving a translucent mass or skeleton of SiOz. 2. With a small amount of soda, silica effervesces and forms a clear mass. The equation is: Na2C03+Si02 = Na2Si03 + C02. 3. Some silicates dissolve in HNO3 or HCl and on evaporation leave a gelatinous mass or a slime of silicic acid. 4. For insoluble silicates a soda fusion must be made. The finely powdered mineral is fused on platinum foil, or a spiral loop of platinum wire, with three parts of soda. The fused mass is dissolved in dilute HNO3 and carefully evaporated just to dry- ness. Add dilute HCl and boil. Filter off the insoluble silica. 204 INTRODUCTION TO THE STUDY OF MINERALS The filtrate contains the bases, which are commonly Al, Fe, Ca, and Mg. The following is a scheme of separation: Add NHjOH and NH.Cl (boiling) Fe(OH)3,Al(OH)3 AKOH), is soluble in KOH. Add (NHJ.C^O, (hot) CaC^O, Add Na^HPO, (cold) MgNH,P0j-6H,0. 5. For the detection of alkalies in silicates the very finely powdered substance is intimately mixed with five parts of CaCOg and one part of NH^Cl and fused for some time on platinum foil. The sintered mass is digested in hot water and filtered. NH^OH and (NH4)2C03 are added to the filtrate. The precipitate is filtered off and the filtrate evaporated to dryness. The residue is ignited until all the ammonium salts are volatilized. Dissolve the residue in a little water. Add HjPtClj and alcohol. A yellow precipitate indicates K. Evaporate filtrate to dryness and test flame for Na. Silver, Ag. 1. With soda on charcoal in R.F., silver minerals yield malle- able metallic globules of silver, which may be tested as under 3. 2. For silver cupellation see page 189. 3. Nitric acid solutions of silver minerals on the addition of HCl give a white curdy precipitate which changes to violet on exposure to light aijid is soluble in NH^OH. Sodium, Na. 1. Intense yellow flame masked by a blue glass. This is a delicate test so that only an intense and prolonged coloration indicates sodium as an essential constituent. 2. Sodium in insoluble silicates may be detected by the method given under Silicon, note 5. THE CHEMICAL PROPERTIES OF MINERALS 205 Strontium, Sr. 1. Strontium compounds give a crimson flame, especially with HCl. 2. Dilute HjSO^ gives a white precipitate, SrSO^, with dilute strontium solutions. Sulfur, S. .4. Sulfids and Sulfo-salts 1. The finely powdered substance fused with three parts of soda on a sheet of mica or platinum foil gives a mass which stains a moistened silver coin. Tellurids and selenids give the same test. 2. In the closed tube some sulfids give a yellow sublimate of sulfur. 3. In the open tube sulfids give off SOj, a colorless gas with the odor of burning matches. 4. A few sulfids dissolve in HCl with the evolution of HjS. {e.g., sphalerite.) 5. Sulfids are oxidized to sulfates by nitric acid with the evolu- tion of brown-red fumes of NOj. The solution may be tested as under 7 below. B. Sulfates 6. A sulfate, powdered and thoroughly mixed with 3 parts of soda and a little charcoal powder, is fused on charcoal in R.F. The fused mass will stain a moistened silver coin. Sulfids give the same test so it is necessary to try the test on mica or platinum first. 7. Sulfate solutions with BaClj, give a white precipitate which is insoluble in HCl. 8. Sulfate solutions will give microchemical gypsum with a calcium salt (calcite dissolved in HCl is convenient) . See Calcium, note 1, page 197. Tellurium, Te. 1. A powdered tellurid added to hot concentrated HjSO^ gives a fine red-violet coloration. 206 INTRODUCTION TO THE STUDY OF MINERALS 2. Tellurids give a pale bluish-green flame coloration and a white sublimate on charcoal. 3. On plaster with bismuth flux a purplish sublimate is ob- tained with tellurids. Tin, Sn. 1. Tin compounds heated on charcoal in O.F. give a straw- colored coating, SnOj. .On addition of Co(N03)2 solution and heating in R.F., a bluish-green coloration results. 2. Tin compounds fused on charcoal with soda and a little sulfur in strong R.F. give malleable metallic buttons of tin which are oxidized by HNO3 to a white insoluble powder, HjSnOj. Titanium, Ti. 1. The NaPOg bead is violet in R.F. and colorless in O.F. 2. Fused with soda, dissolved in HCl, and the solution heated with metallic tin, titanium compounds give a violet colored solu- tion due to the formation of TiClj. The solution is usually turbid due to the formation of metatitanic acid, HjTiOj. 3. To the substance fused with KHSO^ a solution of hydrogen peroxide is added when a yellow coloration results. This is a very delicate test. Tungsten, W. 1. The NaPOj bead is blue in R.F., colorless in O.F. Iron interferes and gives a red bead in R.F. 2. NaPOj beads treated on charcoal in R.F. with tin are dis- solved in HCl with the addition of metallic tin to a deep blue solution. 3. With soluble tungstates HCl gives a yellow residue, WO3, which is soluble in ammonia. Uranium, U. 1. The NaPOg bead is a fine green in R.F. and yellowish-green in O.F. THE CHEMICAL PROPERTIES OF MINERALS 207 Vanadium, V. 1. The NaPOj bead is a fine green in R.F. and light yellow in O.F. 2. In closed tube with KHSO^ vanadates give a yellow mass. Water (see Hydrogen). Zinc, Zn. 1. On charcoal with soda, zinc compo'unds give a white coating which is yellow when hot. 2. Zinc minerals when moistened with Co(N03)2 solution and intensely ignited assume a bright green color due to the forma- tion of cobalt zincate. Zinc silicates give a blue color like aluminum compounds, but if tried on charcoal the sublimate will turn green. 3. (NHJjS precipitates in alkaline solutions ZnS, remarkable as being the only insoluble white sulfid. Zirconium, Zr. 1. A HCl solution of a soda fusion turns turmeric paper orange color. This test is like that for borates, the absence of which must be proved. PART V THE DETERMINATION OF MINERALS 1. Introduction Until one has had considerable experience with minerals it is necessary to determine many of them by physical or chemical tests. If there is no clue to the mineral, it is better to use determinative tables than to make tests at random. In this part of the book are six tables to aid in determining minerals by (1) crystal form, (2) structure or cleavage, (3) color, (4) specific gravity, (5) optical tests, and (6) blowpipe and chemical tests. The more common minerals are distinguished by larger type. In making tests the homogeneity and purity of minerals is important. If possible the impurity should be removed, if not, the effect of the impurity must be taken into account. As physical tests involve little, if any, waste of material, they should be made before the chemical tests. All the tests must, of course, be made on the same kind of material. The following remarks will help in the use of the tables. It must be emphasized that these or similar tables do not deter- mine a mineral, but simply give one a clue to the determination. The description of the suspected mineral should always be looked up and additional confirmatory tests made. 2. Crystal Form The crystal form is highly characteristic and many minerals may be determined by examination of crystals alone. But as the crystal habit is often variable and as large isolated crystals are comparatively rare, a fairly comprehensive knowledge of crys- 210 INTRODUCTION TO THE STUDY OF MINERALS tallography is necessary. The simple reflection gonometer described on page 81 may be used to advantage in determining crystals. It should be noted that isomorphous minerals have nearly identical angles and that for all isometric crystals the angles between corresponding faces are identical. Cleavage observations are of great value in orienting a crystal. For crystalline aggregates the first half of Table 2 is used. 3. Physical Tests Typical specimens of minerals are most easily recognized by simple physical tests. Of these, cleavage is one of the most satis- factory. Cleavage should be recognized by surfaces with step- like arrangement rather than by cracks through the. specimen, as these may be due to other causes. The second half of Table 2 is a list of minerals with prominent cleavage. The horizontal divisions of Tables 2 and 3 are based upon hardness. The top division includes minerals scratched by the finger-nail, the lower division, those not scratched by the knife, while the middle division includes those scratched by the knife, but not by the finger-nail. Table 3 is a list of minerals arranged by color (secondarily by hardness) . The metallic colors are listed separately. For some minerals the color is characteristic and practically constant. For others there is great variation due to impurities and to isomor- phous replacement. Fluorite, for example, is white, colorless, violet, blue, green, pink, yellow, and brown. The minerals with metallic luster are quite constant in color, though it may be con- cealed by surface tarnish. A fresh fracture should always be used. Metallic minerals are opaque even on the thinnest edges. The specific gravity is one of the most constant properties of minerals. For pure massive specimens without cleavage it is one of the best means of identification. Table 4 is a list of minerals arranged according to increasing specific gravity. The numbers given represent the average value for pure minerals. In using these tables it is well to allow one or two tenths (for the heavy THE DETERMINATION OF MINERALS 211 minerals even more) on each side of the observed value as replace- ment or invisible impurities may cause some variation. After some experience one can judge roughly by the "heft" the ap- proximate specific gravity of a mineral if it is not too small. Taking quartz (sp. gr. 2.67) and calcite (sp. gr. 2.72) as standards, gypsum (sp. gr. 2.3) feels light, while topaz (sp. gr. 3.5) feels heavy and barite (sp. gr. 4.5), very heavy. 4. Optical Tests The determination of minerals by optical tests is at present limited to those that are fairly transparent in thin slices or frag- ments. But many minerals which are apparently opaque become transparent or translucent when thin. For many non-metallic minerals the optical determinations are easier to make and more satisfactory than blowpipe and chemical determinations. Then besides, some distinctions which are impossible by chemical methods are easy to make with the microscope. For example, the distinctions between quartz and chalcedony, calcite and aragonite, augite and hornblende. The author's method' for optical determinations consists of reducing the mineral to coarse powder by pounding it on an anvil rather than by grinding. The largest fragments that go through a 100-mesh sieve are examined in some liquid such as oil of cloves on the stage of a polarizing microscope, and the shape, color, pleochroism, index of refraction, interference colors, extinction, elongation, twinning, etc., are noted. The Becke test (see page 117) is used for the determination of the index of refraction. The most convenient liquids are oil of cloves, ?i=1.53; bromoform, 71=1.59; a-monobromnaphthalin, n=1.66; and meth3dene iodid, n = 1.74. If permanent slides are desired, a solution of Canada balsam in xylol is used. The xylol gradually evaporates, leav- ing the balsam (n = 1.54). The following outline will serve as an introduction to the study of optical mineralogy: 1 School of Mines Qimrlerly, vol. 27, pp. 340-359, 1906. 212 INTRODUCTION TO THE STUDY OP MINERALS Suggested Outline of Slides to Illustrate the Optical Properties of Minerals Form. Calamine crystal. Measure angle and determine hOl face. Alunite crystal. Triangular cleavage fragments — fluorite. Rectangular cleavage fragments — anhydrite. Rhombic cleavage fragments — calcite. Prismatic cleavage fragments — actinolite. Acicular cleavage fragments — woUastonite. Flat platy cleavage fragments — orthoclase. Irregular fragments — quartz. Inclusions. Irregularly arranged — topaz. Regularly arranged — labradorite, phlogopite. Index of Refraction and Relief. High relief, n < oil of cloves — fluorite. Low relief, re = oil of cloves — orthoclase. High reUef, w>oil of cloves — garnet. Pleochroism. Pink to red — erythrite. Blue to purple — glaucophane. Pink to pale green — hypersthene. Relief varies with the direction — calcite. (High relief when long diagonal of rhomb is parallel to the vibration plane of the lower nicol.) Isotropic. Amorphous — opal . Isometric — fluorite. Anisotropic — anhydrite, quartz, apatite, etc. Interference Colors. Selenite wedge — (made by shaving down a cleavage flake of gypsum). Low-order colors — apatite. Bright colors — anhydrite. High-order colors — calcite. Extinction. Parallel — woUastonite, wavellite. Symmetrical — calcite, dolomite. Oblique, small extinction angle — hornblende, tremolite. Oblique, large extinction angle — augite, diopside. Elongation. Parallel to faster ray — stilbite. Parallel to slower ray — wavellite, woUastonite. THE DETERMINATION OF MINERALS 213 Aggregate Polarization — chalcedony, serpentine. Optical Anomalies. Isometric mineral with double refraction — Icucite. Berlin blue interference color — chlorite. Twinning. Simple — gypsum. Polysynthetic — plagioclase. Crossed — microcUne. Interference Figures. Uniaxial positive. Alunite crystals; brucite cleavages. Uniaxial negative. Calcite, basal parting; wulfenite, tabular crystals. Biaxial positive, small axial angle. Chlorite cleavage. Biaxial positive, large axial angle. Topaz and heulandite cleavages. Biaxial negative, small axial angle. Biotite, phlogopite, and talc cleavages. Biaxial negative, large axial angle. Muscovite, lepidolite, margarite, and cyanite cleavages. Biaxial, normal to optic axis, showing axial bar. Epidote, (001) cleavage; diopside, (001) parting. Table 5, in which minerals are arranged according to the shape of fragments and index of refraction with respect to four liquids, is used for most minerals. In each division the minerals are arranged according to increasing double refraction, isotropic min- erals coming first. For minerals with good cleavage, which do not crush easily, the list of cleavable minerals just given under the heading "inter- ference figures" may be used. Cleavage flakes of at least 3 mm. should be examined in convergent polarized light. For minerals that are soluble in water, the most satis- factory method is to recrystallize from a saturated water solu- tion. The solution should not be heated, but simply allowed to evaporate in the open air. Several drops are placed on a glass slip, and after about fifteen minutes the slide may be examined. See borax (Fig. 509, p. 367), carnallite (Fig. 434, p. 303), 214 INTRODUCTIUN TO THE SJ^.^DY OF MINERALS ^.^psomttfeand melanterite (Vigf32^, p. 377), halite (Fig. 427, p. ^|j)S^;'^ainite (Fig. 517, p. 374), miraljilito (Fig. TjlS, p. 375), sal- - amittoniac (Fig. 429, j). 299), soda-niter (Fig. 506, p. 305), sylvite 55e(Fig. 428, p. 299), and trona (Fig. 498, p. 350). 5. Blowpipe and Chemical Tests These tests are valuable as confirmatory tests and are often necessary in the case of massive minerals when ph5'sical tests fail. Blowpipe tests are satisfactory for most of the ordinary acids and bases. Among the exceptions are tests for calcium, magnesium, aluminum (fusible minerals), and phosphates, wet tests being necessary for the recognition of these elements. One great ad- vantage of the blowpipe tests over wet tests is the difficulty of getting many minerals into solution. As a last resort it may be necessary to make the regular ciuali- tative separations, and for this reason an outline of a qualitative scheme, especially adapted to minerals, is given on pages 194—5. For a few minerals nothing short of a quantitative anah'sis suffices for their determination. In some cases a simple water determination will locate a mineral. For example, a careful water determination of a zeolite (the zeolites are very similar chemically) will greatly aid in its identification. The Penfield method of heating the mineral in a hard glass tube and actually weighing the water given off may be employed. Table 6 helps one determine a mineral by simple blowpipe and chemical tests. The main divisions are those of the prominent acid radicals. The two largo divisions are (A) non-metallic luster and (B) metallic luster, liut if the luster is in doubt it may be disregarded, in which case the sulfitls of division 7 must be considered with divisiiin 8, and the remainder of division 7 with division 9. Blank forms similar to that given on page 228 are very con- venient foi- reporting the results of ileterniinations. The lilank space in the upper right-hand corner is for sketches of crystals,, crushed fragments, etc. TABLE 1 Minerals Arranged According to Crystal System and Habit TABSl Minerals Arranged AccoRDiNG!»rrf Monoclinic Orthorhombic Pseudo- orthorhombic Tetragonal Pseudo li tetragon 9 AZITRITE ANGLESITE BIOTITE APOPHYLLITE Boumonite t BIOTITE ARSENOPYRITE GYPSUM Wulfenite CHLORITE BARITE MUSCOVITE COLEMANITE CALAMINE GYPSUM CELESTITE Tabular HEULANDITE CERUSSITE ^ Habit MUSCOVITE ORTHOCLASE PHLOGOPITE Spodumene TITANITE Wolframite WOLLASTONITE Chryaoberyl Columbite MARCASITE OLIVINE DATOLITE ANGLESITE DATOLITE APOPHYT.T.TTE TITANITE SULFUR cassiterite chalcopyrite veIuvianite . Pyramidal Wulfenite ZIRCON Habit AUGITE ANDALUSITE HORNBLENDE APOPHYLTTTE ANDALUSUi Aegirite ANGLESITE STILBITE CASSITERITE AUGITE Borax ANHYDRITE TITANITE RUTILE DIOPSIDE Pi COLEMANITE BARITE TREMOLITE VESUVIANITE NATR0LI1 ,} DATOLITE CELESTITE WERNERITE TOPAZ DIOPSIDE CERUSSITE ZIRCON EPIDOTE Enargite Prismatic GYPSUM Epsomite Habit HORNBLENDE ORTHOCLASE Spodumene TITANITE TREMOLITE Vivianite WOLLASTONITE MANGANITE NATROLITE OLIVINE STAUROLITE STIBNITE TOPAZ 216 E 1 :o Crystal System and Habit Hexagonal Pseudo- hezagonal Isometric Pseudo- isometric Alonite APATITE 1 BERYL Brucite 1 CALCITE CORUNDUM Covellite GRAPHITE HEMATITE ILMENITE Molybdenite BIOTITE CHALCOCITE CHLORITE Chrysoberyl LEPIDOLITE MUSCOVITE PHLOGOPITE Polybasite PYRRHOTITE Stepbanite Trona Cubic Habit Argentite Boracite Cerargyrite Cobaltite CUPRITE FLUORITE GALENA HALITE PYRITE Smaltite Sylvite APOPHYLLITE CALCITE CHABAZITE Cryolite HEMATITE QUARTZ CALCITE CHABAZITE CINNABAR CORUNDUM DOLOMITE . HEMATITE 1 Proustite Pyrargyiite QUARTZ RHODOCHROSITE SIDERITE Soda Niter ARAGONITE CERUSSITE Witherite Octahedral Habit Boracite CHROMITE Cobaltite CUPRITE Diamond FLUORITE Franklinite GALENA GOLD MAGNETITE PYRITE SPHALERITE SPINEL CALCITE CHALCOPYRITE 'E APATITE BERYL CALCITE CINNABAR CORUNDUM ARAGONITE HORNBLENDE lolite Strontianite Dodecahedral Habit Boracite CUPRITE GARNET MAGNETITE Mimetite NEPHELITE Proustite Trapezohedral Habit ANALCITE GARNET LEUCITE CALCITE Pyromorphite QUARTZ TOURMALINE Vanadinite Tetrahedral Habit Boracite SPHALERITE TETRAHEDRITE CHALCOPYRITE Pyritohedral Habit Cobaltite PYRITE 217 TABLE 2 Minerals Arranged According to Structure and Cleavage (Including Parting) MiNEBALS A: Stbtjctubu Fibrous Columnar Foliated Mammillary Acicular Bladed Micaceous Botryoidal Stalactitic Pisolitic OoUtiUtili Brucite GYPSUM STIBNITE ■PYROLUSITE STIBNITE Brucite CHLORITE GYPSUM GRAPHITE - BAUXITE Vivianite Molybdenite Orpiment TALC Finger Nail ARAGONITE BARITE ARAGONITE AR AGONITE ARAGONITE B B CALCITE CALCITE BIOTITE Aisenic BAUXITE CELESTITE ^Crocoite T.TilPlDOLlTE CALAMINE CALCITE (JUPRITE CYANITE Margarite CALCITE HEMATITE MALACHITE Gothite MUSCOVITE " "Hydrozincite LIMONITE MANGANITE MANteA-NITE PHLOGOPITE MALACHITE MiUerite SERPENTINE OPAL NATROLITE Strontianite SMITHSONITE PectoUte SERPENTINE STILBITE WavelUte WOLLASTONITE Knife ACTINOLITE ACTINOLITE ALBITE CASSITERITE CHALCEDONY!JY!J HEMATITE BERYL HEMATITE CHALCEDONY HORNBLENDE CYANITE HEMATITE LIMONITE Diaspora LTMONITE TOURMALINE EPIDOTE OPAL TREMOLITE HORNBLENDE PBEHNITE Hubnerite TOURMALINE TREMOLITE VESUVLANITE PSILOMELANE Wolframite Zoisite 220 LE 2 iED According to ID Cleavage (Including Parting.) One Two directions Two directions Three directions Four or more direction at right angles at oblique angles directions Brucite (a) At right angles CHLORITE GRAPHITE Sylvite GYPSUM Molybdenite (b) Two at right angles, Orpiment -• the third at an oblique STIBNITE angle TALC GYPSUM Vivianite APATITE (a) At oblique angles CALCITE APOPHYT.T.TTE CALCITE FLUORITE BIOTITE DOLOMITE SPHALERITE CELESTITE RHODOCHROSITE COLEMANITE - SIDERITE CYANITE (b) At right angles I.EPIDOLITE -^ Alabandite Margarite ANHYDRITE MUSCOVITE Cryolite PHLOGOPITE GALENA HALITE (c) Two at right angles, the third at an oblique angle BARITE CELESTITE CYAOTTE ALBITE ACTINOLITE CORUNDUM Diapaore .- AUGITE Ajnbjygonite- MAGNETITE DIOPSIDE DIOPSIDE Enargite — WERNERITE EPIDOTE LABRADORITE HORNBLENDE Gothite-^ MICROCLINE TITANITE ^^ HEMATITE OLIGOCLASE TREMOLITE Spodumene ^ ORTHOCLASE TOPAZ RHODONITE WoKramite Spodumene -^ WERNERITE " 221 TABLE 3 Minerals Arranged According to Color. — Non-metallic JIANGBE Red (or pink) Sp. gr. Yellow Sp. gr- Green Sp. gr. Blue gt Laumontite 2.3 SULFUR 2.0 Gamierite 2.6 Chalcanthite 2. BAUXITE 2.5 GYPSUM 2.3 Vivianite 2.6 Vivianite 2. Erythrite 2.9 Orpiment 3.4 TALC 2.7 Realgar 3.5 CHLORITE 2.7 HEMATITE 5.2 Cerargyrite 5.5 Finger Nail CHABAZXTE 2.1 Copiapite 2.1 CHRYSOCOLLA 2.1 AUophane IT CALCITE 2.7 OPAL 2.1 OPAL 2.1 CHRYSOCOLLA 2. DOLOMITE 2.8 SERPENTINE 2.5 Wavellite 2.3 Chalcanthite 2., LEPIDOLITE 2.8 CALCITE 2.7 SERPENTINE 2.5 Lazurite 2.< Margarite 3.0 ARAGONITE 2.9 MUSCOVITE 2.9 LEPIDOLITE 2.! FLUORITE 3.2 FLUORITE 3.2 FLUORITE 3.2 FLUORITE 3.; APATITE 3.2 TITANITE 3.5 APATITE 3.2 CYANITE 3.f RHODOCHROSITE 3.5 LIMONITE 3.8 Atacamite 3.7 AZURITE 3.i SPHALERITE 4.0 SPHALERITE 4.0 Brocbantite 3.9 CELESTITE 3.< HEMATITE 5.2 SMITHSONITE 4.4 MALACHITE 3.9 Zinoite 5.5 BARITE 4.5 Olivenite 4.3 Proustite 5.6 Stibiconite 5.2 SMITHSONITE 4.4 CUPRITE 6.0 Scheelite 6.0 ' Pyromorphite 6.8- Croooite 6.0 Wulfenite 6.7 Wulfenite 6.7 Mimetite 7.2 Vanadinite 6.8 CINNABAR 8.0 Knife Orthoolase 2.6 OPAL 2.1 OPAL 2.1 Sodalite Ya QUARTZ 2.6 QUARTZ 2.6 MICROCLINE 2.6 lolite 2.( CHALCEDONY 2.6 CHALCEDONY 2.6 QUARTZ 2.6 QUARTZ 2.( WERNERITE 2.7 BERYL 2.7 CHALCEDONY 2.6 CHALCEDONY 2.< Chondrodite 3.1 Chondrodite 3.1 OLIGOCLASE 2.6 BERYL 2. TOURMALINE 3.1 VESUVIANITE 3.4 BERYL 2.7 Turquoia 2. ANDALUSITE 3.2 TOPAZ 3.5 Turquois 2.7 TOURMALINE 3. RHODONITE 3.5 WILLEMITE 4.1 PREHNITE 2.9 Glaucophane 3. SPINEL 3.6 DATOLITE 2.9 CYANITE 3. GARNET 4.0 ACTINOLITE 3.1 CORUNDUM 4. CORUNDUM 4.0 HORNBLENDE 3.2 WILLEMITE 4.1 TOURMALINE 3.1 RUTILE 4.2 DIOPSIDE AUGITE OLIVINE EPIDOTE VESUVIANITE CYANITE ChryBOberyl GARNET WILLEMITE 3.2 3.3 3.3 3.4 3.4 3.6 3.7 4.0 4.1 224 ORDINvz xu ouLiUK.— -i> ON-METALLIC White, colorless or nearly so Sp. gr. Gray Sp. gr. Brown Sp. gr. Black Sp. gr. rabilite 1.5 GYPSUM 2.3 BAUXITE 2.5 GRAPHITE 2.1 I'Ammoniao 1.5 Cerargyrite 5.5 PHLOGOPITE 2.8 WAD 3.0 exite 1.6 WAD 3.0 PYROLUSITE 4.8 ssomite 1.7 LIMONITE 3.8 arax 1.7 Ivite 2.0 piolite 2.0 rPSUM 2.3 da Niter 2.3 RUCITE 2.4 lumontite 2.3 AUXITE 2.5 i.LC 2.7 mOPHYLLITE 2.8 CALCITE 2.7 OPAL 2.1 SERPENTINE 2.5 DOLOMITE 2.8 STILBITE 2.2 CALCITE 2.7 ANHYDRITE 2.9 HEULANDITE 2.2 BIOTITE 2.9 Ankerite 3.0 SERPENTINE 2.5 WAD 3.0 TREMOLITE 3.0 CALCITE 2.7 Alabandite 4.0 Triphylite 3.5 PHLOGOPITE 2.8 SPHALERITE 4.0 SIDERITE 3.8 BIOTITE 2.9 MANGANITE 4.3 Witherite 4.3 WAD 3.0 Pyrargynte 5.8 >o numerous to SMITHSONITE 4.4 APATITE 3.2 Melaconite 5.9 mention Scheelite 6.0 TITANITE 3.5 ANGLESITE 6.2 LIMONITE SIDERITE SPHALERITE Gothite BARITE Monazite Pyromorphite Vanadinite 3.8 3.9 4.0 4.2 4.3 4.5 5.1 6.8 6.8 PAL 2.1 QUARTZ 2.6 OPAL 2.1 QUARTZ 2.6 EUCITE 2.5 CHALCEDONY 2.6 QUARTZ 2.6 CHALCEDONY 2.6 LBITE 2,6 ORTHOCLASE 2,6 CHALCEDONY 2.6 TOURMALINE 3.1 BTHOCLASE 2.6 ALBITE 2.6 HORNBLENDE 3.1 Glauoophane 3.1 ICROCLINE 2.6 OLIGOCLASE 2.6 TOURMALINE 3.1 HORNBLENDE 3.2 LIGOCLASE 2.6 NEFHELITE 2.6 Chondrodite 3.1 AUGITE 3.3 [JARTZ 2.6 LABRADORITE 2.7 Sillimanite 3.2 Aegirite 3.5 BCALCEDONY 2.6 WERNERITE 2.7 ENSTATITE 3.3 SPINEL 3.6 EPHELITE 2.6 TREMOLITE 3.0 Diallage 3.3 GARNET 3.8 ERYL 2.7 Spodumene 3.1 Axinite 3.3 LIMONITE 3.8 iBRADORITE 2.7 Lawsonite 3.1 VESUVIANITE 3.4 Allanite 3.9 "ERNERITE 2.7 ANDALUSITE 3.2 STAUROLITE 3.7 PSILOMELANE 4.2 oracite 2.9 Sillimanite 3.2 LIMONITE 3.8 RUTILE 4.2 ATOLITE 2.9 Zoidte 3.3 CORUNDUM 4.0 Columbite 5.6 ElEHNITE 2.9 CORUNDUM 4.0 GARNET 4.0 CASSITERITE 7.0 mblygonite 3.0 RUTILE 4.2 REMOLITE 3.0 ZIRCON 4.7 iWBonite 3.1 CASSITERITE 7.0 NDALUSITE 3.1 lodumene 3.2 lOPSIDE 3.2 aspore 3 i. DPAZ (Continued.) amond 225 TABLE 3. — Continued. Minerals Arkanged According to Color.- -Metallic MetaUlc, black Sp. gr. Metallic Sp. gr. Metallic Sp. gr. to dark gray white to light gray brass bronze, red GRAPHITE 2.1 STIBNITE 4.5 Bismuth 9.8 STIBNITE 4.5 Molybdenite 4.7 Molybdenite 4.7 Bismuthinite 6.4 PYROLUSITE 4.8 Sylvanite 8.0 ' Jamesonite 5.7 Biamuth 9.8 Polybasite 6.1 Mercury 13.6 ,T ;, , Bismuthinite 6.4 Argentite 7.3 Finger Nail 4.0 Arsenio 5.7 CHALCOPYRITE SPHALEE ITE 4.2 Alabandite 4.0 Antimony 6.6 Stannite 4.5 MANGANITE 4.3 Iron 7.5 PYRRHOTITE 4.6 Enargite 4.4 Bismuth 9.8 BORNITE 5.2 Stannite 4.5 SILVER 10.5 Millerite 5.5 TETRAHEDRITE 4.7 Platinum 15 to CUPRITE 6.0 CHALCOCITE 5.7 19 Niccolite 7.5 BouTQonite 5.8 COPPER 8.8 Melaconite 5.8 Calaverite 9.0 Pyrargyrite 5.8 Bismuth 9.8 Stephanite 6.2 GOLD 15 to 19 Knife 3.8 ARSENOPYRITE 8.0 MARCASITE LIMC NITE 4.9 Allanite 3.9 Smaltite 6.2 PYRITE 5.0 PSILOMELANE 4,2 Lollingite 7.1 Cobaltite 6.1 RUTILE 4.2 CHROMITE 4.4 ILMENITE 4.7 MAGNETITE 5.1 Franklinite 5.1 HEMATITE 5.2 Columbite 5.6 CASSITERITE 7.0 Wolframite 7.4 Urani nite 9.0 - BLANK FORM FOR REPORTING MINERALS Date No Name.. Form CleoDage Luster and Color Hardness Streak Spec. Grav Other characters Associates Mineral suspected- Optical characters of crushed fragments and cleavages. Fusibility Flame coloration .. Closed tube Open tube On Coal alone On coal with soda . Borax bead NaPO^ bead Solubility Wet tests Miscellaneous tests Division in scheme Summary of impartant characters _ Mineral- TABLE 4 Minerals Arranged According to Specific Gravity TABLE 4. Minerals Akkangei 1.5 2.6 3.2 Mirabilite ALBITE APATITE Sal-ammoniac Alunite DIOPSIDE CHALCEDONY FLUORITE 1.6 loUte HORNBLENDE Camallite Kaolinite Sillimanite Ulexite MXCROCLINE ORTHOCLASE 3.3 1.7 NEPHELITE AUGITE Borax OLIGOCLASE Axinite Epsomite QUARTZ ENSTATITE yivianite OLIVINE 1.9 Zoisite AUophane 2.7 Melanterite BERYL 3.4 CALCITE CALAMINE 2.0 LABRADORITE Diaspore Sepiolite Pectolite EPIDOTE SULFUR TALC Hypersthene Sylvite Turquois WERNERITE VESUVIANITE 2.1 3.5 CHABAZITE 2.8 Aegirite CHRYSOCOLLA CHLORITE Diamond Copiapite DOLOMITE Orpiment GRAPHITE- LEPIDOLITE Realgar HALITE MUSCOVITE RHODOCHROSITl HydromagneBite PyropliyUite TITANITE Kainite PHLOGOPITE TOPAZ OPAL WOLLASTONITE Triphylite STILBITE TRONA 2.9 3.6 ANHYDRITE CYANITE 2.2 ARAGONITE GARNET ANALCITE BIOTITE RHODONITE Chalcanthite Boraoite SPINEL HEULANDITE Erythrite NATROLITE DATOLITE 3.7 PREHNITE Atacamite 2.3 Chrysoberyl APOPHYLLITE 3.0 Hydrozincite GYPSUM Amblygonite STAUROLITE Laumontite Ankerite Strontianite SodaUte Cryolite Soda-Niter Margarite 3.8 Wavellite TREMOLITE AZURITE WAD GARNET 2.4 LIMONITE Brucite 3.1 SIDERITE COLEMANITE ANDALUSITE Lazurite ACTINOLITE 3.9 Chondrodite Allanite 2.6 Glaucophane Brochantite BAUXITE Lawsonite CELESTITE Garnierite MAGNESITE MALACHITE LEUCITE Spodumene SERPEI-JTINS TCTT"".'.T — " I According to Specific Gravity 4.0 S.2 6.8 Alabandite HEMATITE Pyromorphite CORUNDUM Stibiconite Vanadinite GARNET SPHALERITE 5.3 6.9 4.1 5.4 7.0 WILLEMITE 5.5 CASSITERITE 4.2 Cerargyrite 7.1 CHALCOPYRITE MiUerite Lollingite PSILOMELANE Zincite RUTILE 7.2 5.6 Hubnerite 4.3 Columbite Mimetite Gothite Proustite MANGANITE 7.3 Olivenite 5.7 Argentite Witherite Arsenic CHALCOCITE 7.4 4.4 Jamesonite Niocolite CHROMITE Wolframite Enargite 5.8 SMITHSONITE Boumonite 7.5 Pyraigyrite GALENA 4.5 Melaconite Iron BARITE Stannite 5.9 8.0 STIBNITE CINNABAR 6.0 Sylvanite 4.6 ARSENOPYRITE Covellite CUPRITE 8.8 PYRRHOTITE Crocoite Calaverite ZIRCON SoheeUte COPPER 4.7 6.1 9.0 ILMENITE Cobaltite Uraninite Molybdenite Polybasite TETKAHEDRITE 9.8 6.2 Bismuth 4.8 Smaltite PYROLUSITE Stephanite 10.5 SILVER 4.9 6.3 MARCASITE ANGLESITE 13.6 Mercury 5.0 6.4 PYRITE Bismuthinite 15 to 5.1 6.5 19 BORNITE CERUSSITE GOLD Franklinite MAGNETITE 6.6 Platinum Monazite Antimony A -7 Table 5 Minerals Arranged According to Optical Tests TABLE 5. Minerals Aehan< Triangular Fig. 388. Rectangular Fig. 389. Rhomb Fig. 39 « >Oil of Cloves Isotropic FLUORITE Cryolite CHABAZlTi Isotropic Alunite APOPHYLLITE n >Oil of Cloves «< Bromof oim ANHYDRITE nap Isotropic « >Bromofonn »< o-Monobrom- hthalin BARITE CELESTITE BARITE CELESTIT According to Optical Tests Prismatic Fig. 391. Acicttlar Fig. 392. Platy (not previ- ously included) ^& Fig. 393. Irregular Fig. 394. Allophane ANALCITE Laiurite LEUCITE OPAL Sodalite STILBITE XJlezite HEULANDITE LEUiCITE HEULANDITE GYPSUM ORTHOCLASE Cryolite Hydromagnesite NATROLITE MICROCLINE Sepio ite GYPSUM ALBITE CHALCEDONY Laumontite GYPSUM NATROLITE Brucite NEPHELITE Brucite CHLORITE CHRYSOCOLLA Wavellite SERPENTINE PHLOGOPITE CHLORITE WERNERITE Wavellite TALC LEPIDOLITE OLIGOCLASE LABRADORITE Amblygonite Gamierita NEPHELITE CHALCEDONY BERYL loUte SERPENTINE ARAGONITE QUARTZ Alunite BIOTITE WOLLASTONITE WOLLASTONITE MUSCOVITE APATITE Glaucophane TREMOLITE Margarite Turquois Pyrophyllite PectoUte TOPAZ BARITE CALAMINE ACTINOLITE CALAMINE CELESTITE TOURMALINE COLEMANITE TOPAZ TREMOLITE ANDALUSITE PREHNITE TOURMALINE Vivianite Chondrodite ANDALUSITE PREHNITE ACTINOLITE DATOLITE (Continued.) Table 5.— Continued Minerals Arranged According to Optical Tests TABLE 5.— Continued. Minerals Aju" Triangular Rectangular Rhombic Isotropic «> a-Monobrom-naphthalin H < Methylene lodid CALCITE DOLOMITE Ankerite MAGNESITE Isotropic SPHALERITE Diamond Alabandite n >Methylene lodid RHODOUHJiOat SIDERITE SMITHSONITE ''''laRANGED According to Optical Tests Prismatic Acicular Platy (not previ- ously included Irregular Boracite SPINEL Stibiconite Zoidte TriphyUte ENSTATITE VESUVIANITE Hypeisthene Axinite RHODONITE STAUROLITE CYANITE WILLEMITE Diaspore OLIVINE HORNBLENDE Allanite AUGITE CYANITE DIOPSIDE Sillimanite Spodumene Diaspore a Erythrite en Strontianite B Witherite s: Hubnerite | CUPRITE GARNET HEMATITE LIMONITE SPHALERITE SPINEL ; ,iiITE Zincite Atacamite Brochantite EPIDOTE Aegirite LIMONITE Gothite Crocoite CERUSSITE . Pyromorphite CORUNDUM Chrysoberyl Zincite Scheelite Mimetite ANGLESITE Realgar EPIDOTE Monazite ZIRCON Vanadinite CASSITERITE CERUSSITE CINNABAR Proustite Pyrargyrite SULFUR Wulfenite AZURITE TITANITE RUTILE TABLE 6 Minerals Arranged According to Blowpipe and Chemical Tests. TABLE 6. Minerals Akbangbd Accoi A. MIHERALS WITH . Division i — Carbonates. The mineral effervesces with HCl either in the cold or upon heating. Note 1. Calcite is a common impurity in other minerals especially gypsum, apatite, and wollastonite. Note 2. Some sulfida, e.ff., sphalerite, effervesce in HCl also. The gas evolved, however, is HjS recognized by its odor. The mineral is heated in the closed tube. The minerals with name give water in tube. X opposite the closed Fusibility (a). Turns dark on heating in the closed tube. 3 MALACHITE Cu X 3 AZURITE Cu X 5 SIDERITE Fe 7 RHODOCHROSITE Mn 7 Ankerite CaMgF (6.) Turns yellow on heating in the closed li CERUSSITE Pb tube. 7 SMITHSONITE Zn, 7 Hydrozintnttf ' Zn" X (c.) No decided color change on heating. 7 CALCITE Ca 7 ARAGONITE Ca 7 DOLOMITE CaMg 3 Witheiite Ba 7 Strontianite Sr 7 MAGNESITE Mg 7 Hydromagnesite Mg X 14 Trona Na X Division 2 — Sulfates. A water solution of the soda fusion gives a white ppt. with BaCh, which is insoluble ir HCl. Note 1. It is always necessary to add HCl to prove the presence of a sulfate foi the excess of NazCOg always gives a ppt. of BaCO:. Note 2. A water solution of the fusioi is taken. (o.) Soluble in water (decided taste). 1 Epsomite Mg X 1 Melanterite Fe" X li Mirabilite Na X 2 Kainite KMgCl X 3 Chalcanthite Cu X 5 Copiapite Fe"' X MG TO Blowpipe and Chemical Tests H-METALIC LUSTER Fusibility 6.) Soluble in HCl. 34 GYPSUM Ca z 34 ANHYDRITE Ca 34 Brochantite Cu X 7 Alunite Kal X c.) Insoluble in Ha. 4 BAEITE Ba (Anhydrite is soluble with difficulty.) 4 CELESTITE Sr 24 ANGLESITE Pb 34 ANHYDRITE Ca livision 3 — Silicates. Insoluble in NaFO: bead. Confirm by dissolving soda fusion in dilute HNO3. On evaporatine the solution, gelatinoua siUca separates. The silica is dehydrated by carefully heating to dryness. Dilute ECl is added. Silica is filtered off. The metals are in the filtrate and may be tested for by the scheme on page . rhe finely powdered mineral is heated in HCl for several minutes The solution is filtered off and evaporated on a watch-glass. a.) Gelatinizes with HCl. 2 DATOLITE CaB X 24 NATROLITE NaAl z 24 Laumontite CaAl X 24 Allanite AlFeCaCe X 34 Lazurite NaAIS 4 SodaUte NaAlCl 4 NEPHELITE NaAl 5 WILLEMITE ZnMn 6 CALAMINE Zn X 7 OLIVINE MgFe 7 Allophane Al X 7 Chondrodite MgF 7 WILLEMITE Zn 5.) Decomposed by HCl without gelatini- 2 APOPHYLLITE KCa X zation. 2 PREHNITE CaAl X 24 Pectolite CaNa X 3 CHABAZITE CaNaAl X 3 STILBITE CaNaAl X 3 HEULANDITE CaNaAl X 3 ANALCITE NaAl X 3 WERNERITE CaAlNaCl 4 WOLLASTONITE Ca 4 TITANITE CaTi 44 LABRADORITE CaAlNa 5 SERPEKTINE Mg,Fe X 5 SepioUte Mg X 7 CHRYSOCOLLA Cu X 7 Gamierite Ni.Mg X 7 LEUCITE KAl (Continued.) TABLE 6.— Continued Minerals Arranged According to Blowpipe and Chemical Tests TABLE 6. — Continued. Minerals Abbange A. MINERALS WITH HOH- Division 3 — Silicates — Continued Fusibility c. Insoluble (or nearly so) in HCl. 2 LEPIDOLITB T.iKAl *Mean3 soluble in HCl after fusion (alone, not 2i PREHNITE CaAl with Na2C03). 2i Axim'te CaAlB 3 *GARNET AIFeCaMg 3 Glaucophane NaAlMgFe 3 Aegirite FeNa 3 *VESUVIANITE CaAl 3 ■WBRNERITE CaNaAlCl 3i RHODONITE Mn 3i Spodumene LiAl 3i TOURMALINE MgAlB 34 *EPIDOTE CaAIFe 3i Zoisite CaAl 4 TITANITE CaTi 4 TREMOLITE CaMg. 4 ACTINOUTE CaMgFe 34 HORNBLENDE AlCaMgFe 4 DIOPSIDE CaMgFe 4 ATJGITE AlCaMgFe 4i ALBITE NaAl 44 OLIGOCLASE NaCaAl 44 LABRADORITE CaNaAl 44 PHLOGOPITE MgFeAlK ! a Margarite CaAl 5 MUSCOVITE KAl 5 BIOTITE FeKAl 5 ORTHOCLASE KAl 5 MICROCLINE KAl 5 Hypeisthene MgFe 5 ♦TOURMALINE FeAlB 54 TALC Mg 54 CHLORITE AlMgFe 54 BERYL BeAl 6 ENSTATITE Mg 7 CYANITE Al 7 Kaolinite Al 7 QUARTZ 7 CHALCEDONY 7 OPAL Infusible ■ 7 TOPAZ AlF 7 STAUROLITE AIFeMg 7 ANDALUSITE Al 7 Sillimanite Al 7 PYROPHYLLITE Al 7 Zircon Zr 246 ; According to Blowpipe and Chemical Tests HETALLIC LUSTER.— Condnued Division 4 — Phosphates and Arsenates. — The HNO3 solution of the soda fusion gives a yellow ppt. with (NB()jMoOj (with phosphates on gentle heating and with arsenates on boiling.) Fusibility 2i 2i 2i 7 7 2 2 li 54 24 7 Vivianite Triphylite Olivenite Turquois Wavellite Amblygonite Pyromorphite Mimetite APATITE Erythrite Monazite FeP FeLiMnP CuAs AlCuP AlP LiAlP PbPCl PbAsCl CaPCl CoAs CeLaDiP Division 5 — Chromates, Vanadates, and Molybdates. The NaPOj bead in R. F. is bright green. (Note other colors for future use.) o. The NaPOs bead in O. F. is green. 7 14 CHROMITE Crocoite FeCr PbCr 6. The NaPOj bead m O. F. is yellow 14 Vanadinite PbClV c. The NaPOs bead in O. F. is colorless. Wulfenite PbMo Division 6. — Chlorids. The NaPOs bead saturated with CuO (or malachite) gives a blue flame when heated with the mineral. a. Soluble in water (decided taste). 14 14 14 14 HALITE Sylvite Sal-ammoniac CaroalUte Na K NH4 KMg h. Insoluble in water. 1 2 34 Ceraiygyrite Atacamite Boracite Ag Cu MgB 247 (Continued.) TABLE 6.— Continued Minerals Arranged According to Blowpipe and Cliemical Tests TABLE 6. — Continued. Minerals Akbangbd A. MINERALS WITH NOH-METALIC LUSTER— wmitrjued Division 7 — Mot previously included. Mostly oxida and hydroxids, a few sulfids, borates, etc. Fusibility Division 8 — Sulfids and Sulf The soda fusion made on mica stains a moistened silver co (a) On charcoal R.F. gives a garlic odor (AS) 1 1 1 Realgar Orpiment Proustite AsS AsS AsAgS (Sulfids from previous division tite, sphalerite, and cinnabar FusibiUty (a) On charcoal R.F. gives a garlic odor (As) 2 2i 1 (6) On charcoal gives dense white fumes (Sb) and white coating 7 Stibiconite Sb (c) On charcoal R.F. gives magnetic residue (Fe) 5i Si 5i HEMATITE LIMONITE Goethite Fe Fe Fe X X (6) On charcoal gives white coating near assay and dense white fumes (Sb) li (d) On charcoal R.F. gives metallic globules but no magnetic residue 3 3 7 CUPRITE Mela'conite CASSITERITE Cu Cu Sn (e) Borax bead tests for Mn. 7 7 7 4 7 WAD PYROLUSITE PSILOMELANE Hiibnerite Zincite Mn Mn Mn MnW ZnMn X X (c) On charcoal R.F. gives a magnetic resi- due 3 3 3 2 2i li 2 5 (/) Fusible minerals, not previously included 1 1 1 li li u 14 li 2 3 5 5 SULFUR Niter Soda Niter CINNABAR Borax Ulexite COLEMANITE Cryolite Boracite FLUORITE SPHALERITE Scheelite S KN NaN HgS NaB CaNaB CaB NaAIF MgCIB CaF ZnS CaW X X (d) On charcoal R.F. gives metallic glob- ules, but no mag- netic residue 2 li 2i 2i 1 1 1 (a) Infusible minerals, not previously included 7 7 7 7 7 7 7 7 7 CORUNDUM SPINEL Chryaoberyl BAUXITE Diaspore RUTILE CASSITERITE Bruoite Diamond Al MgAl BeAI Al Al Ti Sn Mg C X X X (e) Borax bead test for Mn. 3 (/. g) Not previously included 7 icoRDiNG TO Blowpipe and Chemical Tests ttlHERALS WITH METALIC (OR SUB-METALLIC) LUSTER. ilts. platinum foil) with O. F. realgar, orpiment, prous- Division 9— Hot previously included. Mostly ojdds hydroxida, and metals. Fusibility SENOPYRITK alUte irgite FeAa CoAs CuAs (a) On charcoal R.F. gives garlic odor (As). 1 2J 2 2 Arsenic Smaltite NiccoUte Lollingite As CoNiAs NiAs FeAs BNITE TRAHEDRITE imonite lesonite argyrite irbaaitc Sb CuSb CuPbSb PbSb AgSb AgSb AgCuSb (&) On oharcoal gives dense white fumes. 1 Antimony Sb (c) On charcoal R.F. gives magnetic residue. 5* 5i 5i 7 7 7 4 2i HEMATITE LIMONITE MAGNETITE Iron ILMENITE Franklinite Wolframite AUanite Fe Fe Fe Fe FeTi FeMnZn FeMnW AIFeCaCe X RITE RCASITE RRHOTITE ALCOPYRITE RNITE unite erite lALERITE Fe Fe Fe CuFe CuFe CuSnFe Ni ZnFe (d) On charcoal R.F. gives metallic globules, but no magnetic residue. 1 2 3 3 3 7 Bismuth SILVER, COPPER CUPRITE Melaconite CASSITERITE Bi Ag Cu Cu Cu Sn LENA entite ALCOCITE ellite muthinite iverite Pb Ag Cu Cu Bi AuTe AgAuTe (e) Boras bead tests for Mn. , 7 7 7 PYROLUSITE MANGANITE PSILOMELANE Mn Mn Mn X X (/) Fusible minerals not previously included. 5i Columbite FeMnNb mndite Mn (ff) Infusible minerals not previously included. 7 7 7 7 7 GRAPHITE RUTTLF, CASSITERITE Uraninite Platinum C Ti Sn U Pt ybdenite Mo PART VI THE DESCRIPTION OF IMPORTANT MINERALS Introductory The modern classification of minerals is primarily a chemical one, the, minerals being arranged according to the acid radical, and secondarily a crystallographic one, for in any chemical division minerals of similar "(jhemical composition, which are related crystallographically, are placed together in a group. Thus, the sulfates barite, (BaSO^); celestite, (SrSOJ; and an- glesite, (PbS04), constitute the barite group, as they all crystal- lize in the orthorhombic system, and though the habit may vary, the angles between correspon.ding faces are almost identical. The facts stated in discussing isomorphism and isomorphous mixtures (see page 173) are the principles used in classifying minerals. This is the so-called systematic classification and is regarded as the most scientific one. Minerals are sometimes arranged according to a metallic classification, except for the silicates. Though there may be some advantages in this method, it separates many very similar minerals and unites many unlike ones. It is also inconsistent in that silicates are classified on a different basis from the other minerals. All mineral specimens which have essentially the same chem- ical composition and the same crystal form constitute a mineral species. Those specimens of any mineral species which have the same peculiarity of color, impurity, crystal habit, or mode of aggregation constitute a variety. These varietal distinctions are 253 254 INTRODUCTION TO THE STUDY OF MINERALS based upon non-essential properties, and the present tendency in mineralogy is to use varietal names (in this sense) as little as possible. Varieties should be based upon occurrence, association, and origin (see numbered list under the heading Occurrence of each mineral). About a thousand distinct kinds of minerals or mineral species are known. Most of these are very rare, many of them being found at only one or two localities. In this book two hundred minerals are considered. These include all the common minerals, most of those of economic importance, and others which are intended to give the student a comprehensive view of the mineral kingdom as a whole. The derivation of mineral names deserves a little space at this point. The following names are in honor of prominent scientific men: biotite (Biot, French physicist), brucite (Bruce, an early American mineralogist), dolomite (Dolomieu, French geologist), gothite (Goethe, the German poet), millerite (Miller, English crystallographer), scheelite (Scheele, Swedish chemist), smith- sonite (Smithson, founder of the Smithsonian Institution), wollastonite (Wollaston, English chemist). The following are geographical names based upon prominent localities: andalusite (Andalusia, a province of Spain), aragon- ite (Aragon, ancient kingdom in Spain), anglesite (Anglesea in Wales), bauxite (Beaux in France), copiapite (Copiapo in Chile), ilmenite (Ilmen mountains in the Urals) , labradorite (Labrador) , muscovite (Moscow in Russia), strontianite (Strontian in Scot- land), vesuvianite (Mt. Vesuvius in Italy). The following are derived from the Latin or Greek names for colors: albite (white), azurite (blue), crocoite (saffron), cyanite (blue), celestite (sky-blue), chlorite (green), erythrite (red), hematite (blood), leucite (white), rhodonite (rose-red), rutile (reddish) . As will be recognized, the following names are based upon the chemical composition: alunite, argentite, arsenopyrite, barite, calcite, chromite, cobaltite, cuprite, hydrozincite, magnesite, THE DESCRIPTION OF IMPORTANT MINERALS 255 molybdenite, natrolite, siderite, sodalite, stannite, stibnite, titan- ite, uraninite, wolframite, zincite. It will be noticed that most mineral names end in -ite (the suffix -ites or -ih's, signifying a quality, use, or locality of the mineral). Names in use before this custom was adopted are quartz, opal, topaz, garnet, mica, diamond, galena, beryl, zircon, gypsum, and hornblende. It is interesting to note that at one time a binomial nomen- clature like that at present used for plants and animals was employed for minerals. Thus, barite was known as Baralus ponderosus and celestite as Baralus prismaticus. The order followed in the description of the important minerals is practically that of Dana's System of Mineralogy {6th edition) except that silicates are placed last. SYNOPSIS OF THE CLASSIFICATION OF MINERALS 1. ELEMENTS 2. SULFIDS, ARSENIDS, AND TELLURIDS 3. SULFO-SALTS 4. HALOIDS 5. OXIDS 6. ALUMINATES, FERRITES, ETC. 7. HYDROXIDS 8. CARBONATES 9. PHOSPHATES, ARSENATES, ETC. 10. NITRATES AND BORATES 11. SULFATES 12. TUNGSTATES AND MOLYBDATES 13. SILICATES 256 1. ELEMENTS A. Non-metals Diamond C GRAPHITE C SULFUR S B. Semi-metals Arsenic As Antimony Sb Bismuth Bi C. Metals GOLD Au SILVER Ag COPPER Cu Mercury Hg Platinum Pt Iron Fe Diamond, , C Form. Diamond is nearly always found in small loose crys- tals with rounded faces and curved edges. It crystallizes in the isometric system, probably in the hexte- trahedral class. Though many of the isometric forms have been observed the only common one is the octahedron. Fig. 395 represents a typical crystal with grooved edges and triangular markings. Spinel twins of diamond are rather common. Cleavage. Perfect octahedral. This fact enables the diamond-cutter to save considerable work. H. = 10 (the hardest known substance). Color. Diamond is usually colorless or pale yellow, though 257 Fig. 395. — Diamond crystal. Sp. gr. +3.5 258 INTRODUCTION TO THE STUDY OP MINERALS brilliantly colored blue, green, and red stones are known. One variety known as carbonado is black and opaque. Luster. The luster is the brilliant luster known as adaman- tine. Optical Properties. The index of refraction is very high (« = 2.42). which accounts for its brilliancy. The "fire" of the diamond is accounted for by its strong dispersion, the index of refraction for the red end of the spectrum being 2.402, while for the violet end it is 2.465. Diamonds are transparent to X-rays, while imitations are opaque. Chemical Composition. Pure carbon. Upon heating the diamond in an atmosphere of oxygen it is converted into COj. On heating out of contact with air it is converted into graphite. Blowpipe Tests. Infusible. Insoluble in acids. Uses. On account of its great hardness, brilliancy, and rarity, diamond stands as the gem mineral, par excellence. Among the famous historic diamonds are the Kohinoor, 186 carats; the Regent, 137 carats; the Star of the South, 254 carats; the Imperial, 457 carats; and the Excelsior, 969 carats. Of colored diamonds the most famous are the Tiffany (yellow), the Hope (blue), and the Dresden (green). The largest diamond on record is the Cullinan, found in 1905 at the Premier mine in the Transvaal. This diamond weighed 3025 carats (621.2 grams or about 1.37 pounds avoirdupois). Diamonds are also used as an abrasive in cutting and polishing precious stones, glass, and other materials. The center of the diamond cutting industry is Amsterdam. Several mines within an area of ten square miles at Kimberley, South Africa, have furnished the world's principal supply of diamonds since their discovery in 1867. A black opaque non-cleavable variety of diamond is used for diamond drills. It is found only in Bahia, Brazil, and is known as carbonado or "black diamond." Occurrence. 1. In volcanic necks and dikes of a rock known as kimberlite (locally called "blue-ground"). Kimberlite is an ELEMENTS 259 altered peridotite composed of fragments of pyrope, pyroxene, biotite, olivine, etc., in a matrix of serpentine. The origin of the diamond is in doubt, but many believe it to be of igneous origin. Diamonds have recently been found in peridotite dikes in Pike County, Arkansas. 2. In alluvial deposits associated with heavy minerals. Among these localities may be mentioned southern India, (where dia- monds were first found), the states of Bahia and Minas Geraes, Brazil (one locality is known as Diamantina), and scattered localities throughout the United States. In the Great Lakes region diamonds are found in glacial drift. In California small diamonds have frequently been found in sluice-boxes along with gold. 3. In meteorites from Canon Diablo (Arizona) minute dia- monds have been found. Moissanite, SiC, the same as the arti- ficial carborundum, is also found there. A peridotite meteorite which fell at Novo-Urei, Russia in 1886 also contained diamonds. Moissan obtained small diamonds by cooling in water a block of soft iron saturated with carbon. GRAPHITE, C Fonn. Graphite occurs occasionally in six-sided tabular crystals, but inore often in foliated masses, minute disseminated scales, or earthy lumps. Cleavage. Cleavage in one direction. H = l-2. Sp.gr. ±2.1 Color. Dark gray to black. Luster, metallic. Streak, gray. Opaque. Sectile. Chemical Composition. Graphite is a modification of carbon. It grades from pure carbon to earthy varieties which yield a large amount of ash on combustion. Blowpipe Tests. Infusible. Insoluble in acids. With KCIO3 and cone. HNO3, graphite is converted into a yellow transparent scaly substance called graphitic acid. This distinguishes graphite from amorphous carbon. 260 INTRODUCTION TO THE STUDY OF MINERALS Uses. Graphite is used in the manufacture of lead pencils (varying hardness is due to admixed clay), lubricants for ma- chinery, refractory crucibles, and electrical supplies. Austria and Ceylon are the chief producers. Artificial graphite is now made from anthracite in the electric furnace at Niagara Falls. Occurrence. 1. In crystalline limestones. Franklin, New Jersey. 2. In schists and gneisses often as an essential constituent. 3. In true fissure-veins (Ceylon). 4. In meteorites. Paramorphs of graphite after diamond, called cliftonite, are also found in meteorites. SULFUR, S Form. Sulfur occurs in crystals, incrustations, dissemina- tions, and compact masses. The crystals are good examples of the orthorhombic system. Usual forms: p{lll}, c{001}, n{011}. Fig. 396. FiQ. 397. and s{113}. The habit is usually pyramidal, being determined by {111}. Interfacial angles: pp(lll : iTl) =94° 52', cp- (001 : 111) =71° 40',. cs(001 : 113)=45° 10', pn(001:011) = 62° 17'. Fig. 396 is the common type of crystal and Fig. 397 ELEMENTS 261 shows the crystals formed by the evaporation of a carbon bisulfid solution. H.=2. Sp.gr. ±2.0 Color. Yellow, sometimes with orange, brown, or green tinge. Optical Properties. 71^(2.24) —n„(l. 95) =0.29. Fragments are irregular with high-order interference colors. Chemical Composition. Sulfur, often with such impurities as clay and asphaltum. Blowpipe Tests. Fuses easily (114° C.) and burns with a blue flame, giving off sulfur dioxid. Insoluble in acids. Soluble in carbon bisulfid and in methy- lene iodid. Uses. Sulfur is used in the manufacture of gunpowder, matches, for vulcanizing rubber, and for the production of sulfur dioxid, which is used in paper manufacturing and bleaching. It was formerly used in the manufacture of sulfuric acid, but that is now made from pyrite. The island of Sicily and southwestern Louisiana are the principal sources of sulfur. At the latter local- ity the sulfur is obtained by dissolving it in superheated steam and pumping the solution to the surface. Occurrence. 1. As a sublimate in the crevices around volcanoes (called solfataras). Formed by the incomplete oxida- tion of hydrogen sulfid. 2H2S 4- O^ = 2S -f- 2H2O. 2. In sedimentary rocks formed by the decomposition of such sulfates as gypsum. At Girgenti, Sicily, sulfur occurs in marl associated with gypsum, celestite, and aragonite. 3. As a decomposition product of sulfids such as galena, sphalerite, and pyrite. 4. Formed by some bacteria in sulfate bearing waters. ARSENIC GROUP— HEXAGONAL The three semi-metals or metalloids, arsenic, antimony, and bismuth are isomorphous, for they crystallize in the scalenohedral class of the hexagonal system and have rhombohedral cleavage 262 INTRODUCTION TO THE STUDY OF MINERALS with angles of about 92°-94° and parting parallel to {0lT2} (like calcite). Crystals are very rare. Arsenic, As Form. Arsenic usually occurs in mammillary crusts made up of concentric layers or in granular masses, rarely in small imper- fect crystals. Cleavage may be noticed in coarse-grained specimens. H = 3i. Sp.gr. ±5.7 Color. Tin white, tarnishing black on exposure. Luster, metallic. Chemical Composition. Arsenic, usually with a little anti- mony. Blowpipe Tests. On charcoal easily volatile with the charac- teristic arsin odor, giving a white sublimate some distance from the assay. Oxidized by HNO3 to arsenic acid. Occurrence. 1. As a vein mineral often associated with silver and cobalt minerals. Schneeberg, Saxony, is a prominent locality. Antimony, Sb Form. Antimony occurs in massive lamellar or granular crys- talline aggregates, occasionally cleavable, but rarely crystallized. Cleavage often prominent. H. = 3i. Sp.gr. ±6.6 Color. Tin- white (does not tarnish easily like arsenic) . Chemical Composition. Antimony, usually with some arsenic. Blowpipe Tests. On charcoal easily volatile, giving dense white fumes (without odor) and a white sublimate near the assay. Oxidized by HNO3 to HSbOg, a white residue. Occurrence. 1. As a vein mineral often associated with stib- nite and with silver minerals. ELEMENTS 263 Bismuth, Bi Form. Bismuth occurs in small cleavable masses or in em- bedded dendritic forms. Cleavage often prominent. H. = 2|. Sp.gr. ±9.8 Color. Pale reddish tin-white. Luster, metallic. Chemical Composition. Bismuth. Blowpipe Tests. Easily fusible (1) to a malleable globule which is brittle on the edges. With bismuth flux on charcoal gives a bright red sublimate. Soluble in HNO3. The solution diluted with water gives a white precipitate, (BiO)N03. Uses. Native bismuth is the chief sovi.rce of the bismuth of commerce which is used principally in alloys. Occurrence. 1. As a vein mineral, usually associated with sil- ver and cobalt minerals. Schneeberg, Saxony, is the principal locality. GOLD, Au Form. Though usually finely disseminated through rock and only apparent on assaying, gold also occurs in rolled grains and scales, occasionally in large nuggets, also in crystals and imper- fect crystal aggregates. Like many of the metals, gold crystal- lizes in the isometric system, the octahedron being the only com- mon form. Sheets of gold (leaf-gold) with raised triangular markings arc not uncommon. H. = 2| to 3. Sp. gr. 15 to 19 (according to fineness). Color. Deep to pale yellow. The pale yellow varieties contain silver. Luster, metallic. Very malleable. Chemical Composition. Gold, usually alloyed with more or less silver, sometimes also with Bi, Cu, Fe, Pd, and Rh. The following analyses give an idea of the variation in the chemical composition: 264 INTRODUCTION TO THE STUDY OF MINERALS Analyses of Gold Au Ag Misc. Sp. Gr. South Australia 93.5 87.4 79.9 66.4 6.5 12.1 20.1 33.2 18.8 Urals Peru Cu = 0.1 17.4 16 6 Verespatak SiOj = 0.4 15.0 Gold with more than 20 per cent, of silver is called electrum and is very pale yellow in color. Blowpipe Tests. Fusible at 3. With mercury forms an amal- gam. Soluble in aqua regia, the silver bearing varieties leaving a residue of AgCl. Uses. The uses of gold are well-known. Occurrence. 1. In quartz veins along with pyrite, chalco- pyrite, sphalerite, arsenopyrite, etc. 2. In placers along streams (ancient river-channels in some cases) associated with heavy minerals such as magnetite, ilmen- ite, garnet, zircon, platinum, etc. Prominent localities are Alaska, California, Brazil, Colombia, Urals, and Australia. 3. In quartzite-conglomerate in the Rand, Transvaal, South Africa. The origin of the gold is in doubt. 4. As a secondary mineral in gossan. At Cripple Creek, gold is found in the oxidized zone pseudomorphous after calaverite. SILVER, Ag Form. The characteristic occurrence of silver is in wires, thin sheets, skeleton crystals, dendritic groups, and masses. The cube is the only common crystal form, but the crystals are very small. H.=2i. Sp.gr. + 10.5 Color. Tin-white to pale yellow, but is often tarnished some- times resembling copper. Luster, metallic. Malleable. Chemical Composition. Silver, often with some gold and copper. ELEMENTS 205 Blowpipe Tests. Easily fusible (2) to a malleable button. Soluble in HNO3. HCl gives a white precipitate (AgCl), which turns violet on standing and is soluble in NH^OH. Uses. Native silver is the source of some silver. The Cobalt (Ontario) district now furnishes about one-fifth of the world's supply of silver, the silver being mostly in the form of the native metal. Occurrence. 1. In veins along with bismuth, cobalt minerals, and other silver minerals. Batopilas, Mexico, and Cobalt, Onta- rio, are prominent localities. 2. As a secondary mineral with argentite or galena or in the gossan and formed from other silver minerals. Leadville, Colo- rado, and the Coeur d'Alene district in Idaho are prominent localities. In contrast with gold, silver is never found in placers. COPPER, Cu Form. Copper is found in small disseminated grains, in sheets, and occasionally in large masses. Copper crystallizes in the iso- metric system, but the crystals are usually distorted and made up of dendritic groups. The forms {100}, {111}, {110}, and {210} can sometimes be made out. H. = 2i. Sp.gr. + 8.8 Color. Copper-red, often tarnished and also incrusted with alteration products such as cuprite and malachite. Metallic luster. Chemical Composition. Copper, often with a little silver. Blowpipe Tests. Fusible (3) to a malleable globule. Soluble in dilute HNO3 to a green solution with the evolution of NO2. Also soluble in NH^OH, giving a deep blue solution. Uses. Native copper is an important source of copper in but one locality (Upper Peninsula of Michigan). 266 INTRODUCTION TO THE STUDY OF MINERALS Occurrence. 1. As a secondary mineral in many copper mines formed by the reduction of copper compounds in solution, which in turn were formed by the oxidation of chalcopyrite. 2. In amygdaloidal cavities of diabase associated with calcite, datolite, prehnite, epidote, zeolites, and sometimes with silver. The Upper Peninsula of Michigan is the type locality. Mercury, Hg Form. Mercury occurs as small globules in seams and cavities associated with cinnabar, occasionally in little pools. Mercury is unique in being the only mineral which is liquid at ordinary temperatures. Color, Tin-white. Sp. gr. + 13.6 Chemical Composition. Mercury. Blowpipe Tests. Volatile. Forms an amalgam with gold, silver, and copper. Soluble in HNO3. Occurrence. 1. Probably a secondary mineral formed by the decomposition of cinnabar. Found at New Almaden, Cali- fornia. Platinum, Pt Form. Platinum is found in rounded grains, scales, and irregu- lar lumps. Cubic crystals are very rare. H. Sp. gr. 15-19 (21, if pure) Luster, metallic. Malleable. Some Color. Light steel gray, varieties are magnetic. Chemical Composition. Platinum, often alloyed with iron and metals of the platinum group (Ir, Os, Pd, Rh). Analyses: Pt Fe Pd Rh Ir Os Cu Urals 80.9 84.8 79.8 2.3 8.3 9.4 1.6 1.0 0.3 11.1 2.1 3.4 tr 1.0 4.3 1.0 1.1 1.0 0.6 0.3 S-0.8 Colombia California ELEMENTS 267 Blowpipe Tests. Infusible. Soluble in aqua regia. From this solution KCl precipitates KoPtClg, a yellow crystalline powder composed of minute octahedra. Uses. Native platinum is the only source of platinum. The Ural mountains furnish the main supply. Occurrence. 1. In placer deposits along with gold, magne- tite, ilmenite, zircon, diamond, etc. Prominent localities are the Ural Mts., Colombia, British Columbia, northern California, and southern Oregon. 2. In peridotites with chromite, olivine, and serpentine. These rocks are the original source of the platinum of placers. Urals and British Columbia. Iron, Fe Form. Iron is found in compact or spongiform masses and in disseminated grains. Iron crystallizes in the isometric system, but distinct crystals are exceedingly rare. Many meteoric irons have an octahedral structure which is revealed by etching with HNO3. H.=4§. Sp.gr. + 7.5 Color. Steel-gray to iron-black, often covered with iron-rust. Luster, metallic. Attracted by the magnet. Chemical Composition. Iron, often alloyed with nickel. Contains small amounts of Co, Cu, C, etc. Fe Ni Co Cu s Mn c Silicates, Insol. Terrestrial iron, Green- land. Meteoric iron, Siberia. 91.7 88.0 1.7 10.7 0.5 0.5 0.1 0.1 0.1 tr. 0.1 1.4 0.1 3.9 0.5 Blowpipe Tests. Infusible. Amber borax bead in O.F., bottle green in R.F. 268 INTRODUCTION TO THE STUDY OF MINERALS Soluble in HCl with the evolution of hydrogen. Iron is copper-' coated by a solution of copper sulfate. Occurrence. 1. In meteorites, either as the main constituent or associated with silicates such as olivine. 2. In basalts, perhaps formed from the molten magma. The only prominent locality is Disco Island, Greenland. 2. SULFIDS, ARSENIDS, AND TELLURIDS Realgar, AsS Orpiment, AsjSj / STIBNITE, SbjSa \ Bismuthinite, ^hS, Molybdenite, MoS^ Argentite, AgjS GALENA, PbS CHALCOCITE, Cu^S SPHALERITE, ZnS Alabandite, MnS CnmABAR, HgS Covellite, CuS Millerite, NiS Niccolite, NiAs PYRRHOTITE, FeS f PYRITE, FeSj \ Cobaltite, CoAsS [ Smaltite, (Co,Ni)As2 f MARCASITE, FeS^ j ARSENOPYRITE, FeAsS [ Lollingite, FeAsj Sylvanite, AgAuTe, Calaverite, AuTcj The sulfids and their analogues, the selenids, tellurids, arse- nids, etc., are derivatives of HjS, HjSe, HjTe, HjAs, etc. They may be considered as salts of these acids or as sulfanhydrids of sulfo-salts just as the oxids are anhydrids of the oxy-salts. They are mostly sulfids of the heavy metals. The sulfids may be divided into two classes: (1) the sulfids of the semi-metals, and (2) the sulfids of the metals. Minerals under each of these are arranged according to increasing number of sulfur atoms in the formula. Prominent isomorphous groups are indicated by brackets. 269 270 INTRODUCTION TO THE STUDY OF MINERALS Realgar, AsS Form. Realgar occurs incrusting or disseminated, and is usually massive, rarely showing distinct crystals. Crystals are monoclinic and short prismatic in habit. H. = H-2. Sp.gr. +3.5 Color. Red, becoming orange-red on exposure to light. Lus- ter, resinous. Translucent. Optical Properties. 7i>1.93. Double refraction strong. Frag- ments are irregular and pleochroic (yellow to orange). Chemical Composition. Arsenic sulfid, AsS; (As = 70.1 per cent.). Blowpipe Tests. On charcoal fuses (at 1) and volatilizes, giving off fumes of arsin and sulfur dioxid. In the closed tube gives a red sublimate (AsS). Soluble in KOH. Occurrence. 1. As a vein mineral often associated with orpiment. At Felsobanya, Hungary, realgar occurs in veins with barite, arsenic, orpiment, stibnite, quartz, pyrite, etc. 2. As a sublimation product in solfataras, and in the workings or dumps of burning coal mines. Orpiment, AsjSg Form. Orpiment occurs in foliated or granular masses, in powdery incrustations, and in small indistinct orthorhombic crystals. Cleavage. Perfect cleavage in one direction (010). H. = 1J. Sp.gr. ±3.5 Color. Yellow. Cleavage plates are flexible and have a pearly luster. Optical Properties. n>1.93. Double refraction, strong. Cleavages have extinction parallel to striations, but give no interference figure. Chemical Composition. Arsenic sulfid, AsjSj; (As =60.9 per cent.). SULFIDS, ARSENIDS, AND TELLURIDS 271 Blowpipe Tests. Like those of realgar except that the closed tube sublimate is yellow. Soluble in KOH and in aqua regia. Occurrence. Like that of realgar. STIBNITE, SbjSg Form. Stibnite is found in prismatic or acicular crystals, in columnar or bladed aggregates, and in granular masses. Crys- tals are orthorhombic (bipyramidal class). Usual forms {llOj, {010), {111}, {113}, {121}. The habit is long prismatic with {110} as the dominant form (110:lT0 = 89° 34'). Crystals are often highly modified and always vertically striated. Cleavage. Perfect in one direction parallel to the side pina- coid (010). Also parting parallel to (001). H.=2. , Sp.gr. ±4.5 Color. Lead gray. Luster, brilliant metallic on fresh sur- face. Streak, lead gray. Chemical Composition. Antimony sulfid, Sb2S3; (Sb =71.4 per cent.). Blowpipe Tests, On charcoal easily fusible (1) giving a green- ish-blue flame, dense white fumes, and a white sublimate close to the assay. In the open tube gives SOj and a white non-vola- tile sublimate (SbjOJ on the under side of the tube. In the closed tube gives a dark red sublimate of antimony oxysulfid (Sb,S,0). Decomposed by HNO3 with the separation of a white precipi- tate, (HSbOg). Uses. Stibnite is the principal source of antimony. Occurrence. 1. As a vein mineral associated with pyrite, sphalerite, galena, cinnabar, and realgar in a gangue of quartz, barite, or calcite. Prominent localities for specimens of stibnite are Felsobanya, Hungary and Shikoku, Japan. At the latter locality magnificent crystals over a foot in length are found. 272 INTRODUCTION TO THE STUDY OF MINERALS Bismuthinite, BijSg Form. Bismuthinite is isomorphous with stibnite, and greatly resembles it. Cleavage. In one direction parallel to the length. H. = 2. Sp.gr. +6.4 Color. Lead gray with a peculiar yellowish tarnish. Chemical Composition. Bismuth sulfid, BijSj; (Bi = 81.2 per cent.). Some varieties contain Se. Blowpipe Tests. Easily fusible (1) , giving on charcoal a metal- lic button (malleable, but brittle on the edges) and a yellow coating. Soluble in HNO3. On diluting the solution with water a white precipitate is formed. Occurrence. 1. As a vein mineral associated with bismuth and chalcopyrite especially. Molybdenite, MoSj Form. Molybdenite is usually found in foliated masses or in disseminated scales, and occasionally in hexagonal crystals of tabular habit. Cleavage. One direction parallel to {0001}. H. = li. Sp.gr. +4.7 Color. Bluish lead gray. The streak on glazed porcelain (or glazed paper) has a greenish tinge. Cleavages are flexible and sectile. Chemical Composition. Molybdenum sulfid, M0S2; (Mo =60.0 per .cent.). Blowpipe Tests. Infusible. On charcoal gives a white subli- mate which is copper-red near the assay. Green NaPOg bead in R.F., colorless in O.F. Decomposed by HNOg with the formation of a white sublimate (M0O3) which is soluble in NH.OH. Uses. Chief source of molybdenum, which is used as an alloy with steel. SULFIDS, ARSENIDS, AND TELLURIDS 273 Occurrence. 1. In pegmatites and surrounding rocks, espe- cially granite. 2. In tin-stone veins with cassiterite, wolframite, topaz, etc. 3. As a contact mineral between limestones and granites asso- ciated with epidote, chalcopyrite, etc. Argentite, AgjS Form. Occurs massive, incrusting, more rarely in rough crys- tals. The crystals are isometric, the only common form being the cube. Cubes are often arranged in parallel position. H.=2i. Sp.gr. ±7.3 Color. Dark lead gray, almost black. Luster, metallic. Very sectile. Chemical Composition. Silver sulfid, AgjS; (Ag = 87.1 per cent.). Blowpipe Tests. Easily fusible (IJ). On charcoal yields a malleable button of silver. Soluble in HNO3 with the separation of S. HCl gives a white precipitate, AgCl, soluble in NH^OH. Uses. Argentite, called silver glance by miners, is an impor- tant ore of silver on account of the high silver content. Occurrence. 1. As a vein mineral associated with other silver minerals. Freiberg, Saxony, furnishes fine specimens. GALENA, PbS Form. Galena occurs in well-formed crystals as well as in cleavable and granular masses. Crystals are isometric (hexocta- hedral class). The usual forms are the cube {100), the octa- hedron {111), more rarely the dodecahedron {110) and the tris- octahedron {221}. The habit is usually cubic, cubo-octahedral, or octahedral. Figs. 398 to 402 show a variety of habits. Con- tact or penetration twins with (111) as twin-plane and poly- synthetic twins with (441) as twin-plane (see Fig. 239, p. 66) are sometimes observed. 274 INTRODUCTION TO THE STUDY OF MINERALS Cleavage. Perfect cubic cleavage. H. = 2i. Sp.gr. + 7.5 Color. Lead gray, often tarnished. Metallic luster. Chemical Composition. Lead sulfid, PbS ; (Pb = 86.6 per cent.) . May contain Zn, Ag, free sulfur, and other impurities. Blowpipe Tests. Easily fusible at 2, giving on charcoal a malle- able button, a yellow sublimate (PbO) near the assay, and a white sublimate (PbSO^) farther from the assay. Fig. 398. Fig. 399. Fig. 400. Fig. 401. Fig 402. Decomposed by HCl with the separation of PbClj, a white crystalline precipitate soluble in hot water. Decomposed by HNO3 with the separation of S and PhSOj. Uses. Galena is the most important ore of lead; argentiferous galena is one of the most important silver ores. Occurrence. 1. As a vein mineral associated with sphalerite, chalcopyrite, pyrite, etc., often with barite as a gangue mineral. In the north of England galena occurs with fluorite, barite, cal- cite, and sphalerite in veins in Sub-carboniferous limestone. 2. As a metasomatic replacement of limestone. 3. In sedimentary rocks such as limestones, shales, and sand- stones often associated with sphalerite. In southeastern Mis- souri galena is disseminated through Ordovician limestone. CHALCOCITE, Cu^S Form. Usually fine-grained compact masses, rarely in pseudo- hexagonal orthorhombic crystals, which are sometimes twinned. H. = 2i Sp.gr. + 5.7 SULFIDS, ARSE NWS, AND TELLURIDS 275 Color. Dark lead gray with black tarnish. Metallic luster. Chalcocite is sub-sectile, (i.e., can be cut with a knife, but not so readily as argentite). Chemical Composition. Cuprous sulfid, CujS; (Cu = 79.8 per cent.) . A little iron is usually present. Blowpipe Tests. Fuses at 2. Unaltered in the closed tube. In R.F. on charcoal gives metallic copper. Soluble in HNO3 giving brown-red fumes, residue of S, and a green solution. Uses. Chalcocite is a valuable ore of copper on account of the high percentage of copper. It is a prominent mineral in the ores at Butte, Montana. Occurrence. 1. As a vein mineral along with other sulfids. 2. As a product of secondary enrichment. Butte, Montana. 3. As a replacement of lignite. Sierra Oscura Mts., New Mexico. SPHALERITE, ZnS Form. Sphalerite crystallizes in the hextetrahedral class of the isometric system. Crystals are usually distorted and diffi- Fia. 403. Fia. 404. cult to decipher. The common habits are tetrahedral and dodec- ahedral, the usual forms being a[ 100), djllOj, o{lll},Oi{lll}, and m { 3 11 } . Figs. 403 and 404 represent typical crystals. Crys- tals are often twinned on the (HI) face. 276 INTRODUCTION TO THE STUDY OF MINERALS Cleavage. Prominent dodecahedral at angles of 60° and 90°. H.=3ito4. Sp.gr. ±4.0 Color. The color of sphalerite varies from white to black, depending upon the amount of iron present. The usual color is yellowish-brown or reddish-brown. The luster is adamantine, and the streak pale yellow to brown. ' ■' ' " '-^ Optical Properties. n = 2.37. Fragments are triangular, pale yellow, and isotropic (dark between crossed nicols). The high index of refraction accounts for the adamantine luster. Chemical Composition. Zinc sulfid, ZnS; (Zn=67.0 per cent.). Iron replaces zinc isomorphously as shown in the formula (Zn,Fe)S. Cadmium is another common impurity. The rare elements indium and gallium were discovered in sphalerite. The following are typical analyses: Zn Fe S 67.5 .S2.2 65.2 0.5 32.8 63.4 3.6 33 4 58.2 8.2 33.4 50.0 15.4 33.3 Misc. White; Franklia, New Jersey . Yellow: Schemnitz, Hungary . Brown: Roxbury, Connecticut Dark brown: Westphalia Black: Felsobanya, Hungary . Cd = tr Cd = 1.5 Cu = 0.1; Pb=tr Cd = 03; Pb = 1.0 Blowpipe Tests. Difficultly fusible (5). On charcoal gives a white sublimate, which is yellow when hot. This sublimate heated intensely with cobalt nitrate solution gives a green color (cobalt zincate). The presence of Cd is indicated by an irides- cent coating on charcoal. Soluble in HCl with the evolution of HjS. Uses. Sphalerite is the most important ore of zinc. The Joplin district of southwest Missouri is the principal locality in this country. Occurrence. 1. As a vein mineral associated with galena principally. . 2. As an accessory mineral in sedimentary rocks. SULFIDS, ARSE N IDS, AND TELLURIDS 277 Alabandite, MnS Form. Usually massive, rarely in cubic crystals. Cleavage. Imperfect cubic. H.=3^to4. Sp.gr. + 4 Color. Black or brownish-black with submetallic luster. The streak is olive-green, a distinctive test for this mineral. Optical Properties. Fragments are square or irregular, trans- lucent green, and dark between crossed nicols. n> 1.93. Chemical Composition. Manganese sulfid, MnS; (Mn=63.1 per cent.). Blowpipe Tests. Fusible at 3. Unaltered in the closed tube. After roasting gives bead tests for Mn. Soluble in HCl with the evolution of H^S. Occurrence. 1. As a vein mineral associated with rhodo- chrosite and sulfids. Tombstone, Arizona, is the only prominent American locality for this mineral. CINNABAR, HgS Form. Cinnabar often occurs in minute crystals in cavities. The crystals are hexagonal of variable habit. The common forms are {lOTO), {lOTl}, {0001}, and {4043}. Cinnabar usually occurs disseminated through the rock and in massive and earthy forms. Cleavage. Prismatic. Even in massive varieties reflections from minute cleavage planes may be seen. H.=2^. Sp.gr. +8.0 Color. Scarlet to dark red, black when impure. Luster, adamantine in typical specimens. Streak, vermilion. Optical Properties. n^(3. 20) -71^(2.85) =0.35. Fragments are red, and irregular with high order interference colors. The very high index of refraction accounts for the adamantine luster. Basal sections of cinnabar are not dark between crossed nicols, 278 INTRODUCTION TO THE STUDY OF MINERALS but become dark when one nicol is rotated 325° for a plate 1 mm. thick, using monochromatic red light. This property, possessed also by quartz, is called rotary polarization. Chemical Composition. Mercuric sulfid, HgS; (Hg = 86.2 per cent.). Clay and organic matter are often present as im- purities. Blowpipe Tests. Volatile if pure. In the closed tube with dry soda cinnabar gives a sublimate of metallic mercury (little drops when rubbed with a wire). Soluble in HNO3. Uses. Practically the only ore of mercury. The principal producing localities are Almaden in Spain, Idria in Austria, New Almaden and New Idria in California. Occurrence. 1. In stock-work deposits, i.e., scattered through the rock in stringers, etc., rather than in distinct veins. In the Coast Ranges of California cinnabar is associated with serpentine. Covellite, CuS Form. Tabular hexagonal crystals are rare, the mineral usually occurring as a dissemination or incrustation. H. = l|to2. Sp.gr. +4.6 Color. Deep indigo blue, almost black. Luster, metallic pearly to dull. Chemical Composition. Cupric sulfid, CuS; (Cu = 66.4 per cent.). . Blowpipe Tests. Fusible at 2h. In the closed tube gives a sublimate of sulfur (distinction from chalcocite). On charcoal burns with a blue flame, gives off SOj, and leaves a button of copper. Soluble in HNO, to a green solution. Occurrence. 1. A characteristic mineral of the zone of sec- ondary sulfid enrichment. Butte, Montana, furnishes good specimens. In many mines it occurs as an incrustation on chalcopyrite. SULFIDS, ARSENIDS, AND TELLURIDS 279 Millerite, NiS Form. Millerite occurs in acicular crystals and in fibrous crusts. H.=3i. Sp.gr. + 5.5 Color. Brass yellow. Luster, metallic. Chemical Composition. Xickelsulfid, NiS; (Ni = 64.7 per cent.). May also contain Co, Fe, and Cu. Blowpipe Tests. Fusible at 2 to a magnetic globule which gives a borax bead in O.F., violet when hot. Soluble in HXO3 to a green solution. Occurrence. 1. As a vein mineral associated with cobalt and silver minerals. 2. As a secondary mineral with nickeliferous pyrrhotite. Gap Mine, Pennsjdvania. 3. In sedimentary rock.s in cavities penetrating calcite crys- tals. St. Louis, Mo. and Keokuk, Iowa. Niccolite, NiAs Form. Niccolite practically always occurs in a massive form without cleavage. R. = oh Sp.gr. +7.4 Color. Pale copper red. Metallic luster. Chemical Composition. Nickel arsenid, XiAs; (Ni=43.9 per cent.). May also contain S, Sb, and Co. Blowpipe Tests. Easily fusible (2) to a magnetic residue giving off arsin fumes. Soluble in HNO3 to a green solution. Occurrence. 1. As a vein mineral with cobalt and silver ores. PYRRHOTITE, FeS (j) Form. Pse«d-&hexagonal ertberhoHi4aie crystals of tabular habit are known, but are very rare. Usually massive and with- out cleavage. 280 INTRODUCTION TO THE STUDY OF MINERALS H. = 4 (Pyrite is 6) . Sp. gr. + 4.6 Color. Bronze-yellow. Luster, metallic. Pyrrhotite is at tracted by the magnet. Chemical Composition. Ferrous sulfid, FeS, with a little ex- cess of sulfur. (Fe = 63.6 per cent.). Often contains nickel up to 5 per cent. Blowpipe Tests. Fuses at 3 to a magnetic globule, giving fumes of SOj. In the closed tube gives little or no sulfur (dis- tinction from pyrite). Soluble in HNO3. Uses. The nickeliferous pyrrhotite of Sudbury, Canada, is an important ore of nickel. Occurrence. 1. In basic igneous rocks. Some large depos- its are due to magmatic segregation. 2. As a vein mineral. 3. In crystalline limestones. PYRITE GROUP— ISOMETRIC Pyrite, FeSj; smaltite, CoAsj) chloanthite, NiAsj; cobaltite, CoAsS; and gersdorfhte, NiAsS, constitute an isomorphous group as they crystallize in the diploid class of the isometric system in cubes and pyritohedrons, and have a hardness of 5J to 65 and a specific gravity of 5 to 6.5. There are also intermediate com- pounds such as (Co,Fe)As2, (Ni,Fe)As2, and (Co,Fe)AsS. The general formula, then, can be written (Fe,Co,Ni)(As,Sb,S)2. PYRITE, FeSj Form. Pyrite is often well crystallized and furnishes the typi- cal example of the diploid class of the isometric system. The most common forms are the cube a{100), pyritohedron e{210}, octahedron o{lll}, diploid s{321}, diploid {421}, and trapezo- hedron {211}. The habit is nearly always cubic, pyrito- hedral, or octahedral Figs. 405 to 412 represent typical crystals. SULFIDS, ARSENIDS, AND TELLURWS 281 The cube faces are commonly striated as in Fig. 405. The {hkO} forms are so characteristic of pyrite that they are called pyritohedrons. Of these the pyritohedron {210} is the most common. Penetration twins of the pyritohedron with the a-axis as the twinning axis are occasionally found. (See Fig. 241, page 66). H.=6to6*. Sp.gr. ±5.0 ^^ /^^^ ^ Fig. 405. Fig. 406. Fig. 407. Fig. 408. Fig. 409. Fig. 410. Fig. 411. Fig 4X2. Color. Brass yellow. Luster, metallic. Streak, greenish to brownish-black. Chemical Composition. Iron disulfid, FeSj; (Fe=46.6 per cent.). May contain Cu, Co, Ni, As, and Au. The copper is probably in the form of chalcopyrite. The following are typical analyses : Fe S Cu 44.2 54.1 0.05 46.4 51.4 1.0 44.5 53.4 2.4 Misc. French Creek, Fenn, Amsberg, Germany. Cornwall, Fenn. . . '. . Mn = 0.5; Co = 0.1; As=0.6 282 INTRODUCTION TO THE STUDY OF MINERALS Blowpipe Tests. Fusible at 3 to a magnetic globule. On char- coal pyrite burns with a blue flame, giving off SOj. In the closed tube it gives a sublimate of sulfur and leaves a magnetic residue. Insoluble in HCl, but soluble in HNO3 with the separation of S. Uses. Pyrite is used extensively in the manufacture of sul- furic acid, which is the basis of many chemical industries. Py- rite is also an important low-grade copper ore at many localities, the copper being present as disseminated chalcopyrite. Pyrite is also often gold-bearing. Occurrence. 1. As a vein mineral associated with other sulfids. 2. As an original mineral in igneous rocks. 3. As a secondary mineral in igneous rocks, especially in the country rock around ore-deposits. 4. As a dissemination in sedimentary rocks, such as shales and limestones, often replacing organisms. 5. As bedded deposits in metamorphic rocks. 6. As a contact mineral with hematite and magnetite. Lindgren states that the occurrence of iron sulfids and iron oxids together is characteristic of contact deposits. Cobaltite, CoAsS Form. Cobaltite usually occurs in crystals, which are identi- cal with those of pyrite, the cube, pyritohedron, and octahedron being the only common forms. H.=5i. Sp.gr. ±6.] Color. Reddish silver-white. Luster, metallic. Chemical Composition. Cobalt arsenid-sulfid, CoAsS; (Co = 35.4, As =45.3, S = 19.3). Usually contains iron and occasionally nickel. Blowpipe Tests. On charcoal gives arsin odor and fuses at 2J to a gray, feebly magnetic globule, which colors a borax bead SULFIDS, ARSE NWS, AND TELLURIDS 283 deep blue. Unaltered in the closed tube (distinction from smaltite) . Soluble in HNOg to a rose-colored solution with the separation of sulfur. Occurrence. 1. In fahl-bands of schists and gneisses. Tunaberg, Sweden, is the principal locality. Smaltite, (Co,Ni)As2 Form. Smaltite is usually massive without any cleavage, but occasionally is found in cubic crystals. H. = 5:< Sp. gr.+6.2 Color. Tin white to steel gray. Luster metallic. Chemical Composition. Cobalt-nickel arsenid, (Co,Ni)As2, varying from CoAsj (Co = 28.2 per cent.) to NiAsj (Ni = 28.1 per cent.). Iron and sulfur are usually present in small amounts. The following analyses illustrate the range in composition. Misc. Atacama, Chili Wittichen, Germany Schneeberg, Saxony Co Ni Fe A3 S 24.1 12 4.1 70.8 0.1 10.1 8.5 5.1 69.7 4.7 4.2 .24.9 0.7 68.4 1.1 Cu = 0.4 Cu = 0.9; Bi = Bi = 0.2 1.0 Blowpipe Tests. On charcoal gives off arsin and fuses at 2J to a magnetic globule, which colors the borax bead blue. In the closed tube gives an arsenic mirror if strongly heated. Soluble in HNOg to a rose-red solution. Uses. Smaltite is the chief ore of cobalt. Schneeberg, Saxony, and Cobalt, Ontario, are the principal localities. Occurrence. 1. As a vein mineral usually with silver and bismuth and in a gangue of calcite. Smaltite is often coated with erythrite, a cobalt arsenate called "cobalt bloom." MARCASITE GROUP— ORTHORHOMBIC The following minerals: marcasite, FeS^; arsenopyrite, FeAsS; loUingite, FeAsz; glaucodot (Co,Fe)AsS; safflorite, CoAsj; and 284 INTRODUCTION TO THE STUDY OF MINERALS rammelsbergite, NiAsj, constitute an isomorphous group parallel to the pyrite group. They are orthorhombic in crystallization, have a hardness of 5 to 6^, and are tin-white to brass-yellow in color. Only the first three of these minerals are considered as the others are rare. MARCASITE, FeS^ Form. Marcasite occurs in orthorhombic crystals, in crystal- line aggregates, and in rounded concretionary masses. Crystals are usually tabular in habit and often elongated in the direction of the a-axis. Figs. 413 to 415 represent typical crystals with Fio. 414. Fig. 415. the forms: c{001}, ??i(110}, •y{013}. mm(110:lT0) =74° 55'. Twins with ot{110} as twin-plane are common. H.=6to6i Sp.gr. ±4.9 Color. Pale brass yellow with a greenish tinge. Luster metallic. Chemical Composition. Iron disulfid, FeS^; (Fe = 46.6 per cent.). Analyses often show small amounts of As. Blowpipe Tests. The same as for pyrite. Uses. If found in sufficient quantity, marcasite could be used in the manufacture of sulfuric acid. Occurrence, 1. In sedimentary rocks, often in concretions. 2. As a vein mineral, but not so common as pyrite. SULFIDS, ARSENIDS, AND TELLURIDS 285 ARSENOPYRITE, FeAsS Form. Arsenopyrite is found in well-formed crystals, as well as in disseminated grains and compact masses. The crystals are orthorhombic, similar in habit and angles to marcasite. Figs. 416 and 417 represent typical crystals with the forms m{110), cjOOll andM|014l. Fia. 416. Fig. 417. H. = 5i to 6. Sp. gr.+6.0 Color. Tin- white to light steel-gray. Luster, metallic. Chemical Composition. Iron arsenid-sulfid, FeAsS; (Fe = 34.3, As =46.0, S = 19.7). Often contains cobalt. Blowpipe Tests. On charcoal fuses (at 2) to a magnetic glob- ule and gives off arsin. In the closed tube on gentle heating gives a red sublimate (AsS), but on further heating an arsenic mirror is formed. Soluble in HNO3 with the separation of S. Uses. Arsenopyrite is the chief source of the white arsenic (AsjOg) of commerce. At Deloro, Canada, arsenopyrite is a gold ore. Occurrence. 1. As a vein mineral. Arsenopyrite is found in the quartz veins of the mother lode of California. Lollingite, FeAs2 Form. Usually massive, orthorhombic crystals similar to arsenopyrite being very rare. H.=5i. Sp.gr. ±7.1 Color. Tin-white to steel-gray. Luster, metallic. 286 INTRODUCTION TO THE STUDY OF MINERALS Chemical Composition. Iron arsenid, FeAsa (Fe = 27.2 per cent.)- Usually contains some sulfur. Blowpipe Tests. On charcoal fuses (at 2) to a magnetic glob- ule and gives off arsin. In the closed tube gives an arsenic mirror. Usually gives a faint sulfur test on a silver coin. Soluble in HNO3. Occurrence. 1 As a vein mineral. Sylvanite, AuAgTe^ Form. Sylvanite occasionally is found in distinct monoclinic crystals but usually in branching crystal aggregates or in dis- seminated grains. Cleavage. In one direction (parallel to 010). H.=2. Sp.gr. +8.0 Color, silver-white to steel-gray. Luster, metallic. Brittle. Chemical Composition. Gold and silver tellurid, AuAgTe4 (Au = 24.2 per cent.; Ag = 13.3 per cent.). Blowpipe Tests. Easily fusible (at 1) on charcoal to a gray bead coloring the flame bluish-green and giving a white subli- mate. With soda it is reduced to a pale yellow button. The powdered mineral dropped in hot concentrated HjSO^ gives a purplish-red solution. Soluble in aqua regia with the separation of AgCl. Uses. An important ore of gold and silver, ft is found in Boulder County, Colorado, in West Australia, and in Transylva- nia, Hungary. Occurrence. 1. As a vein mineral. In Transylvania the veins occur in dacites. Calaverite, AuTBj Form. Calaverite usually occurs in small striated elongated crystals along seams. The crystals are complex monoclinic, some of the faces having very high indices. Fracture, subconchoidal. No cleavage. SULFIDS, ARSENIDS, AND TELLURIDS 287 H.=2i. Sp.gr. ±9.0 Color. Pale brass yellow, resembling pyrite. Metallic luster. Chemical Composition. Gold tellurid, AuTejj (Au = 44 per cent.)- Always contains some silver, usually from 2 to 4 per cent. I Blowpipe Tests. Easily fusible (at 1) on charcoal to a yellow button of gold, giving dense white fumes and coloring the flame bluish-green. With hot cone. H2SO4 gives a purplish-red solution. Soluble in aqua regia with the separation of a little AgCl. Uses. An important ore of gold. At Cripple Creek, Colorado, it is the chief source of gold. It also occurs in West Australia. The name is derived from Calaveras County, California, but it is a very rare mineral at this locality. Occurrence. 1 As a vein mineral. At Cripple C'reek, fluorite is a common associate. 3. SULFO-SALTS r CHALCOPYRITE, CuFeSj \ Stannite, CUsFeSnS, BORNITE, CujFeS. Jamesonite, Pb^FeSbuSu Bournonite, PbCuSbSj / Pyrargyrite, AgjSbSa "(^ Proustite, AgjAsSj TETRAHEDRITE, CUjSbSj Stephanite, AgsSbS^ Polybasite, (Ag,Cu)ieSb,Sji Enargite, CUjAsS, Under the sulfo-salts are included certain compounds of sulfur, salts of hypothetical acids which may be derived from ordinary oxygen acids by replacing S for 0, these two elements being very similar chemically. Three classes of these compounds may be distinguished: (1) Sulfoferrites, derivatives of HjFeSg analogous to ferrous acid, H3re03. (2) Sulfarsenites and sulfantimonites, derivatives of HjAsSg and HjSbSg analogous to arsenious acid, HgAsOj and antimonous acid, HaSbOg. (3) Sulfarsenates and sulfantimon- ates, derivatives of H3ASS4 and H3SbS4, analogous to arsenic acid, H3ASO4, and antimonic acid, H3Sb04. There are also condensed acids derived from the above men- tioned acids by the subtraction or addition of HjS, just as con- densed acids may be derived from oxygen acids by the subtrac- tion or addition of Hfi. Thus chalcopyrite, CuFeSj, is a salt of HFeS^ derived from HjFeSg (H3FeS3-H2S=HFeS2). Jameso- nite is a salt of H5Sb3S7(3H3SbS3— 2H2S). Stephanite is a salt of HjSbS, (HgSbSg-t-HjS). Polybasite is a salt of H^Sb^S,, (2H3SbS3 + 5H2S). 288 SULFO-SALTS 289 The sulfo-salts are sontietimes considered as double sulfids. Thus tetrahedrite, CugSbSj, is written SbjSj-SCujS. About sixty sulfo-salt minerals are known, but most of them are rare. CHALCOPYRITE, CuFeS^ Form. Chalcopyrite occurs in crystals, in masses, and dis- seminated through the rock. The crystals belong to the tet- ragonal system, scalenohedral class, but are pseudotetragonal and pseudo-octahedral in form. Fig. 418 represents a common type of crystal with the forms pjlll) and 2{201}. This resembles a tetrahedron, but is distin- guished by the striations. H.=3ito4. Sp.gr. ±4.2 Color. Brass yellow often with iridescent p^^ ^jg tarnish. Metallic luster. Chemical Composition. Cuprous sulfoferrite, CuFeSj; (Cu = 34.5 per cent.). Variations from this formula are usually due to admixed pyrite. Blowpipe Tests. On charcoal fusible (at 2) to a magnetic globule which heated with soda gives a copper button. In the closed tube decrepitates and gives a sublimate of sulfur. Soluble in HNOj to a green solution with the separation of sul- fur. Uses. Chalcopyrite is the principal ore of copper. Occurrence. 1. As a vein mineral associated with pyrite, galena, sphalerite, tetrahedrite, etc. 2. In basic igneous rocks with pyrrhotite, perhaps as a mag- matic separation. Sudbury, Canada. 3. Disseminated through massive pyrite. 4. In fahlbands of schists and gneisses. 5. As a contact mineral with magnetite or hematite. 290 INTRODUCTION TO THE STUDY OP MINERALS Stannite, Cu2FeSnS^ Form. Stannite, though usually massive, is sometimes found in crystals which are similar to chalcopyrite in form. H.=4. Sp.gr. ±4.5 Color. Gray-bronze. Metallic luster. Chemical Composition. Copper sulfoferrite-sulfostannite, CujFeSnS^, like chalcopyrite, but with half of the iron replaced by tin. (Sn = 27.6 per cent.). It usually contains zinc. Blowpipe Tests. Fusible (at IJ) on charcoal, giving a straw- colored sublimate of SnOj near the assay. With Na2C03 on charcoal (R.F.) gives metallic globules. Soluble in HNO3 to a green solution with the separation of S and HjSnOg (white residue). Occurrence. 1. As a vein mineral it occurs in Cornwall and Bolivia, where it is associated with other sulfo-salts and with cassiterite. BORNITE, Cu^FeS, Form. Bornite occurs in masses, very rarely in rough cubic crystals. H.=3. Sp.gr. ±5.1 Color. A brownish-red bronze with purple tarnish. Metallic luster. Chemical Composition. Copper sulfoferrite, CujFeS^; (Cu = 63.3 per cent.). Analyses vary widely due to intermixture with chalcopyrite and chalcocite. Blowpipe Tests. Fusible (at 2J) on charcoal R.F. to a magnetic globule. In the closed tube gives a faint sublimate of sulfur. Soluble in HNO3 to a green solution with the separation of S. Uses. Bornite is an important ore of copper. It occurs at Butte, Montana, intimately associated with chalcocite. Occurrence. 1. As a vein mineral associated with chalcocite and chalcopyrite. SULPO-SALTS 291 2. As a contact mineral between limestones and igneous rocks. 3. As a product of secondary enrichment. Butte, Montana. Jamesonite, Pb^FeSboSj^ Form. Jamesonite occurs in delicate capillary crystals and in columnar and compact masses. Cleavage. In one direction transverse to the length of the crystals. H.=2,V. Sp. gr.±5.7 Color. Lead-gray. Metallic luster. Chemical Composition. Lead sulfantimonite, Pb^FeSb^Si^; (Pb = 40.3 percent.). Blowpipe Tests. On charcoal easily fusible (at 1) giving white and yellow coatings. In the closed tube gives a yellow subli- mate of sulfur and a dark-red sublimate of SbjSjO. With soda on charcoal gives a lead button. Soluble in HCl with the evolution of HjS. On cooling the solution needle crystals of PbClj separate. Decomposed by HNO3 with the separation of a white residue. Occurrence. 1. As a vein mineral. Bournonite, PbCuSbSj Form. Bournonite occurs in well-defined orthorhombic crys- tals and also in fine-grained masses. The crystals are usually twinned and bear some resemblance to cog-wheels, hence the name "cog-wheel ore." H.=2ito3. Sp.gr. ±5.8 * Color, Dark gray to nearly black. Metallic luster. Chemical Composition. Cuprous lead sulfantimonite, PbCu- SbSj. Blowpipe Tests. On charcoal easily fusible (at 1) giving both a yellow and a white coating. The residue fused with soda gives a malleable button. Decomposed by HNOg, leaving a white residue. 292 INTRODUCTION TO THE STUDY OF MINERALS Occurrence. 1. As a vein mineral associated with sulfids and other sulfo-salts. RUBY SILVER GROUP— HEXAGONAL Pyrargyrite, AgjSbSj, and proustite, AgjAsSg, constitute an isomorphous group, for they both crystallize in the ditrigonal pyramidal class with almost identical interfacial angles. They both have a very high index of refraction and very strong double refraction. Pyrargyrite often contains As, while proustite often contains Sb, but the two compounds do not seem to mix in all proportions (Miers). Pyrargyrite, AggSbSj Form. Pyrargyrite often occurs in small well-defined crystals belonging to the ditrigonal pyramidal class. The habit is usually prismatic, the hexagonal prism { 1120} , being the dominant form. Opposite ends of the crystal are differently terminated. Fig. 419 represents a typical crystal, the striations indicating the hemi- morphic character. H. = 2i. Sp.gr. + 5.8 Color. Dark red to black. Translucent red on thin edges. Streak purple-red. Luster metallic-adamantine. Optical Properties. ?i^(3.08) - n„(2.88) = 0.20. Fragments are irregular and red in color, with high order interference colors. Chemical Composition. Silver sulfantimonite, AgjSbSg; (Ag = 59.9 per cent.). Arsenic replaces antimony to some extent. Blowpipe Tests. On charcoal fuses easily (at 1) to a globule of silver sulfid giving a white sublimate. This globule with soda in R.F. gives a silver button. Heated intensely in the closed tube gives a slight red sublimate. n Fig. 41i. SULPO-SALTS 293 Decomposed by HNO3 with the separation of sulfur and a white residue. Uses. A valuable ore of silver. Occurrence. 1. As a vein mineral often formed by secondary enrichment. Proustite, AgjAsSj Form. Crystals resemble those of pyrargyrite. Also massive and in crusts. H. = 2to2i. Sp.gr. + 5.6 Color. Red. Translucent in large pieces. Streak vermilion red. Luster, adamantine. Optical Properties. n^(3.08) - n„(2.79) = 0.29. Fragments are irregular and red in color, with high-order interference colors. Chemical Composition. Silver sulfarsenite, AggAsSg,- (Ag = 65.4 per cent.). Antimony often replaces part of the arsenic. Blowpipe Tests. On charcoal fuses easily (at 1) to a globule and gives off fumes of arsin. The globule heated with soda gives a silver button. In the closed tube gives a yellow sublimate of AS2S3. Decomposed by HNO3 with the separation of sulfur. The solution treated with (NH4)2MoO< and boiled, will yield a yellow precipitate of ammonium arsenomolybdate. Uses. Proustite is a valuable ore of silver occurring with other silver minerals. Occurrence. 1. As a vein mineral often formed by secondary enrichment. Guanajuato, Mexico and Chaiiarcillo, Chili, are prominent localities. TETRAHEDRITE, CUjSbSg Form. Tetrahedrite occurs in masses and also in crystals belonging to the hextetrahedral class of the isometric system. The common forms are the tetrahedron o{lll}, the tristetrahe- 294 INTRODUCTION TO THE STUDY OF MINERALS dron n{211}, and the dodecahedron d{nO}. Figs. 420-423 represent typical crystals. H. = 3i-4. Sp.gr. ±4.8 Color. Dark iron-gray. Metallic luster. Brittle. Chemical Composition. Copper sulf antimonite( CugSbSj; (Cu = 46.8 per cent. Sb=29.6, S=23.0). The copper is often re;^ced Fig. 420. Fig. 421. Fig. 422. Fia. 423. by Fe, Zn, Ag, or Hg and the antimony by arsenic. The follow- ing analyses illustrate the variation in chemical composition. Cu Sb S A3 Fe Zn Ag 45.4 28.8 24.5 tr 1.3 38.0 23.9 25.8 2.9 0.8 7.3 0.6 36.7 20.7 25.3 6.5 1.2 6.9 2.9 30.7 17.8 26.5 11.5 1.4 2.5 10.5 32.8 30.2 24.9 5.9 0.1 Misc. Fresney d'Oisana . . Kapnik, Hungary. Maohetillo, Chili . . Cabarrus Co., N. C. Poracs, Hungary . . Pb=0.1 Hg = 5.6 Blowpipe Tests. Easily fusible (at IJ) giving dense white fumes and a white sublimate near the assay. The residue heated with soda in R.F. gives metallic copper. In the closed tube gives a dark red sublimate (SbjSbjO). ^ Soluble in HNO3 to a green solution with the separation of sulfur and a white residue, HSbOg. Uses. Tetrahedrite is an ore of copper and is known to miners as "gray copper." Argentiferous tetrahedrite or freiber- gite is an ore of silver. Occurrence. 1. As a vein mineral often associated with chal- .SULFO-SALTS 295 c'opyrite. Cornwall, England, and Oruro, Bolivia, are prominent localities. 2. In fahlbands of schists, fahlerz being the German name for tetrahedrite. Stephanite, AgjSbS^ Form. Stephanite occurs disseminated, compact massive, more rarely in crystals. The crystals are orthorhombic, but pseudohexagonal and short prismatic in habit. H. = 2*. Sp.gr. +6.2 Color. Dark gray to black. Very brittle. Metallic luster. Chemical Composition. Silver sulfantimonite, AgsSbS^; (Ag = 68.5 per cent.). Blowpipe Tests. On charcoal easily fusible (at 1) to a globule giving dense fumes and a white sublimate of SbjOj. The globule heated with soda R.F. gives a silver button. Decomposed by HNO3 with the separation of sulfur and a white residue. Uses. A valuable ore of silver. It was a prominent mineral in the Comstock Lode of Nevada. Stephanite is known to miners as "brittle silver." Occurrence. 1. As a vein mineral. Polybasite, (Ag,Cu) jjSbjS^ Form. Polybasite usually occurs in monoclinic pseudo-hexa- gonal crystals of tabular habit. The basal pinacoid has triangu- lar striations which distinguishes it from other similar minerals. H. = 2i. Sp.gr. +6.1 Color. Iron black. Metallic luster. Optical Properties. n>1.93. The thinnest fragments are deep red translucent. Chemical Composition. Silver-copper sulfantimonite (Ag,Cu) ,j- SbjSji; (Ag = about 70 per cent.). Polybasite usually contains arsenic and grades into pearceite, (Ag,Cu)j5As2Sji. 17 296 INTRODUCTION TO THE STUDY OF MINERALS Blowpipe Tests. On charcoal fuses easily (at 1) to a globule and gives a white sublimate. The globule heated with soda gives a metallic button. In order to get pure silver it is neces- sary to cupel the button. Decomposed by HNO3 with the separation of sulfur and a white residue. Uses. Polybasite is an ore of silver. It occurs at Tonopah, Nevada, and at several mines in Colorado. Occurrence. 1. As a vein mineral, usually due to secondary enrichment. Enargite, CUgAsS^ Form. Enargite occurs in columnar masses and also in pris- matic orthorhombic crystals. Cleavage. In two directions at angles of 82° to each other. H.=3. Sp.gr. ±4.4 Color. Dark gray to black. Chemical Composition. Copper sulf arsenate, CU3ASS4; (Cu = 48.3 per cent.). Usually contains a little antimony and a little iron. Blowpipe Tests. Easily fusible at 1, sulfur dioxid masking the odor of arsin. In the closed tube gives a sublimate of sulfur. In the open tube deposits minute octahedral crystals of AsjOj with adamantine luster. Soluble in HNO3. Uses. Enargite is an ore of copper, occurring at Butte, Montana, and at many localities in South America. Near Butte white arsenic is recovered from smelter smoke. Occurrence. 1. As a vein mineral. At Tintic, Utah, enargite is the original source of several copper arsenate minerals. 4. HALOIDS A . Normal anhydrous haloids HALITE NaCl Sylvite KCl Sal-ammoniac NH,C1 Cerargyrite AgCl FLtJORITE CaF^ Cryolite SNaFAlFj sic and hydrous haloids Atacamite Cu,(0H)3Cl Carnallite KMgClj-GH^O The haloids comprise chlorids, bromids, iodids, and fluorids, which are salts of HCl, HBr, HI, and HF respectively. Com- paratively few haloids occur in nature, but several of them are very common minerals. The haloid minerals may be assembled in two groups. HALITE, NaCl Form. Halite occurs in crystals, and in cleavable, granular, and fibrous masses. Crystals are isometric, usually cubes (Fig. 424), sometimes hopper-shaped (Fig. 425), rarely in octahedrons or cubo-octahedrons (Fig. 426). Cleavage. The perfect cubic cleavage is a marked feature of halite. A dodecahedral {110} parting is developed by pressure applied on the cube-edges by a hammer or in a vise (Fig. 307, page 106). H.=2^. Sp.gr. + 2.1 Color. Colorless and white. Often reddish, gray, or deep blue in patches. Optical Properties. Isotropic. w = 1.54. Recrystallizes from a water solution in squares, often hopper-shaped (Fig. 427), which are dark between crossed nicols and have low relief in oil of cloves. 9.07 298 INTRODUCTION TO THE STUDY OF MINERALS Chemical Composition. Sodium chlorid, NaCl; (Na = 39.4 per cent.)- Halite may contain MgCl2,MgS04,CaCl2, and CaS04. It is to these impurities that the deliquescence of table salt is due. Blowpipe Tests. Fuses easily (1) giving an intense yellow flame. With CuO in NaPOg bead gives an azure-blue flame. Soluble in water. Fig. 424. Fig. 425. Fig. 426. Fig. 427. Uses. Halite is the chief source of sodium compounds used principally in the manufacture of soap and glass. It is also used extensively as table salt and as a preservative. Salt brines furnish bromin. Salt is obtained (1) directly by the mining of rocksalt as at Petite Anse, Louisiana, and Lyons, Kansas; (2) by pumping brine to surface and evaporating as at Syracuse, New York, and Hutchinson, Kansas; and (3) by solar evaporation as at Great Salt Lake, Utah. Occurrence. 1. Occurs in beds associated with anhydrite, gypsum, and occasionally with other chlorids and sulfates. These deposits are formed by the evaporation of sea-water. Important localities are Stassfurt, Germany; Wieliczka, Poland; Cheshire, England; western New York; Saginaw, Michigan; and central Kansas. Sylvite, KCl Form. Sylvite occurs in cleavable and granular masses and in well-formed cubic or cubo-octahedral (like Fig. 426) crystals. Etch-figures indicate that sylvite belongs to the gyroidal class of the isometric system. HALOIDS 299 Cleavage. Perfect cubic cleavage. Color. Colorless or white. Taste, sharp saline. H. = 2. Sp.gr. ±2.0 Optical Properties. Isotropic. n=1.49(< oil of cloves). Re- crystallizes from water solution in square crystals with a tendency toward skeleton crystals (Fig. 428). Chemical Composition. Potassium chlorid, KCl; (K = 52.4 percent.). It may contain NaCl. Blowpipe Tests. Fuses easily (1^) coloring the flame violet. With CuO in NaPO, bead gives an azure-blue flame. Soluble in water. Uses. Used as a fertilizer and a source of potassium salts. Fig. 428. — Sylvite recrystallized. Fig. 429. — Sal-ammoniac recrystallized. Occurrence. 1. In salt beds with halite, anhydrite, kainite, and carnallite. It is mostly a secondary mineral formed from carnallite (KHgClj-eHjO). Stassfurt, Germany. 2. As a volcanic sublimate on lava. Vesuvius. Sal-ammoniac, NH^Cl Form. A rare mineral occurring as an incrustation. The usual crystals are distorted dodecahedrons. The biting taste is the easiest method of recognizing the mineral. H. = 1J. Sp.gr. + 1.5 Tests. Optically isotropic, n = 1.64. Recrystallizes from water solution in branching crystal aggregates (Fig. 429). Volatilizes 300 INTRODUCTION TO THE STUDY OF MINERALS without fusion. Heated in the closed tube with KOH or CaO (made by heating calcite) it gives off NHg. Soluble in water. Occurrence. 1. Occurs as a volcanic sublimate. Vesuvius. 2. Occurs on coal-mine dumps (heat is due to spontaneous combustion). Leavenworth, Kansas. Cerargyrite, AgCl Form. Cerargyrite usually occurs as a thin crust or seam, but small cubic crystals are also found. H. = li. Sp.gr. ±5.5 Color. Gray, greenish, or violet. Very sectile. Optical Properties. Isotropic. n = 2.06. May be hammered to a thin sheet which is translucent, but dark between crossed nicols. A few drops of NH^OH will give minute octahedral crystals (AgCl). Chemical Composition. Silver chlorid, AgCl; (Ag = 75.3 per cent.). Blowpipe Tests. On charcoal fuses easily (1), giving a silver button. A fragment touched with a NaPOj bead saturated with CuO gives an intense azure-blue flame. Insoluble in acids, but soluble in NH^OH. Uses. An ore of silver in Idaho, Nevada, Mexico, and Chili. The miner's name is "horn-silver." Occurrence. 1. A secondary mineral in the upper part of ore- deposits. It is formed by the action of chlorid-bearing waters on other silver minerals, and therefore is prominent in arid regions. FLUORITE, CaF, Form. Fluorite usually occurs in cleavable masses, but also often in distinct crystals. The crystallization is isometric (hexoctahedral class). Usual forms: a {100}, /{310}, t{421}, d{110),o{lll}. The habit is practically always cubic, the other forms being subordinate. Octahedral crystals are rare. At some localities apparent octahedra are built up of minute cubes HALOIDS 301 in parallel position. Figs. 430 to 433 represent typical crystals of fluorite. Fig. 433 is a penetration twin with the cube-diagonal as twin-axis. Vicinal faces with high indices such as {32-1-0} are often found on these crystals. Cleavage. One of the most important characters of fluorite is the perfect octahedral cleavage. On a cube this will show as triangular faces at the vertices, or at least as cracks in this direc- tion as in Fig. 433. H.=4. Sp.gr. +3.2 Color. Usually some tint or shade of violet, green, or yellow. It is also white, colorless, and pink. The color is probably due to hydrocarbons. Some crystals from Cumberland are green by transmitted light, but blue by reflected light. This property. ~^ r^^ V. aJ l Fig. 430. Fig. 431. Fig. 432. Fig. 433. also possessed by some aniline colors such as red-ink, is known as fluorescence, a name derived from the mineral fluorite. Some varieties of fluorite are also phosphorescent, that is, after being heated, continue to emit light in the dark. Optical Properties. Isotropic. n = 1.43, hence high relief in oil of cloves. Fragments are triangular and dark between crossed nicols. (See Fig. 387.) Chemical Composition. Calcium fluorid G&iF^; (F = 48.9 per cent.). Free fluorin has been detected in some fluorite. Fluor- ite is the only common compound of fluorin occurring in nature. Blowpipe Tests. In the closed tube decrepitates. Fuses (at 3) to an enamel coloring the flame red. Soluble in HjSOi with evolution of HF, which etches glass. Uses. The main use of fluorite is a flux in iron smelting and 302 INTRODUCTION TO THE STUDY OF MINERALS foundry work. Western Kentucky and southern Illinois are the principal sources of fiuorite in this country. Minor uses are the manufacture of enamels, opalescent glass, and hydrofluoric acid. Moissan in his work on fluorin used vessels made of fiuorite. Occurrence. 1. As a vein mineral associated with galena, sphalerite, calcite, and barite. Typical localities are fissure veins in the limestone of western Kentucky and lead mines in the north of England, where magnificent museum specimens are found. 2. In tin-stone veins associated with cassiterite, apatite, topaz, and lepidolite. Zinnwald, Bohemia. 3. In limestones. St. Louis, Missouri. Cryolite, SNaFAlFg Form. Massive and in pseudo-cubic (monoclinic) crystals often in parallel position. Cleavage. Imperfect in three directions at nearly right angles. H.=2i. Sp.gr. +3.0. Color. White, sometimes brown. Translucent. Optical Properties. n^= 1.36. Low relief in water (for water, n = 1 .33) . Double refraction very weak. Fragments are roughly rectangular or irregular. Interference colors are first order gray. Chemical Composition. Sodium aluminum fiuorid, 3NaF- AlFg ; (Al = 12.8 per cent., Na = 32.8). Blowpipe Tests. Easily fusible (1) giving an intense yellow flame. Soluble in HjSO^ with the evolution of HF. Uses. Formerly used as a source of aluminum, but now used as a bath in the electrolytic production of aluminum from bauxite. Also used in the manufacture of sodium and aluminum salts at Natrona, Pennsylvania. The mineral is shipped from Greenland. Occurrence. 1. In granite-pegmatites. The most important locality is Ivigtut, in southern Greenland, where an immense vein-like mass of cryolite occurs in gneiss. At St. Peter's Dome in El Paso County, Colorado, cryolite occurs in veins in granite. HALOIDS 303 Atacamite, Cu2(OH)3Cl Form. Atacamite occurs in crystal aggregates, and in massive and incrusting forms. Crystals are orthorhombic and prismatic in habit. Cleavage in one direction. H.=3to3i. Sp.gr. + 3.7. Color, bright to dark green. Streak, pale green. Optical Properties. ?!,.(1.88) - n„(1.83) = 0.05. Fragments are prismatic with parallel extinction and are green in color. Chemical Composition. Basic cupric chlorid, Cu2(OH)3Cl or ruClj-SCuCOH)^; (H20 = 12.7 per cent.). Blowpipe Tests. The principal blowpipe test is the blue flame given without the use of HCl. Fusible at 3^. In the closed tube gives water which has an acid reaction. Soluble in acids. Occurrence. 1. A secondary mineral found especially in the desert regions of Chili and Lower California. Carnallite, KHgClg-GH^O Form. Massive or granular. Crystals (orthorhombic) are very rare. Cleavage. No cleavage, but has con- choidal fracture. H. = l. Sp.gr. + 1.6. Color. Colorless or reddish. Luster, greasy. Very deliquescent. Optical Properties. w^(1.49) -n„(1.46) = 0.03. Recrystallized from water solu- tion it forms in order (1) isotropic squares ^^^ ^3^ of KCl, (2) rectangular twinned crystals of KMgClj-eHjO, and (3) streaked aggregates of MgCl^ (Fig. 434). Chemical Composition. Hydrous potassium magnesium chlo- rid KMgClg-eHjO or KCl-MgCl^-eH^O; (KCl = 26.8 per cent.) (H2O = 39.0 per cent.). 304 INTRODUCTION TO THE STUDY OF MINERALS Blowpipe Tests. Fusible at 1^, coloring the flame violet. With CuO in NaPOa bead gives an azure-blue flame. Gives abundant water in the closed tube. Soluble in water. Uses. Carnallite is used as a fertilizer and in the manufacture of potassium salts. KCl crystallizes out of a water solution of carnallite. Occurrence. 1. In salt beds associated with anhydrite, halite, sylvite, and kainite. Stassfurt, Prussia, is the only promi- nent locality. 5. OXIDS QUARTZ, SiO, CHALCEDONY, SiO^ ICE, H.O CUPRITE, Cu^O Zincite, ZnO Melaconite, CuO CORUNDUM, A1,0, HEMATITE, Fe,0 CASSITERITE, SnO^ RUTILE, TiO, PYROLUSITE, MnO, Among the oxids are some of the most common and widely distributed minerals. The oxids of silicon are placed first and after them the monoxids RjO and RO, the sesquioxids, RjOa; ^^^^ the dioxids, ROj, in order. The minerals of the spinel group, sometimes considered as double oxids of the type RORjOg, are, in this book, placed in a separate division, the aluminates, ferrites, etc. QUARTZ, SiOj Form. Crystals of quartz are very common, both large and small, loose and attached. There are crystalline aggregates of various kinds as well as massive, granular, and compact varieties. Quartz crystallizes in the trigonal trapezohedral class of the hexagonal system. c = 1.099. Usual forms: r{1011}, zjOlll}, m{10T0), s{1121[, a;{5161}. Interfacial angles: wir(10T0:10Tl) = 38° 13'; rz(10ll:0111)=46° 16'; ms(1010:1121)=37° 58'; ma;(10l0:5161)=12° 1'; mTO(10T0:0lT0) =60° 0'. Figs. 435- 438 represent typical crystals. The habit varies from prismatic to pyramidal. The two rhombohedrons r and z are often in equal 306 INTRODUCTION TO THE STUDY OF MINERALS combination (Figs. 435, 438) and are apparently a hexagonal bipyramid. The s face at alternate vertices proves the trigonal character. Figs. 375 and 376 are more complex (page 152). Cleavage, practically absent (an imperfect cleavage parallel to r is occasionally noticed) . H.=7. Sp.gr. + 2.66. Color, more often white or colorless, but may be any color. Luster, vitreous. Transparent to translucent, rarely opaque. Fig. 435. Fig. 436. Fig. 437. Fig. 438. Optical Properties. n^(l. 553) -n„(1.544) = 0.009. Double refraction rather weak. Fragments are irregular, with low relief in oil of cloves (n>oil of cloves) and upper first order interference colors. In thick basal ( J. to c-axis) sections quartz shows rotary polari- zation, i.e., in monochromatic light a section only becomes dark by rotating one nicol. For red light the angle of rotation is 13° for each millimeter of thickness. Chemical Composition. Silica or silicon dioxid, SiOj. Varia- tions in analyses are due to inclusions such as chlorite, tourma- line, rutile, etc. Blowpipe Tests. Infusible even on the thinnest edges. When fused with an equal volume of soda, it effervesces and gives a colorless glass. Insoluble in a NaPOj bead. 1.93. Fragments are prismatic or irregular, yellow or orange, and non-pleochroic. Chemical Composition. Zinc oxid, ZnO; (Zn = 80.3 per cent.). Usually contains manganese to the extent of about 4 per cent. Blowpipe Tests. Infusible. Gives the bead tests for Mn and the Co(N03)2test for Zn. Soluble in HCl. Uses. A mixture of zincite, franklinite, and willemite mined in Sussex Co., New Jersey, is used as a source of zinc white (ZnO) . Occurrence. 1. Occurs with franklinite and willemite in a crystalline limestone in Sussex Co., New Jersey. This deposit is probably due to the metamorphism of a sedimentary limestone containing calamine with some manganese and iron minerals. Melaconite, CuO Form. Melaconite is found massive or earthy, rarely in crys- talline scales. H. =3 to 4. Sp. gr. +6.0 (variable). Color, black. Luster, dull or earthy. Opaque. Chemical Composition. Cupric oxid, CuO; (Cu = 79.8 per cent.). It is often mixed with MnOj. OXIDS 311 Blowpipe Tests. Infusible. Gives flame and bead tests for copper. Soluble in HNO3 to a green solution. Occurrence. 1. As a secondary mineral in the oxidized zone of copper mines. (Not as common as cuprite.) ■ HEMATITE GROUP— HEXAGONAL Corundum and hematite form a perfect isomorphous group, although intermediate compounds are lacking. With them is sometimes placed ilmenite, but it is more properly considered a ferrous metatitanite, FeTiOj. It belongs to a different crystal class, the trigonal rhombohedral class. CORUNDUM, AI2O3 Form. Corundum is found in rough loose crystals, in cleav- able masses and disseminated through rock in small crystals or grains. The crystals belong to the scalenohedral class of the A= Fig. 445. Fig. 446. Fig. 447. hexagonal system. Usual forms: c{0001j, rjlOll}, a {1120), n{2243}. Interfacial angles^ cr(0001:10Tl) =57°_34',_rr(10Tl : Tl01)=93° 56', cn(0001:2243)=61° 11'; nn (2243 : 4223) =51° 58'. Habit prismatic (Fig. 446), tabular (Fig. 445), and steep pyramidal (Fig. 447). The trigonal character is shown by the 312 INTRODUCTION TO THE STUDY OF MINERALS r faces at alternate vertices and by the triangular striations on the base c. Cleavage. There is often parting parallel to c and r. The rhombohedral parting greatly resembles cubic cleavage {rr = 93° 56'). H.=9. Sp.gr. +4.0 Color. Bluish-gray is the most common color, but brown, red, pink, bright blue, and white colors are not at all rare. Usually translucent. Luster varies from vitreous to adamantine. Optical Properties. w^(1.767) - n„(1.759) = 0.008. Double refraction rather low. Fragments are irregular with low order interference colors and index of refraction greater than methylene iodid. Large deep colored fragments or small crystals are often pleochroic. Chemical Composition. Alumina or aluminum oxid, AljOj,- (Al = 52.9 per cent.) . Emery is a dark-colored mixture of corun- dum with magnetite, hematite, or spinel. Blowpipe Tests. Infusible. Heated with Co(N03)2 solution, it becomes deep blue. Insoluble in acids. Decomposed by fusion with KHSO4. Uses. Certain varieties of corundum are valuable gems. Ruby, the transparent red corundum, is even more valuable than diamond. Sapphire is the blue transparent corundum. The best rubies come from Burma and the best sapphires from Ceylon. Corundum is also used as an abrasive either as the pure cleav- able mineral or as the mixture known as emery. Occurrence. 1. In certain igneous rocks such as syenites and nepheline syenites in which an excess of AI2O3 has crystallized out as corundum, just as an excess of SiOj crystallizes as quartz in granites. At Craigmont, Ontario, corundum occurs in a syenite and is mined as an abrasive. 2. In peridotites along the borders of adjacent rocks. In North Carolina the country rocks are gneisses, but the mode of origin is doubtful. 3. In crystalline limestones (Burma, New York, New Jersey). OXIDS 313 Emery is associated with limestone at Naxos, Greece, and per- haps is the metamorphic equivalent of bauxite. 4. In sands and gravels. The gem-bearing gravels of Ceylon furnish sapphire and other varieties of corundum. HEMATITE, Fe^Oa Form. Hematite is found in a variety of forms: small crystals in cavities, micaceous, fibrous, oolitic, compact, and earthy. Crystals are hexagonal, usually tabular or low rhombohedral in habit (Figs. 448, 449). The island_of Elba furnishes good crystals with the forms m{10T4}, rflOTl} and n(2243} repre- sented in plan by Fig. 450. Fig. 448. Fig 449. Fig. 450. H.=6. Sp.gr. ±5.2 Color. Iron-black to dark red. Streak, brownish-red. Luster, metallic to dull. Opaque, but translucent red in thin scales. These scales are dark between crossed nicols (basal sections). Chemical Composition. Ferric oxid, FejOg; (Fe = 70.0 per cent.) . The iron is sometimes partly replaced by Ti and Mg. Blowpipe Tests. Fusible with difficulty (5^). On charcoal in R.F. becomes magnetic. Gives bead tests for iron. Slowly soluble in concentrated HCl. Uses. Hematite is the principal ore of iron, the Lake Superior district furnishing the principal supply. (See page 476.) Occurrence. 1. As a metasomatic replacement of cherty 314 INTRODUCTION TO THE STUDY OF MINERALS iron carbonate. This origin is assigned to the Lake Superior hematite. 2. As a metasomatic replacement of oolitic limestone produc- ing oolitic hematite, the well known Clinton iron ore of New York and central Alabama. 3. As an alteration product of other iron minerals. The fibrous pencil-ore of England is supposed to be formed by the dehydration of limonite. Martite is a pseudomorph of hema- tite after octahedral crystals of magnetite. 4. As a pigment in many minerals and rocks, giving a red color. The red color of orthoclase, in many cases at least, is due to minute scales of hematite. RUTILE GROUP— TETRAGONAL Cassiterite (SnOj) and rutile TiOj) together with plattnerite (PbOj), polianite (MnOj), zircon (ZrSiO^ or ZrOj-SiOj), and thorite (ThSi04 or ThOj'SiOj) are isomorphous, all being tetrag- onal and dioxids of tetravalent metals. CASSITERITE, SnO^ Form. Cassiterite is found in crystals, crystalline and reni- form masses, pebbles, and grains (stream-tin). Crystals are tetragonal and prismatic or pyramidal in habit. Twins are common. H. = ej. Sp. gr. + 7.0 (very heavy) . Color, black or brown. Luster, adamantine. Optical Properties, n^ = (2.09) -n„(L99) =0.10. Double re- fraction strong. Fragments are irregular with high order inter- ference colors and high relief even in methylene iodid. Some varieties are pleochroic. Chemical Composition. Tin oxid, SnOj; (Sn = 78.6 percent.). Blowpipe Tests. Infusible. Fused with soda, sulfur, and a little powdered charcoal gives a metallic button and a straw- OXIDS 315 colored coating near the assay. The coating heated with Co- (N03)2 solution assumes a bluish-green color. Insoluble in acids. Uses. Cassiterite is practically the only source of tin. The Malay States lead in the production of tin. Occurrence. 1. In tin-stone veins associated with topaz, wol- framite, arsenopyrite, lepidolite, and fluorite, granite being the country rock. Zinnwald, Bohemia, is a prominent locality. 2. In greisen (quartz-muscovite rock) and other rocks affected by the intrusion of pegmatites, but rare in the pegmatites them- selves. 3. In sands and gravels. (Stream-tin). RUTILE, TiOj Form. Rutile is found in embedded grains or crystals, as acicular inclusions or in a massive form. Crystals are tetragonal and usually prismatic in habit. Usual forms: pjlll), e{101}, k^- k>\ Fig. 451. Fia, 4.52. FiQ. 453. Fig. 454. a{100}, m{110}. Interfacial angles: 7)m(lll :100) =47° 40'; ea- (101:100) =57° 13'; ee(101:011) =45° 2'. Figs. 451 to 454 repre- sent various types of twinned crystals with e{ 101) as twin-plane. H.=6-6i. Sp.gr. +4.2.. Color. Red, brownish-red to black. Streak, pale brown. Luster, metallic-adajnantine. 316 INTRODUCTION TO THE STUDY OF MINERALS Optical Properties. n^(2.90) - w„(2.62) =0.28. Fragments are yellow and irregular with high order interference colors and high relief even in methylene iodid. Chemical Composition. Titanium oxid, Ti02; (Ti = 60.0 per cent.). Iron is usually present. Blowpipe Tests. Infusible. Gives a violet NaPOa bead in R.F. Insoluble in acids. Uses. Rutile is used as coloring matter for porcelain and as a source of ferro-titanium. Nelson County, Virginia, is an impor- tant locality. Occurrence. 1. As an accessory constituent of apatite veins occurring in gabbros. Norway. 2. As an accessory constituent of igneous rocks often occurring as acicular inclusions in quartz. 3. As a secondary mineral in various rocks such as gneisses, schists, and clays. The rutile is set free by the decomposition of titanium bearing silicates, especially the pyroxenes. Rutile is also an alteration product of titanite and occurs as a paramorph after brookite (an orthorhombic form of TiOs) . PYROLUSITE, MnOj Form. Pyrolusite occurs in fibrous and columnar forms, in acicular crystals, in crusts, in masses, and along seams in dendritic forms. Crystals are prismatic but indistinct and perha,i)s always pseudomorphous after manganite. H. = 1 to 2 (very soft). Sp. gr. ±4.8. Color, black. Streak, black. Luster, metallic to dull. Opaque. Chemical Composition. Manganese dioxid, MnOj,* (Mn=63.2 per cent.). It usually contains about 2 per cent, of H2O. Blowpipe Tests. Infusible. In closed tube gives little or no water. Gives manganese bead tests. Soluble in HCl with the evolution of ehlorin. OXIDS 317 Uses. Pyrolusite is one of the ores of manganese, but it is usually mixed with psilomelane or manganite. It is also used in the manufacture of chlorin and has other minor uses. Occurrence. 1. In residual clays formed by the decomposition of limestone, there being a concentration of the manganese oxid. In many cases the pyrolusite is due to the dehydration of man- ganite. 2. Along seams of various rocks as a secondary mineral in dendritic forms. 6. ALUMINATES, FERRITES, ETC. SPINEL, MgAl^O, MAGNETITE, FeFe^Oj Franklinite, (Zn,Mn)Fe204 CHROMITE, FeCr^O, Chrysoberyl, BeAljOj These minerals are sometimes considered as oxids, but they may also represent salts of certain unfamiliar acids. Spinel is magnesium metaluminate derived from HAlOjCHgAlOj — HjO). Magnetite is ferrous metaferrite, iron acting both as an acid and as a base. Chromite is ferrous metachromite, derived from HCrO^ (HgCrOj-H^O). SPINEL GROUP— ISOMETRIC All the above enumerated minerals except chrysoberyl belong to the spinel group, which is one of the best known examples of isomorphism, for many intermediate compounds exist. The minerals of this group are isometric and usually crystallize in octahedrons. Besides the minerals mentioned there are also hercynite (FeAljO^), gahnite (ZnAljO^), magnesioferrite (Mg- Fe204), and jacobsite (MnFejOJ. The general formula, then, is: (Mg,Fe,Mn,Zn) ( Al,Fe,Cr,Mn) jO^. The following analyses, which have been recalculated in the present form from the original analyses, illustrate the range and variation in composition. Analyses of Minerals of the Spinel Group Mg Fe" Mn" Zn AhOi FesOj MnsOi CraOj ! Misc. 14.8 14.5 7.8 1.3 3.6 2.9 18.3 21.5 80.2 62.0 72.3 1.3 11.1 14.1 1.8 12.5 75.4 70.0 4.8 7.9 0.6 56.8 62.7 Spinel, picotite. . . Spinel, pleonaste. . Magnetite 0.3 8,? 17.5 27.3 9.6 SiO. — 2 9 Chromite * 8.4 0.1 4.2 * After deducting 3.3% serpentine. 318 ALUMINATES, FERRITES, ETC. 319 SPINEL, MgAljO, Form. Spinel is practically always found in crystals or grains, usually disseminated, but sometimes loose in sands and gravels. Crystals are isometric, the octahedron being the only common form (Fig. 455) . Contact-twins with {111} as twinning plane are so common that this twin-law is known as the spinel law (Fig. 456). H.=8. Sp.gr. + 3.6. Fig. 455. Fig. 456. Color, black and dark shades of gray, brown, and green; also red and blue. Usually translucent. Luster, sub-adamantine. Optical Properties. Isotropic. n = 1.72. Fragments are ir- regular and dark between crossed nicols. The usual color of the fragments is green (pleonaste) and coffee-brown (picotite). Chemical Composition. Magnesium metaluminate, MgAl204 or MgO-AljO,; (Mg=17 per cent.). The magnesium is often replaced by ferrous iron and the aluminum by chromium and ferric iron. The iron-bearing spinel is called pleonaste and the chrome-bearing spinel, picotite. Blowpipe Tests. Infusible, but the color may change on heat- ing. Turns blue when heated with cobalt nitrate solution. Insoluble in hydrochloric and nitric acids. Decomposed by fusion with potassium acid sulfate. Uses. A red variety called spinel-ruby is used as a gem. 320 INTRODUCTION TO THE STUDY OF MINERALS Occurrence. (1) As a contact mineral in crystalline limestone associated with phlogopite, chondrodite, corundum, and graphite. Amity, New York, is a prominent locality. (2) As an accessory mineral in various igneous and meta- morphic rocks. Pleonaste occurs with emery; picotite, with serpentine. (3) In the gem bearing gravels of Ceylon. (Ruby spinel.) MAGNETITE, FeFe^O, (Fe^O,) Form. Magnetite occurs in loose and attached crystals, in compact and granular masses, and in the form of sand. Crystals belong to the hexoctahedral class of the isometric system. The only common forms are the octahedron o, the dodecahedron d, and the trapezohedron m{311}. The habit is octahedral, more rarely dodecahedral, but almost never cubic. Figs. 457, 458, and 459 represent typical crystals. FiQ. 457. Fig. 458. Fm. 459. Cleavage. Some specimens have octahedral parting. H.=6. Sp.gr. + 5.1. Color, black. Opaque. Metallic luster. Strongly attracted by the magnet and sometimes is a magnet itself {lodestone) . Chemical Composition. Ferrous metaferrite, FeFcjO^ or FeO'FejOj (Total iron = 72.4 per cent.)i May contain Mg, Mn, or Ti. Blowpipe Tests. Fusible with difficulty (5J). Gives bead tests for iron. Soluble in concentrated HCl. ALUMINATES, FERRITES, ETC. 321 Uses. Magnetite is an important ore of iron, mined in New York, New Jersey, and Pennsylvania. In Scandinavia it is the principal iron ore. Occurrence. (1) A very common and widely distributed accessory constituent of igneous rocks. (2) In ore-deposits due to magmatic segregation. The Scan- dinavian magnetite has this origin. (3) As a contact mineral between igneous rocks and lime- stones, often occurring with pyrite. (4) In lenses and layers in schists and gneisses. (5) As an alteration product of iron-bearing silicates in more or less altered igneous rocks. (6) As the main constituent of the so-called black sands which are prominent on the Pacific Coast. Franklinite, (Zn,Mii)Fe204 Form. Franklinite occurs in disseminated crystals or in granu- lar aggregates. The crystals are usually octahedrons, modified by the dodecahedron (like Fig. 444, p. 309). H. = 6. Sp.gr. ±5.1. Color, black. Opaque. Luster, metallic. Streak, dark brown. Slightly magnetic. Chemical Composition. Zinc-manganese metaferrite, (Zn,Mn) Pe^O^ or (Zn,Mn)0-Fe203; (ZnO = 17 to 25 percent., MnO=10 to 16 per cent.). Some analyses show ferrous iron and manganic manganese. A typical analysis is given on page 318. Blowpipe Tests. Infusible. In O.F. the borax bead is ame- thyst (Mn) , while in R.F. it is green (Fe) . On charcoal with soda gives a white coating of ZnO and a magnetic residue. Soluble in HCl with the evolution of a little chlorin. Uses. Franklinite mined in Sussex County, New Jersey^ is used for the production of zinc white. The residue is used for the production of spiegeleisen, an iron-manganese alloy. 322 INTRODUCTION TO THE STUDY OF MINERALS Occurrence. 1. In crystalline limestone with willemite, zin- cite, and rhodonite. Sussex County, New Jersey, is practically the only locality for this mineral. This deposit was probably formed by the metamorphism of a sedimentary limestone contain- ing calamine and some manganese mineral. CHROMITE, FeCrjO, Form. Chromite occurs disseminated and in compact masses, rarely in octahedral crystals. H.=5i. Sp.gr. +4.4. Color, black. Streak, dark brown. Luster, submetallic or metallic. Opaque. Optical Properties. Isotropic n>1.93. Thin fragments are irregular, usually translucent brown, and dark between crossed nicols. Chemical Composition. Ferrous metachromite, FeCrjO^ or FeO-CrjOj (Fe = 24.8 per cent.) . Mg, Al, and ferric Fe are usually present. Two typical analyses are given on page 318. Blowpipe Tests. Infusible. Gives chromium bead tests. Fused with soda gives a magnetic mass. Insoluble in acids. Decomposed by soda with the formation of sodium chromate, which is soluble in water. Uses. Chromite is the only source of the salts of chromium such as potassium chromate, potassium dichromate, and lead chromate. Chromite bricks are used as a furnace-lining for certain kinds of smelting. Ferro-chrome is an alloy used in mak- ing chrome-steel. Occurrence. (1) In peridotites as an original constituent. Ore-deposits may be due to magmatic segregation. (2) In serpentine rocks probably derived from chromium- bearing olivine in the process of the serpentinization of peridotite. Chrysoberyl, BeAIjO^ Form. Chrysoberyl usually occurs in distinct orthorhombic crystals wiiich are often pseudohexagonal on account of twinning. ALUMINATES, FERRITES, ETC. 323 Fig. 460 is a drawing of a penetration trilling with slight reen- trant angles. H. = 8J. Sp.gr. +3.7. Color usually green. One variety (alexandrite) is emerald- green, but red by transmitted light. Optical Properties. ?t^(1.75) -n^(1.74) =0.01. Fragments are irregular with first-order interference colors. Fig. 460. Chemical Composition. Beryllium metaluminate, BeAL^O^ or BeOAljOj. (Be = 7.1 per cent.) Blowpipe Tests. Infusible. Heated with cobalt nitrate solu- tion gives a deep blue color. Chrysoberyl, if finely powdered, is soluble in a NaPOj bead. Insoluble in acids. Uses. Several varieties of chrysoberyl are used as gems. Alexandrite, mentioned above, is a valuable gem. True cat's eye is a fibrous variety of chrysoberyl. Occurrence. 1. In granite, gneisses, and schists. 2. In sands and gravels. Ceylon. 7. HYDRO XIDS OPAL, SiO^icH^O Stibiconite, Sbfi^JLjO f Diaspore, Al(OH),Al203 j Gothite, Fe(dH)3Fe203 [ MANGANITE, Mn(OH)3Mn203 LIMONITE, 2Fe(0H)3Fe203 BAUXITE, AljO,xBfi BRUCITE, Mg(0H)2 PSILOMELANE, iMnO^(Ba.,K^)OKjO{?) WAD, MnOj+HjO (impure) The hydroxids or hydrous oxids are in part normal hydroxids such as Mg(0H)2. Others may be derived by subtracting water from the normal compounds. For example, 2Fe(OH)3 — H20 = FejOg-HjO, gothite; 4Fe(OH)3-3H20 = 2Fe203-3H20, limonite. The formulae may be written in various ways. Limonite may be written 2Fe203-3H20, HjFe.Og, Fe.OaCOH)^, or 2Fe.(OH)3-Fe203. OPAL, SiO^xHjO t Form. Opal usually occurs in seams and cavities, but is also disseminated and massive. It is one of the few minerals that never crystallizes. The glassy variety, called hyalite, has a botryoidal or mammillary surface. Fracture, conchoidal. No cleavage. Very brittle. H. = 5i to 6^. Sp. gr. + 2. 1 (very light) . Color, white, colorless, or almost any color. Usually translu- cent. Luster, more or less greasy. Hyalite is vitreous. Optical Properties. Isotropic. n=lA5. Fragments are ir- regular, dark between crossed nicols, and have high relief in oil of cloves with index of refraction less than oil of cloves (Becke test). Opal is often intimately mixed with chalcedony. 324 HYDROXlDS 325 Chemical Composition. Hydrated silica, SiOjxHjO, with water varying from 3 to 12 per cent. The following are typical analyses : Si02 H2O AI2O3 Fe'iOa CaO MgO Waltsch, Bohemia 95.5 91.9 88.7 83.7 3.0 5.8 8.0 11.5 1.4 1.0 0.8 0.2 0.9 Faroe Islands Meronitz, Bohemia 3.6 0.5 1.6 1.5 0.7 Blowpipe Tests. Infusible, but becomes opaque. In the closed tube yields water. Insoluble in the ordinary acids. Soluble in HF and also soluble in KOH. Uses. The opal with play of colors known as precious opal and also the red or fire-opal are well known gems. The best precious opals are found in New South Wales and in Hungary, while fire-opal is found principally in Mexico. Occurrence. 1. As a secondary mineral in cavities and along the seams of igneous rocks. Fire-opal and precious opal occur principally in trachyte, while hyalite occurs in any of the igneous rocks. 2. As siliceous sinter or geyserite formed around the hot springs and geysers. Yellowstone National Park is a prominent locality. 3. As the principal constituent of diatomaceous earth. Dia- toms and radiolaria secrete casts of opal silica. Stibiconite, Sb^O^HjO Form. Stibiconite occurs massive or as a coating. It is never found crystallized, but is sometimes pseudomorphous after stibnite. H.=4to5. Sp.gr. +5.2. Color, pale yellow. Luster, dull. 326 INTRODUCTION TO ^HE STUDY OF MINERALS Optical Properties. w>1.65, n<1.74. Fragments, irregular, pale yellow, mostly isotropic. Chemical Composition. Hydrated antimony tetroxid, SbjO^-HjO; (Sb = 74.5 per cent., H20 = 5.6 per cent.). Blowpipe Tests. Infusible. In the closed tube gives water. Insoluble in HCI. Occurrence. 1. A secondary mineral often found with stibnite and resulting from its oxidation. Diaspore, Al(OH)3Al203 Form. Diaspore usually occurs in bladed crystal aggregates, rarely in distinct orthorhombic crystals. Cleavage, in one direction (010) parallel to the length. H.=6i. Sp.gr. ±3.4. Color, colorless, white, gray, and pale colors. Luster, pearly to vitreous. Optical Properties. w^=1.72. Fragments are prismatic with bright interference colors, parallel extinction, and negative elon- gation. Chemical Composition. Aluminum hydroxid, Al(OH)3-Al203 or AljOj-HjO; (H2O = 15.0 per cent.). Blowpipe Tests. Infusible. When heated with cobalt nitrate solution it becomes blue. In the closed tube decrepitates and gives water at a high temperature. Insoluble in acids. Occurrence. 1. Occurs with corundum or emery and with margarite. Chester, Massachusetts, is the only prominent American locality. Gothite, Fe(OH)3Fe20 Form. Gothite is found in small acicular crystals, in bladed crystal aggregates, and in scaly or fibrous masses. Crystals are orthorhombic, but are usually too minute to decipher. Cleavage, in one direction parallel to the length. HYDROXIDS 327 H. = 5i Sp.gr. +4.3. Color, yellowish-brown to nearly black. Streak, yellowish- brown like that of limonite. Luster. Optical Properties. w>1.93. Thin fragments are prismatic, and translucent brown with parallel extinction. Chemical Composition. Iron hydroxid, re(OH)„-Fe203 or FcjOj-HjO; (H2O = 10.1 per cent.). Usually contains a little manganese. Blowpipe Tests. Fusible with difficulty (5^). In the closed tube turns red and gives off water. Iron bead tests. Soluble in HCl. Uses. As an ore of iron it is classed as brown hematite along with limonite. Occurrence. 1. In iron-ore deposits along with limonite and hematite. 2. As inclusions in various minerals such as feldspars, quartz, etc. MANGANITE, Mii(OH)3Mn203 Form. Manganite is found in prismatic crystals and in colum- nar and fibrous masses. Crystals are orthorhombic, prismatic in habit, and striated parallel to their length. Cleavage, in one direction (010) parallel to the length of the crystal. H. = 4. Sp.gr. ±4.3. Color. Iron-black or dark gray. Streak, dark brown. Luster, submetallic. Opaque. Chemical Composition. Manganese hydroxid, Mn(OH) 3-Mn203 or Mn203-H20; (H2O = 10.3 per cent.). Blowpipe Tests. Infusible. In the closed tube gives water. Bead tests for manganese. Soluble in HCl with the evolution of chlorin. Uses. Manganite is an ore of manganese occurring along with pyrolusite and psilomelane. 19 328 INTRODUCTION TO THE STUDY OF MINERALS Occurrence. 1. As a vein mineral. Ilefeld in the Hartz Mts. is a prominent locality. 2. As a secondary mineral in residual clays associated with psilomelane. ' LIMONITE, 2Fe(OH)3Fe203 Form. Limonite occurs in mammillary, botryoidal, and stalactitic forms and in fibrous, compact, pisolitic, nodular, porous, and earthy masses. It is never crystallized, but is often pseudomorphous after other iron minerals, especially pyrite. H. = 5i. Sp.gr. +3.8. Color, yellow, brown, or black. Streak,__^llowish3brown. Luster, submetallic to dull. Optical Properties. n> 1.93. Fragments are either prismatic and acicular with parallel extinction or irregular and isotropic. They sometimes show a spherulitic cross. Chemical Composition. Ferric hydroxid, 2Fe(OH)3-Fe203 or 2Fe203-3H20; (Fe = 59.8 per cent.; H20 = 14.5 per cent.). Often impure from the presence of manganese oxid, phosphates, clay, sand, and organic matter. Blowpipe Tests. Fusible with difficulty (5+). When heated in R.F. becomes magnetic. In the closed tube turns red and gives water. Iron bead tests. Soluble in HCl. Uses. Limonite if a prominent ore of iron and in the United States ranks next to hematite in importance. Occurrence. 1. As a secondary mineral in veins and ore- deposits formed by the oxidation of pyrite. Constitutes an important part of the gossan or " iron-hat." 2. As a metasomatic replacement of limestone. 3. As bog-iron ore formed by the oxidation of FeH2(C03)2 which is in solution in marshes. 4. As a pigment and stain in various rocks. HYDROXIDS 329 BAUXITE, Al^OgxH^O Form. Bauxite occurs in pisolitic forms, more rarely in clay- like masses, but never in crystals. H. = lto3. Sp.gr. ±2.5. Color, white, yellowish-white, pale red, or brownish-red. Lus- ter, dull and earthy. Optical Properties, n about 1.57. Fragments are irregular, isotropic in part, and have aggregate structure in part. Chemical Composition. Bauxite is probably not a definite mineral, but a mixture of AljOg-HjO, A1203-3H20, and iron hy- droxids with hydrous aluminum silicate. The aluminum varies from 20 to 40 per cent. Some analyses correspond to Al20g-2H20 (Al = 39.1 per cent.). Bauxite usually contains both iron (up to 15 per cent.) and silica (up to 30 per cent.). Blowpipe Tests. Infusible. Heated with cobalt nitrate solu- tion it becomes blue. In the closed tube gives abundant water. Soluble in HCl with difficulty. Uses. Bauxite is now practically the only ore of aluminum. It is mined in Geoi'gia, Alabama, and Arkansas. Occurrence. (1) In beds formed by the decomposition of igneous rocks, in Arkansas from nepheline syenites. 2. In beds formed by thermal springs. The Georgia- Alabama deposits are accounted for in this manner. Brucite, Mg(0H)2 Form. Brucite is occasionally found in crystals, but more often in foliated masses and sometimes in fibrous seams. Crys- tals are hexagonal and tabular in habit with the basal pinacoid and several rhombohedrons. Cleavage, in one direction parallel to (0001). H. = 2i. Sp.gr. + 2.4. Color, white or greenish-white. Luster, pearly or silky. Optical Properties. 71^.(1.58) — n„(1.56) =0.02. Cleavage flakes 330. INTRODUCTION TO THE STUDY OF MINERALS give a positive uniaxial interference figure without rings unless very thick. The fibrous variety gives acicular fragments with parallel extinction and negative elongation. Chemical Composition. Magnesium hydroxid, Mg(0H)2 or MgOHjO; (H2O = 31.0 per cent.). Often contains Fe and Mn. The manganese is due to the isomorphous replacement of Mn(0H)2. Blowpipe Tests. Infusible, but glows. Heated with cobalt nitrate solution it turns pink. In the closed tube yields water and becomes opaque. Soluble in HCl. Occurrence. 1. As a secondary mineral in serpentine often associated with magnesite and dolomite. Texas, Pennsylvania, and Hoboken, New Jersey, are prominent localities. PSILOMELANE, 4Mn02(Ba,K2)OH20? Form. Psilomelane is found in reniform, botryoidal, and mammillary forms. It is also compact massive and is one of the few minerals that never crystallizes. H.=5i. Sp.gr. ±4.2. Color, black. Streak, brownish-black. Luster, submetallic to dull. Opaque. Chemical Composition. Impure hydrous manganese dioxid, perhaps 4Mn02-(Ba,K2)0-H20.; (MnOj = 70 to 90 per cent; H20 = 3 to 9 per cent.). It usually contains barium and potas- sium and sometimes lithium. Blowpipe Tests. Infusible. In the closed tube gives water and also oxygen. Manganese bead tests. Soluble in HCl with the evolution of chlorin. Uses. Psilomelane is an important ore of manganese and is also used as a source of chlorin. Occurrence. 1. As a secondary mineral in residual clays. Batesville, Arkansas. HYDROXIDS 331 WAD, MnOj + HjO (impure) Form. Wad occurs in compact layers or in earthy, more or less porous, masses. It has never been found crystallized. H. = lto3. Sp. gr. 3 to 4.5. Color, brown to black. Streak, brown. Luster, dull. Chemical Composition. Impure hydrous manganese dioxid; (HjO^ 10 to 20 per cent.). It usually contains Fe and may also contain Cu, Co, Li, or Ba. Blowpipe Tests. Infusible. Gives water in the closed tube and also oxygen. Manganese bead tests. Soluble in HCl with the evolution of chlorin. Uses. Wad is used as a paint. Cobalt-bearing wad from New Caledonia is used as a source of cobalt. Occurrence. (1) As a secondary mineral due to the weather- ing of other manganese minerals. (2) In bog deposits, often associated with limonite. 8. CARBONATES A. . Normal Anhyd- rOUS Carbonates CALCITE, CaC03 Calcite Group DOLOMITE, Ankerite, MAGNESITE, SIDERITE, CaMg(C03), Ca(Mg,Fe)C03 MgCO, FeC03 RHODOCHROSITE, ■ivrTiC03 SMITHSONITE, ZnC03 ' ARAGONITE, CaCO, Aragonite Group Strontianite, Witherite, CERUSSITE, SrCOj BaC03 PbCOj B. Acic i, Basic, and Hydrous Carhojiates MAL71 AZTJI Hydro Trona Hydrc LCHITE JTE zincite magnesite Cu,(0H),C03 CU3(0H),(C03), Zn3(OH)4C03 HNa3H(C03),-2H20 Mg,(0H),(C03)3-3H,0 The carbonates are not many in number, but they include some of the most common minerals with which the mineralogist has to deal. All the important normal carbonates fall into two well-defined isomorphous groups, the calcite group (rhombohe- dral) and the aragonite group (orthorhombic) . These two groups are said to be isodimorphous, as calcite and aragonite are dimor- phous. CALCITE GROUP— HEXAGONAL The calcite group of rhombohedral carbonates is a well characterized group of familiar minerals. These minerals crystallize in rhombohedral and scalenohedral crystals with 332 CARBONATES 333 cleavage parallel to the faces of a rhombohedron of about 75° All except dolomite belong to the ditrigonal scalenohedral class of the hexagonal system. Dolomite belongs to the rhombohe- dral class, but is similar to the other minerals in angles and other properties. All the minerals of this group are uniaxial and opti- cally negative and have very strong double refraction. Many isomorphous mixtures are known and some of them have special names (see mesitite below). The following analyses are representative of the minerals mentioned and illustrate isomorphism. These analyses are recorded in the form of the metals and the carbonate radical, CO3, instead of the usual method (oxids of the metals and COj). Analyses of Minerals of the Calci te Grc up Ca Mg Fe" Mn Zn COa Misc. 40.0 34.8 21.7 21.2 20.3 0.5 12.0 8.9 6.1 29.2 19.1 16.3 1.9 0.2 0.3 0.3 1.3 7.7 13.4 12.. 5 20.7 46.6 36.7 29.7 0.3 0.1 59.5 55.6 64.8 61.1 60.3 69.6 66.9 62.9 51.8 52.5 35.1 52.0 47.3 H2O-0.1 Calcite 5.3 0.4 HjO-O.S Insol. =0.2 Dolomite, (Brown-spar) 0.1 H20-1.4 Magnesite (Breunnerite) 1.4 0.1 0,1 0.4 0.1 0.4 0.3 H?0-1.2 Siderite . . Siderite Siderite. (Clay-ironstone) . . . Rhodochrosite Smithsonite 1.4 8.2 2.8 46.4 51.3 * t 'Si02 = 12.3, Al203=3.2, H20 = 16 tCd-0.7, S=0.1, SiO2 = 0.1. CALCITE, CaCOj Calcite has played a very prominent part in the history of mineralogy. The discovery of cleavage in calcite led to the establishment of crystallography as an exact science by the Abb6 Haiiy, and the discovery of double refraction in calcite opened up the whole subject of crystal optics. The invention of 334 INTRODUCTION TO THE STUDY OF MINERALS the Nicol prism, which is made of calcite, has made possible the identification of the fine-grained rocks. Form. Calcite is found in well defined crystals, which are often large in size, in crystalline crusts and druses, in cleavable masses, in various imitative forms, such as stalactitic, pisolitic and oolitic, in granular masses, and sometimes in fibrous forms. Calcite is the type example of the ditrigonal scalenohedral class of the hexagonal system. In number of forms and variety of their combinations, calcite is unsurpassed among minerals. Over 300 well established forms, most of them rhombohedrons and scalenohedrons, are known. c = 0.854. Usual forms (in order Fig. 461. Fig. 462. Fig. 463. Fig. 464. of tjieir abundance): _to{ 1010}, _c{0001h e{0112}, /{0221J, rjlOTl}, M{4041}, j;{2m}, a{1120}, 2/{3251_}, <{2J34}. Inter- facial_an_gles: ee(0112:1012) =45° 3^; em{0112:1010} =63^ 45'; rr(1011:1101)=74° 55;; rm(1011 : 1010) =45°_23J';//(0221 :2021) = 101° 9';/m(0221:0lT0)=26° 53^; MM(4041 : 4401) =114° 10'; Mm(4041:1010) = 14°_13';_i;i;(2131:2311)=75° 22'; ot(2131: 3121) =35° 36'; ot(2131: 1231)_=47^1'; o(10Tl:2131) =29° l^'; TOt;(1010: 2131) =28°_4'j^ 2/2/(3251: 5231) =45° 32'jj/!/(3251 :3521) = 70° _59'; |i(2134:2314)=4P 55',;_ «(2134:3124) =20° 36^'; OTm(1010:0110) =60° 0'; ma(1010: 1120) =30° 0'. Figs. 461-480 represent typical calcite crystals. As can be seen, the habit is variable. Figs. 461-464 are simple forms. Fig. 461, e{0112}, is an obtuse rhombohedron while Fig. 462, CARBONATES 335 Fig. 469. Fio. 470. Fio. 471. Fig. 472. Fig. 477. Fig. 478. FiQ. 479. Figs. 465-480. — Caloite Crystals. Fig. 480. 336 INTRODUCTION TO THE STUDY OF MINERALS /{0221), is an acute rhombohedron. Fig. 464 is a pseudo-cubic rhombohedron A {0332} with the angle M(0332:3302) =91° 42'. The dotted lines in each case represent the cleavage which is a great help in orienting a crystal. The unit rhombohedron r{1011} alone is rare, but it is very frequently the dominant form as in Figs. 465 and 466. The combination em (Figs. 157 and 158) is said to be the most frequent combination. Figs. 469- 472 are common types. The bottom of Fig. 470 represents cleavage. The faces of e{0112} are very often striated as in Fig. 472. Figs. 473 to 476 represent plans of common types of calcite crystals. The trigonal symmetry is very apparent. Four twinning laws are known for calcite. (1) {0001} as twin-plane. Fig. 477 represents a scalenohedron twinned accord- ing to this law. (2) {0112} as twin-plane. This is often poly- synthetic twinning with striations parallel to the long diagonal (Fig. 478). (3)J10Tl) as twin-plane. Fig. 479 represents the combination { 1010 j , {0lT2} twinned according to this law. The vertical axes of the two parts of the crystal are almost at right angles. (4) {0221} as twin-plane. This, the rarest type of all, is represented by Fig. 480. Cleavage, perfect rhombohedral in three directions at angles of 74° 55'. There is often parting parallel to {0112} and this is sometimes better developed than the cleavage itself. H.=3. Sp.gr. ±2.72 Color. Usually colorless, white, or amber, but may be any color. Luster, vitreous. Optical Properties. n^(1.658) - ?i„(1.486) =0.172. The strong double refraction is one of the most prominent characters of calcite. It may be observed in Iceland spar, the clear trans- parent cleavable variety. Fragments are rhombic (Fig. 481) with symmetrical extinction and very high order interference colors. The rhombs often have striations parallel to the long diagonal. These are due to polysynthetie twinning produced by pounding the fragments. The relief varies with the direction. As shown in Fig. 481, the rhombs have a high relief when the long CARBONATES 337 diagonal is parallel to the vibration plane of the lower nicol. It gives the microchemical gypsum test with dilute H2SO4 (Fig. 387, p. 198). Chemical Composition. Calcium carbonate, CaCO,; (Ca = 40 per cent.). The common replacing elements are Fe, Mg, and Mn. Clay, sand, bitumen, and other mechanical impurities may be present. Blowpipe Tests. Infusible, glows, and gives a yellowish-red flame coloration. In the closed tube whitens and gives off COj, leaving a residue of CaO. Easily soluble in large fragments in cold dilute HCl with vigorous effervescence. In concentrated solutions HjSO^ gives a white crj'stalline precipitate. Uses. Limestones are extensively used for building and ornamental stones, in the manufacture of cement, as ballast and road material and as a flux in smelt- p,,,. ^si.-caidte fragment, ing. Iceland spar is used in optical ap- paratus, especially the polarizing microscope. Occurrence. 1. As a vein mineral, often forming the gangue of ores. The north of England furnishes fine crystallized speci- mens of calcite. 2. As travertine, calcareous tufa, and cave-deposits (stalactites and stalagmites). Calcium carbonate is soluble in carbonated water, the compound CaH2(C03)2 being formed. On the escape of CO2, due to release of pressure, calcite crystallizes out. 3. As a biogenic mineral forming limestones, organisms such as molluscs, brachiopods, corals, and crinoids contributing their shells and other hard parts. 4. As a secondary mineral in cavities of the basic igneous rocks, especially basalt, often associated with the zeolites. Iceland spar occurs in large cavities in basalt in Iceland. 5. As a secondary mineral in seams and cavities of sedimen- tary rocks, especially limestone. In the Joplin district fine 338 INTRODUCTION TO THE STUDY OF MINERALS amber colored calcite crystals occur in chert breccia in the zinc mines along underground water courses. Crystal Cave at Joplin is completely lined with calcite crystals from 1 to 2 feet in length. It is probable that the calcite was formed when the cave was completely filled with a water solution of calcium carbonate. 6. As the principal constituent of crystalline limestones, which were formed from sedimentary limestones by metamorphism. The crystalline limestones often contain diopside, tremolite, woUastonite, garnet, spinel, graphite, etc. DOLOMITE, CaMg(C03)2 Form. Dolomite is found in crystals, in crystal druses, in cleavable masses, and in granular masses. The crystals belong to the rhombohedral class of the hexagonal system and have a lower grade of symmetry than calcite crystals though this is not often apparent on inspection. The only com- mon kind of dolomite crystals is the simple unit rhombohedron. Fig. 482. Fig. 483. Fig. 484. Fig. 485. often curved and more or less saddle-shaped (Fig. 482). Fig. 483 is a rhombohedron modified by cjOOOl} and M{4041}. Crystals like Fig. 484 with c{0001} and M{4041} are found em- bedded in anhydrite and gypsum. Several twinning laws are known for dolomite, but the onlj^ common one is polysynthetic twinning with {0221} as twin-plane which gives rise to striations on the cleavage faces parallel to both the short diagonal and the long diagonal of the rhomb (Fig. 485) . This fact can often be used to distinguish dolomite from calcite. Cleavage, rhombohedral like calcite. CARBONATES 339 H. = 3ito4. Sp.gr. =2.8 Color, white, pink or gray, but rarely colorless. Luster, pearly or vitreous. It is often called pearl spar. Optical Properties. n^(1.682)-n„(l. 503) =0.179. Double re- fraction very strong. Fragments are rhombic with symmetrical extinction and very high order interference colors. The relief varies with the direction as in calcite. Gives the microchemical gypsum test with dilute HjSO^. In fragments it is distinguished from calcite by the absence of striations parallel to the long diagonal. Chemical Composition. Calcium magnesium carbonate, CaMgCCOj)^; (Ca = 21.6 per cent; Mg=13.1 percent.). It often contains iron and thus grades into ankerite. Analyses are given on page 333. Dolomite is a double salt with equal molecular quantities of CaCOj and MgCOg, and not an isomorphous mixture of these two compounds. Blowpipe Tests. Infusible and colors the flame yellowish red. Large fragments are only slightly attacked by cold dilute HCl. Dolomite can thus be distinguished from calcite. In concen- trated solutions H2SO4 gives a white crystalline precipitate. Uses. Dolomitic limestones, like ordinary limestones, are used for building and ornamental purposes and also for furnace linings. Occurrence. 1. As a secondary mineral in cavities of lime- stones. The pink dolomite of the Joplin district is a prominent example of this occurrence. 2. As the essential constituent of dolomitic or.magnesian lime- stones. These limestones are formed from ordinary limestones by the process known as dolomitization. This consists of the partial replacement of calcium carbonate by magnesium carbon- ate in a way not fully understood. As there is a shrinkage of about 10 per cent., these limestones are often porous. 3. As a vein mineral often associated with calcite as in the north of England. 340 INTRODUCTION TO THE STUDY OF MINERALS 4. As the principal constituent of the crystalline dolomitic limestones. These consist either of dolomite or of a mixture of dolomite and calcite. Other characteristic minerals are tremo- lite, phlogopite, chondrodite, olivine, spinel, serpentine, and talc. Serpentine and talc are secondary minerals formed from the other silicates. Ankerite, Ca(Mg,Fe)(C03)2 Form. Ankerite occurs in cleavable masses, rarely in small rhombohedral crystals resembling dolomite. Cleavage, rhombohedral. H = 3ito4. Sp.gr. ±3.0. Color, white to gray or brown. Optical Properties. Similar to calcite and dolomite. Frag- ments are rhombic with symmetrical extinction and very high order interference colors. Gives the microchemical gypsum test with dilute HjSO^. Chemical Composition. An isomorphous mixture of calcium, magnesium, and ferrous carbonates. Ca(Mg,Fe)C03. It often con- tains manganese. Some analyses approximate 2CaC03-MgC03- FeC03, which has been called normal ankerite. Blowpipe Tests. Infusible. In the closed tube darkens. Becomes magnetic when heated on charcoal. Soluble in hot HCl with effervescence. Occurrence. 1. As a vein mineral, especially with the iron ores. Antwerp, New York. MAGNESITE, MgC03 Form. Magnesite occurs in cleavable or compact porcelain- like masses. It sometimes has a botryoidal surface, but is very rarely found in crystals. Cleavage. Rhombohedral cleavage is sometimes prominent. Color, white or gray. Luster, vitreous to dull. CARBONATES 341 H.=4to5A. Sp.gr. ±3.1. Optical Properties. n^(l. 717) -n^(l. 515) =0.202. Fragments are rhombic with symmetrical extinction or irregular with aggre- gate structure. The interference colors are very high order. Chemical Composition. Magnesium carbonate, MgCOj; (Mg = 28.8 per cent.). Iron and calcium are often present. The mas- sive varieties often contain magnesium silicate. Blowpipe Tests. Infusible. Turns pink when heated with cobalt nitrate solution. Soluble in hot HCl with effervescence. Uses. Calcined magnesite is made into bricks for furnace lining. The carbon dioxid formed is liquefied and forms a valu- able by-product. Magnesite is also used in paper-making and has other minor uses. Occurrence. 1. In veins in serpentine (compact massive variety) resulting from the action of carbonated waters on olivine or on serpentine. Porterville, California and Red Mt. near Livermore, California. 2. In talc schists (cleavable variety) at several localities in the Alps. SIDERITE, FeCOj Form. Siderite is found in small crystals in cavities, in cleav- able masses, in botryoidal crusts, and in compact masses. The crystals are varied in habit, the com- mon forms being the unit rhombohedron (10Tl:Tl01 =73° 2i'), the rhombohedron (0lT2|, and the rhombohedron {0221}, the latter often modified by the pinacoid {0001}. Fig.' 486. Fig. 486 is the unit rhombohedronr{10Tl}. Cleavage, rhombohedral. H.=3ito4. Sp.gr. ±3.8. Color, various tints and shades of brown and gray. Optical Properties. n^(1.87) -n^(1.63) =0.24. Strong double refraction. Fragments are rhombic with symmetrical extinc- 342 INTRODUCTION TO THE STUDY OF MINERALS tion and very high order interference colors. In methylene iodid the relief changes with the direction, but in both posi- tions the index of refraction is greater than that of methylene iodid. Chemical Composition. Ferrous carbonate, FeCOg; (Fe = 48.3 per cent.). Ca, Mg, and Mn are usually present in small amounts as replacing elements. Clay-ironstone is an impure massive siderite containing argillaceous material (see analysis, p. 333). Blowpipe Tests. Fuses with difficulty. In the closed tube darkens. Heated on charcoal it becomes magnetic. Soluble in hot HCl with effervescence. The solution gives tests for ferrous iron. Uses. Siderite is one of the minor ores of iron. The clay- ironstone variety has been mined extensively in England and to some extent in Ohio, Pennsylvania, and Maryland. Occurrence. 1. As a vein mineral. 2. As clay-ironstone concretions in shales. 3. As a secondary mineral in cavities of basalt. This variety is called spherosiderite as it is botryoidal. RHODOCHROSITE, MnCOj Form. Rhodochrosite occurs in rhombohedral crystals and in cleavable masses. The unit rhombohedron with (lOTl :Tl01) =73° 0' is the only common kind of crystal (Fig. 487). Cleavage, rhombohedral. H.=4. Sp.gr. ±3.5. Color, pink, red or brownish-red. ^'°- ^^7. Optical- Properties. ?i^(1.82) -n„(1.60) = 0.22. Double re- fraction strong. Fragments are rhombic with symmetrical extinction and have very high order interference colors. Chemical Composition. Manganous carbonate, MnCOg ; (Mn = 47.8 per cent.). Calcium and iron are usually present. CARBONATES 343 Blowpipe Tests. Infusible. In the closed tube it darkens and decrepitates. The borax bead in O.F. is amethyst color. Soluble in warm HCI with effervescence. Uses. Rhodochrosite is sometimes an ore of manganese. Occurrence. 1. As a vein mineral. At Butte, Montana, it is the gangue of silver ores. 2. As a secondary mineral produced from rhodonite (Camp Bird Mine, Telluride, Colorado) and manganese phosphates ( Branch ville, Connecticut). SMITHSONITE, ZnCOj Form. Smithsonite usually occurs in mammillary or botry- oidal incrusting forms and in porous masses. Crystals of smithsonite are comparatively rare and as a rule are much rounded. Cleavage, imperfect rhombohedral and often curved. H. = 5. Sp.gr. +4.4. Color, white, gray, yellow; sometimes blue or green. Optical Properties. Strong double refraction. Fragments are rhombic with symmetrical extinction and very high order inter- ference colors. Index of refraction is greater than methylene iodid. Chemical Composition. Zinc carbonate, ZnCOjj (Zn = 52.1 per cent.). Blowpipe Tests. Infusible. Heated with cobalt nitrate solu- tion on charcoal gives a green coating. In the closed tube turns yellow. Soluble in HCI with effervescence. Uses. Smithsonite is one of the ores of zinc and is often asso- ciated with calamine, the silicate. It is called "dry-bone" in the Wisconsin-Illinois-Iowa zinc district. Occurrence. 1. As a secondary mineral formed from sphaler- ]•+« T+ niar> ^""inva Qp a mf>tsiRr»Tnntif< Tpnlacement of limestone, 344 INTRODUCTION TO THE STUDY OF MINERALS and is often pseudomorphous after calcite and dolomite. Marion County, Arkansas, is a prominent locality. ARAGONITE GROUP— ORTHORHOMBIC The aragonite group is another isomorphous group, though not so well defined as the calcite group. The crystals are ortho- rhombic, but pseudohexagonal (110: 110 = 62° — 64°) and are usually prismatic in habit. Twinning on the unit prism {110} is very common and also accounts for the pseudohexagonal char- acter of some crystals. Optically the minerals are biaxial with a small axial angle. The double refraction is very strong as in the calcite group. ARAGONITE, CaCOj Form. Aragonite occurs in crystals, in columnar and fibrous masses and in incrusting and stalactitic forms. Fibrous masses are especially common for aragonite, but rare for calcite. m Fig. 488. Fig. 489. Fig. 490. Fig. 491. Fig. 492. The crystals are usually prismatic or acicular in habit, and belong to the bipyramidal class of the orthorhombic system. Usual forms: ??j{110}, 6{010), fcjOll}. Interfacial angles: mTO(110:lT0)=63° 48'; m6(110:010) =58° 6'; 6fc(010:0in = CARBONATES 345 54° 13|'. Figs. 488 and 489 are drawings of typical crystals. The pseudohexagonal character can be seen from Fig. 490. Twins with m{110} as twin-plane are common. Fig. 491 is a contact twin and Fig. 492, a penetration trilling with slight reentrant angles. Cleavage, imperfect parallel to the length of the crystals (010 face). Calcite has cleavage oblique to the length of the crystals. H.=3i. Sp.gr. ±2.9. Color, colorless, white or amber. Luster, vitreous, faint resinous on fracture. Optical Properties. n^= (1.68) -?z^(l. 53) =0.15. Double re- fraction very strong. Fragments are prismatic with parallel extinction, negative elongation, and very high order interference colors. Gives the microchemical gypsum test with dilute HjSO^. Chemical Composition. Calcium carbonate, CaCOg; (Ca = 40.0 per cent.). It often contains a small amount of SrCOj. Blowpipe Tests. Infusible, but becomes opaque and falls to pieces when heated. Soluble in cold dilute acids with effervescence. In a concen- trated solution dilute HjSO^ gives a crystalline precipitate of CaS04-2H20. Finely powdered aragonite (also strontianite and witherite) heated in a test-tube with cobalt nitrate solution be- comes lilac colored, while calcite is practically unchanged. Occurrence. 1. As a secondary mineral in basic igneous rocks such as basalt. 2. As a secondary mineral in seams and cavities of limestones along with calcite. 3. In clays and marls along with gypsum. Prominent localities are Girgenti in Sicily and the Pyrenees. 4. As a hot-spring deposit. Carlsbad, Bohemia. Aragonite forms above 30° C. as a crust in a tea-kettle. 5. As the mother-of-pearl layer of mollusc shells. 6. As a secondary mineral in serpentine, produced by the decomposition of the pyroxene of the original peridotite, 346 INTRODUCTION TO THE STUDY OF MINERALS Strontianite, SrCOg Form. Strontianite occurs in acioular crystals and also in columnar and fibrous masses. The crystals are pseudohexago- nal orthorhombic and resemble those of aragonite. H.=3ito4. Sp.gr. +3.7. Color, colorless, white or pale colors. Optical Properties, n/1.67) -n„(1.52) =0.15. Like those of aragonite, but gives faint test, if any, for microchemical gypsum. Chemical Composition. Strontium carbonate, SrCOg; (Sr = 59.3 per cent.). Usually contains some CaC03, which may be detected by the microchemical gypsum test. Blowpipe Tests. Infusible but swells up and gives a crimson flame when heated after moistening with HCl. Soluble in HCl with effervescence. In dilute solutions, II2SO4 gives a finely divided white precipitate which distinguishes strontianite from aragonite. Uses. The strontium hydroxid used in sugar refining is made from strontianite. Occurrence. 1. As a secondary mineral in veins along with celestite and barite. 2. In veins in calcareous marl. Hamm in Westphalia, Germany, is a prominent locality. Witherite, BaCOj Form. Witherite occurs in granular or columnar masses in crystalline druses, and in distinct crystals. The crystals are pseudo- hexagonal twins of pyramidal habit often re- F 493 sembling quartz crystals. (See Fig. 493.) H.=3i. Sp.gr. +4.3. Color, white or gray. Luster, faint resinous or vitreous. Optical Properties. n^(1.67) -n„(1.52) = 0.15. The optical properties are like those of aragonite. CARBONATES 347 Chemical Composition. Barium carbonate, BaCOg) (Ba = 69.6 per cent.). Blowpipe Tests. Easily fusible giving a yellowisli-green flame. Soluble in HCl with effervescence. In dilute solution H2SO4 gives a finely divided white precipitate. Occurrence. 1. In veins with galena. Barite is a secondary mineral. The North of England is the only prominent locality for witherite. CERUSSITE, PbCOg Form. Cerussite occurs in fibrous and reticulated forms, in compact masses, and often in crystals which are orthorhombic and pseudo-hexagonal. The habit is usually parallel to the side pinacoid 6{010) (Fig. 495) or pyramidal with p{lll) and i{021) in about equal development (Fig. 494) . In these figures m { 110 } , aflOOi, rjl30|, klOll}. Fig. 494. Fig. 495. Fig. 496. Twinning (m{110} as twin-plane) is common both as simple contact twins like Fig. 496 and as interpenetrant twins like Fig. 497. H.=3i. Sp.gr. +6.5. Color, colorless, white, gray, and other pale colors. Luster, adamantine. Optical Properties. n^(2.07) -n„(1.80) =0.27. Double refrac- tion very strong. Fragments are mostly irregular with very high 348 INTRODUCTION TO THE STUDY OF MINERALS order interference colors and index of refraction greater than methylene iodid. With HNO3, octahedral crystals of lead ni- trate form. Chemical Composition. Lead carbonate, PbCOg; (Pb = 77.5 per cent.). Blowpipe Tests. Fuses easily. In the closed tube decrepitates and becomes yellow. Heated on charcoal in R.F. it gives a metal- lic button and a yellow coating. Soluble in HNO3 with effervescence. Soluble in hot HCl, but on cooling needle crystals of adamantine luster (PbClj) separate out. Uses. Cerussite is an ore of lead and sometimes is argentiferous. Occurrence. 1. As a secondary mineral derived from galena. It occurs especially in the gossan of ore-deposits. Prominent localities are Broken Hill, New South Wales, and Coeur d'Alene district, Idaho. In the Joplin district cerussite pseudomorphs after galena are found. MALACHITE, Cu2(OH)C03 Form. For malachite the characteristic occurrences are mammillary crusts, fibrous masses, and acicular crystals. The crystals are monoclinic, but are usually very small and indistinct. H.=3Jto4. Sp.gr. ±3.9. Color, emerald green. Optical Properties. w^=1.88. Fragments are prismatic with oblique extinction (23°). Interference colors are masked by the green color of the mineral. Arrowhead twins like gypsum are common among the fragments. Chemical Composition. Basic copper carbonate, Cu2(OH)C03 or CuCOg-CuCOH)^; (Cu = 57.4 per cent.; H20 = 8.1 per cent.). Blowpipe Tests. Easily fusible (3) giving a green flame which is made blue by HCl. In the closed tube it turns black and gives off H2O. Soluble in acids with effervescence. 349 Uses. Malachite is sometimes an ore of copper, together with azurite, cuprite, and chrysocolla which are collectively called oxidized ores. Polished malachite with a concentric fibrous structure is used as an ornamental stone. Occurrence. 1. As a secondary mineral in the upper oxidized zone of copper deposits. It constitutes the so-called "copper- stain." Malachite is often associated with azurite and is some- times pseudomorphous after it. Bisbee, Arizona, is a prominent locality. AZURITE, CUj (OH), (003)2 Form. Azurite occurs in crystals in crystalline coatings and nodular groups of crystals. Crystals are monoclinic, prismatic class, and are usually short prismatic or tabular in habit, often highly modified. The best specimens come from Chessy in France, hence chessylite, the French name for azurite. H.=4. Sp.gr. ±3.8. Color, deep azure blue. Optical Properties. n>1.83. Fragments are irregular, blue in color, but not pleochroic. Interference colors are masked. Chemical Composition. Basic copper carbonate, Cu3(OH)2- (003)2 or 2CuC03-Cu(OH)2; (Cu = 55.2 per cent., H20 = 5.2 per cent.). Blowpipe Tests. The same as for malachite. Uses. Azurite is one of the so-called oxidized copper ores. Occurrence. 1. As a secondary mineral in the oxidized zone. It is usually associated with malachite and has been formed after the malachite. Bisbee, Arizona, is a prominent locality. Hydrozincite, Zn3(OH)^C03 Form. The characteristic occurrence of hydrozincite is in concentric crusts. It also occurs in stalactites and in earthy masses but has never been found in crystals. H. = 2J. Sp.gr. ±3.7. 350 INTRODUC Color, white. Luster, dull. Optical Properties. n>1.74. Fragments show aggregate structure (often banded) and give bright interference colors. Chemical Composition. Basic zinc carbonate, Zn3(OH)4C03 or ZnC03-2Zn(OH) 2 ; (Zn = 60.8 per cent. ; HjO = 11. 1 per cent.) Blowpipe Tests. Infusible. In the closed tube gives off water and turns yellow. On charcoal with cobalt nitrate solution it turns bright green. Soluble in HCl with effervescence. Occurrence. 1. As a secondary mineral resulting from the decomposition of other zinc minerals. Often occurs in layers with smithsonite or calamine. Santander, Spain is a prominent locality. Trona, Na3H(C03)2-2H20 Form. Trona occurs in crystals and crystalline crusts. The crystals are monoclinic and tabular in habit. H.=2ito3. Sp.gr. ±2.1. Color, white or colorless. Optical Properties. n^=1.51. Recrys- tallizes from water solution in rosettes. (Fig. 498.) Chemical Composition. Hydrous sodium acid carbonate, Na3H(C03)2-2H20 or NaC03-NaHC03-2H20; (H20 = 19.9 per cent.). Blowpipe Tests. Easily fusible (IJ) giv- ing an intense yellow flame. On gentle heating in the closed tube gives off water, but does not melt. Soluble in water. Effervesces with HCl. Occurrence. 1. Trona is formed by the evaporation of the water in soda lakes. Owens Lake, Inj^o County, California, is a prominent locality. Fig. 498. Trona recrystallized. CARBONATES 351 Hydromagnesite, Mg,(OH),(C03)3 SHjO Form. Hydromagnesite occurs in crystalline crusts, thin seams, and earthy chalk-like masses. Crystals, which are very small, are usually like Fig. 499, and are either orthorhombic or monoclinic with /3 = 90°. H. = lto3. Sp.gr. ±2.1. Color, white. Luster, often pearly. Optical Properties. n^=1.53. Fragments and crystals are prismatic with parallel e.xtinction, negative elongation, and low- order interference colors. Chemical Composition. Hydrous basic magnesium carbonate, Mg4(OH)2(C03)3 SHjO 01 3MgC03-Mg(OH)2-3H20; {Ti.fi = 19.8 per cent.). Blowpipe Tests. Infusible. In the closed tube gives off water. Soluble in HCl with effervescence. Occurrence. 1. As a secondary mineral in serpentine often with brucite as an asso- ciate. Alameda County, California. 2. As the product of the dedolomitization (see glossary) of magnesian limestone. Predazzite from Tyrol is a metamorphic limestone consisting of calcite and hydromagnesite. A large deposit of hydromagnesite occurs at Atlin, British Columbia. This deposit is at least five feet thick and is probably a spring deposit. Fig. 499. 9. PHOSPHATES, ARSENATES, ETC. A. Normal Anhydrous Phosphates, etc. Columbite, (Fe.Mn) (Nb.Ta) ^Oe Monazite, (Ce,La)POj Triphylite, Li(Fe,Mn)PO^ f APATITE, Ca„(F„Cl,,0,CO,)(PO,)„ Apatite J Pyromorphite, Pb^Cl (POJ3 Group "1 Mimetite, PbjCKAsOJs [ Vanadinite, Pb^ClCVOJa B. Acid, Basic, and Hydrous Phosphates Amblygonite, LiAl(F,OH)PO, Olivenite, Cu2(OH)As04 Vivianite /Vivianite, Fe3(POj2-8H20 Group \ Erythrite, COaCAsOJj-SHjO Wavellite, Al3(OH)3(POJj-5H20 Turquois, H5[Al(OH)2],Cu(OH)(PO^), With the phosphates are included also the analogous com- pounds, arsenates and vanadates. Columbite properly belongs to another division, the niobates and tantalates, but is placed here for convenience. About 150 phosphate minerals are known but most of them are rare. Basic phosphates of iron and of copper are especially numerous. The compounds are practically all orthophosphates, that is, salts of H3PO4. Columbite, (Fe,Mn) (Nb,Ta) jOj Form. Columbite usually occurs in orthorhombic crystals of short prismatic or tabular habit. Cleavage, fair in two directions at right angles. H.=6. Sp. gr. 5.5 to 6.5. 352 PHOSPHATES, ARSENATES, ETC. 353 Color, black, often iridescent. Luster, submetallic. Chemical Composition. An iron-manganese meta-niobate and meta-tantalate grading from (Fe,Mn)Nb20o to (Fe,Mn)Ta20o. The latter mineral is called tantalite. Blowpipe Tests. Fusible on the edges with difficulty (5 J) . On charcoal with soda in R.F. gives a magnetic residue. The soda bead in O.F. is bluish-green (Mn.). The borax fusion dissolved in HCl with tin gives a deep blue color. Insoluble in acids. Uses. Tantalum, which is used as a filament for incandescent lights, is obtained from columbite and tantalite. Occurrence. 1. In granite-pegmatites, associated with beryl, lepidolite, spodumene, etc. The Etta mine in the Black Hills, South Dakota, is the most prominent locality in the United States. Monazite, (Ce,La)P04 Form. Monazite is usually found in the form of sand and occasionally in small embedded crystals. The crystals are mono- clinic, but are difficult to decipher on account of their small size. Cleavage, usually perfect in one direction. H.= .5. Sp.gr. + 5.1. Color, yellow to yellowish-brown or reddish-brown. Luster, faint resinous. Optical Properties. n^=1.79. Double refraction, strong. Fragments are plates, pale yellow in color, and with rather high interference colors (3rd to 5th order). Chemical Composition. Cerium-lanthanum phosphate. Silica and the rare metals, Th, Pr, Nd, and Y are usually present. Blowpipe Tests. Infusible, but turns gray on heating. The flame coloration with H2SO4 is pale bluish-green. Decomposed by acids. To get the phosphate test make a soda fusion. Uses. Welsbach gas mantles are prepared from the thoria, 354 INTRODUCTION TO THE STUDY OF MINERALS ThOj, obtained from monazite. The principal commercial deposit is near Prado, Bahia, Brazil. Occurrence. 1. In gneisses and granites as an accessory con- stituent. Derby has proved its wide distribution in decomposed gneisses of Brazil. 2. In sands along with other heavy minerals. Occurs in western North Carolina and in the beach sands of Brazil. Triphylite, Li(Fe,Mn)PO, Form. The characteristic occurrence of triphylite is in cleav- able masses. Crystals are rare. Cleavage, perfect in one direction. H. = 4Jto5. Sp.gr. ±3.5. Color, usually bluish-gray. Optical Properties. n=1.69. Double refraction weak. Frag- ments are plates with low-order interference colors. Chemical Composition. Lithium iron-manganese phosphate, Li(Fe,Mn)P04, the two compounds LiFePO^ and LiMnPO^ being isomorphous. Blowpipe Tests. Easily fusible (2J) to a magnetic globule, coloring the flame purplish-red. Soluble in HCl. Occurrence. 1. In granite-pegmatites. Branchville, Connec- ticut, is a prominent locality. A large number of other manga- nese-iron phosphates from this locality were described by Brush and Dana. APATITE GROUP— HEXAGONAL The apatite group is a well-established isomorphous group of minerals, for several isomorphous mixtures are known as given in the analyses below. Besides fluor-apatite 3Ca3(P04)2-CaF2 and chlor-apatite, 3Ca3(P04)2'CaCl2, there is dahllite, 3Ca3(P04) ^•- CaCOg, voelckerite, 3Ca3(P04)2CaO, and a rare mineral called PHOSPHATES, ARSENATES, ETC. 355 svabite, SCaCAsOJ.'CaFj. The following are typical analyses of the minerals of this group : Analyses of Minerals of the Apatite Group Ca Pb PO. AsO< VO4 CI F Misc. Fluor-apatite, Portland . . . 39.5 37.2 .... 55.4 54.2 52.2 57.5 26.0 21.3 15.1 0.8 tr 1.9 0.5 0.2 5.0 3.8 1 1 Chlor-apatite, Norway 3.1 Dahllite (Podolite), Russia, . . 36.5 40.4 . . . 4.6 67.0 0.2 !7.-).l CO3-53,* Voelckerite, Zillerthal 1 0.6 2.0 0=1. 4,t Polysphaerite, Freiberg . Pyromorphite, Schemnitz . . . 0.8 1.9 2.5 2.3 2.5 2.5 2.4 0.1 71.7 70.6 68.2 71.7 2.8 10.0 26.7 16.1 13.7 ■14 7 1.7 ^f^Tnpt,il■p, 'Rnhfimiji. Endlichite, New Mexico .... 0.2 30.1 61,7 - 2.6 *Fe2O3 = 3.0 tH2O = 0.1 APATITE, Ca,„(F„Cl„0,C03) (PC,), Form. Apatite is found in crystals, in massive forms, and in concretions. The crystals are hexagonal and belong to the bipyramidal class as there is but one plane of symmetry, which is horizontal. The habit is usually prismatic with the following forms: to {1010), c{0001), and p{1011}. Interfacial angles: mp(10T0:10Tl)=49° 42'; pp(10Tl :0lTl) =37° 44^'. Figs. 500 to 503 represent usual types of crystals. In Fig. 502, the general form /i{2131} is present in addition to s{1121} and other forms. Cleavage, imperfect basal parallel to {0001 } . H. = 5. Sp.gr. ±3.2. Color, usually reddish-brown or green, more rarely white, colorless, or violet. Optical Properties. n^(1.646) -n„(1.641) =0.005. Fragments are irregular, and colorless with low first order interference colors. Gives the microchemical gypsum test with H^SO^. Chemical Composition. Apatite is an isomorphous mixture of 356 INTRODUCTION TO THE STUDY OF MINERALS Ca,„F,(PO,)e with Ca,„Cl3(P0,)e, Cai„0(PO,)„, and CaioCCOs)- (POJe. For CaioFjCPOJe, Ca = 39.7 per cent., F = 3.8 per cent., P04 = 56.5 per cent. Blowpipe Tests. Fusible on edges (5^) , giving a yellowish-red flame. Soluble in HNO3, sometimes with slight effervescence on heating. The solution gives a yellow precipitate with (NHJj- M0O4 on warming. The best test for calcium is dilute HjSO^, which gives needle crystals of CaS04-2H20. NH^OH gives a white precipitate of calcium phosphate. m Fig. 500. Fig. 501. Fig. 502. Fig. 503. Uses. Massive apatite and phosphate rock are used exten- sively as a source of phosphates for fertilizers. Occurrence. 1. As an accessory constituent of igneous rocks, very common and widely distributed but in small quantities. 2. In veins in gabbro associated with wernerite and rutile. The chlor-apatite of Norway Vogt believes to have been formed by a pneumatolytic process. 3. In clefts of gneisses and schists. St. Gothard, Switzerland, furnishes good specimens. 4. In pegmatites and tin-stone veins. 5. As a metasomatic replacement of limestone. Some mas- sive apatites or phosphorites have been formed in this way. Phosphate Rock, etc. Phosphate rock is an impure massive calcium phosphate near apatite in composition. It occurs in beds, either in irregular PHOSPHATES, ARSENATES, ETC. 357 concretionary masses or in nodules. Besides calcium phosphate, it contains organic matter, varying amounts of iron and alum- inum, and calcium carbonate. The calcium carbonate is pres- ent partly as calcite, but principally as the dahllite molecule, 3Ca3(PO,)2CaC03. Phosphate rock Is used extensively as a source of super- phosphate for fertilizers. The crude minerals are treated with HjSO^. Florida, South Carolina, and Tennessee have important deposits. Large deposits have recently been discovered in Idaho, Wyoming, and Utah. Pyromorphite, Pb^CKPOJa Form. Pyromorphite usually occurs as small crystals and earthy crusts. The crystals are hexagonal and prismatic in habit, c{0001} and m{1010) being the only common forms (Fig. 504). H.=3Ho4. Sp.gr. ±6.8. Color, green or brown. Luster, adamantine. Optical Properties. n^(2.061) -n„(2.049) =0.012. Fragments are irregular, colorless or pale green, and have low first-order interference colors. With HNO3 isometric crystals are deposited. Chemical Composition. Lead chlorid-phos- phate, PbjCUPOJj; (Pb = 76.3 per cent.). In m some varieties Ca replaces Pb (polysphaerite, p. 355) and in others (VOJ replaces (POJ. Blowpipe Tests. Easily fusible (2) on char- coal to a globule with apparent crystal faces. fig. 504. With soda on charcoal yields a lead button. With a NaPOj bead saturated with CuO it gives an azure-blue Bame (CI). Soluble in HNO3. (NHJjMoO^ gives a yellow precipitate with the nitric acid solution. Occurrence. 1. As a secondary mineral formed from galena. Phoenixville, Pennsylvania, is a prominent locality. Both pyro- 358 INTRODUCTION TO THE STUDY OF MINERALS morphite pseudomorphs after galena and galena pseudomorphs after pyromorphite have been noted, the former from the Joplin district and the latter from Huelgoat, France. Mimetite, PbjCKAsOJj Form. Mimetite usually occurs in rounded hexagonal crystals. H.=3i. Sp.gr. ±7.2. Color, yellow, orange, or red. Luster adamantine. Optical Properties. n/2.13) -n„(2.12) =0.01. Fragments are irregular, yellow and have low first-order interference colors. With HNO3 isometric crystals of Pb(N03)2 are deposited. Chemical Composition. Lead chlorid-arsenate, Pb5Cl(AsO.,)3; (Pb = 69.5 per cent.). It grades on the one hand into pyromor- phite and on the other into vanadinite. Blowpipe Tests. Easily fusible (IJ) on charcoal and gives a white sublimate and a metallic button. In the closed tube heated with charcoal it gives an arsenic mirror. With CuO in the NaPOg bead it gives an azure-blue flame (CI). Soluble in HNO3. With (NHJ2M0O4 the nitric acid solution gives a yellow precipitate on boiling (phosphates give the precipi- tate on slight warming) . Occurrence. 1. As a secondary mineral in lead mines. Cum- berland, England. Vanadinite, PbjCICVOJg Form. Vanadinite practically always occurs in small hexago- nal crystals of prismatic habit. The common forms are : c { 0001 } and mjlOTO}, (Fig. 505). The general form {2131}, a hexagonal bipyramid, is sometimes present. H.=3. Sp.gr. +6.8. Color, usually red, but also yellow and brown. Luster, ada- mantine. Optical Properties. n^(2.35) - ?i„(2.30) =0.05. Fragments are PHOSPHATES, ARSENATES, ETC. 359 irregular yellow or orange color, and give bright interference colors. Chemical Composition. Lead chlorid-vanadate, PhjClCVOJa; (Pb = 72.7 per cent.)- It often contains (PO4) and (AsOJ as replacing radicals. Endlichite is between vanadinite and mimetite (see analyses, page 355). Blowpipe Tests. Easily fusible (1^) on char- coal giving a white sublimate and a metallic globule. In the closed tube with KHSO^ gives a yellow mass. The NaPOg bead is green in R.F. and yellow in O.F. It gives the CI test with NaPOg bead and CuO. Soluble in HNO3. Fig. 505. Uses. \'anadinite is one source of the vanadium used as an alloy with steel, and of various compounds used in dyeing and in the manufacture of ink. Occurrence. 1. A secondary mineral formed from galena. Yuma County, Arizona, is a prominent locality. Vanadinite often occurs with wulfenite. Amblygonite, LiAl(F,0H)P04 Form. This mineral usually occurs in cleavable masses. Crys- tals are triclinic, but are rare and imperfect. Cleavage, in two directions at angles of 75^°. H. = 6. Sp.gr. ±3.0. Color, white or grayish white. Optical Properties. n^(1.59) — n^(1.57) =0.02. Fragments are irregular plates, cloudy white, and give bright interference colors. Polysynthetic twinning is common. Chemical Composition. Basic lithium aluminum fluo-phos- phate, LiAl(F,0H)P04. The lithium is often partly replaced by sodium. At Canon City, Colorado, a mineral with the composi- tion NaAl(F,0H)P04, has recently been found. It has been called natramblygonite. 21 360 INTRODUCTION TO THE STUDY OF MINERALS Blowpipe Tests. Easily fusible (2) with intumescence to a white opaque globule, coloring the flame red. In the closed tube water is given off and the tube is etched. Soluble with difficulty in H2SO4. Uses. Amblygonite is a source of lithium salts, phosphates being a valuable by-product. It has been mined in South Dakota. Occurrence. 1. In granite-pegmatites associated with lepi- dolite, tourmaline and spodumene. Pala, San Diego County, California, is a prominent locality. OUvenite, Cu2(OH)As04 Form. Olivenite occurs in prismatic or acicular crystals and in fibrous aggregates. The crystals are orthorhombic, but are rare. H.=3. Sp.gr. +4.3. Color, usually various tints and shades of green. Optical Properties, n about 1.83. Fragments are acicular with parallel extinction, positive elongation, and bright interference colors. Chemical Composition. Basic copper arsenate, CujCOH) AsO^; (H20=3.2 per cent.). It may also contain a little (PO^). Blowpipe Tests. Easily fusible (2). In the closed tube with charcoal gives an arsenic mirror. Soluble in HNO3. (NH4)2Mo04 gives a yellow precipitate on boiling the nitric acid solution. Occurrence. 1. A secondary mineral occurring with copper arsenates and phosphates. In the Tintic (Utah) district it has been formed by the oxidation of enargite, CU3ASS4. VIVIANITE GROUP— MONOCLINIC The following compounds are isomorphous with vivianite and erythrite:Fe3(As04)2-8H20, Mg3(P04)2-8H20, Mg, (AsO,)2-8H20, Ni(As04)2"8H20, and Zn3(As04)2-8H20. Isomorphous mixtures PHOSPHATES, ARSENATES, ETC. 361 are common. These minerals are all secondary, usually occur- ring as incrustations. Erythrite is called "cobalt bloom," while annabergite, the corresponding nickel arsenate, is called "nickel bloom." Vivianite, Fe3(PO,)2-8H20 Form. Vivianite occurs in small crystals, in nodules, and in earthy masses. Crystals are monoclinic with varying habit. Cleavage, perfect in one direction (010). H.=2. Sp.gr. ±2.6. Color, deep blue or bluish-green, but colorless if unaltered. Optical Properties. n^(1.62) — ?i^(1.57) =0.05. Fragments are prismatic with either parallel or oblique (28i°) extinction. Pleo- chroic from blue to colorless or blue to green. Chemical Composition. Hydrous ferrous phosphate, Fe,- (FOJj'SHjO; (H20 = 28.7 percent.). It usually contains some ferric iron as the result of oxidation. Blowpipe Tests. Easily fusible (2) to a black magnetic glob- ule. In the closed tube gives water and whitens. It gives the borax bead test for ferrous iron, a bead made blue with CuO becoming opaque red in a neutral flame. Soluble in HNO3 or HCI. (NH,)jMoO, gives a yellow ppt. Occurrence. 1. As a secondary mineral in veinf associated with pyrite and pyrrhotite. 2. In clay and marl beds sometimes replacing fossils and in soils often around the roots of trees. At Mullica Hill, New Jersey, it replaces fossil belemnites. Erythrite, Co3(AsO,)2-8H,0 Form. This mineral occurs in minute crystals and as an incrustation. H.=2. Sp.gr. ±2.9. Color, pink to red. Optical Properties. n^(1.69) — n„(1.62) =0.07. Fragments are 362 INTRODUCTION TO THE STUDY OF MINERALS prismatic with oblique extinction (31°) and are pleochroic from pale to deep pink. Chemical Composition. Hydrous cobalt arsenate, Co3(AsO J 2' 8H2O; (H20 = 24.1 per cent.). The cobalt is sometimes replaced by nickel and iron. Blowpipe Tests. Easily fusible (2) coloring the flame faint blue. In the closed tube it gives off water and turns blue. In the closed tube with charcoal gives an arsenic mirror. Soluble in HNO3 to a pink solution. With (NH^)2MoO^ this solution gives a yellow ppt. on boiling. Occurrence. 1. A secondary mineral on smaltite and cobal- tite, and hence is called "cobalt bloom." Schneeberg, Saxony. Wavellite, Al3(OH)3(PO,)2-5H20 Form. Wavellite occurs in hemispherical or radiating crystal- line aggregates and in stalactitic forms. Distinct crystals (ortho- rhombic) are very rare. H. = 3ito4. Sp.gr. +2.3. Color, green, white, yellow, or gray. Optical Properties. n^=1.52. Fragments are prismatic and acicular with parallel extinction, positive elongation, and bright interference colors. Chemical Composition. Hydrous basic aluminum phosphate, Al3(OH)3(PO,)2-5H20; (H20 = 26.8 per cent.). Some analyses show a little fluorin. Blowpipe Tests. Infusible, but swells and splits. In the closed tube it gives water. Heated with cobalt nitrate solution in the forceps it turns deep blue. Soluble in HCl. Uses. At Mount Holly Springs, Penn., wavellite has been used as a source of phosphorus. Occurrence. 1. As a secondary mineral along the seams of sandstones, slates, and other rocks. PHOSPHATES, ARSENATES, ETC. 363 Tur quois, H, [Al (OH) ,]„Cu (OH) (PO,) , Form. Turquois occurs in soams and incrustations, but at one locality it has been found in minute triclinic crystals. H.=6. Sp.gr. +2.7. Color, usually bluish-green but varies from blue to green. Optical Properties, n about 1.63. Fragments are irregular, bluish or greeni.sh with aggregate polarization in low first-order interference colors. Chemical Composition. Acid and basic aluminum copper phosphate, H5[Al(OH2)]„Cu(OH)(POJ„ according to Schaller. (H20=19.5 per cent.) Blowpipe Tests. Infusible, but turns dark when heated and gives a green flame which is made blue by HCl. In the closed tube at a high temperature it gives water and turns dark. Soluble in HCl after gentle ignition. Uses. Turquois is used extensively as a gem. Blue stones are more valuable than green ones. Occurrence. 1. In seams of volcanic rocks such as trachytes and rhyolites. Los Cerrillos Mts., New Mexico (in andesite). 10. NITRATES, BORATES, AND URANATES Soda Niter, NaNOj COLEMANITE, CajB,0„.5HjO Boracite, MgjCljBieOjo Ulexite, NaCaBjOg SH^O Borax, Na^B^O, IOH2O Uraninite, UO3, UO^, Pb, etc. Of the few mineral nitrates known, soda niter is the only one of importance. Only a few borates are prominent as minerals. H3BO3 is ortho-boric acid. Borax is a salt of HjB^O, derived thus: 4H3B03-5H20 = H2B,07. Ulexite is a salt of HjBjOj (5H3BO3 -6H2O). Colemanite is a salt of H,B,Oi, (6H3BO3-7HJO). Boracite is 2(Mg3BgO,5)-MgCl2 (8H3B03-9H20 = HeB80,5). These are called tetra-, penta-, hexa-, and octo-boric acids respectively. Soda Niter, NaNOj Form. Soda niter is found in crystalline and granular masses. It crystallizes in rhombohedrons with almost the same angles as the unit or cleavage rhombohedron of calcite (1011:0111=73° 30'). Cleavage, rhombohedral like calcite. H. = lito2. Sp.gr. +2.3. Color, white or colorless. Very deliquescent. Optical Properties. n^(1.587) -?i„(1.336) =0.251. The opti- cal properties are like those of calcite. It recrystallizes from water solution in rhombic shaped crystals (Fig. 506) with sym- metrical extinction and very high-order interference colors. 364 NITRATES, BORATES, AND URA NATES 365 Chemical Composition. Sodium nitrate, NaNOg. lodin may be present in the form of Ca (103)2 or lautarite, a yellow mineral. Blowpipe Tests. Easily fusible (1), giving an intense yellow flame. With KHSO^ in the closed tube it gives red-brown fumes of NO2. Soluble in water. Uses. Soda niter is used as a fertilizer and also in the manu- facture of KNO3. Chili furnishes the world's supply. Occurrence. 1. In arid regions. Occurs in northern Chili and to a slight extent in California and Nevada. Soda niter is probably formed by nitrifying bacteria, which convert nitrogen and ammonia into nitrates. Fig. 506. — Soda niter recrystallized. Fig. 507. — Boric acid. COLEMANITE, Ca^B^O^-SHjO Form. Colemanite occurs in crystals, which often line geodes, and in crystalline and compact masses. The crystals are mono- clinic and are often highly modified. Cleavage, perfect in one direction (010). H.=4^. Sp.gr. +2.4. Color, colorless or white. Optical Properties. 71^.(1.61) —n„(l. 58) =0.03. Fragments are irregular plates with bright interference colors. With HCl pseudo-hexagonal crystals of boric acid separate out (Fig. 507) . Chemical Composition. Hydrous calcium hexa-borate, Ca2BoOii-5H20; (H20 = 21.9 per cent.). 366 INTRODUCTION TO THE STUDY OP MINERALS Blowpipe Tests. Fuses easily (Ih) with exfoliation coloring the flame green. In the closed tube gives water. Soluble in hot HCl. Boric acid separates out on cooling. Uses. Colemanite is the principal source of borax and boracic acid. It is obtained in various places in Southern California. Occurrence. 1. In lake-beds interstratified with Tertiary sediments. Calico, San Bernardino County, California. Boracite, Mg^CljEi^Ojo Form. Occurs in small embedded isometric crystals and in granular masses which resemble marble. The crystals, which are very sharp and model-like, though small, belong to the isometric hextetrahedral class and are among the best representatives of that class obtainable. The usual forms are the cube a{100}, dodecahedron dj 110), positive tetrahe- dron { 1 1 1 ) , and negative tetrahedron Oi{lll}. Fig. 508 is a typical crystal. H. = 5to7. Sp.gr. +2.9. Color, colorless or white. Optical Properties. n^(1.673) — n„- (1.662) =0.011. Fragments are irregu- \\ \V^' lar with low first order interference ' colors and sometimes show polysyn- "'" """ thetic twinning. Boracite, though isometric, shows double refraction. This optical anomaly may be explained by assuming that the boracite substance is dimor- phous, for the double refraction disappears at 265° C. Chemical Composition. Magnesium chlorid octo-borate, Mg,Cl,B,Ao or MgCl,-2(Mg,B,0J. Blowpipe Tests. Easily fusible at 2 with intumescence, color- ing the flame green. With a NaPOg bead saturated with CuO it gives an azure-blue flame. The massive variety gives water in the closed tube. Soluble in HCl. NITRATES, BORATES, AND URA NATES 367 Occurrence. 1. In gypsum, anhydrite, or carnallite. Stass- furt, Germany. Ulexite, JUaCaBfi^SIl^O Form. Ulexite is found in rounded fibrous masses, locally called "cotton-balls." H. = l. Sp.gr. + 1.6. Color, white. Luster, silky. Optical Properties. 7i=1.51. Fragments are acicular with parallel extinction, negative elongation, and low first-order inter- ference colors. Chemical Composition. Hydrous sodium calcium penta- borate, NaCaBjOg-SH^O; (H20=35.5 per cent.). Blowpipe Tests. Easily fusible (1) with intumescence, color- ing the flame an intense yellow. In closed tube gives water. Soluble in hot HCl, H3BO3 separating out on cooling. Occurrence. 1. In playas (the dried-up lakes of arid regions) associated with borax, gypsum, and halite. Esmeralda County, Nevada. Borax, Na^B.Oj lOHjO Form. Borax occurs in crystals and in efflorescent crusts. The crystals are monoclinic and near pyroxene in angles and habit. H. = 2. Sp.gr. + 1.7. Color, white or colorless. Efflorescent. Optical Properties. n^{lA7)—n^{lA5) = 0.02. Recrystallizes from water solution in crystals having (1) low first order interfer- ence colors and oblique extinction and (2) bright interference colors and parallel ex- tinction (see Fig. 509). Chemical Composition. Hydrous sodium ^^^ gpg tetraborate, NajB^O^-lOHjO; (H20 = 47.2 Borax reorystalUzed. per cent.). Blowpipe Tests. Easily fusible (1) to a clear glass giving an 368 INTRODUCTION TO THE STUDY OF MINERALS intense yellow flame. A solution in alcohol burns with a green flame. Soluble in water. Uses. Borax has many uses, but most of the borax of com- merce is now made from colemanite. Occurrence. 1. In lake-deposits in arid regions associated with halite, trona, theuardite, glauberite, hanksite, gypsum, etc. Searles' borax marsh, San Bernardino County, California. The borates are leached from Tertiary lake sediments in near-by hills. Uraninite, UO3, UO2, Pb, etc. Form. Uraninite is very rarely found in isometric octahedral crystals. It is usually massive and sometimes has a botryoidal surface. H. = 5i. Sp. gr. 7.5 to 9.5. Color, dark brown to black. Luster, submetallic or pitch- like. Streak usually olive-green. Optical Properties. w>1.74. Fragments are irregular, some- times translucent brown and isotropic, sometimes opaque. Cheniiical Composition. Uncertain. Analyses show varying amounts of UOj, UO3, Pb, and Th. Also contains helium and radium. Blowpipe Tests. Infusible. The NaPOg bead is yellowish- green in O.F. and green in R.F. Soluble in HNO3. With NH^OH the solution gives a yellow precipitate. Uses. Uraninite is the source of uranium compounds and also of radium compounds. Occurrence. 1. In veins with metallic sulfids. Joachimsthal, Bohemia. 2. In granite-pegmatites. Llano County, Texas. 11. SULFATES .1. Normal Anhydrous Sulfates { BARITE, BaSO, Barite Group | CELESTITE, SrSO. [ ANGLESITE, PbSO. ANHYDRITE, CaSO, Crocoite, PbCrO^ B. Basic and Hydrous Sulfates Kainite, MgSOvKCl-SHjO Brochantite, Cu^(^0H)8S0i Mirabilite, Na^SO ^ lOHjO GYPSUM, CaS0,2H,0 Epsomite, MgS0j-7H,0 Melanterite, FeSO^TH^O Chalcanthite, CuSO^ SH^O Copiapite, Fe^(OH)2(SO,)5l7H20 Alunite, KA1,(0H)„(S0,)2 A large number of sulfate minerals, most of them basic and hydrous salts, are known but comparatively few are of much importance. They are all salts of H2SO4. No sulfites, pyrosul- fates, thiosulfates, or persulfates are known among minerals. BARITE GROUP— ORTHORHOMBIC In habit, angles, and cleavage barite, celestite, and anglesite are similar, and thus constitute an isomorphous group. One would expect to find anhydrite in this group, but it differs in angles and especially in cleavage. There are isomorphous mix- tures of BaSO^ and SrSO^, but they do not seem to mix with PbSO,. 370 INTRODUCTION TO THE STUDY OF MINERALS BARITE, BaSO, Form. Barite occurs in crystals, in crested groups, in lamel- lar, nodular, fibrous, and granular masses. Rhombic bipyramidal class: o:6:c = 0.815: 1:1.313. Usual forms: c{001j, m{110), o{01l!_, d{102}, Z{104}, z{\n], y{l22]. Interfacial angles: TOm(110: 110) =78° 22^'; co(001 :011) =52° 43'; cd(001: 102) =38° 51^'; d(001 : 104) =21° 56J'; C2(001 : 111) = 64° 19'; o!/(011:122)=26° 1'. The habit is usually tabular parallel to {001}, as represented in Figs. 510 to 513, but prismatic crystals are also common. 1.74, n„<1.74. Fragments are pris- matic with parallel extinction. Chemical Composition. Basic copper sulfate, CujCOH)^ S0< or CuSO,-3Cu(OH)2; (H30 = 12 per cent.). Blowpipe Tests. Fusible at SJ. In the closed tube turns black and gives off water. Insoluble in water. Soluble in HNO3 without effervescence (distinction from malachite, which it greatly resembles). Occurrence. 1. A secondary mineral associated with other copper minerals. Morenci, Arizona. Mirabilite, Na^SO.lOHjO Form. In crusts and as a powder. Crystals are very rare. H. = ]\to2. Sp.gr. ±1.5. Color, white. Optical Properties, n about 1.44. Recrystallizes from a water solution in long prismatic crystals with low first order interference colors and parallel extinction (Fig. 518). Chemical Composition. Hydrous sodium sulfate, Na^SO^lOH^O; (H20 = 55.9 per cent.). Blowpipe Tests. Easily fusible (1^) giving an intense yellow flame. In the closed tube gives abundant water. Soluble in water. , J'°- ^is. Till -r r Mirabilite recryatamzcd. Occurrence. 1. In soda lakes. It forms along the shores of Great Salt Lake in Utah during the winter. 2. As an efflorescence in caves and other protected places. GYPSUM, CaSO, 2H2O Form. In form gypsum is variable. It occurs in embedded and attached crystals, in cleavable and crystalline masses, and in fibrous and granular masses, 83 376 INTRODUCTION TO THE STUDY OF MINERALS Monoclinic system, prismatic class. a:6:c = 0.690: 1:0.412; /? = 80° 42'. Usual forms: mjllO), Z{111}, 6{010), e{T03}. Interfacial angles: mm(110:110) =68°_30';JZ(111: 111) =36° 12'; mZ(110:lll)=49° 9'; ae(edge iTO-TTO : T03) = 87° 49'. The habit is usually tabular parallel to the side pinacoid {010} . Figs. 519 to 521 represent typical crystals. Twins with {100} as twin-plane are common (Fig. 522). / Fig. 519. Fig. 520. Fig. 521. "W Fig. 522. Cleavage, perfect in one direction parallel to {010}, also imperfect conchoidal parallel to {100} and fibrous parallel to {111}. A cleavage fragment is oriented with respect to the crystal outline as shown in Fig. 306 (p. 105). H. = 2. Sp.gr. + 2.3. Color, colorless, white, amber, gray, pink, etc. Luster, vitreous, silky, or pearly. Optical Properties. n^(l. 529) -n„(l. 520) =0.009. Fragments are prismatic or acicular or platy with bright interference colors and extinction angles of 0°, 13^-°, or 37^°. Recrystallizes from dilute HCl solution as microchemical gypsum (Fig. 387, p. 198). Chemical Composition. Hydrous calcium sulfate, CaS04-2H20; (H2O = 20.9 per cent.). Massive gypsum may contain CaCOj, clay, sand, or organic matter. Blowpipe Tests. Easily fusible (3) to a white enamel giving SULFATES 377 a yellow-red flame. In the closed tube becomes opaque and gives off water at a low temperature. Easily soluble in dilute HCl (distinction from anhydrite) . Uses. Gypsum is extensively used in the manufacture of plaster and as a fertilizer. Occurrence. 1. In bedded deposits associated with salt and limestone and formed directly by the evaporation of inland seas. 2. As a secondary mineral in various rocks, formed principally by the action of sulfuric acid or ferrous sulfate (produced by the oxidation of pyrite) on calcium carbonate. 3. As a hydration product of anhydrite. At the Ludwig mine in Lyon County, Nevada, the hydration has reached a depth of 400 feet. Epsomite, MgSO^-TH^O Form. Epsomite is found in fibrous masses, in crusts, and occasionally in prismatic crystals belonging to the bisphenoidal class of the orthorhombic system. Fig. 81 (p. 31) represents a typical crystal. H.=2to2i. Sp.gr. + 1.7. Color, colorless or white. Optical Properties. n^(1.46) - /'<,(1.43) = 0.03. Recrystallizes from a water solution in prismatic crystals with parallel extinc- tion and low first order interference colors (Fig. 523). Chemical Composition. Hydrous mag- ^^°- ^^^- „ , „ . Epsomite recrystaluzed. nesium sulfate, MgSO,-7H20; (H,0 = 51. 2.) Blowpipe Tests. Easily fusible (1). It gives abundant water in the closed tube. Soluble in water. Occurrence. 1. As an efflorescence in abandoned mine drifts, caves, and other protected places. New Almaden, California. 2. In lake deposits. Large deposits occur in Albany County, Wyoming, along with mirabilite. 378 INTRODUCTION TO THE STUDY OF MINERALS Melanterite, FeSO^-THaO Form. Melanterite usually occurs in fibrous crusts or in capillary crystals. H. = 2. Sp.gr. + 1.9. Color, white or pale green, becoming yellow on exposure. Optical Properties. Wj.(1.49) — n^(1.47) =0.02. Recrystallizes from a water solution in long prismatic crystals with parallel extinction and low first order interference colors (like epsomite, Fig. 523). Chemical Composition. Hydrous ferrous sulfate, FeSO^-yHjO; (H2O = 45.3 per cent.) . It may also contain Mg, Mn, Cu, and ferric iron. Blowpipe Tests. Easily fusible (1). In the closed tube gives water which has an acid reaction. On charcoal R.F. gives a magnetic residue. Soluble in water. Occurrence. 1. A secondary mineral usually formed by the oxidation of pyrite or marcasite. Cabinet specimens of these minerals sometimes go to pieces because of their alteration to melanterite and sulfuric acid. Chalcanthite, CuSO^-SH^O Form. Chalcanthite occurs as an incrustation or as fibrous seams. Artificial crystals of this substance furnish us among the best examples of triclinic crystals. H.=2i. Sp.gr. +2.2. Color, blue. Optical Properties. nj.(1.54) -n^(1.51) =0.03. Recrystallizes from water solution in pale blue prismatic crystals with oblique extinction (10° to 15°). Chemical Composition. Hydrous copper sulfate, CuSO^-SHjO; (H20 = 36.1 per cent.). It often contains iron. SULFATES 379 Blowpipe Tests. Fusible at 3. In the closed tube turns white, then black, yielding abundant water. Soluble in water. The water solution placed on metallic iron (knife-blade) gives a film of copper. Occurrence. 1. A secondary mineral often found in aban- doned mine drifts. Butte, Montana. Copiapite, Fe,(OH)2(SO,)5l7H20 Form. This mineral usually occurs in granular masses or as an incrustation. H. = 2\. Sp. gr.±2.1. Color, yellow. Taste, metallic astringent. Optical Properties. n^(l. 572) -n„(l. 527) =0.045. Fragments are irregular or plates with bright interference colors and are pleochroic, changing from pale to deep yellow. Chemical Composition. Hydrous basic ferric sulfate, Fe^ (OH) j (SOJ^-HH^O; (H20=31.1 percent.). Blowpipe Tests. Fusible with difficulty (5). In the closed tube yields water which has an acid reaction and leaves a dark red residue (FojOj). On charcoal R.F. becomes magnetic. Partially soluble in water. On boiling the water solution, Fe(0H)3 is precipitated. Occurrence. 1. A secondary mineral formed by the oxidation of pyrite or marcasite. . Large deposits occur in the desert regions of Chili near Copiapo. The name was derived from this locality. Alunite, KAl3(OH),(SO,)2 Form. Alunite occurs in cavities of a siliceous rock or dis- seminated through the rock mass. The crystals are very small and belong to the hexagonal system. The habit is usually tabular with the pinacoid { 0001 ) and the rhombohedron { lOTl } . H. = 4. Sp.gr. ±2.8. Color, colorless, white, or gray. 380 INTRODUCTION TO THE STUDY OF MINERALS Optical Properties. w^(1.59) — n„(1.57) =0.02. Fragments are irregular with bright interference colors. Small crystals are triangular, dark between crossed nicols (basal sections), and give a positive uniaxial interference figure in convergent light. Chemical Composition. Basic potassium aluminum sulfate, K2SO,-Al2(SO,)3-4Al(OH)3; (H2O = 13.0 per cent.). Blowpipe Tests. Infusible and turns blue with cobalt nitrate solution. In the closed tube gives water which has an acid reaction. Soluble in H^SO,. Uses. Alum is obtained from the alunite rock by roasting and leaching with water. Occurrence. 1. In cavities of trachytes and rhyolites, and as a metasomatic replacement of these rocks. Alunite is formed by the action of sulphurous vapors on orthoclase. Musa, Hungary. 12. TUNGSTATES AND MOLYBDATES Wolframite Group Scheelite Group Wolframite, (Fe,Mn)WO, Hiibnerite, (Mn,Fe)WO^ Scheelite, CaWO, Wulfenite, PbMoOj WOLFRAMITE GROUP— MONOCLINIC j Fe Mn WO, Wolframite Wolframite 14.5 7.4 1.1 3.2 11.1 17.9 83.0 81.0 80.4 Hiibnerite Wolframite, (Fe,Mn)W04 Form. Wolframite occurs in crystals and in crystalline aggre- gates. The crystals are monoclinic and are usually tabular parallel to {100}. Cleavage, perfect in one direction parallel to {010}. H. = 5to5-i. Sp.gr. ±7.4. Color, black or dark brown. Luster, sub-metallic. Opaque. Chemical Composition. Iron-manganese tungstate, (Fe,Mn) WO^, with iron predominating. Blowp'pe Tests. Fusible (3) to a magnetic globule. The soda fusion is bluish-green (Mn) . Soluble in aqua regia with the separation of WO3, a yellow residue. Uses. Wolframite is the principal source of tungsten. Occurrence. 1. A vein mineral especially in tin-stone veins associated with cassiterite, scheelite, etc. Zinnwald, Bohemia. 381 382 INTRODUCTION TO THE STUDY OF MINERALS Hiibnerite, (Mn,Fe)WO^ Form. Hubnerite usually occurs in bladed crystalline aggre- gates. Cleavage, in one direction parallel to the length. H. = 5to5i. Sp.gr. +7.2. Color, reddish-brown. Luster, sub-metallic. Optical Properties. n> 1.66, w< 1.74. Fragments are prismatic with parallel extinction. Chemical Composition. Manganese-iron tungstate, (Mn,Fe) WO 4, with manganese predominating. Blowpipe Tests. Fusible at 4. Gives the Mn bead tests. Insoluble in acids. The soda fusion gives a yellow residue (WO3) with HCl. Occurrence. 1. A vein mineral often associated with scheelite. SCHEELITE GROUP— TETRAGONAL ' Besides CaWO^ and PbMoO^ there are also CuWO^, CaMoO^, and PbWO^, which are similar crystallographically. Isomorphous replacement is illustrated by the following analyses: ■Ca Pb WO, MoO, Misc. Scheelite 13.8 14.5 12.9 0.9 53.7 89.9 80.3 81.7 0.4 9.1 43.8 Scheelite Scheelite (Cuproscheelite) . . . Wulfenite Cu = 2.6; SiO2 = 0.7 Cu = 0.3; AlAH- Fe2O3 = 0.5 Scheelite, CaWO^ Form. Scheelite occurs in both crystals and massive form. Crystals are tetragonal and belong to the tetragonal bipyramidal class with but one plane of symmetry, which is the horizontal. The habit is pyramidal with {111} or {101} as the dominant form. Angles: (111:111) =79° 55^'; (101: Oil) =72°40^. Cleavage, distinct parallel to {111} (in four directions) . TUNGSTATE8 AND MOLYBDATES 383 H.=4\to5. ' Sp.gr. ±6.0. Color, white, gray, or pale colors. Luster, sub-adamantine. Optical Properties. 71^.(1.93) -n„(l. 92) =0.01. Fragments are irregular with bright interference colors. Chemical Composition. Calcium tungstate, CaW04;(Ca = 16.9 percent.). The tungsten is often partly replaced by molybde- num and the calcium by copper. Blowpipe Tests. Fusible with difficulty (5). The NaPOj bead is blue in R.F. Decomposed by HCl with the separation of a yellow residue (WO3) which is soluble in NH.OH. Uses. Scheelite is one source of tungsten. Occurrence. 1. In veins, especially in tin-stone veins with cassiterite, fluorite, topaz, etc. Wulfenite, PbMoO^ Form. Wulfenite usually occurs in crystals which are tetra- gonal and usually tabular in habit. Fig. 524 is a plan of the common type of crystal with cj 001} and m{ 102}. H. = 3. Sp.gr. +6.7. Color, yellow, orange, or red. Luster, ada- mantine. Optical Properties. n^(2.40) - n„(2.30) =0.10. Fragments are irregular and yellow with rather high interference colors. Thin tabular crystals give a negative uniaxial interference figure in ^^°- 524, convergent light. Chemical Composition. Lead molybdate, PbMoO^; (Pb = 56.4 per cent.). Blowpipe Tests. Easily fusible (2) on charcoal giving a metallic button. The NaPOj bead is green in R.F. Decomposed by HCl. Uses. Wulfenite and molybdenite are sources of molybdenum. Occurrence. 1. A secondary mineral in veins, often associated with vanadinite. Yuma County, Arizona. 13. SILICATES About a fourth of the known minerals are silicates, though many of them are very rare. They are the most important rock- forming minerals and thus make up the bulk of the earth's crust. Among the important rock-making minerals are the feldspars, the pyroxenes, the amphiboles, and the micas. These, together with quartz, constitute about 87 per cent, of the solid crust of the earth according to F. W. Clarke, while the feldspars alone con- stitute about 60 per cent, of the earth's crust. Many of the silicates are complex in composition and the establishment of chemical formulae of some of them has bafHed the skill of many eminent chemists. In a chemical discussion of the silicates the starting point is 1148104, which is called ortho- silicic acid. The compound HjSiOg, derived thus (H^SiO^ — HjO* =^ HjSiOj) is called metasilicic acid. A large number of ortho- silicates and metasilicates are known among minerals, but many silicates cannot be placed in either of these divisions. So the assumption has been made that other silicic acids are possible. Among them are H^SijOj (21148104 — HjO), diorthosilicic acid; HjSijOj (HaSiOg + SiOj), dimetasilicic acid. In a similar way £[481305, HgSigOio, HgSiOj and HioSijOg may be derived. Minerals corresponding to all these acids are known. Another method formerly used by chemists, and still employed by metallurgists is based upon the ratio of the oxygen of silica in that of the bases. 1138104 may be written 2RO-8i02. Here the oxygen ratio is 1 : 1, so orthosilicates are called unisilicates. Meta- silicates, R8i03 or RO-SiOj, are called bisilicates. Polysilicates have the formula RO-nSiOj and subsilicates, iiRO-SiOj, where n is greater than 2. The difficulty of assigning formulae to many silicate minerals lies in the fact that it is often difficult to decide upon the valence 384 SILICATES 385 and grouping of the basic elements. Many silicates give water when heated in a closed tube, but it is often difficult and sometimes impossible to determine whether hydroxyl (OH), hydrion (H), or so-called water of crystallization (HjO) is present. H2Zn2- SiOj is the empirical formula for the mineral calamine. It may be an acid oxy-orthosilicate, H2(Zn20)Si04; a basic metasilicate, Zn2(OH)2Si03; or an acid salt of H^SiOj, one of the possible silicic acids. Serpentine H^MgaSijOj presents a similar problem, for it may be written H2Mg3(SiO,)2-H20, H3Mg30H(SiO,)2, H2Mg3- (OH)2Si207, or Mg3Si207-2H20." The silicates may be conveniently divided into five groups as follows: A. Feldspars, salts of H4Si30g and isomorphous mixtures of these salts with the orthosilicate CaAl2Si208; B. Metasilicates; C. Orthosilicates; D. Miscellaneous anhydrous silicates, and E. Hydrous silicates. A. FELDSPARS ORTHOCLASE, KAlSiaO, MICROCLINE, KAlSiaOj [ ALBITE, NaAlSiaOj / Plagioclase \ OLIGOCLASE, wNaAlSiaOj + reCaAlSijOs LABRADORITE, mCaAlSi^Og + nNaAlSiaOs The following are typical analyses of the various feldspars: Analyses of the Feldspars KiO NaiO CaO AI2O3 SiOz Miso. Orthoclase 11.7 14.0 13.5 4.3 1.0 1.6 0.5 1.3 18.8 17.9 19.6 64.6 65.7 64.8 BaO=0.4; ign=0.1 Fe!03=tr. ign.=0.2 Albit© 0..5 1.3 1.0 tr 0.6 11.1 8.5 6.2 4.4 1.8 0.2 0.4 4.8 8.1 12.0 16.1 19.3 19.3 23.8 26.6 29.6 31.1 36.8 68.8 61.3 58.0 54.2 46.9 44.0 Fe!Oj = 0.1 Fe2O3=0.4 MgO = 0.1;ign.=0.1 FeiOi-13- H2O = 1.0 MgO=0.2; ign.=0.1 386 INTRODUCTION TO THE STUDY OP MINERALS ORTHOCLASE, KAlSigOg Form. Orthoclase occurs in attached and embedded crystals, in cleavable masses, and disseminated through rock masses. Orthoclase is one of the best examples of the monoclinic prismatic class. Axial ratio: a:&:c = 0.658; 1:0.555; /? = 63° 57'. Usual forms: c{001}, &{010}, xjTOl}, y{20l}, m{110}, 2(130}, o{Tll}, mm Fig. 525. Fig. 526. Fig. 528. w{021}. Interfacial angles :6c(010: 001)= 90° 0';mm(l 10 :lTO) = 61° 13'; mc(110:001)=67° 47'; ca;(001 :T01) =50°_16i'; cy{001: 201) =80° 18'; m2(110:130) =29° 59^'; 6o(010:Tll) =63° 8'; cn(001 :021) =44° 56^'. The habit is usually elongate in the di- rection of the a-axis (Figs. 525, 527) or elongate in the direction pA s "^ \ K \ X ^ m \ (l\ \ Fig. 529. Fig. 530. FiQ. 531. of the c-axis and tabular parallel to {010} (Fig. 526). Fig. 528 is prismatic and pseudo-orthorhombic. There are three common twinning laws for orthoclase: (1) the Carlsbad law in which the c-axis is the twin-axis (usually penetration twins with 6(010} as the composition face), (2) the SILICATES 387 Baveno law in which n{021} is the twin-plane and (3) the Mane- bach law in which c{001} is the twin-plane. These three types of twinning are represented by Figs. 529-531. Cleavage in two directions at right angles parallel to {001} and {010}. There is also imperfect cleavage or parting parallel to {110} which assists in orienting cleavages and imperfect crystals. H.=6. Sp.gr. +2.57. Color, white, colorless, gray, pink, red. Sometimes has a play of colors. Optical Properties. n^(1.526) -n„(l. 519) =0.007. Fragments are plates, usually with one set of parallel straight edges (Fig. 532). The interference colors are middle first order (gray and straw-yellow). Chemical Composition . Potassium aluminum trisilicate, KAlSigOg; (K20 = 16.9, Al203 = 18.4, Si02 = 64.7). Sodium usually replaces part of the potassium. Blowpipe Tests. Fusible with difficulty (5). Insoluble in ordinary acids. p^^, 532 Uses. Orthoclase, and also microcline, Orthooiaae fragments, is used in the manufacture of porcelain and china. A variety called moonstone is used as a gem. Occurrence. 1. In acid plutonic igneous rocks, especially granites and syenites. 2. In acid volcanic igneous rocks, rhyolites and trachytes. This is usually sanidine , a glassy variety of orthoclase. 3. In certain rare basic igneous rocks. 4. In gneisses, partly as a remnant of igneous rocks, partly recrystallized. 5. In cavities and seams of schists and gneisses. This is the variety adularia, a transparent orthoclase of unusual habit (Fig. 528). Adularia contains little, if any, soda (see analysis, p. 385). 388 INTRODUCTION TO THE STUDY OF MINERALS 6. In veins. Vein orthoclase has the same habit as adularia, but is not always transparent. Lindgren uses the name valen- cianite for it. Guanajuato, Mexico. 7. In granite-pegmatites, often in very large crystals. 8. In arkoses or feldspathic sandstones (Portland, Connecti- cut) and in some beach sands (Pacific Grove, California) . MICROCLINE, KAlSigOs Form. In form microcline is practically the same as ortho- clase, but is triclinic with the angle (001:010) =89° 30' instead of 90°. The angles are very close to those of orthoclase. Cleavage. In two directions practically at right angles (89° 30'). H. = 6. Sp.gr. ±2.56. Color, white, gray, reddish, green. The green variety is called amazon-stone. Optical Properties. w^(l. 529) -n„(l. 522) =0.007. Fragments are plates with middle first interference colors and are distin- guished from orthoclase by the "gridiron" structure caused by polysynthetic twinning in two directions at right angles. Chemical Composition. The same as for orthoclase. Blowpipe Tests. The same as for orthoclase. Occurrence. 1. In granite-pegmatites (near Florissant, Colorado). 2. In granites (but not in rhyolites). 3. In gneisses. PLAGIOCLASE GROUP— TRICLINIC This is perhaps the best defined isomorphous group to be found among minerals. There is a perfect gradation in properties from the albite end of the group with the formula NaAlSigOg to the anorthite end of the group with the formula CaAljSijOg. Intermediate members of the group are designated by Ab^An^j, SILICATES 389 Ab meaning the albite molecule and An the anorthite mole- cule. Crystals are triclinic, but with angles near those of ortho- clase. The angle (001:010), for example, varies from 86° 24' for albite to 85° 50' for anorthite, while for orthoclase the correspond- ing angle is 90° 0'. The plagioclases have good cleavage parallel to {001), and imperfect cleavage parallel to {010}. Twinning is rarely absent in the plagioclases. The most common twinning is known as albite twinning, in which {010} is the twin-plane. This is usually polysynthetic and the twin striations show best on the {001} cleavage face and are always parallel to the (001:010) edge (Fig. 534). In pericline twinning, which is also polysyn- thetic, the 6-axis is the twin-axis. This kind of twinning shows Fig. 533. — Pericline twinning. Fig. 534. — Albite twinning. best on the {010} cleavage (Fig. 533), and the angle the striations make with (001:010) edge differs for the various plagioclases. This angle is given in the column of the following tabulation labelled angle of rhombic section. In optical properties the plagioclases are similar. They are all biaxial with 2V varying from 77° to 90°. The indices of refraction vary from 1.54 for albite to 1.58 for anorthite. The double refraction is weak (0.007 to 0.012). Typical analyses are given on page 385. The following tabulation illustrates the continuous variation in properties. 390 INTRODUCTION TO THE STUDY OF MINERALS Extiao- Extinc- Angle of Sp. gr. »r »a tion on 001 tion on 010 rhombic section 1.S40 1.531 ii° 19° + 22° 2.62 1.543 1.534 3 12 + 10 2.64 1.549 1.542 1 H + 3 2.66 1.562 1.555 5 16 - 1 2.69 1.577 1.567 18 29i - 9 2.73 1.684 1.672 27i 334 -10 2.74 1.588 1.576 37 36 -16 2.76 (001:010) Albite Oligoclase . . Andesine . . Labradorite Bytownite . Anorthite. . ,Ab ' jAbGAm JAbaAni JAbiAni JAbiAns JAbiAne iAn 86°24' 86°32 86°14 86°12 se^eo ALBITE, NaAlSijOg Form. Albite occurs in small crystals, in lamellar masses, and intergrown with orthoclase or microcline. The crystals are triclinic, pinacoidal class, and are usually tabular parallel to {010}. The usual forms are the same as for orthoclase. Fig. 534 represents the common type of crystal with c{001}, 6 {010}, m{110}, 3 {110} and a;{101}. Albite, pericline, and Carlsbad twins are all common and sometimes two or more of these are combined on one crystal. Cleavage, perfect parallel to {001} and less perfect parallel to {010}. H.=6. Sp.gr. +2.62. Color, white, colorless or gray. Optical Properties. n^(l. 540) -n„(l. 532) =0.008. Fragments are plates with middle first order interference colors and extinc- tion angles of about 4°(001) and about 20°(010). The index of refraction is less than that of oil of cloves. Chemical Composition. Sodium aluminum trisilicate, NaAl- SigOs,- (Na^O^ll.S, Al203 = 19.5, Si02 = 68.7). A little calcium is present for albite grades into oligoclase. Blowpipe Tests. Fusible (4) to a colorless glass coloring the flame yellow. Insoluble in ordinary acids. SILICATES 391 Occurrence. 1. In granite-pegmatites associated with tour- maline, lepidolite, spodumene, etc. 2. In veins and seams, especially in the metamorphic rocks. 3. In certain soda-rich igneous rocks, usually intergrown with orthoclase or microcline. This intergrowth is known as perthite. OLIGOCLASE, Ab^Anj to AbgAiii Form. Oligoclase occurs in cleavable masses and disseminated through rock masses, but is rarely found in distinct crystals. Cleavage, perfect parallel to { 001 } , less perfect parallel to { 010 j . H.=6. Sp.gr. +2.65. Color, white, colorless, greenish or reddish. Optical Properties, n/1. 547) -n„(l. 539) =0.008. Fragments are plates with middle first order interference colors and extinc- tion angles of about 2°(001) and about 8°(010). The fragments usually show polysynthetic twinning. Chemical Composition. Sodium-calcium aluminum silicate, AbeAuj to AbjAnii (Na2O = 10.0 to 8.7 per cent.; CaO=3.0 to 5.3 per cent.). Blowpipe Tests. Fusible at 4. Insoluble in ordinary acids. Occurrence. 1. In acid and intermediate igneous rocks, such as the granite-rhyolite series and the diorite-andesite series. Strange to say, oligoclase is more common in granites than albite. LABRADORITE, Ab^Ani to AbjAn, Form. Labradorite occurs in embedded crystals and in cleav- able masses, but very rarely in distinct loose crystals. Albite twinning is the common kind of twinning. Cleavage, perfect parallel to {001), less perfect parallel to {010}. H. = 6. Sp, gr.±2.71. 392 INTRODUCTION TO THE STUDY OF MINERALS Color, gray or white, often showing a play of colors which is an optical effect due to minute inclusions. Optical Properties. n^(1.570) -n„(l. 561) =0.009. Fragments are plates with middle first order interference colors and extinc- tion angles of about 12°(001) and about 20°(010). The frag- ments usually show polysynthetic twinning and often minute inclusions arranged in rows. Chemical Composition. Calcium-sodium aluminum silicate, AbiAui to AbiAug; (CaO = 10.4 to 15.3 per cent.; Na20 = 2.8 to 5.7 per cent.). Blowpipe Tests. Fusible at 4. Soluble with difficulty in HCl. Uses. Labradorite rock is used as an ornamental stone. Occurrence. 1. In basic igneous rocks such as gabbros, dia- bases, and basalts, associated with olivine, augite, hypersthene, ilmenite, and magnetite. 2. As the principal constituent of anorthosite, a basic plutonic igneous rock composed practically of labradorite. Adirondack Mts., New York State. B. METASILICATES Pyroxene Group Amphibole Group LEUCITE, ENSTATITE, Hypersthene, DIOPSIDE, AUGITE, Aegirite, Spodumene, WoUastonite, Pectolite, RHODONITE, f TREMOLITE- ACTINOLITE, HORNBLENDE, Glaucophane, KAl(Si03), MgSiOj (Mg,Fe)Si03 Ca(Mg,Fe)(Si03), mCaMgCSiOj) ^ + n (Mg,Fe) (Al„Fe) ^SiO NaFeCSiOj), LLA.l(SiO,)2 CaSiOs HNaCa,(Si03), MnSiOs Ca(Mg,Fe)3(Si03), mCa(Mg,Fe)3(SiO,)3 + n(Al,Fe)(F,OH)Si03 NaAl(Si03),.(Fe,Mg)Si03 SILICATES 393 BERYL, Be3Al3(Si03), CALAMINE, Zn,(OH)2Si03 TALC, H,Mg3(Si03), PyrophyUite, HAl (SiOj) j ILMENITE, FeTiOj LEUCITE, KAlCSiOg)^ Form. For leucite the characteristic form is well-defined embedded crj-stals. The crystals are isometric, the only com- mon form being the trapezohedron {211} (Fig. 535). Cross-sec- tions are eight-sided (Fig. 536) . Fig. 535. Fig. 536. H. = 5Ato6. Sp.gr. +2.5. Color, white or gray. Optical Properties. n=1.50. Isotropic. Fragments are ir- regular and are either dark between crossed nicols or have very low first order interference colors. Chemical Composition. Potassium aluminum metasilicate, KAl (8103)2- A little sodium is sometimes present. Blowpipe Tests. Infusible. Decomposed by HCl. , Occurrence. 1. In certain basic volcanic rocks in which leu- cite takes the place of the feldspars. Leucite is rare in the United States, but is especially common in central Italy. PYROXENE GROUP The minerals listed under metasilicates from enstatite to rhodo- nite inclusive constitute a mineral group, though they are not 394 INTRODUCTION TO THE STUDY OF MINERALS strictly isomorphous, for enstatite and hypersthene are ortho- rhombic and rhodonite is triclinic, while the others are monoclinic. Typical analyses of the more important pyroxenes are given in the following tabulation. Analyses of Pyroxenes MgO FeO CaO AbOa re203 Si02 36.9 3.2 1.3 58.0 29.7 10.1 1.3 58.0 21.3 21.3 3.1 0.4 51.4 17.3 1.9 25.0 0.5 1.0 54.3 16.1 5.0 24.9 1.5 0.6 52.8 10.0 12.3 22.1 2.0 1.3 51.1 16.4 8.4 20.3 3.8 60.2 16,0 4.1 19.0 9.8 4.5 46.9 7.6 9.4 12.3 21.5 3.8 42.2 13.2 4.3 21.3 8.2 3.7 46.5 0.3 9.5 2.0 1.8 23.3 51.4 Misc. Enatatite . . . Bronzite . . . . Hypersthene Diopside. . . . Diopside . . . . Diopside . . . . Diallage . . . . Augite Augite Augite Aegirite . . . ign. =0.8 MnO=1.0 MnO =0.1; ign. =0.3 Na2O=3.0 TiOj = 2.8 NajO = 11.9; TiOj =0.1 ENSTATITE, MgSiOj Form. Enstatite usually occurs in lamellar or fibrous-lamellar masses. The mineral is orthorhombic, but distinct crystals are very rare. Cleavage, distinct in several directions. H. = 5ito6. Sp.gr. ±3.3. Color, bronze, gray, or brown. Luster, metalloidal. Optical Properties. n^(1.67) -?!„(1.66) =0.01. Fragments are -prismatic with parallel extinction, low first order interfereftce colors, and positive elongation. Chemical Composition. Magnesium metasilicate; (MgO = 40.0 per cent.) . Ferrous iron usually replaces part of the magnesium. Ferriferous enstatite is called bronzite. Blowpipe Tests. Fusible on thin edges (6). Insoluble in acids. SILICATES 395 Occurrence. 1. In basic igneous rocks such as peridotites and gabbros. 2. In meteorites. Hypersthene, (Fe,Mg)Si03 Form. Hypersthene usually occurs in cleavable masses or disseminated through rock masses. Cleavage, good cleavage in one direction. H. = 5J. Sp.gr. ±3.4. Color, dark brown or greenish brown. Optical Properties. n^(1.70) - w„(1.69) =0.01. Fragments are prismatic with parallel extinction, bright interference colors, and positive elongation. Hypersthene is usually pleochroic, chang- ing from pink to green. Chemical Composition. Ferrous-magnesium metasilicate, (Fe,Mg)SiO,. Blowpipe Tests. Fusible (5) to a black glass. On charcoal in R.F. becomes magnetic. Soluble with difficulty in HCl. Occurrence. 1. In basic igneous rocks especially gabbros and norites. DIOPSIDE, Ca(Mg,Fe) (SiO,) , Form. In crystals, -granular masses, disseminated through rocks, but rarely fibrous or columnar. Monoclinic system, prismatic class. a:&:c= 1.092: 1 :0.589; /9 = 74° 10^ Usual_ forms :_a{ 100), 6{010), m(llO), c{001}, pjlll), OJ221}, /f{311}, d[10l\. Interfacia]_angles: mm(110: iTO) =92° 50'; pp(lll : iTl) =48° 29'; oo(221 :221) =84° 11' The habit is usually prismatic in the direction of the c-axis. Figs. 537 to 540 represent typical crystals. The cross-section of crystals is characteristic, usually being four- or eight-sided as represented in Fig. 541. 396 INTRODUCTION TO THE STUDY OF MINERALS Cleavage, imperfect in two directions at angles of 87° 10' and 92° 50' (parallel to the unit prism {110}). There is often parting parallel to {001} which is more prominent than the cleavage. The variety diallage has well-defined parting parallel to { 100} . H.=4to6. Sp.gr. +3.2. pXTp IP I P m Fia. 537. Fig. 538. Fig. 539. Fig. 540. Color, white, gray, and various tints of green. Optical Properties. Wj.(1.70) -n„(1.67) =0.03. Fragments are prismatic and colorless or pale green with bright interference colors and an extinction angle of 20°-30°. A thin parting parallel to {001} gives an interference figure consisting of an Fig. 541. — Cross-sections of diopside and augite. axial bar with concentric rings. Diallage has parallel extinction and positive elongation. Chemical Composition. Calcium magnesium-ferrous meta- silicate varying from CaMg(Si03)2 to CaFe(Si03)2. It may also contain small amounts of aluminum, ferric iron, and manganese. SILICATES ' 397 Blowpipe Tests. Fusible at 4 to a colorless or pale green glass. Insoluble in acids. Occurrence. 1. In crystalline limestones as a contact mineral associated with garnet. 2. In schists and other metamorphic rocks, both in the rock mass and in seams. 3. In gabbros and peridotiteg (the variety diallage). AUGITE, wCaMg (SiO 3)2 + ^ (Mg.Fe) (Al.Fe) ^SiO, Form. Augite usually occurs in embedded crystals. The crystals are monoclinic, prismatic class, with the forms: a{100), 6{010!, m{110}, s{Tll!. Interfacial angles: mm(110:lT0) = 92° 50', ss(Tll:ni)=59° 11'. The habit is usually prismatic (Figs. 542-4) and either square or octagonal in outline (see Fig. 541). Twins with a { 100} as twin-plane are common (Fig. 545). ■b Fig. 542 a m. Fig. 543. Fig. 544. 'b', Fig. 545. Cleavage, imperfect in two directions parallel to {110}, and at angles of 92° 50' and 87° 10'. H. = 5i. Sp.gr. +3.3. Color, dark green to black. Optical Properties. n^(1.73) -n„(1.71) =0.02. Fragments are prismatic with bright interference colors and large extinction angles (25° to 40°) . In thin fragments it is only slightly pleochroic, if at all. This usually distinguishes augite from hornblende. 398 INTRODUCTION TO THE STUDY OF MINERALS Chemical Composition. An isomorphous mixture of CaMg (8103)2 and (Mg,re)(Al,Fe)2SiOe in varying proportions. The presence of aluminum and ferric iron distinguishes augite from diopside. Sodium and titanium are sometimes present. Blowpipe Tests. Fusible at 4 to a black glass. Insoluble in acids. Occurrence. 1. In basic igneous rocks, especially basalts and diabases, often as phenocrysts. Bohemia. Aegirite, NaFe(Si03)2 Form. In crystal form aegirite is like augite, but is more apt to occur in long prismatic or acicular crystals. Cleavage, imperfect prismatic in two directions at angles of 93° and 87° H. = 6. Sp.gr. ±3.5. Color, black in thick crystals, in thin crystals green. Optical Properties. n^=1.75. Fragments are prismatic and green in color. The interference colors are bright and the extinc- tion practically parallel. The pleochroism from yellowish-green to bright green is a prominent feature. Chemical Composition. Sodium ferric metasilicate, NaFe- (8103)2. Analyses also show ferrous iron. Blowpipe Tests. Fusible (3) to a black slightly magnetic globule, coloring the flame yellow. Only slightly soluble in HCl. Occurrence. 1. In soda-rich igneous rocks such as nepheline- syenites, phonolites, soda-syenites, and soda-granites. Magnet Cove, Arkansas. Spodumene, LiAl (8103)2 Form. Spodumene occurs in rough monoclinic crystals and in cleavable masses. The habit of the crystals is prismatic and usually tabular parallel to {100}. SILICATES 399 Cleavage, in two directions at angles of 93° and 87°- There is also, at times, parting parallel to { 100} which causes the mineral to break into plates. H.=6i. Sp.gr. +3.1. Color, white, gray, colorless, lilac, greenish. Optical Properties. n^(1.67) — n^(1.65) =0.02. Fragments are prismatic with first order interference colors and oblique extinc- tion of 20° to 25°- Chemical Composition. Lithium aluminum metasilicate, LiAKSiO,)^. Blowpipe Tests. Fuses at 3.5 to a clear glass giving a purple- red flame. Insoluble in acids. Uses. Spodumene has been used to some extent as a source of lithium salts. A transparent lilac variety called kunzite is used as a gem. Occurrence. 1. In granite-pegmatites associated with albite, lepidolite, tourmaline, etc. Pennington County, South Dakota. One crystal from the Etta Mine in this county measured thirty feet in length. WOLLASTONITE, CaSiOg Form. Wollastonite is found in cleavable, columnar, fibrous, and compact masses. Distinct crystals are rare. They are monoclinic and are elongate in the direction of the 6-axis. Cleavage, in two directions (001 and 100) at angles of 84^°. H.=4ito5. Sp.gr. ±2.9. Color, white or gray. Optical Properties. w^(1.63) -n„(1.62) =0.01. Fragments are acicular with parallel extinction and positive elongation. The interference colors are bright. Chemical Composition. Calcium metasilicate, CaSiO,. Blowpipe Tests. Fuses at 4 to a white glass giving a yellowish- red flame. 400 INTRODUCTION TO THE STUDY OF MINERALS Gelatinizes with HCl, and usually effervesces because of ad- mixed calcite. Occurrence. 1. In crystalline limestones at the contact with igneous rocks and often associated with garnet, diopside, etc. Pectolite, HNaCa2(Si03)3 Form. Pectolite occurs in acicular crystals, in fibrous radiat- ing forms, and in compact masses. Crystals are monoclinic, but usually are very minute. H. = 4J. Sp.gr. ±2.7. Color, white. Optical Properties. ?i^(1.63) — n^(1.60) =0.03. Fragments are acicular with bright interference colors, parallel extinction, and positive elongation. Chemical Composition. Acid sodium calcium metasilicate HNaCa2(Si03)3 or H2Si03-Na2Si03-4Ca(SiO)3; (H20 = 2.7 per cent.). Blowpipe Tests. Easily fusible (2J) giving a yellow flame. In the closed tube gives a little water. Decomposed by HCl with the separation of silica. Occurrence. 1. A secondary mineral in cavities of basalts and diabases associated with calcite, zeolites, prehnite, and datolite. Bergen Hill, New Jersey. RHODONITE, MnSiOg Form. Rhodonite is found in cleavable and compact masses and occasionally in crystals. The crystals are triclinic, but similar to diopside and augite in angles. Cleavage, in two directions at angles of 92^° (parallel to 100) and also an additional parting parallel to (100) the angle (100:110) being 48° 33'. H.=6. Sp.gr. +3.6. Color, pink or red, often stained black by manganese oxids. SILICATES 401 Optical Properties. 7t^(1.74)-n„(1.72) =0.02. Fragments are prismatic with bright interference colors and large extinction angles (20 to 25°). Chemical Composition. Manganese metasilicate, MnSiOg. Calcium is usually present and sometimes iron. Blowpipe Tests. Fusible at 3 to a dark glass. Partially soluble in HCl. Uses. Compact rhodonite is used as an ornamental stone, especially in Russia. Occurrence. 1. In veins. 2. In crystalline limestones with willemite, franklinite, and zin- cite. Franklin Furnace, New Jersey. AMPHIBOLE GROUP The amphibole group is parallel to the pyroxene group, but the orthorhombic and triclinic members are so rare that no account of them will be given here. The amphiboles differ from the pyr- oxenes mainly in the prism and cleavage angle, which is 56° instead of 87°. For many of the pyroxenes there are corre- sponding amphiboles, but they cannot be regarded as dimorphous minerals. For example, diopside is CaMg(Si03)2, while the corresponding tremolite is CaMg3(Si03)4. Tremolite-actinolite and hornblende contain a small amount of water of constitution, while diopside and augite, if unaltered, contain none. The follow- ing analyses (except glaucophane) were made by Stanley in order to determine the chemical constitution of the amphiboles. Analyses of Amphiboles CaO MgO 24.1 FeO Ah03 FezOa Si02 Ti02 H2O 1.6 NazO K2O 1 F Tremolite 13.2 0.6 1.8 57.7 0.1 0.5 0.2 , 0,4 Actinolite 12.1 21.2 5.5! 1.2 0.8 56.3 1.8 0.2 0.3 0,1 Hornblende .... 9.8 12.6 10.5 8.3 6.9 43.8 0.8 0.6 3.4 1.3 1,8 Hornblende .... 11. S 11.2 14.3 11.6 2.7 42.0 1.5 0.6 2.5 0.1 0,8 Hornblende .... 12.0 14.2 2.2 17.6 7.2 39.9 1.7 0,4 3.2 0,2 0,1 Glaucophane . . . 2.0 10.3 9.8 9.3 4.4 54.5 0.4 1.8 7.6 0,2 402 INTRODUCTION TO THE STUDY OF MINERALS TREMOLITE-ACTINOLITE, Ca(Mg,Fe)3(Si03), Form. Tremolite and actinolite occur in long prismatic crys- tals and in columnar and fibrous aggregates. Crystals are monoclinic with the prism {llOj and the pinacoid {010}, but rarely have terminal faces. The axial ratios and interfacial angles are like those of hornblende. Characteristic cross-sec- tions are shown in Fig. 546. a Fig. 546. — Cross sections of tremolite, actinolite, and hornblende. Cleavage, in two directions at angles of 56° and 124° parallel to { 110} . The cleavage is more perfect than that of diopside. H.=5i. Sp.gr. +3.0. Color, white or gray (tremolite); pale green to dark green (actinolite) . Optical Properties. n^(1.636) - n„(1.611) =0.025. Fragments are prismatic or acicular with bright interference colors, positive elongation, and extinction angles of 10° to 15°. Chemical Composition. Calcium magnesium-iron metasilicate Ca(Mg,Fe)3 (8103)4. Tremolite has very little, if any, iron, while actinolite runs as high as 6 per cent, in FeO. Aluminum and ferric iron are very low and this is the principal chemical dis- tinction between these minerals and hornblende. Blowpipe Tests. Fusible at 4 to a glass. Insoluble in acids. Uses. The fibrous tremolite is one kind of asbestos. Occurrence. 1. Tremolite occurs especially in crystalline dol- omitic limestones. Lee, Massachusetts. 2. Actinolite occurs especially in schists often associated with talc. p SILICATES 403 HORNBLENDE, mCa(Mg,Fe)3(Si03), + n(Al,Fe) (F,0H)Si03 Form. Hornblende occurs in well-defined crystals, in cleav- ages, in disseminated crystals ind grains, and in bladed aggre- gates. Monoclinic system. Prismatic class. Axial ratio: a:b:c = 0.551:1:0.293; /? = 7_3° 58'. Usual forms: m{110}, 6{010), a{100}, r{011j, p{101}. Interfacial angles: mm(110:lT0) = 55° 49', n{011:0Tl}=31° 32', rp(011 :T01) =34° 25'. Habit short to long prismatic, usually pseu- dohexagonal or rhombic in cross-section. (See Fig. 546.) The common type of hornblende crys- tal is that of Fig. 547. Cleavage, perfect in two directions at angles of 56° and 124°, parallel to {110}. H.=5i. Sp.gr. ±3.2. ^'°- '"■ Color, dark green, dark brown, or black. Optical Properties. n^(1.653) -n„(1.629) =0.024. Fragments are prismatic and green or brown in color. The extinction angle varies from 5° to 20° and the elongation is positive. Pleocjaroism is a marked feature of hornblende. The colors vary from pale to deep green, from yellowish-green to bluish-green, from brown to greenish-brown, or from pale to deep brown. By the pleochroism and the extinction hornblende may easily be distinguished from augite, which it often greatly resembles. Chemical Composition. A complex metasilicate of calcium, magnesium, ferrous iron, aluminum, and ferric iron with fluorin and hydroxyl. The formula given above was established by Pen- field and Stanley (see analyses, page 401). Blowpipe Tests. Fusible at 4 to a black glass. Insoluble in acids. Occurrence. 1. In volcanic igneous rocks such as andesites and certain basalts. 2. In plutonic igneous rocks especially granites, syenites, and diorites, rarely in gabbros and peridotites. 404 INTRODUCTION TO THE STUDY OF MINERALS 3. In diabases and gabbros as an alteration product of augite. This alteration product is called uralite and the process is known as uralitization. 4. In schists and gneisses often forming rock masses, the horn- blende schists and amphibolites. Glaucophane, NaAlCSiOj) j- (Fe,Mg)Si03 Form. Glaucophane occurs in small disseminated crystals and in fibrous masses. Crystals are prismatic in habit with { 100 } , j 10 } , and {110}, but are rarely terminated. The cross-section as seen in thin rock sections is pseudo-hexagonal or rhombic (Fig. 546). Cleavage, parallel to {110}, i.e., in two directions at angles of 56° and 124°. H.=6. Sp.gr. +3.1. Optical Properties. nj.(1.64) — n„(1.62) =0.02. Fragments are prismatic and blue in color with pleochroism from blue to violet. The extinction is practically parallel and the elongation positive. Chemical Composition. Sodium aluminum, iron-magnesium metasilicate, NaAl(Si03)2-(Fe,Mg)Si03. Glaucophane is one of the group known as soda amphiboles. Blowpipe Tests. Easily fusible (3) to a dark glass giving an intense yellow flame. *Insoluble in acids. Occurrence. 1. In schists and gneisses often constituting the main part of the rock. Glaucophane schists and glaucophane gneisses are especially abundant in the Coast Ranges of Califor- nia. BERYL, Be3Al3(Si03)e Form. For beryl the characteristic forms are crystals and columnar masses. Beryl crystallizes in the hexagonal system, and is one of the few examples of the dihexagonal bipyramidal classes. Axial ratio : c = 0.49,8. Usual forms: cjOOOl}, m{ 1010} SILICATES 405 a{1120), pjlOlll, s{1121}. Interfacial angles: cpJOOO 1:1011) = 29° 561.'; cs{ 0001: 1121) =44° 56'; ma(10T0:1120) =30° 0'. The habit is usually prismatic (Figs. 548-550), but sometimes tabular (Fig. 551). Crystals are often very large. H. = 7ito8. Sp.gr. ±2.7. Color, usually various tints of green, but sometimes white, yellow, pink, or blue. Optical Properties. n^(1.570) -n„(l. 564) =0.006. Fragments are irregular with low first order interference colors. Fig. 548. Fig. 549. Fig. 550. Fig. 551. Chemical Composition. Beryllium aluminum metasilicate, Be3Al2(Si03)g. The alkalies, sodium, lithium, and csesium often partly replace beryllium. Blowpipe Tests. Fusible on thin edges (6) . Insoluble in acids. Uses. The deep green variety, emerald, is a valuable gem. A sea-green variety called aquamarine and a yellow variety called golden beryl are also gems. Occurrence. 1. In granite-pegmatites associated with topaz, albite, lepidolite, etc. San Diego County, California. 2. In mica-schists and gneisses. North Carolina. CALAMINE, Zn2(OH)2Si03 Form. Calamine occurs as drusy crystalline coatings, more rarely in botryoidal and stalactitic forms and also massive. Crystals are orthorhombic, pyramidal class. The habit is usually 406 INTRODUCTION TO THE STUDY OF MINERALS ^^ tabular parallel to {OlOj and the two ends of the crystal are differently terminated. Fig. 552 is a typical' crystal with the forms cjOOl), 6{010!, m{110), z{031}, i{30li, e{OlT}, ?;{12T}. H. = 5. Sp.gr. +3.4. Color, colorless, white, and pale colors. Optical Properties. n^(1.64) — n„(1.61) =0.03. Fragments are irregular and prismatic with parallel extinction. Crystals have parallel extinction and positive elongation. The interference colors are bright. Chemical Composition. Basic zinc meta- silicate, Zn2(OH)2Si03 or ZnSiOa-ZnCOH)^; (Zn = 54.2 per cent., H20 = 7.5 per cent.) The common impurities are iron and aluminum. Blowpipe Tests. Fusible on the edges (5). In the closed tube decrepitates and gives off water. Heated with cobalt nitrate solution on charcoal, the assay becomes blue and the sublimate on the coal green. Soluble in HCI, giving a fine jelly. Uses. Calamine is one of the ores of zinc. Occurrence. 1. A secondary mineral usually derived from sphalerite and often associated with smithsonite. It is found in the upper or oxidized zone of mines. Granby, Newton County, Missouri, is a prominent locality. Fig. 552. TALC, H3Mg3(SiO,), Form. Talc is found in scales, in foliated, compact, or fibrous masses. Distinct crystals are exceedingly rare. Cleavage, perfect in one direction. H. = usually 1, bu-t-se-metimes 4. Sp. gr. + 2.7. Color, white, gray, or pale green. Luster, pearly. SILICATES 407 Optical Properties. 71^,(1.59) — n„(l. 54) =0.05. Cleavage flakes give a negative biaxial interference figure with a small axial angle (2E=10°-20°). Chemical Composition. Acid magnesium metasilicate, H.^Mgs- (SiOJ^; (H20 = 4.8 per cent.). Talc usually contains Fe and Al in small quantities. Blowpipe Tests. Fusible (5i) on thin edges. In the closed tube gives water on intense ignition. When heated with cobalt nitrate solution intensely it gives a faint pink color. Not decomposed by acids. Uses. Talc is used for soap, French chalk, talcum powder, and in the form of soapstone as a refractory material. A fibrous variety is used in the manufacture of paper. Occurrence. 1. A secondary mineral occurring as an alteration product of various silicates such as serpentine, enstatite, and actinolite. 2. In schists often forming the rock-masses known as talc- schists and soapstones. Pyrophyllite, HAUSiOj)^ Form. In radiated forms and compact masses, but not in distinct crystals. H. = li. Sp.gr. ±2.8. Color, white, yellow, gray, brown. Luster, pearly. Optical Properties. 71^.(1.59) — n_j(l. 57) =0.02. Fragments are prismatic with parallel extinction and positive elongation. The interference colors are low first order. Chemical Composition. Acid aluminum metasilicate, HAl- (SiOa)^; (H2O = 5.0 per cent.). Blowpipe Tests. Infusible, but often exfoliates. In the closed tube gives water on intense ignition. Heated with cobalt nitrate solution it becomes deep blue. This test distinguishes pyrophyl- lite from talc. Partially decomposed by H2SO4. Occurrence. 1. In schistose metamorphic rocks. 24 408 INTRODUCTION TO THE STUDY OF MINERALS ILMENITE, FeTiOg Form. Ilmenite occurs in tabular hexagonal crystals, in flat plates without definite outline, in disseminated grains, in compact masses, and in the form of sand. The crystals resemble those of hematite in habit and angles, but the crystal class is different, ilmenite belonging to the trigonal rhombohedral class. H. = 5to6. Sp.gr. ±4.7. Color, black. Luster, submetallic or metallic. Streak, black to brownish-red. Slightly magnetic. Chemical Composition. Ferrous metatitanate, FeTiOg; (Fe = 36.8 per cent.), analogous to a metasilicate. Ilmenite usually contains ferric iron and also magnesium. It grades on the one hand into hematite and on the other into MgTiOj, a mineral called geikielite, as the following analyses show. Pyrophanite with the composition, MnTiOj, and senaite with the composition, (Fe,Mn,Pb)Ti03, also belong to the ilmenite group. Analyses of Minerals of the Ilmenite Group FeO MgO MnO TiO, Fe.O, Misc. Hematite . . . 7.6 0.4 9.1 83.4 Ilmenite .... 22.4 0.5 0.3 23.7 53.7 Ilmenite .... 36.5 0.6 2.7 45.9 14.3 Picroilmenite 24.4 14.2 56.1 5.4 Geikielite . . . 6.3 28.5 63.8 1.9 46.9 50.5 1.1 Si02 = 1.6 Senaite 27.0 0.3 10.4 52.1 PbO = 10.9 Blowpipe Tests. Infusible. The soda fusion dissolved in HCl and boiled with metallic tin gives a violet solution. Slowly soluble in HCl. Decomposed by fusion with KHSO4 Occurrence. 1. In igneous rocks, especially diabases. 2. As a magmatic segregation in igneous rocks. 3. As a prominent constituent of sands, especially the "black sands." SILICATES C. ORTHOSILICATES NEPHELITE, NaAlSiO, Sodalite / Sodalite, Na.Al3Cl(SiO.)3 Group. [ Lazurite, Na^3S(SiO.)3 GARNET, R"3R"i,(SiOj3 OLIVUNK, (Mg,Fe),SiO, WILLEMITE, Zn^SiO, WERNERITE, mCa,AleSieO,5 + nNa^AljSijO^jCl VESUVIANITE Ca3Al3(OH,F)(SiO,). ZIRCON, ZrSiO, TOPAZ, Al,F,SiO, Sillimanite, Al^OSiO, ANDALUSITE, Al^OSiO, CYANITE, Al^OSiO. DATOLITE, CaB(OH)SiO, Zoisite, HCa^3(SiO,)3 EPIDOTE, Ca,(Al,Fe)3(OH)(SiOJ3 Allanite, (Ca,Fe)(Al,Ce,Fe)3(OH)(SiO,)3 Axinite, HCa2(Fe,Mn)Al2B(SiO,), PREHNITE, H^,Ca,(SiOj3 Chondrodite, Mg,(F,OH),(SiO.), MUSCOVITE, H,KAl3(SiOJ3 Mica LEPIDOLITE, LiKAl,(OH,F)(Si03)3 Group BIOTITE, (H,K),(Mg,Fe),Al,(SiOJ3 PHLOGOPITE, HJKMg3Al(SiO,)3 409 NEPHELITE, NaAlSiO, Form. Nephelite occurs in embedded crystals or grains and in massive forms. Crystals are hexagonal and short prismatic in habit. The cross-sections are six-sided and rectangular. H. = 5ito6. Sp.gr. +2.6. Color, white, gray, or reddish. Luster, greasy. Optical Properties. w^(l. 543) -n„(l. 538) =0.005. Fragments are irregular with low first order interference colors. Chemical Composition. Essentially sodium aluminum ortho- silicate, NaAlSiO^, but a mixture of this compound with KAlSi04 and NaAlSi.Oc. 410 INTRODUCTION TO THE STUDY OF MINERALS Blowpipe Tests. Fuses at 4 to a colorless glass. Gelatinizes with HCl. Occurrence. 1. In nephelite-syenites, phonolites, and other rare soda rich igneous rocks. It is often associated with ortho- clase, but never with quartz. Magnet Cove, Arkansas. SODALITE GROUP— ISOMETRIC Besides sodalite and lazurite, noselite Na5Al3(S04)(Si04)3 and haiiynite Na3CaAl3(S04)(SiO^)3 belong to this group. Sodalite, Na,Al3Cl(SiO,)3 Form. Sodalite usually occurs in disseminated or massive forms, but rarely isometric dodecahedral crystals are found. Cleavage, indistinct. H.=5ito6. Sp.gr. ±2.3. Color, blue, gray or colorless. Optical Properties. n=1.48. Isotropic. Fragments are ir- regular and colorless, but dark between crossed nicols. Chemical Composition. Sodium aluminum chlorid-orthosili- cate, Na^AljCKSiOJ or 3NaAlSiO,-NaCl. Blowpipe Tests. Fusible with intumescence to a colorless glass. The NaP03 bead with CuO gives an azure blue flame. Gelatinizes with HCl. Occurrence. 1. In soda-rich igneous rocks such as nepheline syenites and phonolites. Lazurite, Na5Al3S(SiOj3 Form. Lazurite usually occurs in compact massive form more or less mixed with calcite, pyrite, and other silicates. This mix- ture is known as lapis lazuli. H. =5 to 5 J. Sp.gr. ±2.4. Color, deep blue. Streak, blue. Optical Properties, n about 1.5. Isotropic. Fragments are SILICATES 411 irregular, deep blue, non-pleochroic, and dark between crossed nicols. Chemical Composition. Sodium aluminum sulfid-orthosilicate, NajAlgSCSiOJa or SNaAlSiO^-NajS. It usually contains calcium and the sulfate radical, due to isomorphous replacement. Blowpipe Tests. Fuses (3) with intumescence to a white glass. Soluble in HCl with gelatinization and with the evolution of HjS. Uses. Lapis lazuli is a valuable ornamental stone. The paint called ultramarine was formerly lapis lazuli, but is now made artificially. Occurrence. 1. As a contact mineral in crystalline limestones associated with diopside and other silicates. Eastern Asia is the principal locality. GARNET, R''R?-(SiOj3 Form. Garnet is found in distinct crystals, which are usually embedded, in granular or compact masses, and in the form of sand. Garnet crystallizes in the hexoctahedral class of the isometric system. The usual forms are the dodecahedron djllO) and the Fig. 563. Fig. 554. Fig. 555. Fig. 556. trapezohedron n{211}. The hexoctahedron {321} is sometimes found, but the cube and octahedron are exceedingly rare forms for garnet. Figs. 553-556 illustrate commonly occurring garnet crystals ranging from dodecahedral habit to trapezohedral habit. Interfacial angles: d(i(110:101)_= 60°; dd(110: iTO) = 90° 0'; rm(211:121)=33°33i';ww(211:211) = 48°lli';dn(110:211)=30°. 412 INTRODUCTION TO THE STUDY OF MINERALS H. = 7. Sp. gr. varies from 3.5 to 4.2. Color, red, brown, yellow, green, black. Optical Properties, n varies from 1.74 to 1.85. Isotropic. Fragments are irregular, colorless or pale red, and dark between crossed nicols. Some varieties have very weak double refraction. Chemical Composition. Garnet is an isomorphous mixture of the following compounds: Mg3Al,(SiOj3 Fe3Al,(SiOj3 Mn3Al2(SiO,)3 Ca3Fe,(SiOj3 Ca3Cr,(SiOj3 It is rare to find a garnet that corresponds exactly to any one of these as can be seen from the following analyses: Grossularite Pyrope Almandite Spessartite Andradite Uvarovite Analyses of Garnets CaO MgO FeO MnO AI2O3 FezOa CriOs Si02 Grossularite Pyrope 33.9 5.0 1.4 2.4 0.5 31.5 31.6 1.7 17.9 3.6 3.7 tr. 1.5 8.1 33.8 29.5 5.7 0.3 0.5 1.1 4.8 37.2 20.2 22.4 22.7 19.2 20.9 2.2 5.7 4.9 5.5 39.5 40.9 37.6 Almandite Spessartite 4.9 35.9 35 7 Andradite 30.4 — 2.0 21.8 35.3 36 9 Blowpipe Tests. All are fusible (3 to 4) except uvarovite, which is infusible. Insoluble before fusion, but after fusion (alone, not with NajCOj) it gelatinizes with HCl (except uvarovite) . Uses. Garnet is used as an abrasive, especially for finishing wood and leather. Some varieties are used for gems. Occurrence. 1. In crystalline limestones especially at con- tacts, associated with wollastonite, diopside, vesuvianite, etc. (grossularite and andradite). 2. In schists and gneisses (almandite). SILICATES 413 3. In eclogites with pyroxenes or amphiboles. 4. In granites and granite-pegmatites (almandite and spessartite) . 5. In peridotites and derived serpentines (pyrope). 6. In nepheline- and leucite-bearing lavas, such as phonolites, etc. {melanite variety of andradite). 7. In seams of chromite as an alteration product (uvarovite). 8. In sands. / d \ OLIVINE, (Mg,Fe)2SiO, Form. For olivine the characteristic occurrences are granular masses or disseminated crystals and grains. Crystals are ortho- rhombic and are usually tabular in habit. Fig. 557 represents a crystal with all the seven type forms of the rhombic bipyramidal class. a{100}, &i010), c|001}, mjllO}, d{101j, ^1021), p[in}. H.=6ito7. Sp.gr. ±3.3. Color, yellowish green to bottle green. Optical Properties. ?i/1.70) -n„(1.66) = 0.04. Fragments are irregular and colorless with bright interference colors. Chemical Composition. Magnesium-iron orthosilicate (Mg,Fe)2Si04, varying from Mg2SiO^ (forsterite) to FejSiO^ (fayalite). The following analyses show the range in composition: ^ Fig. 557. MgO FeO CaO SiO, Misc. Forsterite Olivine 54.4 50.3 44.1 30.6 1.5 8.5 17.5 28.1 65.5 0,8 1.4 42.8 41.2 39.2 38.9 32.4 ign.=0.8 Olivine MnO-1.2 Favalite MnO - 2 1 * fc — 414 INTRODUCTION TO THE STUDY OF MINERALS Blowpipe Tests. Infusible. Gelatinizes with HCl. Uses. Clear transparent olivine is used as a gem under the name peridot. Occurrence. 1. In peridotites with enstatite or diallage. The olivine is usually partially altered to serpentine. An igneous rock composed practically of olivine alone is called dunite. 2. In basalts, diabases, and gabbros. If the olivine is promi- nent, these rocks are called olivine-basalts, etc. 3. In tuffs and volcanic bombs. 4. In meteorites. Pallasite is a meteorite rock with olivine filling the cavities in a spongy mass of iron. WILLEMITE, Zn^SiO, Form. This mineral is usually crystalline massive or granular massive. Crystals are hexagonal and prismatic in habit with the hexagonal prism {1120}, and the rhombohedron {1011). H.=5i. Sp.gr. ±4.1. Color, pale red, yellow to green. Optical Properties. 71^.(1.71) —n^(1.69) =0.02. Fragments are irregular with bright interference colors. Chemical Composition. Zinc orthosilicate, Zn2Si04; (Zn = 58.0 per cent.). Manganese often replaces part of the zinc. Blowpipe Tests. Fusible (5i) with difficulty. With cobalt nitrate solution on charcoal the assay turns blue and the subli- mate on the coal green. Willemite is distinguished from cala- mine by the absence of water in the closed tube. It gives a fine jelly when dissolved in HCl. Uses. Willemite is a source of zinc white and also of spelter. Occurrence. 1 . In crystalline limestone intimately mixed with franklinite and zincite. It perhaps has been formed by the meta- morphism of calamine present in the original sedimentary lime- stone. Franklin Furnace, New Jersey. SILICATES 415 WERNERITE, mCa^Al^SiaO^s +nNa,Al3Sia02,Cl Form. Wernerite occurs in rough crystals, in cleavable, columnar, and massive forms. Crystals are tetragonal (tetragonal bipyramidal class), prismp,tic in habit and often resemble diop- side or augite crystals. The usual forms are a {100), m{110j, r{lll), and {101}. Interfacial angles: r-r(lll :Tll) =43° 45', mr(U0:lll)=58° 12', am(100: 110) =45°. Fig. 558 represents a typical wernerite crystal. Cleavage, imperfect parallel to {lOOj ai^d {llOj, so in four directions in one zone at angles of 45° H. = 5i. Sp.gr. +2.7. Color, white, gray, greenish, or reddish. Optical Properties. n^(1.57) — n^(1.55) = 0.02. Fragments are prismatic with parallel extinction, bright interference colors, and negative elongation. Chemical Composition. An isomorphous mixture of calcium aluminum silicate with sodium aluminum chlorid-silicate in varying proportions. . Blowpipe Tests. Easily fusible (3) to a white glass with intu- mescence coloring the flame yellow. Partially decomposed by HCl. Occurrence. 1. In crystalline limestones at the contact with i, igneous rocks and associated with diopside, garnet, and other Tsilicates. 2. In gabbros along the border of apatite veins. Formed fro.p plagioclase by the pneumatolytic process known as scapo- litization. Fig. 558. VESUVIANITE, CaeAl3(OH,F)(SiOj5 Form. For vesuvianite the characteristic form is striated columnar masses, but crystals belonging to the tetragonal system are also common. The usual forms are m { 1 10 } , c { 001 } , p { 1 1 1 } 416 INTRODUCTION TO THE STUDY OF MINERALS The interfacial angles are cp(001: 111) =37° 13J'; pp(lll:lll) = 50° 39'; owi(100:110)=45° 0'. The habit is prismatic or low pyramidal and the cross-section is usually square. Fig. 559 is a drawing of a typical vesuvianite crystal. H.=6J. Sp.gr. +3.4. Color, green, yellow-brown, or greenish-brown. Optical Properties. n^(l. 723) -n„(l. 722) =0.001. Fragments are irregular with very low firist order anomalous interference colors. Chemical Composition. Basic calcium aluminum orthosili- cate, CajAljCOHjF) (810^)5. Iron replaces part of the aluminum and magnesium, part of the calcium. Blowpipe Tests. Fuses at 3 with intumes- cence to a colored glass. In the closed tube at a high temperature it gives a little water (about 2 per cent.). Slightly decomposed by HCl. After fusion (alone) it gelatinizes with HCl. Occurrence. 1. In crystalline limestones at the contact with igneous rocks and associated with garnet, diopside, wollastonite, etc. Fig. 559. ZIRCON, ZrSiO, Form. Zircon is practically always found in crystals which are either embedded or loose occuring in sands. Zircon is one of the best examples of the ditetragonal bipyram- idal class of the tetragonal system, c = 0.640. Usual forms: m{110},_a{100}, p{lll), it{331}, a;1131). Interfacial angles: pp(lll:lll)=56° 40'; wp(110: 111) =47° 50'; op(100:lll) = 61° 40'; mi<(110:331)=20° 12'. The habit is low pyramidal or prismatic (Figs. 560 to 563). H.=7J. Sp.gr. ±4.6. SILICATES 417 Color, usually brown but also red, yellow, and colorless. Luster, adamantine. Optical Properties. 71^.(1.98) — n„(l. 93) =0.05. Fragments are irregular with high order interference colors. Chemical Composition. Zirconium orthosilicate, ZrSiO^. Blowpipe Tests. Infusible, but loses color. Insoluble in acids. Fig. 560. Fig. 561. Fig. 662. Fig. 563 Uses. Certain Icinds of zircon called hyacinth are used as gems. Zircon is the source of zirconia (ZrOj) used as a glower in the Nernst lamp. Occurrence. 1. In igneous rocks especially the acid rocks rich in soda such as syenites and soda-granites. 2. In sands and gravels. Ceylon. TOPAZ, AljFjSiO^ Form. Topaz occurs in well-defined crystals and in cleavable masses. Topaz crystallizes in the bipyramidal class of the ortho- rhombic system. a:6:c = 0.528:l:0.477. Usual forms: m{110}, l{120}, c{001), /{021}, 2/{041}, m{111), o{221}, J{223}. Inter- facial angles: mm(110:110) =55° 43'; ZZ(120:120) =_86° 49'; m?(110:120)=29° 44'; //(021 :021) =87° 18'; i/2/(041 :041) = 124° 41'; ci (001: 223) =34° 14'; cm(001 : 111) =45° 35'; co(001:221) = 63° 54'. The habit is usually prismatic, with m{ 110} and Z{ 120} about equally developed. Figs. 564-567 represent typical crystals. 418 INTRODUCTION TO THE STUDY OF MINERALS Cleavage, perfect in one direction parallel to {001). H. = 8. Sp.gr. +3.5. Color, colorless, white, yellowish, bluish, reddish. Optical Properties. n^(1.622) - n„(1.613) =0.009. Fragments are plates with irregular outline and low first order positive inter- ference colors. Cleavage flakes give a positive biaxial interference figure. The optical orientation of topaz is a — a, P = h, y = c; hence (010) is the axial plane. \ V, ju ^ m m I FiQ. 564. Fig. 565. Fig. 566. Fig. 567. Chemical Composition. Aluminum fluorid-silicate, AljFjSiO^, or 2AlF3-Al4(SiOj3. Hydroxyl replaces part of the fluorin. Blowpipe Tests. Infusible. With cobalt nitrate solution it gives a deep blue color. On intense ignition in a closed tube some varieties give a little water. Heated with NaPOg in the closed tube etches the tube. Partially decomposed by H2SO4. Uses. Topaz is sometimes used as a gem. Occurrence. 1. In granite-pegmatites and surrounding rocks associated with tourmaline, lepidolite, albite, fluorite, apatite, beryl, etc. El Paso County, Colorado. Sillimanite, Al20Si04 Form. In prismatic and acicular crystals and in fibrous masses. Sillimanite crystallizes in the orthorhombic system, but SILICATES 419 distinct crystals are rare. The prism faces with (110:110 = 88°) are usually the only forms present. H.=6i. Sp.gr. ±3.2. Color, brown, gray, or white. Optical Properties, ?i^(1.68) -n„(1.66) =0.02. Fragments are prismatic or acicular with parallel extinction and positive elonga- tion. The interference colors are bright. Chemical Composition. Aluminum oxid-silicate, Al20Si04 or AlA-Al4(SiO,)3. Blowpipe Tests. Infusible. It turns blue with cobalt nitrate solution. Insoluble in acids. Occurrence. 1. In gneisses and schists, apparently always the result of metamorphism. On heating to 1350° G., andalusite and cyanite change into sillimanite. ANDALUSITE, Al^OSiO, Form. Andalusite occurs in rough, attached or embedded crystals, in columnar masses, and in rolled pebbles. The crystals are orthorhombic and prismatic in habit, the only common forms being {110} and {OOlj with a prism angle of 89° 12' (110: iTO). (Fig. 568.) H. = 7i. Sp.gr. +3.1. Color, usually gray, often with symmetri- cally arranged white or black areas. (See Fig. 568). Optical Properties. n^(1.64) — n^(1.63) = fig. 568. 0.01. Fragments are irregular or prismatic with parallel extinction and colorless or pleochroic from reddish to greenish. The interference colors are low first order. Chemical Composition. Aluminum oxid-silicate, AljOSiO^, or Al,03-Al,(SiO,)3. 420 INTRODUCTION TO THE STUDY OF MINERALS Blowpipe Tests. Infusible. It turns blue when heated with cobalt nitrate solution. Insoluble in acids. Occurrence. 1. In schists and slates as a product of contact metamorphism. Lancaster, Massachusetts. CYANITE, Al^OSiO, Form. For cyanite the characteristic form is bladed crystals or crystal aggregates. Crystals are tri clinic, tabular parallel to { 100) , and elongated in the direction of the c-axis. Cleavage, perfect in one direction parallel to { 100} . There are imperfect cleavages in other directions. H. = 4 J (parallel to the length) and 7 (perpendicular to the length). Sp.gr. +3.6. Color, blue, bluish-gray, green, or white, often colored in spots and streaks. Optical Properties. n^(1.728) -n„(l. 712) =0.016. Fragments are prismatic or acicular with oblique extinction of 30° and bright interference colors. Cleavage flakes give a biaxial interference figure with the axial plane oblique to the edge. Chemical Composition. Aluminum oxid-silicate, AljOSiO^ or AlzOg- Al4(Si04) 3. Sillimanite, andalusite, and cyanite all have the same composition, but differ in physical properties. Blowpipe Tests. Infusible. It turns blue with cobalt nitrate solution. Insoluble in acids. Occurrence. 1. In schists and gneisses as the product of metamorphism. DATOLITE, CaB(OH)SiO, Form. Datolite occurs in crystals, crystalline druses, and crystalline masses. Crystals are monoclinic and are usually com- plex and difficult to decipher. As the angle /? (between the a-and c-axes) is 89° 51', the crystals often appear to be orthorhombic. Fig. 569 is an orthographic projection of a datolite crystal. SILICATES 421 H.=5i. Sp.gr. +2.9. Color, colorless, white, pale green. Optical Properties. n^(1.67P) -n„(1.626) =0.044. Fragments are irregular with bright interference colors. Chemical Composition. Basic calcium boron orthosilicate, CaB- (OH)SiO,; (H20 = 5.6 per cent.). Blowpipe Tests. Easily fusible (2) to a glass, with intumescence coloring the flame green. In the closed tube it gives water. Gelatinizes with HCl. Occurrence. 1. A secondary mineral in cavities of diabases and basalts associated with the zeolites, apophyllite, prehnite, and pectolite. Zoisite, HCa2Al3(SiO^)3 Form. Zoisite is usually found in columnar masses. Orthor- hombic crystals are rare. Cleavage, perfect in one direction (010). H.=6to6i. Sp.gr. +3.3. Color, usually gray. Optical Properties. n^(l. 702) -n„(1.697) =0.005. Fragments are prismatic with parallel extinction, negative elongation, and low first order interference colors which are often Berlin blue. Chemical Composition. Acid calcium aluminum orthosilicate, HCa2Al3(SiO,)3; (H20=4.4 per cent.). Blowpipe Tests. Fusible at 3 to a white glass. In the closed tube it yields water. Insoluble in acids, but after fusion gelatinizes with HCl. Occurrence. 1. In metamorphic rocks, especially hornblende and glaucophane schists. 422 INTRODUCTION TO THE STUDY OF MINERALS EPIDOTE, Ca^CAl.Fe) 3(0H) (SiO,) 3 Form. In crystals, in columnar aggregates, and in granular masses. The crystals are monoclinic, prismatic in habit, and are elongated in the direction of the &-axis instead of the c-axis, as in most monoclinic minerals. Usual forms: a{ 100}, &{010j,c{001), w{120), r(T01}, n{Tn}. Interfacial angles: ac(100:001) =64° 37'; cr(001:T01)=63° 42'; nn(lll :111) =70° 29'. aM(100:210) = 35°29i'. Fig. 570 is a cross-section of a typical crystal, while Fig. 571 is the cross-section of a twin crystal with a {100} as twin plane. Fig. 570. Cleavage, {001} perfect; {100} imperfect. H.=6i. Sp.gr. +3.4. Color, usually pistache-green, but varies from gray through yellow to deep brownish-green and almost black, according to the amount of iron present. Optical Properties. n^(1.767)-n„(l. 730) =0.037. Fragments are irregular or prismatic with parallel extinction. The interfer- ence colors are bright. The deep colored varieties are pleochroic from colorless to pale green. Cleavage flakes give an interference figure consisting of an axial bar with concentric rings. Chemical Composition. Basic calcium aluminum-iron ortho- silicate, Ca2(Al,Fe)3(OH) (810^)3, an isomorphous mixture of CajAlgCOH) (810^)3, a mineral called clinozoisite, andCajFcjCOH)- (SiOJ 3, not yet discovered.- The following analyses illustrate the isomorphism. SILICATES 423 CaO FeO Clinozoisite . . . 2-1.5 Epidote , 23. S Epidote j 23.9 Epidote 23.3 0.3 0.5 0.9 A1,0, FO2O3 SiO^ H,0 32.6 1.7 39.1 29.5 5.7 38.0 2.0 MnO = 0.2 26.5 8.2 39.2 2.2 22.6 14.0 37.8 2.1 Blowpipe Tests. Fusible at 3 to a colored glass with intumes- cence. In the closed tube at a high temperature it gives a little water (about 2 per cent.). Partially decomposed by HCl. After fusion (alone) it gelatinizes with HCl. Occurrence. 1. At the contact between igneous rocks, espe- cially granites, and limestones, often associated with copper minerals. 2. As a weathering product along seams of igneous rocks, especially granites. 3. In schists, especially as the product of intense folding, asso- ciated with hornblende. New York City. Allanite, (Ca,Fe) (Al,Ce,Fe,) 3 (OH) (SiO,) 3 Form. Allanite occurs in rough crystals, in masses, and in embedded grains. Crystals are monoclinic and usually tabular parallel to (100). H. = 5ito6. Sp. gr. from 3 to 4. Color, black. Luster, submetallic. Optical Properties, n^ = 1.68. Fragments are irregular, brownish, and slightly pleochroic. Isotropic or with low first order interference colors. Chemical Composition. A basic orthosilicate similar to epidote, but with calcium partly replaced by ferrous iron and aluminum and ferric iron partly replaced by cerium and other rare earth metals. 25 424 INTRODUCTION TO THE STUDY OF MINERALS Blowpipe Tests. Easily fusible at 2^ to a dark magnetic glob- ule. In the closed tube it gives water, but usually only at a high temperature. Gelatinizes with HCl. Occurrence. 1. As an accessory constituent of igneous rocks, sometimes intergrown with epidote. Axinite, HCa2(Fe,Mn)Al2B(SiOj4 Form. Axinite occurs in crystals and crystalline aggregates. The crystals furnish about the best known examples of triclinic crystals. The habit is usually tabular and the cross-sections shaped-wedge. Fig. 109, page 38, represents a typical crystal. H.=6ito7. Sp.gr. +3.3. Color, violet, brown, smoky gray. Optical Properties. 71^.(1.68) —n^(1.67) =0.01. Fragments are irregular with interference colors. Thick fragments are pleochroic. Chemical Composition. An acid calcium iron-maganese alu- minum boron silicate, HCa2(Fe,Mn) AljBCSiO^)^. Blowpipe Tests. Easily fusible (2^) with intumescence to a dark glass. In the closed tube at a high temperature gives a little water. Fused with KIISO4 and CaFj it gives a green flame. Insoluble in HCl. Occurrence. 1. At the contact of granites with basic lime- rich rocks such as schists and impure limestones. PREHNITE, HjCa^Al^CSiOJa Form. Prehnite is found in crystalline druses and seams. Distinct crystals (orthorhombic) are very rare. The imperfect crystals are usually grouped in mammillary and globular forms, showing a series of ridges. H. =6 to 6 J. Sp.gr. ±2.9. Color, usually pale green or white. Optical Properties. n^(1.65) -n„(1.62) =0.03. Fragments are irregular with bright interference colors. SILICATES 425 Chemical Composition. Acid calcium aluminum orthosilicate, H2Ca2Al3(Si04)3; (H20=4.4 per cent.). Blowpipe Tests. Easily fusible (2) with intumescence to a white glass. In the closed tube it gives water. Decomposed by HCl and after fusion (alone) gelatinizes with HCl. Occurrence. 1. A secondary mineral in cavities of diabases and basalts associated with datolite, pectolite, apophyllite, and the zeolites. Chondrodite, Mg^CFjOH) 2 (810^)2 Form. Chondrodite occurs in disseminated crystals and grains and is also sometimes massive. Crj'stals are monoclinic, equidimensional, but rather complex. H.=6to6J. Sp.gr. ±3.1. Color, red, orange, yellow, brown. Optical Properties. 71^.(1.64) — n„(1.61) =0.03. Fragments are irregular and colorless, or yellow with slight pleochroism. The interference colors are bright. Chemical Composition. Basic magnesium orthosilicate, Mg5(F,OH)2(SiO,)2 or Mg(F,OH)2-2Mg2SiO,. Iron replaces part of the magnesium and hydroxyl part of the fluorin. Blowpipe Tests. Infusible. In the closed tube it gives a little water (about 1.3 per cent.) at a high temperature. In the closed tube with NaP03 etches the inside of the tube. Gelatinizes with HCl. Occurrence. 1. In crystalline limestones with phlogopite, spinel, etc. Sparta, New Jersey. MICA GROUP The micas are acid orthosilicates of aluminum with magnesium, ferrous iron, and the alkalies. On heating at a high temperature they give off from 2 to 5 per cent, of water. The micas are monoclinic, but pseudohexagonal in habit. Dis- 426 INTRODUCTION TO THE STUDY OF MINERALS tinct terminated crystals are very rare. The very perfect cleav- age parallel to { 001 } is the most striking feature of the micas. The micas are optically biaxial with varying axial angle. In the absence of crystal faces cleavage plates of the micas may be ori- ented by means of the percussion figure in connection with the interference figure. A sharp, quick blow with a dull conical point develops a six-rayed star. If the interference figure lies along one of the rays then that ray is the direction of the 6-axis. Fig. 572. Mica of the first class. Fig. 573. Mica of the second class. These are micas of the first class and are represented by Fig. 572. If the interference figure lies between two rays of the percussion figure, then the third ray is the direction of the 6-axis. These are micas of the second class and are represented by Fig. 573. Muscovite and lepidolite are micas of the first class, while biotite and phlogopite are micas of the second class. MUSCOVITE, H,KAl3(SiOJ3 Form. Muscovite usually occurs in cleavages and scaly masses and but rarely in well-defined crystals. The crystals are tabular in habit, and pseudohexagonal or pseudorhombic, but are really monoclinic. Fig. 574 represents a muscovite crystal, the side elevation on the right proving it to be monoclinic. Cleavage, very perfect in one direction parallel to }001j. H.=2to2i. Sp.gr. ±2.8. SILICATES 427 Color, colorless, pale green, or pale brown. Thin sheets are alwaj's transparent. Optical Properties. ?i^(1.597) - n^(1.560) = 0.037. Cleavage flakes give low first order interference colors and in convergent light, a fine negative biaxial interference figure with large axial angle (2E=60° to 75°). Chemical Composition. An acid potassium aluminum ortho- silicate, H2KAl3(SiO^)3; (H20 = 4.5 per cent.). Fig. 674. Blowpipe Tests. Fusible on thin edges (5) and whitens. In the closed tube it gives a little water. Insoluble in acids and not decomposed by HjSO^. Uses. Muscovite is used principally as an insulator for elec- trical apparatus, but has numerous other uses. Occurrence. 1. In granite-pegmatites and granite-aplites. 2. In schists and gneisses, often the main constituent of the mica-schists. 3. In granites. Granite is about the only igneous rock in which muscovite occurs as an original constituent. 4. In sandstones and sands as a detrital mineral. 5. As a secondary mineral derived from other silicates, espe- cially the feldspars. This variety is called sericite and the altera- tion process is known as sericitization. 428 INTRODUCTION TO THE STUDY OF MINERALS LEPIDOLITE, LiKAl^COH.F) (8103)3 Form. Lepidolite usually occurs in scaly masses, rarely in crystals. Crystals are pseudohexagonal like those of muscovite, but are much smaller. Cleavage in one direction. H. = 2ito3i. Sp.gr. +2.8. Color, pale to deep lilac. Optical Properties. n^(1.60) — n^(1.56) =0.04. Cleavage flakes give low first order interference colors and in convergent light a negative, biaxial interference figure with large axial angle (2E=60° to 80°). Chemical Composition. Lithium potassium aluminum fluorid metasilicate, LiKAl^COH.F) (8103)3. Blowpipe Tests. Easily fusible at 2 with intumescence to a white glass, coloring the flame purple. In the closed tube on intense ignition it gives water which has an acid reaction due to the HF formed. Partially decomposed by HCl. After fusion gelatinizes with HCl. Uses. Lepidolite is a source of lithium salts. Occurrence. 1. In granite-pegmatites and surrounding gran- ites associated with tourmaline, albite, muscovite, spodumene, amblygonite, etc. Pala, San Diego County, California. BIOTITE, (H,K)3(Mg,Fe)Al,(SiOj3 Form. Biotite occurs in embedded crystals and disseminated scales and in lamellar masses. Crystals are pseudohexagonal like those of muscovite. Cleavage, very perfect in one direction. H.=2Jto3. Sp.gr. +2.9. Color, black or dark brown. Thin sheets are translucent. Optical Properties. n^(1.60) - n„(1.57) =0.03. Cleavage flakes are almost dark between crossed nicols and in convergent light SILICATES 429 give a negative biaxial interference figure with a small axial angle (2E = 0-20°). It is sometimes practically uniaxial. Chemical Composition. Acid-potassium magnesium-iron alu- minum orthosilicate (H,K)2(Mg,Fe)2Al2(Si04)3. Blowpipe Tests. Fusible on edges (5) and turns white. In the closed tube gives a little water (2 to 4 per cent.) on intense ignition. Decomposed by concentrated HjSO^. Occurrence. 1. In many kinds of igneous rocks, but espe- cially prominent in granites, and in certain dike rocks known as lamprophyrs. 2. In schists and gneisses, sometimes as the dominant mineral. Biotite is often associated with muscovite. PHLOGOPITE, H^KMggAlCSiOJ, Form. Like the other micas phlogopite occurs in crystals, in disseminated scales, and in lamellar masses. The crystals are pseudohexagonal, but are often prismatic in habit as well as tabular. Cleavage, very perfect in one direction. H.=2Jto3. Sp.gr. +2.8. Color, bronze, brown, yellow. Optical Properties. w^(1.60) — n^(1.56) =0.04. Cleavage flakes give very low first order interference colors and in conver- gent light, a negative biaxial interference figure with a small axial angle (2E = 5° to 20°). Chemical Composition. Acid potassium magnesium aluminum orthosilicate, H2KMg3Al(Si04)3. Also contains iron and fluorin. Blowpipe Tests. Fusible (5) on thin edges and whitens. In the closed tube it gives a little water on intense ignition. Easily decomposed by concentrated HjSO^. Uses. Phlogopite is used principally as an insulator in elec- trical apparatus. Occurrence. 1. In crystalline limestones associated with spinel, graphite, chondrodite, etc. 430 INTRODUCTION TO THE STUDY OF MINERALS D. MISCELLANEOUS ANHYDROUS SILICATES lolite, (Mg,Fe).Al3(0H),(Si,0,), TOURMALINE, RJAl3B,(0H)jSi,0„ STAUROLITE, FeAls(OH)(SiOe)j Margarite, HjCaAliCSiO,)^ CHLORITE, H.(Mg,Fe)eAl,(SiO,), SERPENTINE, H^MgjSi^O, Sepiolite, H^Mg^SijOu Kaolinite, H^AljSijOj Lawsonite, HjCaAl2(Si05)2 TITANITE, CaTiSiOs lolite, (Mg,Fe),Al,(0H),(Si,0,)5 Form. lolite occurs in orthorhombic (pseudohexagonal) crys- tals of short prismatic habit. It is also massive. Cleavage, indistinct. H. = 7to7*. Sp.gr. ±2.6. Color, blue to colorless. Luster, vitreous like quartz. Optical Properties. n^(1.54) -n^(1.53) =0.01. Fragments are irregular with low first order interference colors. In thick frag- ments iolite is pleochroic from dark blue to light blue, blue to colorless, or blue to yellowish. Chemical Composition. A basic magnesium-iron aluminum diorthosilicate (Mg,Fe),Al8(OH)2(Si207)5. Blowpipe Tests. Fuses with difficulty (5) and becomes opaque. Partially decomposed by acids. Uses. lolite is sometimes used as a gem. Occurrence. 1. In gneisses. Guilford, Connecticut. 2. As a contact mineral in slates. Japan. TOURMALINE, RJAl3B2(OH)2Si,Oig Form. Tourmaline usually occurs in distinct attached or embedded crystals, and in columnar subradiating aggregates. Tourmaline is the type example of the ditrigonal pyramidal class SILICATES 431 of the hexagonal system. c = 0.447. Usual forms: a {1120}, m- {lOTO}, ?n,{0lT01, r{10Tlj^f j0lT2),_o{0221}, cfOOOl}, <{2131!, m{3251}, x{1232),_ri{0111}, fi{1012j, CiJGOOT}. Interfacial angles:_rwi(1011:1010)=62° 40' j_ emi(0lT2:0lT0) =75^ 30^'; o?7!j(0221:G110)_=44° 3'; ee(0112:T012) =25°_2^; rr(1011 :T]01) = 46°_ 52'^oo (022 1:2021)= 77^ O'j xa;( 1232:1322) =21° 18;^ 2■■^- (1232:3212) =43° 22,V; «(2131 :23Tl) =63° 48'; «(2131 :3T21) = 30° 38i'. The habit is short to long prismatic and the cross-section three,- six-, or nine-sided, very often being rounded triangular like a spherical triangle. The two ends of the crystals are usually differently terminated. Figs. 575 to 578 represent typical tour- maline crystals. Fig. 576. Fig. 578. Cleavage, none. The absence of cleavage distinguishes tour- maline from hornblende. H.=7to7i. Sp.gr. ±3.1. Color, black, brown, green, blue, red, pink, colorless. The exterior and interior and also the opposite ends of a crystal often differ in color. Transparent colored crystals show pleochroism with a dichroscope. Tourmaline is pyroelectric, that is, a crystal which has been heated will, on cooling, develop positive electricity at one end and negative electricity at the other. This may be tested with a fine hair. 432 INTRODUCTION TO THE STUDY OF MINERALS Optical Properties. nj.(1.65) -n„(1.63) =0.02. Fragments are irregular or prismatic with parallel extinction. The interference colors are bright. The black and deep colored varieties are pleochroic (often from blue to smoke-colored), while the light colored varieties are colorless, but in thick fragments or small crystals are also pleochroic. Chemical Composition. A complex silicate of aluminum, boron, iron, magnesium, and the alkalies. No satisfactory formula has yet been established. Penfield gives R,'Al3B2(OH)2Si40ij, in which R^ is iron, magnesium, and the alkalies. The following are typical analyses: Analyses of Tourmaline LizO Na20 HjO FeO MgO AI2O3 B2O3 SiOz Misc. 1. Pink 1.9 2.1 3.4 0.2 42.2 10.6 37.6 2.0 2. Pale green . . 1.8 2.2 3.3 1.5 41.3 10.6 36.7 3.8 3. Brown tr 0.9 3.1 0.9 14.6 28.5 10.4 35.3 Ca=5.1 4. Black tr 2.2 3.6 11.9 4.5 31.1 9.9 34.9 2.2 S. Black tr 2.0 3.6 14.2 1.0 33.9 9.6 35.0 0.6 Three principal varieties based upon composition and color are recognized namely, (1) iron tourmaline, black (analyses 4 and 5), (2) magnesium tourmaline, brown (analysis 3), (3) alkali tour- maline, red, green, or blue (analyses 1 and 2). Blowpipe Tests. The fusibility varies from easy fusibility at 3 (magnesium variety) to infusibility (alkali variety). In the closed tube at a very high temperature it gives water (from 2 to 4 per cent.). Tourmaline gives a green flame when fused with boracic acid flux. Insoluble in acids. After fusion (alone) it gelatinizes with HCl. Uses. Colored tourmaline is used as a gem. Occurrence. 1. In granite-pegmatites often associated with albite, lepidolite, beryl, apatite, fluorite, etc. Pala, San Diego County, California. SILICATES 433 2. In rocks surrounding pegmatites often associated with cassiterite, topaz, etc. In greisen, a quartz-muscovite rock formed from granite by pneumatolysis; in luxullianite, a variety of quartz-porphyry in which the quartz is partly replaced by tourmaline; and in tourmaline schists. Jefferson Count j', Colorado. 3. In crystalline limestones (the brown magnesium variety), associated with spinel, phlogopite, corundum, etc. Hamburg, New Jersey. 4. In veins with copper minerals. 5. In sands. Brazil. STAUROLITE, FeAl5(OH)(SiOe)2 Form. Staurolite crystallizes in the orthorhombic system and is rarely found massive. The habit is prismatic with the forms c{001), m{110}, 6{010}, and r{l01} (Fig. 579). Interfacial angles: mTO(110: iTO) =50° 40'; cr(001:101) = 55° 16'. Cruciform penetration twins with {032j as twin-plane are common. H.=7to7i. Sp.gr. ±3.7. Color, brown. Optical Properties. n^(1.74) -n„(1.73) = 0.01. Fragments are irregular with upper first order interference colors and are pleo- chroic from light to deep yellow. Chemical Composition. Basic iron aluminum silicate, FeAlj- (OH)(SiO„)2, corresponding to the acid HgSiO„(H,SiO,-|-2H20). The iron is partly replaced by magnesium and the aluminum by ferric iron. Blowpipe Tests. Infusible. Partially decomposed by HjSiO^. Occurrence. 1. In mica schists often associated with cyanite, sillimanite, and garnet." Fig. 579. 434 INTRODUCTION TO THE STUDY OF MINERALS Margarite, H^CaAl^CSiOJj Form. Margarite is a micaceous mineral rarely found in well- defined crystals. Like the micas it occurs in lamellar forms and in scaly masses. Cleavage, perfect in one direction. H.=4. Sp.gr. +3.0. Color, gray, pink, white. Luster, pearly. Cleavage laminse are brittle, hence the name brittle mica sometimes used. Optical Properties. n^=1.64. Cleavage flakes in convergent light give a negative biaxial interference figure with large axial angle (2E = 100°-120°). Chemical Composition. Acid calcium aluminum silicate, H2CaAl,(Si05)2; (H^O =4.5 per cent.). Blowpipe Tests. Fusible at 4J and whitens. In the closed tube yields water. Partially decomposed by HC'l. Occurrence. 1. Associated with corundum and emery, prob- ably as an alteration product. Chester, Massachusetts. CHLORITE,* H,(Mg,F^),Al,(SiO,)3 Form. Chlorite crystals are monoclinic but pseudohexagonal in habit, usually resembling crystals of the micas. The mineral also occurs in disseminated flakes and in scaly masses. Cleavage, perfect in one direction. H.=2to2J. Sp.gr. +2.8. Color, green of various tints and shades, varying from almost white to almost black. Cleavage laminee are flexible , but not elastic. Optical Properties. 11^=1.58. Fragments are irregular, green in color with faint pleochroism, and very low (often Berlin blue) first order interference colors. Cleavage flakes in convergent light * Chlorite ia really the name of a group of minerals, but on account of the difficulty of distinguishing them they are all included under one heading. SILICATES 435 give an interference figure which is either uniaxial, or biaxial with a small axial angle (2E = 0°-60°). Chemical Composition. Acid magnesium-iron aluminum sili- cate. The composition varies for different chlorites; for one of them (clinochlore) the formula is H8(Mg,Fe)5Al2(SiOg)3. In some varieties chromium and ferric iron partly replace the aluminum. Blowpipe Tests. Fusible with difficulty (5^). In the closed tube it gives about 12 per cent, of water at a high temperature. Decomposed by HjSO^. Occurrence. 1. A secondary mineral in igneous rocks, formed by the alteration of such silicates as biotite, hornblende, augite, etc. 2. In schists, often forming independent rock masses, the chlorite-schists. SERPENTINE, H.MggSijOo Form. Serpentine has never been found in crystals, though it often occurs pseudomorphous after other crystallized minerals. It is usually compact or granular massive, but also occurs in fibrous, columnar, and lamellar forms. H. = 3to4. Sp.gr. ±2.5. Color, green of various tints and shades, from greenish-white to greenish-black. It is also often yellow, brown, or red and the color is not apt to be uniform, but is in spots and streaks. Optical Properties. n^=1.57. Fragments are irregular, or prismatic or acicular with parallel extinction and positive elonga- tion. The interference colors are low first order. The irregular fragments show aggregate structure between crossed nicols. Chemical Composition. Acid magnesium silicate, H4Mg3Si20g; (HjO = 12.9 per cent.) . Part of the magnesium is usually replaced by ferrous iron. Some analyses show a little aluminum and some a little calcium. 436 INTRODUCTION TO THE STUDY OF MINERALS Analyses of Serpentine MgO FeO CaO A1,0, Fe,03 SiO, H^O Misc. 42.6 0.1 0.1 0.3 42.0 14.7 36.5 1.9 5.1 42.9 13.2 NiO = 0.6 41.2 2.4 41.3 14.5 36.8 7.2 2.6 41.6 12.7 Cr203 = tr. Blowpipe Tests. Fusible with difficulty (6). In the closed tube gives water at a high temperature. Decomposed by HCl. Uses. Serpentine is used as an ornamental stone. Fibrous serpentine or chrysotile is one of the minerals included under the term asbestos, tremolite being the other. Occurrence. 1. An alteration product of olivine and to a less extent of bronzite, forming the metamorphic rock serpentine from original peridotite. 2. A secondary mineral in seams and veins and on the border of serpentine rocks. Chrysotile, the fibrous variety, and antigo- rite, the lamellar variety, occur in this way. 3. An alteration product of diopside and olivine in crystalline limestones, thus forming the rocks known as ophicalcites. Sepiolite, H^Mg^SijOio Form. Sepolite occurs in compact earthy masses, and occa- sionally in fibrous seams. H.=2to2^. Sp.gr. +2.0. Color, white, yellowish white, grayish white. Sepiolite floats on water when dry. Optical Properties, n about 1.54. Fragments are irregular with low first order interference colors and aggregate structure. Chemical Composition. Acid magnesium silicate, H^MgaSigOio; (H20 = 12.1 per cent.). SILICATES 437 Blowpipe Tests. Fusible on thin edges (5) . In the closed tube yields water. Heated with cobalt nitrate solution it becomes pink. Gelatinizes with HCl. Uses. Sepiolite or meerschaum is used for tobacco pipes. Occurrence. 1. Associated with serpentine. (Asia Minor.) Kaolinite, H^Al2Si20g Form. Kaolinite is sometimes found in minute pseudo-hexa- gonal (monoclinic) crystals of tabular habit. Fig. 580 represents crystals found by the author at Argentine, Kansas, in a dolomitic limestone. The usual occurrence of kaolin- ite is in clayJi^masses. H.=2to2J. Sp.gr. + 2.6. Color, white, grayish, yellowish, etc. Luster, pearly to dull. Optical Properties. n=1.55. Fragments are irregular and show aggregate structure between crossed nicols. Chemical Composition. Acid aluminum silicate, H^AljSijO,; (H20 = 14.0 per cent.). Iron is often present in small amounts. Blowpipe Tests. Infusible if pure. Heated with cobalt nitrate solution it becomes deep blue. In the closed tube gives water. Insoluble in acids. Uses. Kaolin, a mixture of kaolinite and other aluminum silicates with more or less quartz, feldspar, etc., is used in the manufacture of porcelain, china, and pottery. Occurrence. 1. A secondary mineral formed from the feld- spars either by carbonated water or by pneumatolysis. The association at some localities of kaolinite with fluorite and cassiter- ite proves its origin by pneumatolysis. 2. In sedimentary rocks formed by the alteration of aluminous silicates. Fig. 580. Kaolinite crystals (x 500). 438 INTRODUCTION TO THE STUDY OF MINERALS Lawsonite, H^CaAlj (8105)2 Form. Lawsonite usually occurs in tabular or prismatic orthorhombic crystals which are disseminated through the rock. Cleavage, fair but not prominent. H.=8. Sp.gr. ±3.1. Color, white or gray. Optical Properties. n^(1.68) — n„(1.66) =0.02. Fragments are plates with parallel extinction or are irregular. The interference colors are bright. Chemical Composition. Acid calcium aluminum silicate, JE.CaAl^CSiOJ^; (H26 = 11.4 per cent.). Blowpipe Tests. Fusible to a blebby glass with exfoliation at first, but after that it is infusible and glows. In the closed tube it yields water at a high temperature. Insoluble in HCl, but after fusion (alone) it gelatinizes. Occurrence. 1. In glaucophane schists and related rocks. It has been formed from the anorthite molecule of plagioclase. Tiburon Peninsula, California is the type locality of lawsonite. 2. In gabbros and diorites as an alteration product of plagio- clase. TITANITE, CaTiSiOs Form. Titanite occurs in attached crystals, and in dissemi- nated crystals and grains. Crystals are monoclinic of varied habit, but are usually acute rhombic in cross-section. Usual forms: cjOOl}, m{110}, wjlllj. Interfacial angles: mm(110: Tl0)=66° 29'; rm(lll:lTl)=43° 49'; cm(001 :110) =65° 30'; cn(001:lll)=38° 16'. Cleavage. There is sometimes prominent parting in two direc- tions at angles of 54°. H.=5to5i. Sp.gr. +3.5. Color, varying tints and shades of yellow and brown. Luster, adamantine or subadamantine. SILICATES 439 Optical Properties. ((^('J.OO) — ?t„(l.S8) =0.12. Fragments are irregular and slightly pleochroic with very high order interference colors. Chemical Composition. Calcium titanate-silicate, CaTiSiOj, a salt of H^SijOj in which one atom of silicon is replaced by one of titanium. Blowpipe Tests. Fusible at 4 to colored glass. Gives a violet XaPOa bead in R.F. Partially soluble in HCl. Occurrence. 1. An accessory constituent of igneous rocks, very common and widely distributed. 2. In clefts and seams of metamorphic rocks. 3. Disseminated through metamorphic rocks and probably formed from titaniferous pyroxenes. Coast Ranges of California. E. HYDROUS SILICATES Zeolite Group APOPHYLLITE, HEULANDITE, STILBITE, Laumontite, CHABAZITE, ANALCITE, NATROLITE, Gamierite, Allophane, CHRYSOCOLLA, (H,K),Ca(Si03)2-H20 H,CaAl,(Si03),-3H,0 H.(Na„Ca)Al,(Si03)„-4H,0 H,CaAl2(Si20,)2 2H20 (Ca,Wa,)Al,(Si03),-6H,0 KaAl(Si03)2H20 Na^AljSijOio^H^O H,(Ni,Mg)SiO.H,0 Al^SiOjSHjO CuSiO,-2H,0 ZEOLITE GROUP Under the zeolites are included a number of hydrous silicates of aluminum with calcium and the alkalies, which are somewhat similar to the feldspars except for the water of crystallization. They are characterized by low specific gravity (2 to 2.5) and moderate hardness (3 to 5 J) . They are all decomposed by HCl with the separation of slimy silica and are easily fusible (2 to 3) with intumescence, hence the name from the Greek word meaning to boil. 440 INTRODUCTION TO THE STUDY OF MINERALS The zeolites are secondary minerals found in cavities of such basic igneous rocks as basalts and diabases. Table Mt. at Golden, Colorado, and Bergen Hill, New Jersey, are prominent localities for zeolites. APOPHYLLITE, (H,K)2Ca(Si03)2H20 Form. Usually occurs in distinct crystals in cavities and along seams. Apophyllite crystallizes in tetragonal crystals of varying habit. Usual forms: a{100}, 2/{310}, p{lll}, c{001}. Interfacial angles: cp(001:lll) =60° 32'; ap(100:lll) =52° 0'; pp(lll:lTl)=76° 0'; 02/(100:310) = 18° 26'. (Figs. 581-584). Cleavage, in one direction parallel to {001}. /^==^=^' Fig. 581. FiQ. 582. Fig. 583. Fig. 584. H. = 4Jto5. Sp.gr. ±2.3. Color, colorless or white. Luster of (001) face, pearly; of other faces, vitreous. Optical Properties, n/1. 535) -n„(l. 533) =0.002. Fragments are square or rectangular and are either dark between crossed nicols or have low first order interference colors. Cleavage flakes give a positive uniaxial interference figure in convergent light. Chemical Composition. A hydrous acid calcium metasilicate, (H,K)2Ca(Si03)2-H20. A little potassium replaces part of the hydrogen and some analyses show a little fluorin. Blowpipe Tests. Easily fusible (2) with exfoliation to a white enamel. In the closed tube it yields water (about 16 per cent.). SILICATES 441 Decomposed by HCl. Occurrence. 1. In cavities of basic igneous rocks. West Paterson, New Jersey. HEULANDITE, H,CaAl2(Si03)„-3H20 Form. Heulandite crystallizes in the monoclinic system. Usual forms: 6{010), c{001},J{201}, s{201}, m{110}. Angles: c<(001:201)=63° 40'; cs(001:201) =66° 0'; 6m(010: 110) =^68° 2'. The habit is usually thick tabular parallel to {010}. The unsymmetrical outline of Fig. 585 is characteristic. Cleavage, perfect in one direction parallel "H to {010}. \ H. = 3ito4. Sp.gr. ±2.2. Color, colorless, white, pale brown, red- dish. Luster pearly on the (010) face. Optical Properties, n/1.505) - n„(1.498) = 0.007. Fragments are plates with low ^^^ ggg first order interference colors. Cleavage flakes give a positive biaxial interference figure in convergent light. Chemical Composition. Hydrous acid calcium metasilicate, H,CaAl2(Si03)e-3H20; (H20=14.8 per cent.). The calcium is usually partly replaced by small amounts of sodium, potassium, and strontium. Brewsterite is a similar isomorphous mineral with the calcium mostly replaced by strontium and barium. Blowpipe Tests. Easily fusible (3) with exfoliation to a white enamel. In the closed tube it gives water. Decomposed by HCl. Occurrence. 1. A secondary mineral in cavities of basic ig- neous rocks. STILBITE, H,(Ca,Na2)Al2(Si03),.4H20 Form. Stilbite usually occurs in indistinct crystals or in sheaf-like aggregates. Crystals are monoclinic but are pseudo- 442 INTRODUCTION TO THE STUDY OF MINERALS orthorhombic by twinning. The symmetrical outline of Fig. 586 is typical of stilbite. Cleavage, in one direction rather perfect. H. = 3Jto4. Sp.gr. + 2.1. Color, white, yellow, brown. Optical Properties. n^(1.500) -n„(1.494) =0.006. Fragments are prismatic with parallel extinction and negative elongation. The interference colors are upper first • order. Chemical Composition. Hydrous acid calcium-sodium aluminum metasilicate, H,(Ca,Na2) Al^CSiOg) 6-4H20 ; (H^O = 17.2 per cent, if Ca:Na = 6:l). Blowpipe Tests. Easily fusible (3) with exfoliation to a white enamel. In the closed tube yields water. Decomposed by HCl. Occurrence. 1. A secondary mineral in cavities and seams of igneous rocks, especially basalts and diabases. Fig. 586. Laumontite, U^CaAl^iSi^Oj) ^ 2R2O Form. Laumontite occurs in small crystals, and in fibrous and columnar forms. The crystals are monoelinic and prismatic in habit with acute terminations. Cleavage, parallel to the length of the crystals. H.=3to4. Sp.gr. ±2.3. Color, white or reddish. Luster, somewhat pearly. Often crumbles on exposure. Optical Properties. n^(l. 525) -n^(l. 513) =0.012. Fragments are prismatic with low first order interference colors and oblique extinction (30°). Chemical Composition. Hydrous acid calcium aluminum diorthosilicate, H4CaAl2(Si207)2-2H20; (H20 = 15.3 per cent.). SILICATES 443 Blowpipe Tests. Fuses at 2^ with swelling to a white enamel. In the closed tube it yields water. Occurrence. 1. As. a secondary mineral in igneous rocks, especially amygdaloidal basalts and diabases. CHABAZITE, (Ca.Na^) Al^CSiOj) .-eH^O Form. Chabazite practically always occurs in distinct rhom- bohedral crystals which are cube-like (10Tl:Tl01) =8.5° 14' (Fig. 587) . Penetration twins with c as the twin-axis are common. H.=4A. Sp.gr. +2.1. Color, white, colorless, pink, red. Optical Properties. 71^(1.488) -n„(l. 485) =0.003. Fragments are nearly square rhombs or are irregu- lar. The interference colors are low first order. Chemical Composition. Hydrous cal- cium-sodium aluminum metasilicate, (Ca,Na2) AljCSiOj) .-GHjO. A little potas- sium is usually present. Blowpipe Tests. Fuses at 3 with in- tumescence to a white glass. In the closed tube yields water (about 21 per Fig. gg?. cent.). Decomposed by HCl. Occurrence. 1. A secondary mineral in cavities and seams of igneous rocks associated with the other zeolites. ANALCITE, NaAl(Si03)2H20 Form. Analcite occurs in attached crystals or in druses lining cavities and seams. It is isometric in crystallization, the only common form being the trapezohedron, {211}, the same form that is common on garnet (Fig. 588). H. = 5to5i. Sp.gr. +2.2. Color, colorless or white. 444 INTRODUCTION TO THE STUDY OF MINERALS Optical Properties. n=1.48. Isotropic. Fragments are irregu- lar and dark between crossed nicols. Chemical Composition. Hydrous sodiupi aluminum metasili- cate, NaAI (Si03)2-H20; (H20 = 8.2 per cent.). Blowpipe Tests. Fusible at 3^ to a colorless glass. In the closed tube gives water. Soluble in HCI with gelatinization. Occurrence. 1. A secondary mineral in seams and cavities of basic igneous rocks. 2. As an original constituent of certain dia- bases, called teschenites. Fig. 588. The NATROLITE, NajAl^SijOio^H^O Form. This mineral occurs in divergent crystal groups or in fibrous masses. Crystals are orthorhombic but apparently tetragonal (110:110 = 88° 45'). The habil is long prismatic or acicular terminated by the low bipyramid {lllj Fig. 589 represents a typical natrolite crystal, side face, b, proves it to be orthorhombic. H. = 5. Sp.gr. + 2.2. Color, colorless or white. Optical Properties. n^(1.488) -n„(1.475) =0.013. Fragments are prismatic or acicular with parallel extinction, positive elongation, and bright inter- ference colors. Chemical Composition. Hydrous sodium alu- minum silicate, Na2Al2Si30j„-2H20; (H20 = 9.5 per cent.). Blowpipe Tests. Easily fusible (2^) to a color- less glass giving a yellow flame. In the closed tube it yields water. Decomposed by HCI with gelatinization. Occurrence. 1. A secondary mineral occurring in cavities of basalts and diabases. m Fig. 589. SILICATES 445 Garnierite, H2(Ni,Mg)SiO,H20 Form. Garnierite has never been found in crystals. It occurs in earthy masses. H. = 2to3. Sp.gr. + 2.5. Color, bright green to pale green. Optical Properties, n about 1.59. Fragments are irregular, greenish in color, and show aggregate structure in polarized light. Chemical Composition, A hydrous acid nickel-magnesium orthosilicate, H2(Ni,Mg)SiO,-H20. Blowpipe Tests. Infusible. Heated on charcoal becomes mag- netic. In the closed tube blackens and yields water. The borax bead is violet when hot. Partially decomposed by HCl. Uses. Garnierite is one of the chief ores of nickel. The French colony of New Caledonia is the only important locality. Occurrence. 1. A secondary mineral associated with serpen- tinized peridotites; probably an alteration product of nickel- bearing olivine. Riddles, Oregon. Allophane, AljSiO^ SHjO Form. Allophane is one of the very few amorphous minerals. It usually occurs in incrusting forms sometimes with a mammil- lary surface and resembles hyalite. H. = 3. Sp.gr. + 1.9. Color, white, colorless, pale blue. Very brittle. Optical Properties. n=1.49. Isotropic. Fragments are irregular, colorless, and dark between crossed nicols. Chemical Composition. Hydrous aluminum silicate, AljSiOj- 5H2O; (H20 = 35.7 per cent.). Some varieties contain copper. Blowpipe Tests. Infusible. In the closed tube gives a large amount of water at a low temperature. With cobalt nitrate solu- tion it gives a deep blue color. Soluble in HCl with gelatinization. 446 INTRODUCTION TO THE STUDY OF MINERALS Occurrence. 1. A secondary mineral occurring in seams and incrustations. CHRYSOCOLLA, CuSiOg 2H2O Form. Chrysocolla is never found in crystals but occurs in seams and in incrustations, sometimes having a mammillary surface. H. = 2to4. Sp. gr. + 2.1. Color, bluish-green or greenish-blue. Optical Properties, n about 1.57. Fragments are irregular, isotropic in part, but usually show aggregate structure in polarized light. Chemical Composition. Hydrous copper metasilicate, CuSiOg- 2H2O; (Cu = 36.1 per cent., H2O = 20.5 per cent.). Blowpipe Tests. Infusible. Colors the flame green. In the closed tube it blackens and gives water. Decomposed by HCl without gelatinization. Uses. Chrysocolla is one of the so-called oxidized ores of copper. Occurrence. 1. A secondary mineral often associated with malachite, azurite, and cuprite, and usually found in the upper workings of mines. Gila County, Arizona. PART VII THE OCCURRENCE, ASSOCIATION, AND ORIGIN OF MINERALS 1. DISTRIBUTION AND OCCURRENCE The earth's crust is composed, for the most part, of rocks which are made up of various minerals. Minerals forming the main bulk of rocks are comparatively few in number being limited to a dozen or so kinds, such as feldspars, pyroxenes, amphiboles, micas, olivine, quartz, and calcite. The great majority of minerals are found in rocks in small amounts, the so-called acces- sory constituents, or in veins, which are sometimes very rich in rare and valuable minerals, or as secondary minerals developed in cavities and other favorable places. Some minerals are common in quantities the world over, for example, quartz, calcite, feldspars, pyrite, sphalerite, and hema- tite. Some are widely distributed but occur in small quantities, for example, zircon and titanite. Others occur in large quantities at a very few localities, for example: cryolite (West Greenland), franklinite (Sussex County, New Jersey) and carnallite (Stassfurt, Germany). Minerals, unlike plants and animals, are not arranged in geographic zones, nor are they necessarily characteristic of any particular geological age except locally. The only zones that can be recognized are vertical zones depending upon physical conditions of temperature and pressure. Thus Van Hise recog- nizes three zones: (1) from the surface to the level of ground- water, (2) zone of ground-water, (3) zone below the ground-water. Minerals occur in rocks or in mineral deposits such as veins, stockworks, contact deposits, etc. Rocks may be defined as 447 448 INTRODUCTION TO THE STUDY OF MINERALS mineral aggregates which make up an essential part of the earth's surface. They are usually divided into three large groups, (1) igneous (those formed from fusions), (2) sedimentary (those laid down in water), and (3) metamorphic (original igneous or sedi- mentary rocks more or less modified by heat, pressure, or chemi- cal action.) Though some minerals such as quartz and pyrite are common to all these groups, many are restricted in their occur- rence. Leucite, for example, is found only in igneous rocks, while glaucophane is restricted to the metamorphic rocks. 2. ASSOCIATION AND ORDER OF SUCCESSION The minerals of rocks and other mineral deposits occur together in more or less definite association one with another. Many of the associations are so characteristic that the experienced miner- a-logist makes use of the facts in determining minerals. On the other hand, the determination of minerals helps to decide the kind of rock or mineral deposit and so is important to the geologist, petrographer, and mining engineer. The following are some of the characteristic mineral associations: lepidolite, tour- maline, spodumene, beryl, and albite in granite-pegmatites; wolframite, cassiterite, scheelite, topaz, and fluorite in tin-stone veins; wollastonite, vesuvianite, wernerite, garnet, and calcite in limestone contacts; tremolite, phlogopite, chondrodite, spinel, and dolomite in crystalline dolomitic limestones; zeolites, calcite, datolite, prehnite, and apophyllite as secondary minerals in basic igneous rocks. Not only the association but also the order of succession is often characteristic. In most granites the following order may be made out in a thin section: (1) magnetite, (2) biotite, (3) feldspar, and (4) quartz. In granites and igneous rocks generally, it is the solubility rather than the fusibility that governs the order of crystallization. Thus, quartz, though the most infusible, separates out last because it is most soluble under the given conditions. THE ORIGIN OF MINERALS 449 In the Joplin lead and zinc district of south-west Missouri the minerals crystallizing free in cavities usually exhibit the follow- ing sequence : (1) dolomite, (2) galena, (3) sphalerite, (4) chalco- pyrite, (5) marcasite, (6) calcite. In veins quartz is usually the first mineral to crystallize out, though it may also occur in a second generation. Minerals formed subsequent to the main rock mass are called secondary. For example, chalcedony in rhy- olites, and cerussite in veins with galena, are secondary minerals. 3. ORIGIN OF MINERALS Minerals may originate in various ways. Some have been formed from water solution (veins, spring deposits, secondary minerals in cavities) either by concentration of solutions or by chemical reactions. Some have been formed by separation from a molten magma (minerals of igneous rocks, such as granites, rhyolites, basalts, etc.). Others have been formed by organisms and still others by the chemical readjustment incident to metamorphism. To determine the method of formation and source of the material is an important part of mineralogical investigation. Many minerals have been formed in several different ways and this fact is often indicated by a peculiarity of habit. Thus ortho- clase is usually formed from fusion as in granites and rhyolites, but vein orthoclase (valencianite) has a pseudo-orthorhombic habit quite different from that found in igneous rocks. 4. PARAGENETIC VARIETIES The present tendency in mineralogy is to do away with the old varietal names based upon non-essential properties and to substi- tute for them varieties based upon the mode of occurrence and possible origin. On account of the difSculty of always determin- ing the origin it is convenient to base the varieties principally upon the mode of occurrence. Paragenesis is a term used for the association of minerals with special reference to their occurrence and origin. So these varieties may be designated by the term 450 INTRODUCTION TO THE STUDY OF MINERALS paragenetic varieties. It is not possible to establish paragenetic varieties for all minerals at present and in many cases there may be a difference of opinion. The number of varieties also depends upon the subdivisions of rocks and mineral deposits used, which in turn depend upon the purpose in view. As an example of paragenetic varieties let us take the mineral calcite. (a) As a vein mineral, often as the gangue of metallic ores. (b) As travertine, calcareous tufa, and cave deposits. (c) As a biogenic mineral forming limestones. (d) As a secondary mineral in basic igneous rocks. (e) As a secondary mineral in seams and cavities of sedimen- tary rocks. (f) As the recrystallized material of crystalline limestones. 5. SYNTHESIS OF MINERALS Besides filling up gaps in isomorphous groups and furnishing better material for study the synthesis of a mineral often gives a clue to its origin in nature. Most minerals have been produced artificially ; even the diamond has been made in minute crystals in three or four different ways. The methods of mineral synthesis differ greatly, the apparatus varying from a test-tube to the electric-furnace. A general method is that of the sealed tube. A hard glass tube containing the proper substances is sealed up and heated in a bomb furnace for several days or several weeks if necessary. Water vapor under pressure plays an important part in the reaction, as it also seems to in nature. An example of this method is the production of covellite (CuS) by heating powdered sphalerite (ZnS) in a water solution of CUSO4, using an atmosphere of COj. After a few hours a blue black powder (covellite) appears. The reactioii is ZnS + CuS04 = CuS-|-ZnS04. In the Joplin district the author has found covellite pseudomorphous after sphalerite, so that this reaction has doubtless taken place in nature. Geologic time is partly compensated for by increased temperature and pressure. The basic igneous rocks are easily reproduced and such minerals THE ORIGIN OF MINERALS 451 as olivine, pyroxene, leucite, and plagioclase crystallize out easily from a molten mass of the proper constituents. French mineralogists and chemists have been especially active in mineral synthesis. The work of Fouqu6 and L6vy in mineral synthesis is a classic. Intermediate between the naturally occurring minerals and the so-called artificial minerals are the minerals which have been formed on mine-tools, prehistoric implements, old coins, etc. Man has unintentionally furnished part of the material, but has not directed the conditions of the experiment, hence the term accidental synthesis. The author has identified cuprite, mala- chite, azurite, and cerussite on buried Cliinese coins of the seventh century found at Kiukiang, China. A classic example is that of the old Roman baths at the thermal springs of Bourbonne-les-Bains in France described by Daubree. Here bronze coins thrown in the spring as votive offerings were incrusted with such minerals as chalcocite, chalcopyrite, bornite, and tetrahedrite. Zeolites were also formed in the conduits. 6. ALTERATIONS, PSEUDOMORPHS, AND REPLACEMENTS Many minerals show evidences of alteration, especially around the border. The more common chemical changes involved are oxidation (sulfids to sulfates, sulfids to oxids, arsenids to arse- nates, etc.), reduction (sulfates to sulfids, sulfids to metals, oxids to metals) , and carbonation (sulfids to carbonates) . There are also more complex changes which have received special names. The following may be mentioned as prominent alterations, many of them occurring as pseudomorphs: Galena to cerussite, sphalerite to smithsonite, pyrite to limonite, smaltite to erythrite, cuprite to malachite, olivine to serpentine (serpentinization) , augite to hornblende (uralitization), and orthoclase to sericite, a variety of muscovite (sericitization). A psetidomorph is one mineral with the form of another, a false form, as the name indicates. Thus limonite, which never crys- tallizes, is often found in cubes. As may be seen by breaking 452 INTRODUCTION TO THE STUDY OF MINERALS the crystals, the cubes were originally pyrite and were changed to limonite by oxidation. This is an example of a limonite pseudomorph after pyrite. Pseudomorphs may be divided into four classes as follows: 1. Alteration pseudomorphs in which there is either loss, gain, or interchange of some constituent. Examples of the three cases: copper after azurite, malachite after cuprite, cerussite after galena. 2. Paramorphic pseudomorphs or paramorphs. A pseudo- morph of one dimorphous mineral after another is called a para- morph. Example, calcite after aragonite. 3. Substitution pseudomorphs. An interchange of substance not involving alteration. Example, chalcedony after calcite. 4. Incrustation pseudomorphs. If one mineral incrusts another and then the original mineral is dissolved there remains a cavity preserving its original shape which may afterward be filled by some other mineral. Example, quartz after fluorite. The replacement of a fossil by a mineral substance is called a petrifaction. Quartz, chalcedony, calcite, limonite, and pyrite are the more commonly occurring minerals found as petrifactions. Metasomatic replacement is a geological term which implies the replacement or substitution of a rock or vein by ore-bearing solutions. One of the best examples is fine granular galena with the form and structure of limestone and formed by the replace- ment of the limestone by galena molecule by molecule so that the structure is retained. Metasomatic replacement is im- portant in the study of ore-deposits. List of Mineral Occurrences According to occurrence the principal minerals may be listed and discussed under eight headings as follows : Igneous Rocks Organic Deposits Volcanic Exhalations Veins Pegmatites Spring, Lake, and Sea Deposits Clastic Rocks Metamorphic Rocks THE ORIGIN OF MINERALS 453 7. IGNEOUS ROCKS The igneous rocks are the result of the cooling and crystalliza- tion of a molten mass called a magma. As these rocks are found at great depths and often represent the oldest known part of the earth's surface, they are supposed to have been the original crust of the earth upon which the younger sedimentary rocks have been laid down. Igneous rocks are usually dense crystalline rocks without much structure. They occur in deep-seated masses of indeterminate shape called batboliths ; in dikes, which are comparatively narrow, nearly vertical, wall-like rock masses; in sills, which are like dikes, but are parallel to the bedding planes of sedimentary rocks; in laccolites, lens-shaped masses, the intrusions of which have forced overlying rocks into huge domes; in surface flows, due to the outpouring of lava from volcanoes; and in volcanic necks, rock masses occupying the throats of former volcanoes. Batholiths, dikes, sills, and laccolites are intrusive, that is, have forced their way into other rocks from below. They are also called plutonic rocks, which means deep-seated. Surface flows and volcanic necks, on the other hand, are extrusive, that is, have reached and often have spread out over, the surface. They are also called volcanic rocks. Unless they are glasses due to quick cooling of the magma, igneous rocks are aggregates of minerals. The bulk of rock- masses is composed of a comparatively few minerals. The important minerals of igneous rocks may be indicated thus: 1. Principal minerals. Feldspars, pyroxenes, amphiboles, biotite, olivine, nepheline, leucite, and quartz. 2. Accessory minerals. Magnetite, apatite, zircon, titanite, and ilmenite. 3. Secondary minerals (due to alteration or infiltration). Quartz, chalcedony, opal, calcite, zeolites, chlorite, epidote, sericite (muscovite), hornblende, and serpentine. The separation of minerals from the molten magma takes place according to the same physicochemical laws that govern 454 INTRODUCTION TO THE STUDY OF MINERALS water solutions. The minerals separate out in order of solubility rather than of fusibility. Thus in a granite composed of quartz, orthoclase, and biotite, quartz, the most infusible mineral, is the last to separate out. The igneous rocks are classified on the basis of texture and occurrence, mineral components, and chemical composition. The following table gives the essential features of the commonly accepted classification. Thus syenite is a rock composed essen- tially of orthoclase (it may have small amounts of other minerals Chemical composition Acid Si02, 80-65% Intermediate Si02, 65-52% Basic Si02, 52-40% Volcanic Texture porphyritic Rhyolite Trachyte Andesite Basalt (rare) Plutonic Texture granular Granite Syenite Diorite Gabbro Peridotite Essential Minerals Orthoclase Plagioclase No feldspars Hornblende Pyroxene Quartz Olivine also) without quartz, has granular texture, has a moderate silica percentage, and occurs in deep-seated masses. As rocks are mixtures of minerals the definite lines of the classification do not exist in nature. Rocks intergrade in all directions and many are difficult to classify. There is good evidence to show that some rocks are the result of a splitting up of an original magma into several magmas. This process is known as magmatic differentiation. These sepa- rate magmas are characterized by differences in the minerals or by differences in the chemical composition. Some ores such as magnetite, ilmenite, and chromite are supposed to have originated in this way. These ores are considered to be ultra-basic rocks THE ORIGIN OF MINERALS 455 formed by segregation of the metals from a large rock mass while in "the form of a magma. 8. VOLCANIC EXHALATIONS Among the gases given off by volcanoes and fumaroles are H^O, CO., H.S, SO2, and HCl. These, reacting upon each other and upon the minerals of the surrounding rocks, giVe rise to such minerals as sulfur, sylvite, sal-ammoniac, gypsum, sassolite (H3BO3), and hematite, minerals which crystallize in the pores of the lava. Hematite is formed thus: 2FeCl3 + 3H20=Fe20g + 6HC1. Vesuvius is a prominent locality for a great variety of minerals which have been formed in this way. In burning coal mines and coal-mine dumps, such minerals as sulfur, sal-ammoniac, orpiment, and realgar are sometimes formed by sublimation. 9. PEGMATITES Pegmatites are dikes or veins that have been formed during the last stages in the consolidation of plutonic rocks through the agency of mineralizers or dissolved vapors. They probably represent a residual mother liquor which has been injected into fissures. Among the characteristics of pegmatites are large crystals, simultaneous crystallization of minerals, variability in different parts of the vein, and the presence of rare elements and unusual minerals. There are pegmatites corresponding to most of the known plutonic rocks, but they are especially prominent in granites. In this country granite-pegmatites are prominent in New England, in South Dakota (Black Hills), and in San Diego County, California. The following are the characteristic minerals of granite-pegmatites: quartz, orthoclase, albite, muscovite, lepidolite, tourmaline, spodumene, amblygonite, topaz, beryl, and columbite. In Norway gabbro-pegmatites contain apatite, rutile, ilmenite, hornblende, and wernerite. In southern Nor- way a host of rare minerals has been found in the nepheline- syenite pegmatites. 27 456 INTRODUCTION TO THE STUDY OF MINERALS 10. CLASTIC ROCKS The clastic rocks are those which are made up of fragments of preexisting rocks, usually more or less sorted by water. They may be distinguished from igneous rock by their fragmental nature, banded appearance, and often by the presence of fossils. Tuffs are fragmental igneous rocks formed by the explosive action of volcanoes and are often interbedded with lavas. They are composed of angular fragments of gl^ss and of rock-frag- ments. They are named according to the rock to which they correspond, rhyolite-tuffs, andesite-tuffs, etc. Clays, sand, and gravel are, in a geological sense, unconsoli- dated rocks. Clays are made up principally of hydrated silicates such as sericite and kaolinite, as well as of minute fragments of quartz and feldspars. The most prominent mineral of sands and gravels is quartz, but it may be accompanied by such minerals as feldspars, garnet, and magnetite. Locally almost any mineral may be found in a sand. On the Hawaiian beaches the sand is largely olivine from the decomposing basalt. Shales, sandstones, conglomerates, breccias, and limestones are the common consolidated clastic rocks. They are consolidated partly by the pressure of overlying rocks and partly by the cementing material which is usually calcite, quartz, chalcedony, or limonite. Shales are consolidated clays with the same minerals as the latter. Sandstones are consolidated sands with quartz as the prominent mineral. A feldspathic sandstone is called an arkose. A conglomerate is a consolidated gravel made up of rounded pebbles, while in a breccia the pebbles are more or less angular. Quartz is the most prominent mineral of con- glomerates and breccias. Limestones are, as a rule, due to the accumulation of organic remains such as shell fragments, crinoid stems, etc., and have been comminuted so as to form a calcareous mud. The clastic rocks are usually porous, hence percolating solu- tions carrying dissolved mineral matter often deposit minerals THE ORIGIN OF MINERALS 457 in clefts, seams, and other cavities. The principal secondary minerals so formed are calcite, dolomite, barite, aragonite, quartz, chalcedony, gypsum, pyrite, and vivianite. 11. ORGANIC DEPOSITS The principal example of a rock formed by organisms is lime- stone. Molluscs, brachiopods, corals, crinoids, and other animals secrete calcium carbonate from the sea-water to form shells and other external skeletons. The calcium carbonate recrystallizes to form calcite or in some cases aragonite. Silica, principally in the form of opal, is secreted by sponges, radiolaria, and diatoms. Probably some chert is formed by the recrystallization of silica in the form of chalcedony. A variety of opal called geyserite is also secreted by the algae of certain hot springs. Limonite is formed in part by organisms. Sulfur is sometimes produced by certain bacteria from sulfate bearing waters. Niter is produced by bacteria from ammonia and other nitrogenous substances such as guano. The Chili saltpeter deposits have perhaps been formed in this way and then have been recrystallized. 12. VEINS Under favorable conditions of temperature and pressure almost all inorganic substances are soluble in certain kinds of water. Silica is soluble in heated alkaline water. Water charged with carbon dioxid will dissolve carbonates. Sulfids are soluble in alkaline waters. Most veins are supposed to have been produced by the opening of narrow fissures, clue principally to faults, and by the subsequent filling of the fissures. Percolating solutions would deposit minerals either by. simple concentration or by reactions with other solutions whereby precipitates are formed. The effect of crystallization would tend to widen the vein. Veins differ greatly in width and extent. Some are known to continue downward for 3,000 feet. In the Hartz Mts., Germany, veins have been traced for ten miles, but these are exceptional 458 INTRODUCTION TO THE STUDY OF MINERALS cases. Veins usually possess a banded structure and are more or less symmetrical, the symmetry being due to the fact that the solutions deposited their contents equally on both walls of the fissure. Fig. 590 illustrates a symmetrical vein from Freiberg. Fig. 591 is a drawing of a tin-stone vein from Zinnwald. A broad zone of rock with narrow more or less parallel anastomos- ing veins is known as a lode, though some writers use lode as a synonym of vein. Fig. 590. — Vein (after von Weissen- bach). a, c, sphalerite; b, e, fluorite; d, barite; /, calcite. Fig. 591. — Tin-stone vein (after Beck). a, greisen; b, lepidolite; c, fluorite; k, cassi- terite; w, wolframite. The principal gangue minerals of veins are quartz, calcite, dolomite, siderite, rhodochrosite, fluorite, and barite. The principal ore minerals are pyrite, chalcopyrite, sphalerite, galena, arsenopyrite, tetrahedrite, argentite, ruby silvers, and gold. The exposed oxidized portion of a vein or lode is known as a gossan or "iron-hat." Gossans are usually more or less cellular rusty outcrops which extend to varying depths. The minerals of a gossan are mostly secondary and are usually oxidation products of the original suliids of the vein. Among the promi- nent minerals may be mentioned limonite, hematite, cerussite, anglesite, malachite, azurite, cuprite, copper, chrysocolla, smith- sonite, calamine, cerargyrite, silver, and gold. There is often a concentration of the precious metals in the gossan. THE ORIGIN OF MINERALS 459 In a zone between the gossan and the unaltered sulfids such minerals as ehalcocite, covellite, bornite, and the ruby silvers are often found. This deposit constitutes a zone of secondary sulfid enrichment and results from reactions between the sulfates of the gossan (in solution) and tlie unaltered sulfids below. 13. SPRING, LAKE, AND SEA DEPOSITS Calcareous tufa and travertine are banded deposits of calcium carbonate formed around springs and in limestone caverns as the result of the evaporation of carbonated water holding calcium carbonate in solution. Cave-deposits such as stalactites and stalagmites belong to the same category. The evaporation of ocean water in closed inland seas gives rise to beds of rock-salt, gypsum, and anhydrite and in exceptional cases as at Stassfurt, Germany, to such minerals as carnallite and polyhalite. Sylvite and kainite are secondary minerals in the Stassfurt deposits. The borax and soda lakes of the arid regions of California and Nevada furnished on evaporation borax, ulexite, hanksite, the- nardite, and similar minerals. The source of the boracic acid is unknown. Among other minerals formed in the lakes of arid regions are trona from Owens Lake, California, mirabilite from Great Salt Lake, Utah, and epsomite from lakes in Albany County, Wyoming. 14. METAMORPHIC ROCKS Certain prominent rocks which will not fall in any of the pre- ceding divisions are called metamorphic rocks. They are usually more or less banded crystalline rocks which have been formed from igneous or sedimentary rocks by recrystallization due either to the heat of an igneous intrusion, or to the pressure resulting from mountain-making. The metamorphism due to igneous intrusion is called contact metamorphism because the effects, though often striking, are 460 INTRODUCTION TO THE STUDY OF MINERALS confined to the immediate vicinity of the intruded mass. By con- tact metamorphism an impure siliceous limestone becomes a crystalline limestone with new silicate minerals. Shales are changed to a dark compact rock called hornfels. Bituminous coal or lignite may be changed to anthracite or even to coke. Contrasted with contact metamorphism is regional metamor- phism by which rock-masses over large areas are affected. By regional metamorphism shales become slates; limestones, marbles; sandstones, quartzites; while schists and gneisses are produced from various kinds of rocks. Schists or finely laminated metamorphic rocks contain mus- covite, biotite, chlorite, hornblende, actinolite, glaucophane, talc, quartz, and garnet. They are named according to a promi- nent constituent as hornblende schist, chlorite schist, etc. Gneisses are coarsely banded rocks grading into the schists. They contain the same minerals as the schists except that the feldspars are more prominent. Quartzites are metamorphic sandstones with a firm siliceous cement of quartz or chalcedony so that on fracture the grains are broken through as well as the cement. Slates are dark-colored, fine-grained metamorphic rocks derived from shales. New rock cleavage, which has no relation to the original bedding plane, has been developed and is called slaty cleavage. The minerals of slates, as far as they can be recognized, are quartz, chlorite, muscovite, pyrite, and andalusite. The dark color is due to carbonaceous matter. Crystalline limestones are developed from ordinary limestones by metamorphism. If the original limestone was pure it is simply a case of recrystallization, large anhedra of calcite often forming. The characteristic minerals of crystalline limestones are calcite, wollastonite, garnet, vesuvianite, graphite, and diop- side. In the dolomitic or magnesian limestones calcite, dolomite, tremolite, phlogopite, chondrodite, graphite, and spinel are found. These minerals with the exception of calcite and dolo- mite are formed from the impurities of the original limestone. THE ORIGIN OF MINERALS 461 Serpentines arc altered peridotites (basic plutonic igneous rocks composed essentially of olivine and some variety of pyrox- ene). The mineral serpentine has been derived from olivine principally, according to the equation 2Mg2Si04 + C02-l-2H20 = MgjH^SijOg + MgCOg. The prominent minerals in serpentine are olivine (original), pyroxene (original), chromite, magnetite, mag- nesitc, hydromagnesite, dolomite, aragonitc, brucite, opal, and the mineral serpentine found in seams as well as in the main rock mass. 462 INTRODUCTION TO THE STUDY OF MINERALS Mineral Products of the United States (Amounts over $100,000 in round numbers according to the United States Geological Survey) 1910 1911 1912 1913 Coal Pig iron Clay products Copper Petroleum Gold Stone Natural gas Cement Lead Silver Zinc Sand Lime Phosphate rock Aluminum Salt Gypsum Mineral waters Slate Zinc oxid Sulfur Asphalt Mineral paints Glass sand Antimonial lead. . . . Borax Sand-lime brick Quicksilver Pyrite Talc and soapstone. Tungsten ores Grindstones Fibrous talc Bauxite Feldspar Fluorspar Graphite Mica Precious stones Fuller's earth Oilstones, etc Quartz Manganiferous ores. . Peat Infusorial earth Barite Garnet Miscellaneous Total $2,003,745,00C $629,557,000 425,115,000 170,116,000 137,180,000 127,896,000 96,269,000 76,520,000 70,756,000 68,752,000 32,756,000 30,854,000 27,268,000 19,521,000 13,895,000 10,917,000 8,956,000 7,900,000 6,623,000 6,357,000 6,237,000 5,325,000 4,605,000 3,080,000 2,175,000 1,517,000 1,338,000 1,202,000 1,169,000 958,000 958,000 864,000 807,000 796,000 728,000 716,000 502,000 430,000 377,000 337,000 296,000 294,000 229,000 194,000 187,000 140,000 130,000 122,000 113,000 806,000 PART VIII THE USES OF MINERALS The magnitude of the mineral industry of this country may be appreciated by a perusal of the table on the opposite page, giv- ing the value of the mineral products for the year 1910 (blank spaces are provided for subsequent years). The total value reaches a little over two billion dollars. In this discussion of economic mineralogy the minerals are grouped in three classes: (A) Those used in the natural state (after concentration if necessary), (B) those used for the extrac- tion of metals and alloys, and (C) those used in the chemical industries. Under the various heads the chief minerals, the producing localities, the treatment, uses, and production are given. Coal, petroleum, gas, clay products, cement, and stone are not treated as they are not definite minerals. A. MINERALS USED IN THE NATURAL STATE The minerals used in the natural state are discussed in the following order: Precious stones, ornamental stones, abrasives, refractory materials, glass, porcelain, and pottery, paints and graphic materials, fertilizers, fluxes, lubricants, paper-making materials, plaster, mica, and sulfur. 1. PRECIOUS STONES The qualities that make minerals valuable as precious stones are color, luster, dispersion, and hardness. Precious stones are sold by the carat, which is equal to about 205 milligrams. The proposed metric carat is 200 milligrams. 464 INTRODUCTION TO THE STUDY OF MINERALS The four great gems that rank above all the others are dia- mond, emerald, ruby, and sapphire. Diamond. Diamonds were first obtained in India, and later Brazil furnished the main supply, but in recent years the Kim- berley mines in South Africa have supplied about 95 per cent, of the total output of the world. In South Africa diamonds occur in a peculiar serpentine breccia known as "blue ground," which fills what appears to be volcanic necks. The diamond bearing rock is allowed to weather for some time and then the material is concentrated by means of greased shaking tables, the diamonds alone adhering. In Brazil, India, and most other localities, diamonds occur in alluvial sands and gravels. In the United States diamonds have been found in the glacial drift of Wisconsin, Michigan, and Illinois and in placer-deposits in California and North Carolina. Recently diamonds have been discovered in Pike County, Arkansas, in a decomposed peridotite somewhat like that of South Africa. About a thousand stones in all have been found and some of them are of very fine quality. Emerald is a deep green variety of beryl. A flawless emerald is very rare and commands a far higher price than a diamond, a one-carat stone bringing over $1000. Colombia, South America, is the principal locality for the emerald. Ruby is a deep red variety of corundum and like the emerald is considered more valuable than the diamond. A few have been found in North Carolina, but the finest ones come from Burma. Sapphire is the blue transparent variety of corundum. Ceylon and Siam have furnished the principal supply, but they have also been found at several localities in North Carolina and Mon- tana. At Yogo Gulch, Montana, sapphires and other varieties of corundum occur disseminated through a dike of basic igneous rock, which is exposed on the surface for over four miles. Varieties of corundum other than ruby and sapphire are dis- tinguished by the prefix oriental. Thus oriental emerald is green corundum and oriental amethyst, purple corundum, etc. THE USES OF MINERALS 465 Spinel. An intense red variety known as ruby-spinel is the principal one used as a gem. It is found in Ceylon and Burma. Hiddenite is an emerald green variety of spodumene found in North Carolina. Kunzite is a lilac colored variety of spodumene found in the pegmatites of San Diego Co., Cal. It was named in honor of Dr. George F. Kunz, the leading gem expert in this country. Tourmaline. Pink, red, and green varieties of tourmaline suitable for gems are found in San Diego Co., Cal., and in Maine and Connecticut. They occur in pegmatites associated with beryl, topaz, and kunzite. Chrysoberyl. There are several varieties used as gems, the cat's-eye with opalescent striations, and alexandrite, which is green by daylight and red by artificial light. Zircon. Zircon occurs in a variety of colors and some are used as gems. The red and orange varieties are called hyacinth and the colorless, smoky, or yellowish varieties, jargon. They are found in the gem-bearing gravels of Ceylon. Garnet. Both pyrope and almandite are used as gems as well a,s some rare varieties. A purplish-red variety known as rhodo- lite occurs in North Carolina. The ordinary gem garnet is pyrope. It is extensively mined in Bohemia. Turquois. The best quality of turquois is obtained from Persia, but at present the United States furnishes the principal supply. Turquois is found in the Los Cerillos Mts. and the Burro Mts. of New Mexico and also at various localities in Arizona and California. Topaz is found in the Urals, in Brazil, in the Thomas Mts., Utah, and in San Diego Co., California. Beryl. Besides emerald the gem varieties include golden beryl, pink beryl, and aquamarine, a pale bluish-green stone. Pink beryl and aquamarine are found in San Diego Co., California. Rose colored beryl of gem quality recently found in Madagascar has been named morganite. Olivine. The gem variety is known as peridot. Peridot has 466 INTRODUCTION TO THE STUDY OF MINERALS been found at several localities in northern Arizona, occurring in a peridotite. Opal. Precious opal is found in New South Wales and in Hungary and fire-opal in Mexico. At all of these localities opal occurs as a secondary mineral in volcanic rocks. Minor Gems. Among minor gems may be mentioned epidote, lapis-lazuli, moonstone (a variety of feldspar), iolite, diopside, vesuvianite, andalusite, cyanite, staurolite, axinite, benitoite, and numerous varieties of quartz and chalcedony such as ame- thyst, rose quartz, smoky quartz (called smoky topaz), chryso- prase, moss-agate, and bloodstone. Domestic Production of Precious Stones. The production of precious stones in the United States for 1910 amounted to $296,000, in the following order: turquois ($86,000), sapphire ($53,000), tourmaline ($46,000), kunzite ($33,000) variscite, a hydrous phosphate of aluminum from Utah ($26,000), chryso- prase ($9,000), and calif ornite ($8,000). Contrast this amount with the imports into the United State. which amounted in 1910 to about $43,000,000, mostly diamonds. 2. ORNAMENTAL STONES The following minerals are used extensively for vases, table- tops, mantels, and other interior decorations. Jade is a valuable ornamental stone, consisting either of a variety of actinolite or tremolite called nephrite or of a distinct mineral of the pyroxene group called jadeite. It is a compact, very tough material of a green color and is highly prized by the Chinese. Jade has not been found in this country, but a related substance called californite, a compact variety of vesuvianite, has been discovered at several localities in California. The pro- duction of californite in 1910 was about $8,000. Serpentine, a compact mottled or streaked rock of various shades of green is used for mantels, wainscotings, and the like. It is quarried in Vermont, Pennsylvania, and Washington. THE USES OF MINERALS 467 Malachite from the Urals is used extensively in Russia. Rhodonite, also from the Urals, is a favorite ornamental stone in Ilussia. Lapis-lazuli, a mixture of lazurite, calcite, pyrite, amphibole, and diopside, found in Persia, India, and other localities is a much valued ornamental stone known in ancient times. Mexican onyx or onyx marble is a banded variety of calcite or aragonite formed from water solutions. Mexican onyx is found in Arizona and California, but the principal supply comes from Mexico. It is used for interior decorations such as soda fountains. Alabaster is a translucent variety of gypsum used for statuary. The alabaster of antiquity was onyx marble. Marble exceeds all the other ornamental stones in amount and value. It is quarried principally in Vermont and Tennessee, though large deposits are known in other states. Verde-antique or ophicalcite is crystalline limestone with mot- tlings or streaks of serpentine. Vermont is the principal source. Chalcedony. Silicified wood is made into polished slabs for table-tops. 3. ABRASIVES Corundum. Canada is the principal producer of corundum. In the vicinity of Craigville, Ontario, corundum occurs in syenite. The rock is crushed and the impurities removed by washing and scouring. North Carolina and Georgia have been the only domes- tic producers. The domestic production in 1910 together with that of emery amounted to only 1,000 short tons, valued at $15,000. Emery is an intimate mechanical mixture of corundum and magnetite or hematite, sometimes also with spinel. Emery is obtained chiefly in Asia Minor, and on the island of Naxos in the Grecian Archipelago. Chester, Massachusetts, and Peekskill, New York, have produced some emery. Garnet as an abrasive is mined principally in Warren County, 468 INTRODUCTION TO THE STUDY OF MINERALS New York. It is crushed and used as garnet sand-paper for dressing wood and leather. In 1910 the domestic production was 3,800 short tons, valued at $113,000. Quartz. Crystalline vein quartz is used in various industries as an abrasive material. Diamond. Diamond powder is used as abrasive in cutting and polishing gems. A black, almost opaque variety of diamond called carbonado is the most valuable abrasive known. It is used extensively in diamond drills. A large well-set diamond drill is worth about $10,000. Carbonado is found only in Bahia, Brazil, as a constit- uent of sands and gravels. It is harder than the gem diamond, and is worth as much per carat as the ordinary straw yellow diamond. Other Abrasives. Among other abrasives are whetstones and grindstones in the production of which Ohio leads; millstones and buhrstones used in grinding cereals, paints, and other mineral pro- ducts; diatomaceous earth, a porous material composed of the siliceous remains of diatoms; volcanic ash, a fine comminuted natural glass; tripoli, a decomposed chert found in southwestern Missouri; flint-pebbles, used in tube-mills for fine grinding of ores. Artificial abrasives include carborundum, a silicon carbid. Sic, made in the electric furnace at Niagara Falls, by fusing together silica, coke, and sawdust, and alundum, artificial Al20g, made in the electric furnace from bauxite. 4. REFRACTORY MATERIALS The following minerals are used as furnace linings, covering for boilers and steam-pipes, and as fire-proof building materials. Asbestos. There are two different minerals known as asbestos, one a fibrous tremolite and the other fibrous serpentine or chry- sotile. The chrysotile is the stronger of the two, but has a shorter fiber. Asbestos is either used by itself or mixed with magnesia, saw dust, Portland cement, etc., and is made into rope, cloth, boards, and blocks. Canada leads in the production of as- THE USES OF MINERALS 469 bestos. Georgia is the principal domestic producer. In 1910 the production in the United States amounted to 3700 short tons valued at $68,000. The imports, on the other hand, amounted to about a million dollars. Talc. The massive variety known as steatite or soapstone is sawed into slabs and used for stove-linings, washtubs, switch- boards, laboratory tables, etc. Virginia is the principal pro- ducer. In 1910 the domestic production was 79,000 short tons valued at $864,000. Graphite is used in the manufacture of crucibles, being mixed with clay and sand. Austria and Ceylon lead in the production. In 1910 the domestic production was 51,000 short tons valued at .$377,000. The imports were about six times that amount. Diatomaceous earth is a good non-conductor of heat on ac- count of its porous nature. Chromite, calcined magnesite, and bauxite are used in the manu- facture of bricks for furnace linings. 5. GLASS, PORCELAIN, AND POTTERY The minerals mentioned are used either for the body of the ware or for the glaze. Feldspar. Orthoclase or microcline is used in the manufacture of porcelain and chinaware. It is obtained principally from peg- matites in Maine, Pennsylvania, and Connecticut. The domestic production in 1910 was 81,000 short tons valued at $502,000. Quartz is mixed with feldspar in the manufacture of porcelain. It is obtained in pegmatites along with feldspar. Glass Sand. Sand, or sandstone nearly free from iron, is used in the manufacture of glass, the other constituents being soda- ash and limestone. Pennsylvania leads in the production. In 19J10 the domestic production was 1,461,000 short tons valued at $1,517,000. Fluorite is used to some extent in the manufacture of opalescent glass and enamel. Halite furnishes the glaze of pottery. 470 INTRODUCTION TO THE STUDY OF MINERALS Clays are used extensively in the manufacture of pottery. Pottery to the value of $33,784,000 was produced in the United States in 1910. 6. PAINTS AND GRAPHIC MATERIALS Graphite is used in the manufacture of lead pencils, being mixed with clay to form various grades of hardness; also as stove polish and as an exterior paint. Hematite. The Clinton iron ore and other varieties of hema- tite made into metallic paint are used for bridges, roofing, etc. Ocher, umber, and sienna are earthy varieties of limonite and hematite mixed with clay and other impurities. They are washed, floated, ground, and sometimes roasted, producing a variety of colors. In 1910 the domestic production was 13,000 short tons valued at $139,000. Barite, finely ground and floated, is extensively used as a basis of paint and also as an adulterant of white lead. The leading supply comes from southeastern Missouri. In 1910 the domestic production was 43,000 short tons valued at $122,000. Talc and pyrophyllite are used as tailor's chalk, crayons, and slate pencils. 7. FERTILIZERS Nitrates, phosphates, and potassium are essential to plant growth, hence the use of the following minerals to replenish these constituents in soils. Apatite. In Norway and Canada extensive deposits of apatite occur and are mined to some extent. In the United States apa- tite is recovered as a by-product in the magnetic separation of magnetite in the iron-ore at Mineville, N. Y. Phosphate Rock. The impure calcium phosphate known as phosphate rock is mined principally in the southern states, Florida, Tennessee, and South Carolina being the leading pro- ducers. According to the occurrence the phosphates are classed as hard rock, land pebble, and river pebble. There are THE USES OF MINERALS 471 recent discoveries of extensive deposits of phosphate rock in the area where Idaho, Wyoming, and Utah join. It occurs as a dark colored oolitic rock interbedded with shales, sandstones, and limestones of Upper Carboniferous age. To render the phos- phate available for plant food phosphate rock must be treated with sulfuric acid. This is the principal use of sulfuric acid. The domestic production in 1910 was 2,655,000 long tons valued at $10,917,000. In the world's production the United States is first, followed by Tunis. Potassium salts used as fertilizers are obtained from Stass- furt, Germany. The principal minerals used are kainite and carnallite. Nitrates. The chief source of nitrates is sodium nitrate or Chili saltpeter from the desert regions of northern Chili, where there are extensive deposits. Norwegian saltpeter or calcium nitrate is now made by fixing the nitrogen of the air. Nitric oxid formed at the high tempera- ture of the electric arc is passed into calcium hydroxid solution and calcium nitrate is the result. Calcium cyanamid, CaCNj, is another compound with available nitrogen that is made from the air. Nitrogen, made either by the fractional distillation of liquid air or by passing air over heated copper, is heated with calcium carbid to a temperature of 1100° C, when calcium cyan- amid is the result. Calcium nitrate and cyanamid plants are located in Norway, Italy, and Dalmatia at localities where cheap electric power can be generated by waterfalls. Ammonium sulfate, a by-product in the manufacture of coal- gas, is also a valuable fertilizer. Gypsum or land plaster is used as a fertilizer, especially in arid regions to neutralize black alkali or sodium carbonate. 8. FLUXES In smelting ores it is necessary to add certain constituents to help in the reduction and to combine with impurities to form fusible slags. 472 INTRODUCTION TO THE STUDY OF MINERALS Limestone is the principal flux used in this country. It is used with siliceous ores of various kinds and also with iron ores. Fluorite is a valuable flux, especially in iron-smelting and in foundry work. Southern Illinois and western Kentucky fur- nish the principal supply. In 1910 the domestic production was 69,000 short tons valued at $430,000. A good deal of fluorite is imported from England. Quartz. Silica in the form of quartzite or low grade siliceous ores is used as an acid flux with basic ores. Manganiferous iron-ores are used as a flux in some smelters. Leadville, Colorado, has supplied a notable amount. 9. LUBRICANTS Graphite, talc and mica are the principal lubricants used for machinery. Their value lies in their foliated character, in their ability to stand high temperatures, and in the fact that they fill in irregularities. 10. PAPER-MAKING MATERIALS Barite, magnesite (calcined), clay, and fibrous talc are used in paper-making. Fibrous talc is especially desirable on account of the strength that the fibers impart to paper. This product, called agalite, is mined extensively in St. Lawrence Co., N. Y. In 1910 the domestic production was 71,700 short tons valued at $728,000. 11. PLASTER Gypsum is now used principally in the manufacture of plaster. The rock gypsum is finely ground, calcined so as to drive off a part of the water, and then ground again. Various foreign substances such as glue, hair, or fine wood shavings, called retar- ders are added to prevent quick setting. Plaster is CaSOi'JH 0, but on mixing with water and setting, it becomes gypsum (CaS0,-2H,0) again. THE USES OF MINERALS 473 The United States and France are the world's principal pro- ducers of gypsum. In the United States the states in order of production are New York, Iowa, Michigan, and Oklahoma. The domestic production in 1910 was 2,239,000 short tons valued at $6,523,000. 12, MICA Large quantities of mica are used as an insulating material in the electrical industry. There are many minor uses. India, Canada, and the United States rank first in the production of mica. North Carolina and South Dakota furnisJi the principal domestic supply which in 1910 amounted to 2,476,000 pounds of sheet mica and 4,000 short tons of scrap mica, together valued at $337,000. 13. SULFUR The world's principal supply of sulfur is derived from Sicily. In the United States the largest deposit is a bed of sulfur over 100 feet thick in Calcasieu Parish, Louisiana. On account of the difficulty of sinking a shaft the sulfur is obtained by forcing superheated steam down one pipe and pumping the molten sulfur up through another pipe. Utah and Wyoming also produce some sulfur. In 1910 the domestic production was 255,000 long tons valued at $4,605,000. At present only a small amount of sulfur is imported. B. MINERALS USED FOR THE EXTRACTION OF METALS AND ALLOYS 14. ALUMINUM Chief MineraL Bauxite. Producing Localities. The United States leads in the produc- tion of bauxite. The states in order are Arkansas, Georgia, and Alabama. 474 INTRODUCTION TO THE STUDY OF MINERALS Treatment. An electrolytic method is used. The mineral is first heated with sodium carbonate. Then carbon dioxid is introduced and forms aluminum hydroxid, which on heating gives aluminum oxid. The metal is obtained by the electrolysis of the oxid in a bath of cryolite which protects the metal from the air. Uses. Aluminum has many uses; wire for transmission of electricity, cooking vessels, in various alloys such as aluminum bronze. A mixture of powdered aluminum and iron oxid with the trade name "thermit" is used for welding rails, crank shafts, etc., and for the extraction of such metals as tungsten, titanium, etc., from their ores. Production. The domestic production of aluminum in 1910, 47,734,000 pounds valued at $8,956,000. 15. ANTIMONY Chief Minerals. Stibnite. Producing Localities. France leads in the production of anti- mony. There are antimony deposits in Utah, Arkansas, Nevada and California, but the domestic production in 1910 was nil. Treatment. The ore is fused with scrap iron and the impure metal called regulus is obtained which must be refined. In smelting, antimonial lead is produced and this is often made into type-metal directly. Uses. Antimony is used principally in the manufacture of alloys such as type-metal. Babbitt metal, and Britannia metal. Production. The domestic production of antimonial lead for 1910 was 14,000 short tons valued at $1,338,000. 16. BISMUTH Chief Minerals. Bismuth and bismuthinite. Producing Localities. Bolivia stands first in the production of bismuth. The production in the United States is very slight. In former years Leadville, Colorado, furnished some bismuth. THE USES OF MINERALS 475 Uses. Bismuth is used principally in the manufacture of easily fusible alloys. 17. COPPER Chief Minerals. Copper, chalcocit e, chalcopyrite, bornite, cuprit e, malachite, azurite, and copper-iearing pyrite. Pro3ucing Localities. The United States leads in the produc- tion of copper, its output being greater than the rest of the world combined. The three great copper producing states in order are Arizona, Montana, and Michigan, but Utah and California also produce notable amounts. In Arizona there are four important districts: Bisbee, Clifton-Morenci, Jerome, and Globe. The oxidized ores malachite, azurite, and cuprite were formerly the principal ores in Arizona, but now the sulfids are more important. In Montana the mines around Butte produce chalcocite, bornite, and enargite. In Michigan the ore is native copper disseminated through conglomerates and amygdaloids and runs less than 2 per cent, in copper. Treatment. The native copper of Michigan is crushed and concentrated and the concentrates treated with limestone and slag in a reverberatory furnace. The oxidized ores are smelted in the blast furnace with coke and the proper fluxes. Sulfid ores are first roasted. Then on fusion in a reverberatory furnace a sulfid of copper and iron containing about 50 per cent. copper (called matte) is formed. The molten matte is treated in a converter in which fine streams of air drive off the sulfur and the result is blister copper, which contains about 98.5 per cent, of copper. Most of the copper except that from Michigan (called Lake copper) is refined electrolytically. Uses. Copper is used principally in the electrical industry and in the manufacture of alloys such as brass, bronze, and German silver. Production. In 1910 the domestic production was 1,080,159,- 000 pounds valued at $137,180,000. 476 INTRODUCTION TO THE STUDY OF MINERALS 18. GOLD Chief Minerals. Gold, calaverite, and sylvanite. Producing Localities. The countries of the world in order of their production are Transvaal, United States, Australia, Russia, and Mexico. The states in order are Colorado, Alaska, California, Nevada, and South Dakota. Gold is found mainly in two kinds of occurrences: in quartz veins with pyrite and other sulfids, and in placers along with other heavy minerals. Treatment. Gold-bearing quartz is crushed in stamps and the gold caught on copper plated with mercury . The amalgam formed is removed at intervals and retorted. The residues were formerly treated with chlorin formed by the action of manganese dioxid and sulfuric acid on salt which dissolved the gold as chlorid. The chlorination process is now replaced by the cyanide process by means of which very low grade ores may be treated. A weak solution of potassium cyanid percolates through the finely divided ore and dissolves the gold, which is later precipitated by zinc shavings. Placer gold found in superficial deposits of sand, gravel, and con- glomerate, which are derived by the breaking down of pre-exist- ing rocks with quartz veins, are worked by hydraulic mining, by dredging, by drifting, or by surface placers. The gold is collected by amalgamation. Gold is also obtained from base bullion derived from mixed ores, and in the electrolytic refining of copper. Production. In 1910 the domestic production was 4,657,000 troy ounces valued at $96,269,000 (coining value). The world's production for 1908 was 1443,400,000. 19. IRON Chief Minerals. Hematite, limonite, and magnetite. Producing Localities. The United States leads in the produc- tion of pig iron, while Germany is second and Great Britain third. THE USES OF MINERALS 477 The most important locality in this country is the Lake Superior district comprising the Mesabi, Vermilion, and Cuyuna ranges of Minnesota, the Menominee and Gogebic ranges of Michigan and Wisconsin, and the Marquette and Crystal Falls range of Michigan. Of these the Mesabi range is the most impor- tant producer. About a hundred mines in the Lake Superior district each produce annually 100,000 tons or over of iron ore. Of states, Minnesota is in the lead, producing just about as much as all of the rest of the United States combined. The Hull Rust Mine at Hibbing, Minn., produced 3,190,000 long tons of iron ore in the year 1910. In the Lake Superior region hematite is the important ore mineral. The next most important district is that of central Alabama, hematite being the important ore. This district owes its importance to the convenient location of iron ore, coal, and limestone. In the Adirondack region of New York magnetite is mined. In Virginia limonite is the principal iron ore. It occurs in residual clays overlying limestones. Pennsylvania, New Jersey, Georgia, and Missouri also pro- duce iron ores. In the western states, especially in Utah, are important ore-deposits which will be developed at some future time. But at present the high-grade ores (50 to 60 per cent, iron) of the Lake Superior region overshadow all others. Treatment. The Lake Superior ores are shipped by Lake steamers to the blast furnaces of Pennsylvania and Ohio center- ing around Pittsburg. The ore is smelted in a blast furnace with coke and limestone by which pig-iron is produced. Pig-iron remelted and poured into molds is cast iron which contains about 5 per cent, of carbon and other impurities. Wrought iron is made by stirring molten cast iron in a puddling furnace lined with pure ferric oxid. The carbon of the pig-iron unites with the lining and leaves a very pure iron. Steel is now made in part directly from cast iron in a vessel called a converter. Blasts of air are passed through the molten iron, all the carbon 478 INTRODUCTION TO THE STUDY OF MINERALS being converted into gases. Then weighed quantities of iron or alloys of iron containing known amounts of carbon are added. Steel contains 1 to 2 per cent, of carbon in the form of certain carbids which are necessary to give it hardness and other desirable properties. The open-hearth process is also used in the manu- facture of steel. Production. The domestic production of pig-iron for 1910 was 27,303,000 long tons valued at $425,115,000. 20. METALS USED AS ALLOYS WITH STEEL The following metals, chromium, manganese, molybdenum, nickel, titanium, tungsten, and vanadium, are alloyed with steel to increase its hardness, strength, toughjiess, tenacity, etc. Chromium. Chromite is used. The principal sources of chromite are Asiatic Turkey and New Caledonia. California has important deposits, but the production is slight. Manganese. Pyrolusite, psilomelane, and franklinite are the chief minerals. In the Lake Superior district manganiferous iron ores and in New Jersey the residue from the manufacture of zinc oxid are used in the steel industry. Manganese deposits are also worked in Virginia, Georgia, Arkansas, and California, but in 1908 the imports amounted to over a million dollars. Molybdenum. Molybdenite is mined in this country princi- Tpally at Crown Point, Washington, but the production is very small. Nickel. Nickel-bearing pyrrhotite and garnierite are the chief sources of nickel, which is used in the manufacture of rails and armor plate. Nickel is obtained principally from the pyrrhotite of Sudbury, Canada, and from the garnierite of New Caledonia. Titanium. Rutile is practically the only source of titanium. The only important domestic source of titanium is Nelson Co., Virginia. Vanadium. Carnotite, a mixture of vanadium and uranium compounds; roscoelite, a vanadium mica; and patronite, a newly THE USES OF MINERALS 479 discovered vanadium sulfid, all furnish vanadium. Carnotite and roscoelite occur disseminated in sandstone in western Colorado, while patronite is found only in Peru. Tungsten. The most important tungsten mineral is wolfra- mite, but hlibnerite and scheelite have also been mined. Boulder County, Colorado, contains the largest deposits in this country. Tungsten is used extensively in the manufacture of tool-steel. In 1910 the domestic production of tungsten ores was 1800 short tons valued at $807,000. Treatment. There are two methods for the production of these metals and their ferro-alloys, the electric furnace and the Gold- schmidt process. In the electric furnace the ferro-alloys are obtained directly from their ores by reduction. In the Gold- schmidt process the metals or their ferro-alloys are made by igniting a mixture of powdered aluminum and oxid of the metal. The metal and the aluminum change places, aluminum oxid and the metal being formed. 21. LEAD Chief Mineral. Galena. Producing Localities. In the production of lead the United States ranks first, the principal states in order being Missouri, Idaho, Utah, and Colorado. Treatment. The ore is heated in a blast furnace with coke and limestone, and base bullion, an impure lead often containing silver and gold, is the result. The silver is recovered by the process known as desilverization, metallic zinc being added to the molten lead. At a certain temperature an alloy of zinc and silver is formed as a scum on the surface and is removed. Uses. Lead is used principally in the manufacture of white lead, lead piping and sheeting, and alloys such as type-metal and shot. Production. In 1910 the domestic production was 372,000 short tons valued at $32,756,000. 480 INTRODUCTION TO THE STUDY OF MINERALS 22. MERCURY Chief Mineral. Cinnabar. Producing Localities. Spain leads in the production of mer- cury, with Italy second, and the United States third. In this country California furnishes the bulk of the supply, though Texas is also an important producer. The New Almaden mine in Santa Clara County, California, has in the past been the greatest producer, but at present the New Idria mine in San Benito County, California, is the main source. These localities are named from famous quicksilver mines in Europe, Almaden in Spain and Idria in Austria. Treatment. The ore is heated in retorts or at the large mines in furnaces resembling lime kilns. The mercury vaporizes readily and is condensed under water. Uses. Mercury is used principally in the manufacture of vermilion and in the amalgamation of gold ores. Production. In 1910 the domestic production was 20,000 flasks (75 pounds each) valued at $958,000. 23. PLATINUM Chief Mineral. Platinum. Producing Localities. Russia is practically the only producer of platinum, the mineral occurring in placer deposits in the Ural Mts. Small amounts are recovered in northern California and southern Oregon from the gold placers. Colombia, South Amer- ica, also furnishes some platinum. Treatment. The concentrates are treated with dilute aqua regia which dissolves out most of the impurities. After dissolving in concentrated aqua regia the platinum is precipitated with ammonium chlorid as (NHJjPtClg, which on heating leaves platinum containing a little iridium. Production. In 1910 the domestic production was 773 troy ounces valued at $25,277. THE USES OF MINERALS 481 24. SILVER Chief Minerals. Silver, argentite, pyrargyrite, proustite, stephanite, cerargyrite, and argentiferous galena. Producing Localities. Mexico leads in the production of silver, with the United States second, and Canada third. In the United States the five leading states in order of production are Montana, Colorado, Nevada, Utah, and Idaho. Canada owes its rank as third place to the comparatively new discoveries at Cobalt. Treatment. Most of the silver is obtained from argentiferous galena by the Parke's process. Zinc is added to molten lead and forms a zinc-silver alloy which has a lower melting point than lead. This alloy is skimmed off the molten lead and the zinc is distilled off. The silver is purified by cupellation. Silver, together with gold, is obtained in the electrolytic refin- ing of copper. If the ore does not contain lead or copper the process known as pan amalgamation is used. The essential parts of the process are the reduction of silver by metallic iron, usually the bottom of the vessel, and subsequent amalgamation with mercury. Production. In 1910 the domestic production of silver was 57,137,000 troy ounces valued at $30,854,000. 25. TIN Chief Mineral. Cassiterite. Producing Localities. The Federated Malay States leads in the production of tin, followed by Bolivia and Australia. Cornwall, England, was formerly the principal producer. In the United States a little tin has been produced by Alaska, South Dakota, and South Carolina, but the amount has been very small. Treatment. Tin ore is smelted in a reverberatory furnace with coal. The impure metal obtained is purified by reheating and by continued oxidation. Uses. Tin is used in the production of tin-plate (sheet-iron coated with tin) and various alloys such as bronze. 482 INTRODUCTION TO THE STUDY OF MINERALS Production. In 1910, 59 short tons of cassiterate tin were pro- duced in the United States. The value of the imports, however, amounted to $33,913,000 in 1910. 26. ZINC Chief Minerals. Sphalerite, smithsonite, and calamine. Producing Localities. Germany leads in the production of zinc, with the United States second and Belgium third. Of the states Missouri, Colorado, and Wisconsin are the main producers. The Joplin district in southwestern Missouri furnishes about 6.5 per cent, of the zinc produced in the United States. Treatment. After crushing and concentrating, the zinc ore is roasted. It is then mixed with powdered coke and heated in clay retorts. The zinc volatilizes and forms in condensers at- tached to the retorts. This crude zinc is called spelter. The ore from the Joplin district is shipped to Kansas princi- pally, where natural gas furnishes a convenient fuel for smelting. In Colorado the smelters formerly deducted for zinc in ore, but now Colorado is an important producer. In New Jersey the zinc ores are used for the production of zinc white and very little for spelter. Uses. Zinc is used principally in galvanizing iron and in the manufacture of brass, an alloy of copper and zinc. Production. In 1910 the domestic production of spelter was 252,000 short tons valued at $27,268,000. C. MINERALS USED IN THE CHEMICAL INDUSTRIES 27. ALUMINUM COMPOUNDS The compounds manufactured are alum, aluminum sulfate, and the oxid, which is called alundum. Chief Mineral. Bauxite, cryolite, and alunite. Producing Localities. Bauxite is obtained in Georgia, Ala- bama, and Arkansas, cryolite in Greenland, and alunite in Italy. THE USES OF MINERALS 483 Treatment. Alum is made from bauxite, cryolite, and alunite; aluminum sulfate from bauxite; and alundum from bauxite by, heating in the electric furnace. Uses. Alum is used principally in the manufacture of paper, aluminum sulfate in dyeing (as a mordant to fix the color), and alundum as an abrasive. Production. In 1910 the domestic production of alum and aluminum sulfate was 105,000 short tons valued at $2,000,000, and of alundum, 13,410,000 pounds valued at $804,600. 28. ARSENIOUS OXID Chief Mineral. Arsenopyrite. Producing Localities. Cornwall formerly furnished the princi- pal supply, but now Germany leads with France second. In this country arsenious oxid is produced at Everett, Washington, and from the smelter fumes at Anaconda, Montana. Treatment. Arsenopyrite on roasting gives off arsenious oxid, AS2O3. At the Washoe smelter, Anaconda, Montana, the enargite of the ore furnishes the arsenious oxid. Uses. The principal use of arsenious oxid is in the manufacture of paris green and london purple, valuable insecticides, but there are many minor uses. Production. In 1910 the domestic production was 2,994,000 pounds valued at $52,000. 29. BORAX AND BORACIC ACID Chief Minerals. Colemanite. Producing Localities. San Bernardino, Ventura, and Inyo counties, California, furnish all the colemanite produced. Treatment. Borax is made from colemanite by decomposing it with sodium carbonate. Boracic acid is obtained from cole- manite by the use of sulfuric acid. Most of the California borax is refined at Bayonne, New Jersey. 484 INTRODUCTION TO THE STUDY OP MINERALS Uses. Borax and boracic acid are used in the manufacture of soap, glass, toilet preparations, food preservatives, etc. Production. The domestic production of crude borax in 1910 was 42,300 short tons valued at $1,202,000. 30. BROMIN Bromin is obtained from the bittern or salt brines of Michigan, Ohio, and West Virginia, as a by-product in the salt industry. Bromin is used principally in photography and in medicine. The dornestic production in 1910 was 245,000 pounds valued at $41,000. 31. COBALT COMPOUNDS Smalt, a cobalt silicate, is about the only compound used to any great extent. Chief Minerals. Smaltite and asbolite (cobaltiferous wad). Producing Localities. New Caledonia and Cobalt, Ontario, furnish the principal supply of cobalt. Uses. Smalt is used as a pigment, especially for glass and pottery. 32. CHROMIUM COMPOUNDS Potassium and lead chromates, chrome alum, and chromium oxid. Chief Mineral. Chromite. Producing Localities. New Caledonia and Asia Minor furnish the bulk of the supply. Uses. The chromium compounds mentioned are used in dye- ing, in tanning, and in the manufacture of paints. Production. The domestic production of chromite in 1910 was 205 long tons valued at $2,700. 33. lODIN lodin is obtained principally from the impure sodium nitrate of Chile, where it occurs as a calcium iodate. THE USES OF MINERALS 485 34. LITHIUM SALTS Chief Minerals. Lepidolite, spodumene, and amblygonite. Producing Localities. Pala, San Diego Co., California, and the Black Hills, South Dakota, have furnished the domestic supply. Uses. Lithium carbonate is used for medicinal purposes, especially in artificial mineral waters. Production. In 1908 the domestic production of lithium minerals was 203 short tons valued at $1550. 35. MAGNESIUM COMPOUNDS Magnesia (MgO) and epsom salts (MgSO^-THjO) are the com- pounds used. Chief Mineral. Magnesite. Producing Localities. Austria and Greece furnish the principal supply of magnesite. In California there are several important deposits, Porterville and Red Mt. near Livermore being the principal ones. Treatment. Magnesite on heating gives off carbon dioxid, which is liquefied for use in soda fountains, and leaves magnesia. Epsom salts is made by dissolving magnesite in sulfuric acid. Uses. Magnesia is used in the manufacture of refractory bricks, paper, boiler coverings, etc. Production. In 1910 the domestic production was 12,400 short tons valued at $74,000. 36. NITRATES Chief Mineral. Soda Niter. Producing Localities. Chili furnishes the total supply. Although the exports from Chili amount to about a million tons a year it is said that the deposits will last 200 or 300 years at the present rate of exportation. A little soda niter is found in the desert regions of California. Uses. Soda niter is used as a fertilizer, and in making potassium nitrate. 486 INTRODUCTION TO THE STUDY OF MINERALS 37. POTASSIUM SALTS Potassium chlorid, nitrate, and sulfate. Chief Minerals. Carnallite and kainite. Producing Localities. Germany produces practically the entire supply of potassium salts. Potassium chlorid and sulfate are obtained from carnallite and kainite by solution and recrystal- lization. The nitrate is made by crystallizing a water solution of potassium chlorid and sodium nitrate. Uses. Potassium chloride and sulfate are used as fertilizers, potassium nitrate principally in the manufacture of gunpowder. 38. SODIUM SALTS Chief Mineral. Halite or rock-salt. Producing Localities. Austria-Hungary leads in the produc- tion of salt, followed by the United States and Germany. In this country the states in order of production are Michigan, New York, Kansas, and Ohio. Treatment. Salt occurs in thick bedded deposits often asso- ciated with gypsum or anhydrite. The salt is sometimes mined in the form of rock-salt just as coal would be mined. At other localities water is forced down and the resulting brine is evapo- rated. In some places sea or lake water is evaporated to obtain salt. Soda or sodium carbonate, next to sodium chlorid, is the prin- cipal sodium 'salt used. The Solvay process for the manufacture of soda is as follows: ammonia gas and carbon dioxid are passed into a solution of salt. Sodium bicarbonate is formed, which on heating gives normal sodium carbonate. Caustic soda is made from soda and calcium hydroxid. Uses. Salt is used in the packing-house industry, soda is used in manufacture of glass, and caustic soda in the manufacture of soap. Production. In 1910 the domestic production was 30,000,000 barrels (280 lbs. net) valued at $7,900,000. THE VSE,S OF MINERALS 487 39. STRONTIUM COMPOUNDS Chief Minerals. Strontianite and celestite. Producing Localities. Germany is practically the only pro- ducer. Uses. Strontium hydroxid is used in the refining of beet- sugar and strontium nitrate in pyrotechnics. 40. SULFURIC ACID Chief Mineral. Pyrite. Producing Localities. Portugal leads in the production of pyrite, with the United States second. In this country ahout half of the pyrite is produced in Virginia. Treatment. The ore is roasted in specially designed furnaces. The sulfur dioxid is forced into lead chambers where steam and nitrogen dioxid convert it into sulfuric acid. Uses. Sulfuric acid is the basis of many industries. In order of the amount of acid used, these industries are superphosphate for fertilizer, refining of petroleum, pickling iron and steel, ammo- nium sulfate, alum and aluminum sulfate, manufacture of other acids, blue vitriol, etc. Production. In 1910 the domestic production of pyrite was 238,000 long tons valued at $958,000. For 1908 the imports amounted to $2,264,000. 41. THORIA Chief Mineral. Monazite. Producing Localities. The principal localities are North and South Carolina and Bahia, Brazil. Treatment. The monazite which occurs in the form of sand is decomposed by sulfuric acid. The addition of oxalic acid con- verts sulfates into oxalates. Ammonium oxalate dissolves out the thorium oxalate, which is converted into thoria by heating. 29 488 INTRODUCTION TO THE STUDY OF MINERALS Uses. Thoria is used in making Welsbach gas mantles. Production. The domestic production of monazite and zircon in 1910 was 99,300 pounds valued at $12,000. 42. ZINC OXID Chief Minerals. Franklinite, willemite, and zincite. Producing Locality. Sussex Co., New Jersey, furnishes the only supply of these minerals. Treatment. Zinc oxid is made directly from the ores by heat- ing in a special type of furnace. The residues which contain a large percentage of manganese are made into spiegeleisen and ferromanganese, used in steel making. Uses. Zinc oxid is a valuable paint which does not tarnish like white lead. Production. In 1910 the domestic production was 59,000 short tons valued at $5,325,000. GLOSSARY Absorption. A term referring to the intensity of light in different directions in a crystal. Achroite. A colorless variety of tourmaline. Acicular. Needle-shaped. Acid salt. A compound in which only part of the hydrogen of an acid is replaced by a metal. Acute bisectrix. The lino bisecting the axial angle. Adamantine. Brilliant luster like that of cerussite. Adularia. ^V glassy variety of orthoclase found in vein.s. Agalite. Fibrous talc pseudomorphous after eustatite. Agate. A variegated chalcedony. Alabaster. Translucent massive gypsum. Aggregate polarization. The extinction of a mass of particles or fibers at different times. Aggregate structure. The structure which causes aggregate polarization. Alexandrite. A variety of chrysoberyl. Almandite. A sub-species of garnet containing Fe and Al. Alpha (a). (1) The angle between the 6 and c axes. (2) The shortest indicatrix axis. Alteration. A change in chemical composition due to circulating waters, weathering, etc. Aluminates. Salts of H3AIO3, HalO^, etc. Aluminum minerals. Alunite, amblygonite, andalusite, bauxite, corundum, cryolite, cyanite, diaspore, sillimanite, spinel, topaz, turquois, wavel- lite, and many silicates. Alundum. Artificial corundum used as an abrasive. Alimi-stone. An impure siliceous alunite. Amazon-stone. A green variety of microcline. Amethyst. A purple variety of quartz. Amethyst, oriental. A purple variety of corundum. Ammonium minerals. Sal-ammoniac. Amorphous. Totally devoid of crystalline form or structure. Amphibole. A group of silicates including tremolite, actinolite, hornblende, and glaucophane. A.M1Tfv.441/^iHa1 Poforrinor tn am vp-diiles. 490 GLOSSARY Amygdules. Secondary minerals filling rounded cavities in volcanic rocks. Analyser. The upper nicol. Andesine. A plagioclase between oligoclase and labradorite. Andesite. A volcanic rock composed essentially of plagioclase and horn- blende. Andradite. A sub-species of garnet containing Ca and Fe'" Angle, axial. The angle between the optic axes. Angle, interfacial. The dihedral angle between two faces. Anhedral. Without crystal faces. Anisotropic. Substances with double refraction. Antimonates. Salts of HjSbOj, HSbOj, etc. Antimonites. Salts of HjSbOj, HSbOj, etc. Antimony minerals. Antimony, bournonite, jamesonite, polybasite, pyrar- gyrite, stephanite, stibioonite, stibonite, tetrahedrite. Antimony glance. Synonym of stibnite. Antimony ocher. Synonym of stibiconite. Aquamarine. A pale sea-green variety of beryl. Arborescent. Branching like a tree. Arkose. A feldspathic sandstone. Arsenates. Salts of H3ASO1, HAsOj, etc. Arsenites. Salts of H3ASO3, HAsOj, etc. Arsenic minerals. Arsenic, arsenopyrite, cobaltite, enargite, loUingite, mimetite, niccolite, olivenite, orpiment, proustite, realgar, smaltite. Arsenical pyrites. A synonym of arsenopyrite. Asbestos. Fibrous tremolite or fibrous serpentine. Asterism. A six-rayed star effect seen by holding certain minerals up to the light. Astringent. A taste that puckers the lips. Asymmetric class. The class without any symmetry. Asymmetric dispersion. The dispersion which produces an interference figure without any symmetry of cold distribution. Axes of reference. Coordinate axes to which crystal faces are referred. Axes of symmetry. Lines about which a crystal may be revolved so that it occupies the same position in space. Axial angle. The angle between the optic axes. Axial cross. Coordinate axes or axes of reference. Axial elements. The axial ratio and the angles between the axes. Axial plane. The plane of the optic axes. Axial ratio. The relative lengths on the axes cut off by the face. Barium minerals. Barite, witherite. Barytes. Synonym of barite. Basal pinacoid. The pinacoid |001 \ or (0001 ( . GLOSSARY 491 Basalt. A dense vulcanic rook composed chiefly of plagioclase and augite with a glassy grounduess. Basaltic hornblende. A variety of hornblende found in volcanic rocks. Basanite. A black compact variety of chalcedony. Basic rocks. Igneous rocks comparatively low in silica (SiO2<50%). Basic salts. Compounds in which only part of the hydroxyl of a base is replaced by an acid radical. Bastite. A pseudomorph of serpentine after bronzite. Batholith. A deep-seated mass of igneous rock of indeterminate shape. Baveno twin. An orthoclase twin with {021 [ as twin plane. Becke test. A test for relative indices of refraction. Berlin blue. An anomalous interference color of the first order. Bertrand lens. A small lens inserted in the microscope tube to magnify the interference figure. Beryllium minerals. Beryl and chrysoberyl. Beta (/3). (1) The angle between the a- and c-axes. (2) The indicatrix axis normal to the plane of a and fl. Biaxial. An optical term for crystals of the orthorhombic, monoclinic and triclinic systems. Bipyramid. A double-ended pyramid. Birefringence. The strength of the double refraction denoted by (T-a). Bisectrix. The line bisecting the angle between the optic axes. Bisilicates. The same as metasilicatos, R"Si03 = R"< ).Si02. Bismuth flux. A mixture of 2 parts S, 1 part KI, and 1 part KHSO^. Bismuth minerals. Bismuth, bismuthinite. Bisphenoid. A form apparently consisting of two sphenoids place together symmetrically. Blackband. An impure carbonaceous siderite. Black jack. A synonym of ferriferous sphalerite. Blende. A synonym of sphalerite. Blue-john. A banded fluorite used for ornamental purposes. Bog iron ore. A synonym of limonite. Bog manganese. A synonym of wad. Bone-ash. Calcium phosphate used in silver cupellation. Borates. Salts of H3BO3, HBO^, H^B^O,, etc. Boracic acid flux. A mixture of 1 part fluorite and 4 parts KHSO^. Boron minerals. Axinite, boracite, borax, colemanite, datolite, ulexite. Bort. A variety of diamond without distinct form or cleavage. Botryoidal. Resembling a bunch of grapes. Brachy-axis. The a-axis in the orthorhombic and triclinic systems. Brachy-dome. The form {oA'^i in. Brachy-pinacoid. The form {010 j. »^.-,--!_ A _.,„i, ^«r!o mi nf nnn.rfi" aTiffular fragments cemented together. 492 GLOSSARY Brittle mica. Synonym of margarite. Brittle silver ore. Synonym of stephanite. Bronze mica. Synonym of phlogopite. Bytownite. A basic plagioclase between labradorite and anorthite. Calcareous tufa. A porous calcium carbonate formed by springs. Calc-spar. A synonym of calcite. Calcium minerals. Anhydrite, ankerite, apatite, aragonite, calcite, cole- manite, dolomite, fluorite, gypsum, scheelite, woUastonite and many silicates. Campylite. A mineral intermediate between pyromorphite and mimetite. Capillary. Hair-like. Carbon minerals. Graphite, diamond, and carbonates. Carbonado. A black opaque variety of diamond. Carbonates. Salts of H^COj. Carborundum. An artificial silicon carbid. Carlsbad twin. A penetration twin of the feldspars with c as the twin-axis. Carnotite. A potassium uranium vanadium compound. It is probably a mixture. Center of symmetry. A crystal that has for every face a similar opposite parallel face has a center of symmetry. Cerium minerals. Allanite, monazite. Chalcotrichite. A capillary variety of cuprite. Chalk. A fine-grained calcium carbonate rock formed, by organisms. Chatoyant. . With an opalescent reflection or radiation. Chalybite. A synonym of siderite. Chert. A massive chalcedony rock. Chessylite. A synonym of azurite. Chiastolite. A variety of andalusite with regularly arranged carbonaceous material. Chlorids. Salts of HCl. Chlorin minerals. Atacamite, boracite, apatite, carnallite, cerargyrite, hal- ite, mimetite, pyromorphite, sal-ammoniac, sylvite, sodalite.vanadinite, wernerite. Chlorite. A group of silicates. Chondrules. Rounded nodules occurring in meteorites. Chromates. Salts of HjCrO,. Chrome garnet. Synonym of uvarovite. Chrome iron ore. Synonym of chroraite. Chromites. Salts of HjCrOj, HCrOj, etc. Chromium minerals. Chromite, crocoitc, and uvarovite. Chrysolite. A synonym of olivine. Chrysotile. A fibrous variety of serpentine. GLOSSARY 493 Chrysoprase. An apple-srecn chalcedony. Clastic. Made up of fragments of preexisting rocks. Clay. A mixture of hydrous aluminum silicates with feldspar, quartz, etc. Cleavage. The property of breaking in certain crystallographio directions. Closed forms. A crystal form that encloses space of itself. Clino-axis. The a-axis in the monoclinic system. Clinopinacoid. The form 1010} in the monoclinic system. Clinodome. The form \oKl\ in the monoclinic system. Cobalt minerals. Cobaltite, erythrite, smaltite. Color chart. The interference colors of a wedge placed on a diagram. Colors, interference. The colors of doubly refracting substances as seen in polarized light. Combination. A crystal composed of two or more forms. Columnar. Made up of coarse, more or less parallel, aggregates. Complementary forms. Two forms which combined geometrically produce a form with higher symmetry. Composition face. The face of union of the parts of a twin crystal. Composite symmetry. The recurrence of similar faces by rotation about an axis combined with reflection in a plane. Conchoidal fracture. Curved fracture like the surface of a shell. Concretion. Nodular masses often fantastic in shape. Conglomerate. A rock made up of coarse, rounded fragnjents cemented together. Congruent forms. Two forms which may each be derived from the other by rotation. Conoscope. A polariscope for convergent light. Contact goniometer. A cardboard or metal protractor for measuring angles. Contact metamorphism. Metamorphism produced by the heat of an igneous intrusion. Contact minerals. Minerals formed by contact metamorphism. Contact twin. A twin with definite composition face. Copper minerals. Atacamite, azurite, bornite, bournonite, brochantite. chalcanthite, chalcocite, chalcopyrite, chrysocolla, copper, coveUite, cuprite, enargite, malachite, melaconite, olivenite, stannite, tetrahedrite Copperas. Synonym of FeSO^.TH^O. Copper glance. Synonym of chalcocite. Copper pyrites. Synonym of chalcopyrite. Cordierite. Synonym of iolite. Crossed nicols. Two Nicol prisms with their vibration planes at right angles. Crossed dispersion. The dispersion which produces an interference figure with color distribution symmetrical to the center of the figure. Crystal class. The sum of all crystals with the same symmetry. 494 GLOSSARY Crystallites. Minute hair or fern-like forms which are supposed to be incipi- ent crystals. Cube. A parallelopiped composed of six similar faces at right angles to each other. Cupellation. The removal of lead from lead-silver alloy by volatilization and absorption by bone-ash. Cyclic twin. A repeated twin which tends to form a circle. Dacite. A volcanic rock intermediate between rhyolite and andesite. Decrepitate. Flies to pieces when heated. Deltohedron. An isometric form of 12 faces with the symbol jhhlj. Deliquescent. Absorbing water from the air. Dendritic. Branching like a tree. Diabase. An igneous rock composed essentially of plagioclase and auglte with ophitic texture. Dialloge. A lamellar variety of diopside. Dichroic. A substance with two axial colors. Dichroscope. An instrument for observing pleochroism. Dike. A tabular mass of intrusive igneous rock. Diploid. The general form of the diploidal class. Dioploidal class. The class with the symmetry SAj . 3P . 4A3. Dimorphism. The occurrence of a substance in two modifications which differ in crystallization. Diorite. A plutonic igneous rock composed essentially of plagioclase and hornblende. Dispersion. The divergence of the optical constants for different parts of the spectrum. Dodecahedron. The form jllO} in the isometric system. Dodecants. One of the twelve divisions into which space is divided by four axes of the hexagonal system. Dog-tooth spar. A variety of sharp-pointed calcite crystals. , Dolomitization. The process by which calcium carbonate is converted into dolomite. Dome. A form consisting of two faces astride a plane of symmetry. Domatic class. The crystal class with a plane of symmetry. Double refraction. The property of splitting a ray of light into two rays. Drusy. A surface apparently sprinkled with minute crystals. Dry-bone. A local synonym of either smithsonite (Wis.) or cerussite (Mo.). Ductile. Capable of being drawn into wire. Dunite. An igneous rock composed almost entirely of olivine. Eclogite. A metamorphic rock composed of pyroxene or amphibole with garnet. GLOSSARY 495 Edge. The intersection of two crystal faces. Effervescence. The bubbUng caused by the evolution of such gases as CO^ and H,,S. Efflorescent. Gives up its nature of crystalhzation on standing. Elaeolite. A synonym of nephelite. Elastic. An clastic mineral springing back when bent. Electrum. A natural alloy of gold and silver with over 20% of silver. Elements, axial. The relative intercepts of the unit form and the angles ' between the axes. Elements of symmetry. Axis, plane and center of symmetry. Emerald. The clear green variety of beryl. Emery. A mixture of corundum with either magnetite or hematite. Empirical formula. The simplest method of expressing the chemical com- position without regard to structure or constitution. Enantiomorptaous. Two compounds are enantiomorphous when they are the mirror-image of each other. Endlichite. An isomorphous mixture of mimetite and vanadinite. Erubescite. A synonym of bornite. Essonite. A variety of garnet. Etch-figures. The small geometric figures produced on crystal faces by a solvent. Ether. A hypothetical substance necessary to explain the transmission of light. Exfoliate. To swell up or to spread out like the leaves of a book. Extinction. The getting dark of a, doubly-refracting crystal between crossed-nicols. Extinction angle. The angle between the extinction direction and the crystal outline. Extinction direction. The position of extinction. Extraordinary ray. In doubly refracting substances the ray which produces the image that revolves around the other image. Extrusive. A term applied to igneous rocks which have been poured out on the surface. Face. One of the flat, more or less smooth, surfaces of a crystal. Face color. The color of a crystal when viewed in a particular direction. Facet. The polished surface of a cut-stone. Fahlerz. The German name for tetrahedrite. Felsite. A volcanic igneous rock with fine granular texture and without phenocrysts. Felsitic. A fine granular texture, applied to igneous rocks. Feldspathoid. Lencite, nephelite, and sodahte are called feldspathoids be- • 1-- 4.1 — „fi„„ t„i,„ +!,„ nlace of the feldspars. 496 GLOSSARY Ferro-alloys. Alloys of iron or steel with various elements. Ferro-magnesium. The roek-forming minerals, augite, hornblende, olivine, biotite, etc., which contain Fe and Mg. Fibrous. Made up of fibers. Fire-opal. A variety of opal with red reflections. Flexible. Capable of being bent and remaining so. Flint. A variety of chalcedony. Fluorids. Salts of HF. Fluorin minerals. Apatite, amblygonite, ohondrodite, cryolite, fluorite, lepidolite, topaz. Fluxes. Synonym of fluorite. Foliated. Made up of flat plates. Fool's gold. Synonym of pyrite. Form. The sum of like faces on a crystal. Form, closed. A form which encloses space of itself. Form, open. A form which does not enclose space of itself. Formula weight. The sum of the atomic weights of the elements. Fowlerite. A zinc-bearing variety of rhodonite. Fracture. The manner of breaking when not oleavable. Freibergite. Argentiferous tetrahedrite. French chalk. A kind of talc used by tailors. Friable. Capable of being pulverized by rubbing between the fingers. Fusibility, scale of. A list of minerals ranging from very easily fusible stib- nite to infusible quartz. Gabbro. A basic plutonic rook composed of basic plagioclase and diallage. Galenite. Synonym of galena. Gamma {T). (1) The angle between the a and 6 axes. (2) The longest indicatrix axis. Gangue. The non-metallic part of a vein. Gelatinization. Solubility with the formation of jelly-like silica. General form. The form \hkl\ or hkil in any crystal class. Geode. A hollow concretion usually lined with crystals. Geyserite. A variety of opal silica formed in hot spring deposits. Glance. A general name for metallic sulfids.. Glass, natural. Volcanic rocks such as obsidian, pitchstone, pumice, etc. Gliding. A molecular rearrangement due to twinning. Globular. Made up of more or less complete spheres. Gnomonic projection. A projection made on a plane tangent to a sphere. Gold minerals. Calaverite, eleetrum, gold, sylvanite. Goniometer. An instrument used for measuring crystals. Gossan. The upper oxidized portion of a vein or ore deposit. GLOSSARY 497 Granite. A pliitonic igneous rook composed essentially of quartz and ortho- clase. Granitic or granitoid. The even-grained texture of a granite. Graphic determination of indices. Graphic granite. An intergrowth of quartz and orthoclase. Graphic solution of a spherical triangle. Graphic texture. The texture of graphic granite. Graphitic acid. A substance formed by the action of KCIO3 and cone. HNO3 on graphite. Gravel. A coarse fragmental deposit. Gray copper ore. A synonym of tetrahedrite. Grelsen. A quartz-muscovite rook formed by pneumatolysis. Greenstone. A dense mctamorphic rock. Grossularite. A sub-species of garnet containing Ca and Al. Guano. A phosphatic deposit formed by the leaching of bird and bat e.xcreta. Gypsum wedge. A thin wedge-shaped slice of selenite. Gyroid. The general form of the gyroidal class. Gyroidal class. The crystal class with 6A; ■ 4A, • 3A,. Habit, crystal. The general shape of a crystal as determined by growth in certain directions. Hackly. A term applied to the fracture of metals. Haloids. Salts of HCl, HBr, HI, and HF. Hardness, scale of. Heavy liquids. Liquids such as methylene iodid and potassium mercury iodid used for specific gravity determinations. Heavy spar. Synonym of barite. Hematite brown. Synonym of limonite. Hemi-dome. The form \hol\ in the monoclinic system is sometimes called a hemi-dome. Hemi-prism. The form \hko\ in the triclinic system. Hemi -pyramid. The form \hkl] in the monoclinic system. Hexagonal system. The sum of all classes with a single 3-fold or 6-fold axis of symmetry. Hexahedron. Synonym of a cube. Hexoctahedral class. The class with the symmetry GAj- 4A3- 3A,- 9P. Hexoctahedron. An isometric form consisting of 48 faces. Hextetrahedral class. The class with the symmetry 3A2- 4A3- 6P. Hextetrahedron. An isometric form with 24 faces. Hiddenite. An emerald-green variety of spodumene. Holohedral. A name sometimes applied to the class that has the highest 498 GLOSSARY Hemihedral. The forms with lower symmetry than the holohedral. Hemimorphic. Having two ends of the crystal differently terminated. Horizontal dispersion. The dispersion which produces an interference figure with color distribution symmetrical with respect to a line normal to the axial plane. Hornfels. A dense rock formed by the contact metamorphism of shales. Horn silver. Synonym of cerargyrite. Hyacinth. A variety of zircon. Hyalite. The glassy variety of opal. Hydromagnesite. Hydrous salts. Salts with the so-called water of crystallization. Hydroxids. Compounds of elements with hydroxy! . Hydroxyl. The radical or ion (OH). Iceland spar. Transparent calcite showing double refraction. Icosahedron. One of the regular Platonic solids with twenty faces, an im- possible crystallographic form. Icositetrahedron. A synonym of the isometric trapezohedron. Ideal form. A form in which like faces are of the same size and shape. Idocrase. A synonym of vesuvianite. Igneous rocks. Rooks formed by the consoKdation of molten magma. Imitative forms. The slopes of minerals which resemble other natural objects. Inclined dispersion. The dispersion which produces an interference figure with color distribution symmetrical to the trace of the axial plane. Inclusions. Foreign substances caught when crystals are being formed. Index of refraction. A number which indicates the amount of bending of a ray of light on going from one medium into another. Indicatrix. An ellipsoid, spheroid, or sphere used as an aid in studying crystal optics. Indices, Miller. The reciprocal ratios of intercepts in terms of a selected unit. Indicolite. A deep blue variety of tourmaline. Intercepts. Distances cut off on axes of reference by planes. Interfacial angle. The dihedral angle between two faces. Interference. The result of two light waves travelling along the same path after one has been retarded. Interference colors. The effects of interference as seen in white light. Interference figure. The image produced by certain crystal plates when viewed in convergent polarized light. Intergrowth. An intimate regular arrangement of two substances produced by simultaneous crystallization. GLOSSARY 499 Intrusive. Igneous rocks which have invaded other rocks are called in- trusive. Intumescence. A bubbling up due to escape of water when heating a mineral. Inversion. The operation indicated by an composite axis of two-fold symmetry. Iridescence. A rainbow effect produced by interference of light in thin films. Iron minerals. Ankerite, arsenopyrite, biotite, bornite, chalcopyrite, chromite, columbite, copiapite, franklinite, goethite, hematite, ilmenite, iron, limonite, loUingite, magnetite, marcasite, melanterite, olivene, pyrite, pyrrhotite, siderite, triphylite, irrianite, wolfamite, and some silicates Iron-pjrites. A synonym of pyrite. Isodunorphism. Two parallel isomorphous groups one or more of which in each group is dimorphous. Isomorphism. The property of similar chemical compounds crystallizing in similar forms. Isomorphous mixtures. A solid solution of two or more isomorphous substances. Isotropic. Usually means optically isotropic. This term is synonymous with singly refracting. Jade. A compact tough green or greenish-white ornamental stone com- posed of either an amphibole or pyroxene mineral. Jargon. A pale-colored variety of zircon. Jasper. A red, yellow, or brown variety of chalcedony colored by iron oxids. Jolly balance. A specific gravity balance made of a spiral brass wire. Kaolin. A mixture of kaolinite with other aluminous silicates. Kaolinization. The process by which the feldspars are converted into kaolin. Kidney-ore. A variety of hematite. Kunzite. A variety of spodumene used as a gem. Kyanite. The same as cyanite. Laccolite. A dome-shaped mass of igneous rock the intrusion of which arches the overlying strata. Lamellar. Made up of plates. Laminae. Thin layers. Lapilli. Small fragments of volcanic rocks produced by the explosive 500 GLOSSARY Lead minerals. Anglesite, bournonite, cerussite, orocoite, galena, jameso- nite, mimetite, pyromorphite, vanadinite, wulfenite. Lenticular. Lens-shaped. Light, polarized. Light in which the vibrations are in one plane. Limestone. A sedimentary rock consisting of CaCO^. Limestone, crystalline. A metamorphic rock derived from a sedimentary limestone. Limit form. Any form except { hid \ and | hyil \ . Lithium minerals. Amblygonite, lepidolite, spodumene, triphylite. Lodestone. A variety of magnetite, which is a magnet itself. Luster. A term applied to the quaUty of light reflected from a surface. Made. Synonym of a twin-crystal. Macro-axis. The 6-axis (long) in orthorhombic and triclinic crystals. Macrodome. The form \hol\ parallel to the macro- or 6-axis in the ortho- rhombic and triclinic systems. Macropinacoid. The pinacoid {100} in the orthorhombic and triclinic systems. Macroscopic. The same as megascopic. Magma. The liquid mass from which the igneous rocks originate by cooling and solidification. Magnesium minerals. Actinolite, ankerite, boracite, brucite, carnallite, chondrodite, dolomite, enstatite, epsomite, hydromagnesite, hyper- sthene, kainite, magnesite olivine, sepiolite, serpentine, spinel, talc, tremoite and other silicates. Magnetic pyrites. Synonym of pyrrhotite. Magnetic iron ore. Synonym of magnetite. Malleable. Capable of being hammered out flat. Mammillary. A surface consisting rounded protuberances. Manbach. One of the twinning law of orthoclase. Manganese minerals. Alabandite, franklinite, hiibnerite, manganite, psilomelane, pyrolusite, rhodochrosite, rhodonite. Marble. Any limestone capable of taking a good polish. Martite. A hematite pseudomorph after magnetite.' Massive. Without definite form or shape. Meerschaum. Synonym of sepiolite. Megascopic. Can be seen with the naked eye. Melanite. A variety of andradite garnet. Menacconite. Synonym of ilmenite. Mercury minerals. Cinnabar; mercury. Metals. Elements which replace the hydrogen of acids. Metalloids. Semi-metals such as As, Sb, and Bi. Metasilicates. Salts of H._,Si03. GLOSSARY 501 Metasomatic replacement. The rcijlaccment of a rock body or vein, in which the original structure is retained. Metasomatism. The same as metasomatic replacement. Meteorites. Celestial bodies compdscd of iron, and various silicates which occasionally enter the earth's atmosphere. Methylene iodid. A heavy liquid used in specific gravity determinations. Micas. A group of silicate minerals. Micaceous. Made up of readily separable plates. Microchemical tests. Chemical tests made on a small scale. The form, color, and optical properties of the minute crystals produced are used. Microcosmic salt. The same at salt of phosphorus. Microlites. Minute crystals found in obsidians and related rocks. Microperthite. An intergrowth of albite with orthoclase or microcline. Miller indices. Mathemathical symljols for crystal faces. Mimetic twinning. The tendency of twinning to raise the grade of symmetry and thus to imitate otiier crystals. Mispickel. Synonym of arsenopyrite. Molecular compounds. Compounds such as MgSOjKCl-SHjO which are written as if they consist of separate molecules. Molybdenum minerals. Molybdenite, wulfenite. Molybdates. Salts of H^MoO^. Monochromatic. Light of one color or approximately one wave-length. Monoclinic system. The system which includes crystals with one fixed direction of symmetry. Moonstone. Avariety of orthoclase used as a gem. Morphology, crystal. The same as geometrical crystallography. Mountain cork. A porous variety of tremolite. Mountain leather. A variety of tremolite resembling leather. Mundic. A local miner's name for pyrite or marcosite. Native elements. Elements that occur in nature uncombined. Negative crystal. A cavity within a crystal of the same shape as the crystal. Negative elongation. The length of a crystal is parallel to the faster ray. Negative, optically. Uniaxial crystals in which c = a and biaxial crystals in which the acute bisectrix is a. Nepheline. Same as nephelite. Nepheline-syenite. Same as nephelite-syenite. Nephelite-syenite. A plutonic igneous rock composed essentially of ortho- clase and nephelite. Nickel minerals. Garnierite, millerite, and niocolite. Nicol. A nicol prism. Nicol prism. A piece of Iceland spar arranged so as to produce polarized licrht. 502 GLOSSARY Niobates. Salts of H3Nb04 and HNbO,. Niobium minerals. Columbite. Nitrates. Salts of HNO3. Nitratine. Synonym of soda-niter. Nodular. Consisting of a rounded lump. Non-metallic. Any luster but metallic. Normal salt. A salt in which all the hydrogen of the acid has been replaced by metals. Ndlrite. A gabbro in which the ferro-magnesium mineral is largely hyper- sthene. Obsidian. A natural volcanic glass of vitreous luster. Obtuse bisectrix. The line bisecting the obtuse angle between the optic axes. Obtuse rhombohedron. A rhombohedron with interfacial angle less than 90°. Ocher. A clay colored by iron oxids. Octahedron. An isometric crystal with eight triangular faces at angles of 70° 32'. Octahedral cleavage. Cleavage in four directions parallel to the faces of an octahedron. Omphacite. A bright green variety of diopside occurring in eclogites. Onyx. A banded variety of chalcedony. Onyx marble. A banded variety of calcite or aragonite. Onyx, Mexican. The same as onyx marble. Oolite. A rock made up of minute spheres. OBlitic. Made up of minute spheres. Opalescence. The peculiar milkiness seen in opal. Opalized wood. Wood replaced by opal. Open forms. Crystal forms that do not enclose space of themselves. Ophicalcite. A serpentine veined or mottled with calcite, dolomite or magnesite. Optical anomalies. Abnormalities in the optical properties. Optical character. The designation as to whether optically positive or optically negative. Optical constants. The indices of refraction, axial angle, extinction angle, etc. Optical orientation. The position of a, /?, and T with respect to the axes of reference, a, b, and c. Optic axes. Two directions normal to the circular sections of the indicatrix for biaxial crystals. Optic axis. The direction of the c-axis in uniaxial crystals. Optics, crystal. The subject which treats of the transmission of light in crystals. GLOSSARY 503 Ordinary ray. In doubly refracting crystals the ray which produces the stationary image. Organic deposits. Rocks and other deposits formed by organisms or their remains. Oriental amethyst. A purple variety of corundum. Oriental emerald. A green variety of corundum. Oriental sapphire. The true sapphire, a variety of corundum. Oriental ruby. The true ruby, a variety of corundum. Oriental topaz. A yellow variety of corundum. Orientation, optical. The position of the indicatrix axes a, /?, and T with respect to the axes of reference a, b, c. Orient to a crystal. Is to set up with the axes of reference in their con- ventional position. Ortho-axis. The 6-axis in the monoclinic system. Orthodome. The forms \hol\ and {hol\ in the monoclinic system. Orthographic drawing. A drawing made by perpendicular projectors. Orthopinacoid. The form jlOOj in the monoclinic system. Orthorhombic system. The system which includes crystals with three fixed directions of symmetry. Orthosilicates. Salts of H,SiOj, the normal silicic acid. Oxids. Compounds of elements with oxygen. Oxy-chlorid. A compound of the type R3OCI or R^O • RCl. Oxy-salt. Ordinary salts as distinguished from sulfo-salts. Oxy-sulfid. A compound of the type R,OS or RjO • R^S. Paragenesis. The association of minerals with special reference to the occurrence and origin. Paragenetic varieties. Varieties based upon mode of occurrence and origin. Parallel growth. Two or more crystals with corresponding faces parallel. Parameters. The intecepts of the unit face on the axes of reference. Paramorph. The pseudomorph of one dimorphous mineral after another. Parting. A separation caused by molecular disturbance such as twinning. Peacock copper. Synonym of chalcopyrite. Pearl-spar. A synonym of dolomite. Pearly luster. The luster due to continued reflection from parallel plates. Pedial class. The class without any symmetry. Pedion. A form consisting of a single face. Pegmatites. Coarse vein-like deposits occurring in plutonic igneous rocks especially granite. Pentagonal dodecahedron. Another name for the pyritohedron. Percussion-figure. A six-rayed star developed on the micas by a sharp blow. Pericline. A variety of albite elongated in the direction of the S-axis. 504 GLOSSARY Peridot. The gem variety of alivine. Peridotite. A plutonio igneous consisting essentially of alivine and some pyroxene. Perthite. An intergrowth of albite with orthoolase or microcline. Petrifaction. The replacement of fossils by mineral substances. Petrography. The science treating of rocks especially from the descriptive side. Petrographic microscope. The same as polarizing microscope. Petrographic province. A region in which the igneous rocks are supposed to have a common origin. Petrology. The study of rocks from a broad standpoint. Phantom crystal. A cystal in which an earlier stage of crystallization is marked in some way. Phenocryst. Crystals scattered through a fine ground-mass and formed before the ground-mass. Phonolite. A volcanic rock composed essentially of orthoclase and nepheline. Phosphate rock. An impure massive calcium phosphate. Phosphorite. Practically a synonym of phosphate rock. Picotite. A chrome-bearing variety of spinel. Pinacoid. A form consisting of two parallel faces. Pinacoidal class. The class with a center of symmetry. Pisolitic. Made up of spheres about the size of buck7shot. Pitch-blende. A synonym of uraninite. Placers. Stream deposits of sand and gravel. Plagioclase. A group of soda-lime feldspars. Plaster tablets. Thin rectangular plates used as a blowpipe support. Pleochroism. The property of absorbing different kinds of light in different directions. Pleonaste. An iron-bearing variety of spinel. Plumose. Feather-like. Plutonic rocks. Deep-seated igneous rocks. Poikilitic. A term referring to inclusions, irregularly arranged through a crystal. Polar edges. Polar edges are those that intersect the c-axis. Polarized light. Light in which the vibrations are in one plane. Polarizer. The lower niool. Pole. The projection of a crystal face on a sphere made by a radius normal to the face. Polyhedron. A general name for, a solid bounded by plane faces. Polymorphism. The occurrence of a substance in two or more modifications which differ in crystallization. Polysynthetic twin. A twin made up of three or more parts in which the same face serves as twin-plane. GLOSSARY 505 Positive elongation. The length of the crystal is parallel to the slower ray. Positive, optically. Uniaxial crystals in which (■ = ;- and biaxial crystals in which the acute bisectrix is y. Potash. K.O. Potassium minerals. Alunite, biotite, carnallite, kainite, lepidolite, leucite, microoline, muscovitc, sylvite. Porphyry. An igneous rock with porphyritic tuxturc. Porphyritic. With crystals set in a fine ground-mass. Precious opal. A variety of opal with a play or colors. Primitive circle. The great circle in the plane of projection (stereographic projection). Primitive form. A form from which other forms may be derived. Principal axis. The c-axis in tetragonal and hexagonal crystals. Prism. An open form with faces all in one zone. Prismatic class. The class with the symmetry AjPC. Prismatic habit. Applied to crystals that are elongated in one direction. Projection, gnomonic. A projection made on a plane tangent to a sphere. Projection, Unear. A projection formed by crystallographic planes; shifted so that they cut the unit length of the c-axis, on a horizontal plane. Projection, stereographic. A projection made on a plane though the center of a sphere by projectors from the south pole. Pseudo-hexagonal. Orthorhombic or monoclinic crystals which simulate crystals of the hexagonal system. Pseudomorph. A mineral with the crystal form of another mineral. Pyknometer. A small flask for specific gravity determinations. Pyramid. An open form consisting of several faces meeting in a point. Pyramidal habit. With the general shape of a pyramid. Pyrargjrrite. Pyrites. Synonym of pyrite. Also used for other minerals, e.^r., copper pyrites. Pyritohedron. The form \hko\ in isometric diploid class. Pyroclastic. Made up of fragmental volcanic products. Pyroelectric. The property of developing electricity on heating or cooling. Pyrognostic tests. The same as blow-pipe tests. Pyrope. A sub-species of garnet containing Mg and Al. Pyroxene. A name sometimes used for diopside and augite together. Pyroxene group. A group of metasilicate minerals. Pyroxenite. A plutonic igneous rock composed essentially of some pyroxene. Quarter -undulation. Refers to one-fourth of a wave-length for sodium light. Quart zite. A metamorphic rock made up firmly of cemented quartz grains --. L 1 — A „rorirrQ.oV,(>npd slice nf ouartz used in optical work. 506 GLOSSARY Quicksilver. A synonym of mercury. R. A general symbol standing for some metal as RCO3. Radical. A group of elements such as NH^or SO^. Radium. An element found in uraninite. Rare-earth metals. Cerium, didymium, erbium, lanthanum, thorium and ythrium. Rational indices. The indices of crystal faces are rational numbers usually simple numbers below 6. Reduction. The taking away of oxygen or the change to a lower state of oxidation. Reduction, color tests. Tests made with NaHOj beads and tin. Reflection goniometer. An instrument for measuring interfacial in or coordinate angles. Reflection, total. The turning back of light rays when the critical angle is exceeded. Refraction. The bending of light rays when passing from one medium into another. Refraction, index of. A number to express the amount of refraction. Refractometer. An instrument for determining the index of» refraction. Regular system. Same as the isometric system. Relief. The appearance of a substance as depending upon its index of refraction and that of a substance in which it is embedded. Reniform. Kidney-shaped. Replacement. Substitution of a rock or vein by mineral matter. (2) The substitution of an element or radical by other element or radical. Resinous. The luster of resin. Reticulate. Made up of a net- work. Rhombic bipyramid. A bipyramid with rhombic cross-section. Rhombic prism. A prism -with, a rhombic cross-section. Rhombic system. Same as orthorhombic system. Rhombohedral class. The crystal class with the symmetry JP^. Rhombohedron. A six-faced form resembling a distorted cube. Rhyolite. A volcanic igneous rock composed essentially of quartz and orthoclase with more or less glass. Rock. An essential part of the earth's crust. Rock cystal. The clear transparent variety of quartz. Rock salt. Synonym of halite. Rubellite. The pink or red variety of tourmaline. Ruby. The red transparent variety of corundum. Ruby copper. Synonym of cuprite. Riiby silver. A group name including pyrargyrite and proastite. Rutiliated quartz. Quartz penetrated by needles of rutile. GLOSSARY 507 Salt. Synonym of halite. Salt of phosphorus. Sodium ammonium phosphate, a reagent used in blow- pipe analysis. Salts, acid. Salts in which only a part of the hydrogen of the acid has been replaced by metals. Salts, basic. Salts in which only part of the hydroxyl of the base has been replaced by acid radicals. Salts, normal. Salts in which hydrogen of acid and hydroxyl of base have been replaced. Sandstone. A sedimentary rock composed of consolidated sand. Sanidine. A glassy transparent orthoclase occurring in volcanic rocks. Sapphire. A blue transparent variety of corundum. Satin-spar. A fibrous variety of gypsum. Scalenohedron. A twelve-faced form with the symmetry Aj-SAj-SP and with alternate interfacial angles equal. Scapolite. A synonym of wernerite. Schiller. A bronze-like reflection caused by inclusions. Schist. A metamorphic rock which splits into thin layers. Schistose. Splitting into thin layers. Schorl. An old name for tourmaUne. Secondary. Minerals formed subsequent to those of the main rock mass are said to be secondary. Sectile. Capable of being cut by a knife but not flattened under the hammer. Sedimentary. Laid down in water. Selenite. The cleavable variety of gypsum. Selenite plate. A plate of selenite which gives a purplish-red interference color of the first order with crossed nicols. Semi-opal. Common opal as distinguished from precious and fire opal. Sensitive tint. The purple interference color given by a selenite plate (red of the first order). Sericite. A silvery variety of muscovite, usually secondary. Serpentine (mineral). The mineral. Serpentine (rock). A metamorphic rock formed from a peridotite. Serpentinization. The change of peridotite to serpentine rock. Shale. A clastic rock formed by the consolidation of fine aluminous sedi- ments. Silicates. Salts of silicic acid. Sill. A dike-like igneous rock parallel to sedimentary beds. Silver glance. Synonym of argentite. Silver minerals. Argentite, cerargyrite, polybasite, proustite, pyrargyrite, silver, stephanite, sylvanite. Skeleton crystals. Hollow or imperfectly developed crystals formed by 508 GLOSSARY Slate. Thinly cleavable fine grained metamorphic rocks formed from shales. Smalt. Cobalt silicate used as a pigment. Smoky quartz. A variety of quartz colored by a brownish pigment. Soda. (1) Sodium carbonate, a blowpipe reagent. (2) NajO. Sodalite. A massive talc rock. Sodium metaphosphate. A reagent used in blowpipe analysis, NaPOj. Sodium minerals. Aegirite, albite, analcite, borax, cryolite, glaucophane, halite, lazurite, mirabilite, natrolite, nephelite, pectolite, sodalite, soda-niter, trona ulexite, wernerite. Specfiic gravity. The weight of a substance compared with an equal bulk of water. Specular iron ore. A synonym of hematite. Specularite. A synonym of hematite. Spessartite. A sub-species of garnet containing Mn and Al. Sphene. A synonym of titanite. Sphenoid. A form consisting of faces not astride a plane of symmetry. Sphenoidal class. The class with an axis of 2-fold symmetry. Stalactite. Icicle-like crystalline aggregates. Stalactitic. Referring to stalactites. Steatite. A massive talc rock. Stereogram. A stereographic projection of a crystal. Stereographic projection. A projection made on a plane through the-center of a sphere by projectors from the south pole. Stratified rocks. Rocks made up of layers and usually formed in water. Streak. The color of the powder of a mineral. Streak-plate. A piece of unglazed porcelain for testing the streak. Stream-tin. A variety of cassiterite found in the form of pebbles or sand in placers. Strontium minerals. Celestite, strontianite. Sub-. A prefix indicating a lower quality or degree than the normal. Subsilicates. SiUcates of the type nRO-SiOj (n>2). Sulfantimonates. Salts of HjSbS,. Sulfantimonites. Salts of HjSbSj. Sulfarsenates. Salts of HjAsSj. Sulfarsenites. Salts of H3ASS3. Sulfates. Salts of H^SO,. Sulfids. Salts of H^S or compounds of the metals and semi-metals with S. Sulfo-acids. Acids of the type HjAsSj, HjAsS,, etc. Sulfo-salts. Salts of the type R'jAsS,, in which O of ordinary salts has been replaced by S. Supplementary twinning. Twinning by which a crystal simulates the sym- metry of a crystal class with higher grade in the same svstem. GLOSSARY 509 Symbols of crystal faces. Mathematical expressions for designating the position of crystal faces on coordinate axes. Sjnnmetry. The rhythmical recurrence of faces; edges and vertices. Sj^mmetry, axis of. A line about which a crystal may be revolved so it occupies the same position in space. Symmetry, center of. A crystal that for every face has a similar opposite parallel face has a center of symmetry. Symmetry, composite. The recurrence of faces, etc., by rotation about an axis combined with reflection in a plane. S3?mmetry, plane of. A plane that divides a crystal into two parts so that similar faces occur on opposite sides of the plane. Syenite. A plutonic igneous rock composed principally of orthoclase. Synthesis, mineral. The artificial production of minerals. Systems, crystal. Crystals which have the same kind of axes of reference constitute a system. Symmetrical Dispersion. The dispersion which produces an interference figure with color distribution symmetrical to trace of axial plane and also to a line normal to this. Tabular habit. The crystal habit due to limited growth in one direction. Talc -schist. A talc rock that splits in thin layers. Tantalum minerals. Columbite. Tarnish. A surface layer usually caused by oxidation. Tellurids. Salts ot H,Te. Tellurium minerals. Calaveritc sylvanite. Tenorite. Synonym of melaconite. Tetartohedral. Classes in which the general form has one-fourth the number of faces of the general form of the holohedral class. Tetartoid. The twelve-faced general form of the tetartoidal class. Tetartoidal class. The class with the symmetry SAj- 4A3. Tetartopyramid. The form \hkl\ in the triclinic system. Tetragonal system. The system which includes the crystal classes with a single axis of four-fold symmetry. Tetrahedron. A form consisting of four equilateral triangular faces. Tetrahexahedron. The twenty-four faced form \hko\ of the isometric system. Texture. The mutual relations of the minerals in a rock. Thin section. A paper-thin slice of a rock or a mineral. Thoria. The oxid of thorium, ThO^. Tiger's-eye. A quartz pseudomorph after fibrous crocidolite (a soda amphi- bole). Tin minerals. Cassiterite, stannite. 510 GLOSSARY Tin-stone veins. Veins intermediate between ordinary veins and pegma- tites. Titanium minerals. Ilmenite, rutile, titanite. Total reflection. The turning back of light rays when the critical angle is exceeded. Touch stone. A black variety of chalcedony used for testing the streak of gold and other metals. Trachyte. A volcanic igneous rock composed principally of orthoclase. Tra,p. A dense black igneous rock usually either basalt or diabase. Trapezohedron, hexagonal. The 12-faced form with the symmetry Ag-eAj. Trapezohedron, isometric. The 24-faced form \hkk] in the hexoctahedral diploidal, and gyroidal classes of the isometric system. Trapezohedron, tetragonal. The 8-faced form with the symmetry Aj-4A2. Trapezohedron, trigonal. The 6-faced form with the symmetry Aj-SA^. Travertine. A banded calcium carbonate rock formed from water solution. Triclinic system. Includes the crystal classes without any fixed direction of symmetry. Trigonal pyramidal class. The class with the symmetry Aj. Trigonal trapezohedral class. The class with the symmetry A-^-SA^- Trilling. A twin crystal with three individuals or parts. Trimorphism. The same substance in three different modifications. Trisoctahedron. An isometric form of 24 faces with the symbol \hU}. Tristetrahedron. An isometric form of 12 faces with symbol \hkk}. Troostite. A variety of willemite containing Mn. Truncation. The modification of an edge by a face that makes equal angles with adjacent faces. Tufa, calcareous. A porous deposit of calcium carbonate formed by water solution. Tuff. A fragmental volcanic rock. Tungstates. Salts of H^WOj. Tungsten minerals. Hubnerite, scheelite, wolframite. Turkey-fat ore. A yellow variety of smithsonite containing cadmium. Turmeric paper. Paper used for testing borates and zirconium. Twin-axis. The line about which one part of the twin has apparently been resolved. Twin-crystals. Crystals in which one part is in reversed position with respect to the other. Twin-plane. A plane normal to the twin-axis. Type symbols. General symbol hkl, etc., for crystal faces. Ultrabasic rocks. Igneous rocks with less than about 35 per cent, of silica. Uranium minerals. Uraninite. Unit bipyramid. The form | 111 ( . GLOSSARY 511 Unitfonn. The form illlj or |10Tl|. Unit prism. The form i 1 10 1 . Unit pyramid. The form 1111). Unit series of bipyramids. The forms {hhl\. Unisilicates. Same as orthosilicates, R"Si02 = 2ROj-Si02. Uralite. An actinolite pseudomorph after, or alteration of, augite. Uralitization. The alteration involved in the formation of uralite. Uvarovite. A sub-species of garnet containing Ca and Cr. Vanadates. Salts of HjVOj. Vanadium minerals. Vanadinite. Vein minerals. The minerals of veins, especially the gangue. Veins. Narrow mineral masses formed by water solutions usually in fissures. Vibration directions. The same as extinction directions. Vicinal faces. Crystal faces with high indices adjoining or replacing faces with simple indices. Vitreous luster. A luster like that of ordinary glass or quartz. Volcanic ash. A fine fragmental volcanic deposit. Volcanic breccia. A coarse fragmental volcanic deposit. Volcanic rocks. Igneous rocks which solidified at or near the earth's sur- face. Volcanic tuff. A more or less consolidated fragmental volcanic rock. Vug. A cavity in a rock or vein usually lined with crystals. Wave-length. The distance wave-motion of light is transmitted during one vibration. Weathering. Superficial alteration. Weiss symbols. Crystallographic symbols involving intercepts on the axes of reference. Westphal-balance. A convenient apparatus for determining the specific gravity of a liquid. Wood-opal. A variety of opal formed by the replacement of wood. Wood silicified. Chalcedony or opal formed by the replacement of wood. Yellow ocher. A clay colored by limonite. Zeolites. A group of hydrous silicates. Zinc blende. A synonym of sphalerite. Zinc minerals. Calamine, franklinite, hydrozincite, smithsonite, sphalerite, willemite, zincite. Zinc white. Artificial zinc oxid used as paint. Zirconia. Oxid of zirconium, ZrO^. 512 GLOSSARY Zonal relations. The mathematical relations of face'^symbols and zone symbols. Zone. A belt of planes on a crystal, the intersection edges of which are parallel. Zone axis. The direction of the intersection edges of the faces of a zone. Zone circle. A great circle containing the poles of all the crystal faces in a zone. Zone symbol. A symbol written thus [uvw] to distinguish it from a face or form symbol. INDEX Abrasives, 467 Absorption, 146 Achroite, 430 Acids, 166 Acid salt, 168 Actinolite, 402 Acute bisectrix, 139 Adamantine, 111 Adularia, 387 Aegirite, 398 Agate, 307 Alabandite, 277 Alabaster, 467 Albite, 390 Alexandrite, 466 Allanite, 423 Allophane, 445 Almandite, 412 Alteration, 451 Alum, 483 Aluminates, 318 Aluminum, ores of, 473 tests for, 195 Alundum, 469 Alunite, 379 Amazon-stone, 388 Amblygonite, 359 Amethyst, 305 oriental, 311 Ammonium, tests for, 196 AmorphouB, 68 Amphibole, 401 Amalcite, 443 Analyzer, 124 , Andalusite, 419 Andesine, 390 Andesite, 454 Andradite, 412 Angle, axial, 139 interfacial, 4 Anglesite, 371 Anhydrite, 373 Anisotropic, 137 Ankerite, 340 Anorthite, 390 Antimony, 262 ores of, 474 tests for, 196 Apatite, 354 Apophyllite, 440 Apparatus, blowpipe, 176 Aquamarine, 404. Aragonite, 344 Argentite, 273 Arsenates, 352 Arsenic, 261 tests for, 196 Arsenical pyrites, 285 Arsenopyrite, 285 Asbestos, 468 Association of minerals, 448 Asterism, 78 Asymmetric class, 419 dispersion, 150 Atacamite, 303 Augite, 397 Atomic weights, table of, 164, 165 Axes of reference, 15 of symmetry, 6 Axial angle, 139 cross, 99 elements, 94 514 INDEX Axial plane, 138 ratio, 17 Axinite, 424 Azurite, 349 Barite, 369 Barium, tests for, 196 Barytes, 370 Basalt, 454 Basic rocks, 454 salts, 168 Batholith, 453 Bauxite, 329 Bead tests, 187 Becke test, 117 Berlin blue, 155 Beryl, 404 Beryllium, tests for, 197 Biaxial, 138 Bipyramid, 13 Biotite, 428 Bisectrix, 139 Bismuth, 263 tests, 186 ores of, 474 tests for, 197 Bismuthinite, 272 Bisphenoid, 13 Blende, 276 Blowpipe, 175 analysis, 175 Bog iron ore, 328 manganese, 331 Borates, 364 Boracite, 366 Borax bead tests, 186 mineral, 367 reagent, 178 Bornite, 290 tests for, 197 Botryoidal, 67 Bournonite, 291 Breccia, 456 Brittle mica, 434 silver ore, 295 Brochantite, 374 Bronze mica, 429 Brucite, 329 Bytownite, 390 Calamine, 405 Calaverite, 286 Calcareous tufa, 459 Calcite, 332 Calc-spar, 333 Calcium, tests for, 197 Calculation, crystal, 88 Cane-sugar, 37 Carbonado, 469 Carbonates, 332 tests for, 198 Carborundum, 469 Carnallite, 303 Cassiterite, 314 Celestite, 371 Center of symmetry, 7 Cerargyrite, 300 Cerussite, 347 Chabazite, 443 Chalcanthite, 378 Chalcedony, 307 Chaloocite, 274 ChalcopyfTEe, 289 Charcoal, 176 Chatoyant, 589 Chalybite, 341 Chessylite, 349 Chemical formula, 169 tests, 252 Chlorids, 297 Chlorin, tests for, 198 Chlorite, 434 Chondrodite, 425 Chrome garnet, 412 Chrome iron ore, 422 Chromite, 322 INDEX 51 Chromites, 318 Chromium, ores of, 47S tests for, 199 Chrysoberyl, 322 Chrysocolla, 446 Chrysolite, 413 Chrysotile, 435 Cinnabar, 277 Classification of crystals, 20 of minerals, 253 Clastic, 456 Clay, 456 Cleavage, 103 Closed tube tests, 183 Closed form, 14 Cobalt, tetts for, 199 Colemanite, 365 Color chart, 131 of minerals, 112 Colors, interference, 125 Columbite, 352 Combination, 3 Columnar, 67 Complementary forms, 14 Composition face, 63 Composite symmetry, 7 Conchoidal fracture, 105 Concretion, 67 Conglomerate, 456 Congruent forms, 14 Contact goniometer, 4 metamorphism, 459 twin, 63 Copiapite, 379 Copper, 265 ores of, 475 tests for, 199 Copper glance, 274 pyrites, 289 Cordierite, 430 Corundum, 311 Covellite, 278 Crocoite, 373 Crossed nicols, 124 dispersion, 150 Cryolite, 302 Crystal, 3 class, 21 Crystalline aggregates, 67 limestone, 460 Crystallites, 75 Cube, 57 Cupellation, 190 Cuprite, 309 Cyanite, 420 Cyclic twin, 64 Cyrolite, 302 Datolite, 420 Deltohedron, 61 Dendritic, 67 Determination of minerals, 209 Dialloge, 396 Diamond, 257 Diaspore, 326 Diopside, 395 Dichroic, 147 Dichroscope, 146 Dike, 453 Diploid, 59 Diploidal class, 59 Dimorphism, 172 Dihexagonal bipyramidal class, 45 pyramidal class, 47 Ditetragonal bipyramidal class, 39 pyramidal class, 42 Ditrigonal bipyramidal class, 55 pyramidal class, 50 soalenohedral class, 48 Diorite, 416 Diopside, 395 Dispersion, 149 Dodecahedron, 57 Dolomite, 338 "" 11 516 INDEX Domatic class, 36 Double refraction, 120 Drawing of crystals, 97 Edge, 3 Eteolite, 419 Elastic, 107 Electrum, 264 Elements, 257 axial, 94 chemical, 164 of symmetry, 7 Emerald, 464 Emery, 468 Enantiomorphous, 14 Enargite, 296 Endlichite, 355 Enstatite, 394 Epidote, 422 Epsomite, 377 Erthyrite, 361 Erubescite, 290 Essonite, 415 Etch-figures, 108 Ether, 113 Extinction, 133 angle, 134 direction, 133 Extraordinary ray, 120 Extrusive, 453 Face, 3 Fahlerz, 293 Feldspars, 385 Ferro-alloys, 479 Fertilizers, 470 Fibrous, 67 Flame, oxidizing, 181 reducing, 181 tests, 182 Flexible, 107 Fluorids, 297 Fluorin, tests for, 199 Fluorite, 300 Fluxes, 471 Fool's gold, 280 Form, 10 closed, 14 open, 14 Formula, weight, 400 Fowlerite, 400 Fracture, 105 FrankUnite, 321 Freibergite, 293 Fusibility, scale of, 189 Gabbro, 454 Galena, 273 Galenite, 273 Gangue, 458 Garnet, 411 Garnierite, 445 Gems, 466 General form, 21 Geode, 67 Geyserite, 457 Glass tubing, 177 Glaucophane, 404 Gnomonic projection, 86 Goethite, 326 Gold, 4 Gold, ores, 476 tests for, 264 Goniometer, 78 Gossan, 458 Gothite, 326 Granite, 454 Graphic determination of indices, 90 Graphic solution of a, spherical triangle, 86 Graphite, 259 Graphitic acid, 259 Gray copper ore, 293 Grossularite, 412 Gypsum, 375 Gyroid, 62 Gyroidal class. 62 INDEX 517 Habit, crystal, 74 Halite, 297 Haloids, 297 Hardness, scale of, 107 Heavy liquids. 111 spar, 370 Hematite, 313 brown, 328 Heulandite, 441 Hexagonal .system, 44 Hexoctahedral class, 56 Hexoctahedron, 56 Hextetrahedral class, 60 Hextctrahedron, 60 Hiddenite, 465 Holohedral, 25 Hemihedral, 25 Horizontal dispersion, 150 Hornblende, 403 Horn silver, 300 Hubnerite, 382 Hyacinth, 416 Hyalite, 324 Hydromagnesite, 351 Hydroxids, 324 Hydrozincite, 349 Hyper.sthene, 395 Ice, 308 Iceland spar, 120 Idocrase, 377 Igneous rocks, 453 Ilmenite, 408 Imitative forms, 67 Inclined dispersion, 150 Inclusions, 77 Index of refraction, 115 Indicatrix, 136 Indices, Nuller, 17 Indicolite, 392 Intercepts, 16 Interfacial angle, 4 Interference figure, 140 Intrusive, 453 Inversion, 7 lolito, 430 Iron, 267 Iron-ores, 476 pyrites, 282 tests for, 200 Isomorphism, 173 Isomorphous mixtures, 175 Isotropic, 137 Jade, 466 Jamesonite, 291 Jargon, 378 Jarnierite, 445 Kainite, 374 Kaolin, 437 Kaolinite, 437 Kidney-ore, 313 Kunzite, 465 Kyanite, 382 Labradorite, 391 Laccolite, 453 Lamellar, 67 Law of constancy of interfacial angles, 9 Law of the rationality of the indi- ces, 19 Laumontite, 442 Lawsonite, 438 Lazurite, 410 Lead, ores of, 479 tests for, 201 Lepidolite, 428 Leucite, 393 Light, nature of, 113 polarized, 119 Limestone, 457 crystalline, 460 •: • )rm, 21 518 INDEX Limonite, 328 Lithium, tests for, 201 Lollingite, 285 Lubricants, 472 Luster, 111 Magma, 453 Magmatio differentiation, 454 Magnesite, 340 Magnesium, tests for, 201 Magnetic pyrites, 280 iron ore, 321 Magnetite, 320 Malachite, 348 Malleable, 107 Mammillary, 67 Manganese, ores, 478 Magnanese, tests for, 201 Manganite, 327 Marcasite, 284 Margarite, 434 Measurement, crystals, 78 of axial angle, 160 of index of refraction, 158 of interfaoial angles, 4 Meerschaum, 436 Melaconite, 310 Melanite, 411 Melanterite, 378 Menacconite, 408 Mercury, 481 Metals, tests for, 201 Metasilicates, 392 Metasomatic replacement, 452 Methylene iodid. 111 Mica, production of, 473 Micas, 425 Micaceous, 67 Microcline, 388 Microlites, 75 Microscope, 123 Miller indices, 17 Millerite, 279 Mimetite, 358 Mirabilite, 375 Mispickel, 285 Molecular compounds, 169 Molybdenite 272 Molybdenum, tests for, 202 Molybdates, 381 Monazite, 353 Monochromatic, 115 Monoclinic system, 31 Muscovite, 426 Native copper, 265 elements, 257 gold, 263 iron, 267 platinum, 266 silver, 264 Natrolite, 444 Negative crystal, 78 elongation, 136 Negative, optically, 138 Nepheline, 410 Nephelite, 409 Niccolite, 279 Nickel, ores, 478 tests for, 202 Nicol, 122 prism, 122 Niobium minerals, 352 tests for, 202 Nitrates, 486 tests for, 202 Nitratine, 364 Nodular, 67 Normal salt, 168 Obtuse bisectrix, 139 Occurrence of minerals, 447 Ocher, 470 Octahedron, 57 Oligoclase, 391 Olivenite, 360 INDEX Olivine, 413 Onyx, marble, 467 Mexican, 467 Opal,- 324 Open forms, 14 Open tube tests, 183 Optical anomalies, 154 character, 138 constants, 158 orientation, 156 properties, 113 tests, 211 Optic axes, 138 Optics, crystal, 113 Order of succession, 448 Ordinary ray, 120 Ores, 474 Organic deposits, 457 Origin of minerals, 449 Orpiment, 270 Oriental amethyst, 411 Orientation, optical, 156 Ornamental stones, 467 Orthoclase, 386 Orthorhombic system, 27 Orthosilicates, 409 Oxidizing flame, 181 Oxids, 305 Paints, mineral used as, 470 Paper-making materials, 472 Paragenesis, 449 Paragenetic varieties, 449 Paramorph, 452 Parting, 105 Peacock copper, 289 Pearl-spar, 338 Pearly luster, 111 Pectolite, 400 Pedial class, 38 Pedion, 11 Pegmatites, 455 Peridot, 466 Peridotite, 454 Petrifaction, 452 Petrographic microscope, 123 Phantom crystal, 76 Phlogopite, 429 Phosphates, 352 tests for, 203 Phosphate rock, 356 Phosphorite, 471 Physical properties, 103 tests, 210 Picotite, 319 Pinacoid, 11 Pinacoidal class, 37 PisoUtic, 67 Pitch-blende, 368 Plaster, 472 Platinum, 480 tests for, 266 Pleochroism, 146 Pleonaste, 28 Plutonic rocks, 453 Polarized light, 119 Polarizer, 124 Pole, 82 Polybasite, 295 Polymorphism, 172 Polysynthetic twin, 64 Positive elongation, 136 Positive, optically, 137 Potassium minerals, 188 salts, 487 tests for, 203 Precious stones, 463 Prehnite, 424 Primitive circle, 82 Prism, 13 Prismatic class, 32 habit, 75 Projection, gnomonic, 86 linear, 87 stereographic, 82 520 INDEX Proustite, 293 Pseudomorph, 451 Psilomelane, 330 Pykno meter, 110 Pyramid, 13 Pyrargyrite, 292 Pyrite, 280 Pyrites, 59 Pyroelectric, 431 Pyrolusite, 316 Pyromorphite, 357 Pyrope, 374 Pyrophyllite, 407 Pyroxene group, 393 Pyrrhotite, 279 Qualitative scheme, 194 Quarter-undulation, 135 Quartz, 305 Quartzite, 460 Quartz- wedge, 144 Quicksilver, 266 Reagents, list of, 178 Realgar, 270 Reducing flame, 181 color tests, 189 Reflection goniometer, 80 Reflection, total, 117 Refraction, 115 index of, 115 Refractometer, 159 Refractory materials, 469 Regular system, 55 Relief, 118 Reniform, 67 Replacement, 451 Rhodochrosite, 342 Rhodonite, 400 Rhombic bipyramid, 28 Rhombic prism, 28 system, 27 Rhombohedral class, 53 Rhombohedron, 13 Rhyolite, 454 Right-handed crystals, 152 Rock, 140 Rock salt, 297 Rocks, classification of, 447 Rubellite, 430 Ruby, 464 Ruby copper, 309 Ruby silver, 292 Rutile, 315 Sal-ammoniac, 299 Salt, 297 Salt of phosphorus, 179 Salts, acid, 168 basic, 168 normal, 168 Sand, 456 Sandstone, 456 Sapphire, 464 Satin-spar, 375 Scalenohedron, 13 Scale of fusibility, 189 hardness, 107 Scapolite, 415 Scheehte, 382 Schist, 460 Schorl, 431 Secondary, 449 Sectile, 107 Selenite, 375 Sensitive tint 134 SepioUte, 436 Serpentine (mineral), 435 Serpentine (rock), 461 Shale, 456 Siderite, 341 Silicates, 384 tests for, 203 Sill, 453 SilUmanite, 418 Silver, 481 INDEX r,2i Silver, cupellation, 189 glance, 273 Silver, tests for, 201 Skeleton crystals, 77 Slate, 460 Smalt, 485 Smaltite, 283 Smithsonite, 343 Sodalitc, 410 Sodium salts, 487 Sodium tests for, 204 Solubility tests, 190 Specific gravity, 109 Specular iron ore, 313 Specularite, 313 Spessartite, 412 Sphalerite, 275 Sphene, 438 Sphenoid, 11 Sphenoidal class, 37 Spinel, 319 Spodumene, 398 Stalactitic, 67 Stannite, 290 Staurolite, 433 Steel, 478 Stephanite, 295 Stereographic projection, 82 Stibiconite, 325 Stibnite, 271 Stilbite, 441 Streak, 112 Streak-plate, 112 Striations, oscillatory, 76 twinning, 64 Strontianite, 346 Strontium, tests for, 205 SubsiHcates, 384 Sublimates on charcoal, 185 in closed tube, 183 in open tube, 183 on plaster, 186 Siilfantimonates. 288 Sulfantimonites, 288 Sulfarsenates, 288 Sulfarsenites, 288 Sulfates, 369 tests for, 205 Sulfids, 269 tests for, 205 Sulfo-acids, 288 Salfo-salts, 288 Sulfur, 260 production of, 473 Sulfuric acid, 488 Sylvanite, 286 Sylvite, 298 Symbols of crystal faces, 15 Symmetry, 5 axis of, 6 center of, 7 composite, 7 plane of, 6 Syenite, 454 Synthesis, mineral, 450 Systems, crystal, 25 Symmetrical dispersion, 150 Table of elements with atomic weights, 164 Table of the mineral products of the United States, 462 Table of the 32 crystal classes 22-3 Tables, determinative, 453 Tabular habit, 75 Tal(i, 406 Tcllurids, 269 Tellurium minerals, 269 tests for, 205 Tenorite, 310 Tetartohedral, 20 Tetartoid, 62 Tetartoidal class, 62 Tetartopyramid, 15 Tetragonal bipyramidal class, 39 system, 39 522 INDEX Tetrahedrite, 293 Tetrahedron, 61 Tetrahexahedron, 57 Thoria, 488 Tin, ores of, 482 Tin, tests for, 206 Titanite, 438 Titanium, tests for, 206 Topaz, 417 Total reflection, 117 Tourmaline, 430 Trachyte, 454 Trapezohedron, hexagonal, 13 isometric, 56 tetragonal, 13 trigonal, 13 Travertine, 464 Tremolite, 402 TricUnic system, 37 Trigonal bipyramidal class, 54 pyramidal class, 50 trapezohedral class, 53 Trimorphism, 172 TriphyUte, 354 Trisoctahedron, 56 •Tristetrahedron, 61 Trona, 350 Tufa, calcareous, 459 Tuff, 456 Tungstates, 381 Tungsten, tests for, 206 Turquois, 363 Twin-axis, 63 Twin-crystals, 63 Twin-plane, 63 Two-circle goniometer, 81 Type symbols, 17 Ulexite, 367 Uraninite, 368 Uranium, tests for, 206 Unit series of bipyramids, 28 Unisilicates, 384 Urarovite, 412 Vanadates, 352 Vanadinite, 358 Vanadium minerals, 358 ores of, 478 Vanadium, tests for, 207 Vein minerals, 458 Veins, 457 Vesuvlanite, 415 Vibration directions, 133 Vicinal faces, 77 Vitreous luster, 1 Vivianite, 360 Volcanic rocks, 453 tuff, 456 Vug, 67 Wad, 331 Water of constitution, 169 of crystallization, 169 Wave-length, 114 WavelUte, 362 Weiss symbols, 17 Wernerite, 415 Westphal-balance, 111 Willemite, 414 Witherite, 346 Wolframite, 381 WoUastonite, 399 Wulfenite, 383 Zinc blende, 275 Zinc ores, 483 oxid, 488 Zinc, tests for, 207 white, 488 Zircon, 416 Zinc, tests for, 207 Zoisite, 421 Zone, 5 symbol, 89