BOUGHT WITH THE INCOME FROM THE SAGE ENDOWMENT FUND THE GIFT OF Henrg W. Sage 189X A.S.^^..xjjS.: ....SLS^./.k/jS- 9963 Cornell University Library QE 434.J65 A key for the determination of rock-form 3 19'24""d04H5 493 % Cornell University Library The original of this book is in the Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924004115493 A KEY FOR THE DETERMINATION OF Rock-Forming Minerals in Thin Sections BY ALBERT JOHANNSEN, Ph.D. Member U. S. Geohgical Survey FIRST EDITION FIRST THOUSAND NEW YORK JOHN WILEY & SONS London: CHAPMAN & HALL, Limited 1908 Copyright, 1908 BY ALBERT JOHANNSEN tSift *rten«fit Jrraa JUibrrt grunutuinii ani> (Ilaiiitiatig M«n lork PREFACE. The determination of rocks is largely based upon the iden- tification of their component minerals, and since it is only in coarse, granular rocks that these can be identified megascop- ically, it is commonly necessary to employ microscopical me- thods. A knowledge of the usual appearance of the minerals as they occur in rock sections is not sufficient, because many of them are very similar, and one must be familiar with the apphcation of the optical methods by which they are de- termined to secure accurate results. Tables for the determination of minerals have heretofore been made of secondary importance in text books. In this \'olume the reverse method has been adopted and the de- scriptive tables make up the greater portion of the book. Theoretical discussions in optics have been avoided so far- as possible, and only enough has been given to make clear that which is necessary for the practical determination of the minerals. The material has been condensed as much as is consistent with clearness, and all of the descriptions are given in as concise a form as possible. Owing to the difficulty of having the pages of the index indented by machinery, it was found necessary to extend the leaves to the full width of the book, and to indicate the lines along which the student should cut the pages. After trim- ming along the lines shown, it will be found convenient to form another index guide by cutting off about one-half inch from the upper right hand corners of the pages descriptive of negative minerals. IV PREFACE. In the preparation of this book I have constantly referred to the fourth edition of Professors Rosenbusch and Wiilfing's " Mikroskopische Physiographic der petrographisch wichtigen MineraUen," and to them my thanks are due for their kind- ness in granting me permission to reproduce certain diagrams and figures from their book. Among these are a number of the feldspar diagrams which I have redrawn and, in some cases, slightly modified. For convenience in showing the relation between the percentage proportions of the albite-anorthite molecules and the common names, I have inserted above each diagram these names, limiting them as the best usage seems to warrant. For the mineral descriptions in Part III, I have consulted the works of Rosenbusch, Levy and Lacroix, Dana, Groth, and Hintze. Many of the methods in the chapter on the feldspars are from A. Michel-Levy's "Etude sur la deter- mination des feldspaths" and I am greatly indebted to him for permitting me to reproduce his diagrams and the valu- able color plate published in Levy and Lacroix's "Mineraux des Roches." The student who wishes more complete theoretical dis- cussions of the principles underlying the optical methods, is referred to the works mentioned above and also to Professor Iddings' "Rock Minerals," which has appeared since the manu- script of this book was completed. From the nature of the tables it is very probable that some errors have escaped my notice. I shall be glad to be notified of any such that may be found. Albert Johannsen. U. S. Geological Survey, Washington, D.C, November 26, 1907. CONTENTS. PAGE Preface iii CONTEIJTS V PART I. Introduction 3 Light 3 Isotropic media 3 Anisotropic media 4 a. Uniaxial crystals. ; 4 b. Biaxial crystals 4 Index of refraction 4 Double refraction 5 Monochromatic light 6 Optically uniaxial crystals 6 Positive and negative uniaxial crystals 6 Principal optic section 7 Measure of the double refraction 7 Optically biaxial crystals 7 Interference colors 8 Optic axes 8 Axial angle 8 Bisectrix 8 Plane of the optic axes 9 Optic normal » 9 Optic binormals 9 Positive and negative biaxial crystals 9 Newton's color scale 9 Examination in parallel polarized light 11 Isotropic plates 11 Anisotropic plates ' 11 Plates at right angles to an optic axis 11 Superimposed plates 12 Extinction 12 Determination of the extinction angles. 12 V 'VI CONTENTS. PAGE a. To observe extinction angles 12 b. With a gypsum plate 12 c. With Klein's quartz plate 13 d. With a Bertrand ocular 13 Orientation 14 Determination of the relative indices of refraction of Minerals 14 a. Becke's method 14 h. Schroeder van der Kolk's method 15 Indices of refraction of immersion fluids 16 Table of mean indices of refraction 18 Determination of the order of Birefringence 18 a. Quarter undulation mica plate 19 6. Gypsum plate 19 c. Quartz wedge 19 d. Fedorow's mica compensator 20 e. Parallel nicols 21 /. Amann's birefractometer 21 g. Babinet compensator 21 h. Michel-L^vy comparateur 21 Determination of the relative values of two vibration DIRECTIONS . 90 Determination of the character of elongation of a mineral SHOWING A wedge SHAPED EDGE 23 DiCHROISM 23 Pleochroism 23 ex.'imination in convergent polarized light 24 Interference figures 24 Appearance in uniaxial minerals 24 a. Sections normal to the optic axis 24 6. Sections oblique to the optic axis 24 c. Sections parallel to the optic axis 26 Appearance in biaxial minerals 26 a. Sections perpendicular to the acute bisectrix 26 b. Sections perpendicular to the obtuse bisectrix 27 c. Sections incUned to the bisectrix 28 d. Sections at right angles to an optic axis 29 e. Sections parallel to the plane of the optic axes 29 Locating the points of emergence of the optic axes 30 a. In uniaxial minerals 30 b. In biaxial minerals'. 30 The axial angle 30 Apparent and true optic angle 30 Measurement of the axial angle 31 Dispersion of the optic Axes 32 Optical characteristics of the different systems 33 CONTENTS. VU PAGE a. Orthorhombio system 33 h. Monoclinic system .' 33 Inclined 34 Horizontal 34 Crossed 35 c. Triclinic system 35 Determination of the optical character of crystals 35 Uniaxial crystals 35 a. Quarter undulation mica plate 35 6. Gypsum plate 36 u. Quartz or mica wedge 37 Biaxial crystals 37 a. Mica plate. 37 h. Gypsum plate 38 c. Mica or quartz wedge 39 Optical anomalies 43 Measurements under the microscope 43 Measurement of enlargement 43 Measurement of lengths 43 Measurement of thicknesses 43 Measurement of angles 44 Measurement of the axial ansle 44 PART II. THE MIXERAL GROUPS. Spinel GROtrp 45 Garnet Group 46 Sodalite Group , 47 Carbonate Group 48 Scapolite Group 49 Olivine Group 50 HuMiTE Group 51 Epidote Group 51 Mica Group •. 52 Chlorite Group 53 Zeolite Group 54 Pyroxene and Amphieole Groups 56 Pyroxenes 56 Amphiboles , 59 Feldspar Group , 61 Classification 62 Cleavage 62 Chemical composition 62 VUl CONTENTS. PACB Celsian 63 Hyalophane 64 Orthoclaee 64 Twinning 64 Carlsbad law 64 Baveno law 66 Manebach law . 66 Characteristics 67 Sanidine 68 Microcline 68 Pericline twinning 69 Anorthoclase 69 The plagioclase feldspars 69 Twinning 69 Albite law 69 Determinative methods 70 1. By specific gravities 71 2. By the optical character of the mineral 72 3. By the relative indices of refraction of the feldspar and some known mineral with which it is in contact. ... 72 4. By the index of refraction, according to Schroeder van der Kolk's embedding method 76 5.-6. By the extinction angle on cleavage flakes parallel to (001) or (010) ,, 78 How to recognize the (001) face 81 How to recognize the (010) face 81 7. By the position of emergence of the bisectrix in (010) plates 8) 8. By the extinction angle in Carlsbad twins upon the (010) face 82 9. By the extinction angle on sections at right angles to both (001) and (010) 84 10. By the extinction angle on sections at right angles to either bisectrix 86 11. By the extinction angle on sections at right angles to to the optic normal (b) 88 12. By the extinction angle on sections from the (001), (010) zone 88 13. By the extinction angles on sections from the zone at right angles to (010) 91 14. By the extinction angles on sections from the zone at right angles to (010) when the albite twinning is combined with Carlsbad twinning 93 CONTENTS. IX PART III. PAGE Explanation of the tables 95 Mineral Tables 99 PART IV. Table of mean indices of refraction 510 Table of maximum birefringences 511 Newton's color scale 512 Tables of specific gravities 513 Separation by heavy solutions 514 Indicators 515 Vibration directions in accessories ' 516 Minerals arranged according to crystaUine systems 517 Minerals which occur in needle-like crystals 518 Minerals which occur in fibrous aggregates 518 Minerals which occur in radiating groups of fibres 518 Minerals which occur as cavity fillings 518 Alteration products which occur in minute shreds 518 Alphabetical list of the rock-forming minerals 519 General index 527 Mineral index 537 PART I. INTRODUCTION. Oman res ipsa negat, Contenta docere. Maniucs, Astronomica, 3, 39. ERRATA. Page 16. Anise oil, index of refraction should be 1.56. Page 124. Foselite, should be Noselite. Page 228. Diaspore should be optically positive. Pages 259 and 281. The index words should be printed on the lower half of the sheet. The page should not be indented, but should have a straight edge and be of the width indicated at the top. Page 294. Diaspore should be optically positive. Page 449, line 17. Omit comma. uixi.cfj. Y £iXj± and no interference phenomena are produced. All sections of isotropic substances, therefore, are dark between crossed nicols, and remain so during the rotation of the stage. Isotropic substances are of two classes: amorphous bodies and fluids, and crystals of the isometric system. Amorphous substances have no crystalline character and 3 INTRODUCTION. Light. In a homogeneous medium, light is transmitted in straight lines without change of direction and by means of vibrations at right angles to the direction of movement. The intensity of the light depends upon the amplitude of this movement, and the color of the rate. Extreme red light has about 443,000,000,000,000 waves in a second, while extreme violet has about 702,000,000,000,000. When light is made to vibrate in a single plane, as by pass- ing through a nicol prism, it is said to be polarized. Since the direction of vibration in the nicol prism of the analyzer is at right angles to that in the polarizer, the light is completely cut off in passing through the two. Isotropic Media. Substances in which the velocity of the transmission of light is independent of the direction of vibration are called isotropic. In such a body the form of the wave-surface may be represented by a sphere. Under the microscope the light from the polarizer is unchanged in passing through the mineral section, consequently it is completely cut off by the analyzer and no interference phenomena are produced. All sections of isotropic substances, therefore, are dark between crossed nicols, and remain so during the rotation of the stage. Isotropic substances are of two classes: amorphous bodies and fluids, and crystals of the isometric system. Amorphous substances have no crystalline character and 3 4= INTRODUCTION. allow the light to vibrate with the same ease in all direc- tions. Isometric crystals have crystalline form, but the ease of vibration is the same in all directions. Anisotropic Media. In anisotropic media the velocity of the transmission of light varies with the direction. Anisotropic crystals are uniaxial or biaxial. In a uniaxial crystal the surface of wave velocities is an ellipsoid of rotation; in a biaxial crystal it approaches the form of an ellipsoid of three axes. All substances that are not amorphous or of the isometric system are optically anisotropic. They may be divided into two groups: 1. Uniaxial crystals, or those which have but one optic axis; that is, but one direction along which there is no double refraction. The optic axis coincides with the crystallographic axis c, which is also the direction of either the greatest or least ease of vibration. Tetragonal and hexagonal crystals belong here. 2. Biaxial crystals, or those with two optic axes. To these belong orthorhombic, monoclinic, and triclinic crystals. In the orthorhombic system the axes of maximum and minimum ease of vibration coincide with the crystallographic axes; in the monoclinic and triclinic systems they do not. Index of Refraction. A ray of light passing from one medium to another under- goes various changes; a part of the light does not enter the second medium but is reflected back, and a part is transmitted through the second medium but has its direction changed, that is, it is refracted. Beyond a certain angle, characteristic for each mineral, light is totally reflected and none is refracted. The angle of incidence at which this takes place is called the critical angle. DOUBLE REFRACTION. 5 The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for the same media and is called the index of refraction. It is inversely proportional to the wave velocity. In isotropic media this index is the same in every direction, while in anisotropic media it varies in different directions. Double Refraction. A ray of light entering an anisotropic medium is resolved into two rays vibrating at right angles to each other with differ- ent velocities, although the difference in rate is very slight. When the ray is vibrating parallel to the direction of greatest ease of vibration, it advances fastest; and slowest when parallel to the direction of least ease. In consequence of the two differ- ent hght velocities, there must be, also, two different indices of refraction. As the stage of the microscope is rotated, the axes of light vibrations will coincide with the planes of vibra- tion of the nicols four times, at intervals of 90°, and will pro- duce darkness each time. This property of anisotropic crystals of resolving the light vibrations into two sets at right angles to each other, is called double refraction. Since in anisotropic minerals the rate of vibration is different in different directions, it follows that this difference is still found in the components of these rays after they have passed through the analyzer, consequently one of the waves reaches the eye slightly in advance of the other, although in the same plane after passing the analyzer. The interference of the two produces an interference color. The directions of greatest and least ease of vibration, which are always at right angles to each other, and a third direction at right angles to these two, form the axes of elasticity or vibration axes. They are indicated by the German letters o, i, and c: o is considered the direction of greatest ease of vibra- tion, also called the direction of maximum elasticity; 6 is the direction of intermediate ease of vibration or elasticity: and c is the direction of least ease of vibration. AH of these o INTRODUCTION. velocities vary inversely as the indices of refraction. The planes of which these lines are the intersections are called the principal optic sections. They always contains two axes of elasticity. Monochromatic Light. In many observations, as for dispersion, etc. (see p. 32), monochromatic light is used. This may be produced by the absorption of certain rays by means of glasses or ray-filters, by monochromatic flames produced by vaporizing certain sub- stances, or by means of a monochromator, which breaks white light into the spectrum. For use under the microscope all but the first are too complicated. Optically Uniaxial Crystals. Of the two rays of light resulting from the double refrac- tion of a ray passing through a uniaxial crystal, one vib- rates with equal velocity in every direction. This is caUed the ordinary ray and its wave-surface is a sphere. Its index of refraction is denoted by m. The second ray is called the extraordinary ray and its index of refraction is denoted by £. It passes through the crystal with difi^erent velocities in differ- ent directions, agreeing only in one direction with the ordinary ray. In this direction, consequently, there will be no double refraction, since the values of w and e are equal; in other words, this is an axis of isotropy. In the tetragonal and hexagonal sys- tems this direction of greatest or least ease of vibration coincides with crystaUographic c, which is also an optic axis. AH waves moving at the same angle with this axis have equal velocities, consequently any section of the wave-surface figure at right angles to this axis is a circle, while every other section of the wave-surface is an ellipse. Positive and negative unaxial crystals. From the above it is seen that there are two kinds of uniaxial crystals. In one the extraordinary ray has its greatest velocity along the principal axis; in the other this is the direction of least velocity. OPTICALLY BIAXIAL CRYSTALS. ? These relationships are shown in Figs. 1 and 2, which are sec- tions through the centers of the wave- velocity figures. In Fig. 1 s>co, and in Fig. 2, £oj). Optically negative ( — ) crystals. A uniaxial crystal is con- sidered negative when crystallographic c is the direction of greatest ease of vibration (c=o) and(£li>c, or o = B>c, etc.; the absorption axes being assumed to coincide with the axes of vibration, although they do not always do so. Examination in Convergent Polarized Light. Convergent light is used to separate uniaxial from biaxial crystals, to determine dispersion and the optical character, and to separate isotropic minerals from anisotropic ones cut at right angles to the optic axis. Interference Figures. Appearance in uniaxial minerals, (a) Sections normal to the optic axis. These show a symmetrical dark cross, due to the extinction of all rays coinciding with the planes of the nicols, and a series of concentric rings, alternating light and dark in monochromatic light and showing the colors of the spectrum in white light (Fig. 6). During a rotation of the stage the arms of the dark cross remain station- FiG. 6.— A uniaxial in- ary. An increase in the strength of the terference figure. ^^^^^^ refraction or an increase in the thickness of the slide causes the cross and rings to become more sharply defined and the distance between the rings to decrease. (b) Sections oblique to the optic axis. The center of the cross will appear farther and farther toward the edge of the field of the microscope as the section is more and more oblique to the optic axis (Figs. 7-18), finally being entirely outside the field. On rotating the stage, the center of the cross (the point of emergence of the pptic axis) will move in the same direo- INTERFERENCE FIGURES. 25 Fig. 7. Fig. 8. Fig. 9. Fig. 10. Fui. U. Fig. 12. FiG.s. 7-12. — Uniaxial interference fis-ui-es. The center is toward the edg of the field of tlie microscope. Fig. 16. Figs. 13-18.— Uniaxial Fig. 17. Fig. IS. interference figures. The center is beyond the field of the microscope. 26 EXAMINATION IN CONVERGENT POLARIZED LIGHT. tion as the stage, but the arms will always remain parallel to the cross-hairs. To distinguish such figures from biaxial inter- ference figures showing only part of a bar, one needs only remember that in uniaxial minerals the arms remain parallel to the cross-hairs during the rotation of the stage. (c) Sections parallel to the optic axis. These sections show hyperbolse which are very similar to those seen in biaxial figures, except that they do not appear in the field until the stage is nearly in the 90° position, and then they move in, form a cross, which is seldom sharp, and very rapidly move out again in the direction of the principal axis. During the greater part of the rotation, therefore, they do not show in the field of the microscope. The optic axis lies in the direction toward which the hyper- bola leave the field. Its position may also be determined by the appearance of the interference colors. If the stage is turned 45° from the position in which the bars form a cross, there will appear a certain interference tint in the center of the field. Outward from this center there will be a fall in the color scale toward the quadrants in which the optic axis lies and a rise in the other two quadrants. This holds good for both { + ) and (— ) crystals. (Figs. 19 and 20.) In thick plates, or in Fig. 19. Fig. 20. Figs. 19-20. — Interference figures of uniaxial minerals cut parallel to the optic axes. Fig. 19. Quartz ( + ). Fig. 20. Apatite ( — ). minerals having high double refraction, this is only observable at the center; beyond that, there is a uniform rise in colors. Appearance in biaxial minerals, (a) Sections perpendicular to the acute bisectrix. These show in white light between" crossed nicols, when the axial angle is less than the field of the micro- INTERFERENCE FIGURES. 27 scope and the plane of the axes coincides with the cross-hairs, a blaclv cross, and a series of colored curves (Fig. 24). At the point of emergence of each optic axis there will be a dark spot, and surrounding it a number of rings which unite at the center and form lemniscate curves around the two. On rotating the stage the cross dissolves into two hyperbolre whose poles are the loci of the optic axes. These bars revolve in an opposite direction from the stage and alwaj'S have their con- vex side toward the acute bisectrix. (Figs. 21-2G.) The Fig, 21. Fig. 22. Fig. 23. Fig. 24. Fig. 2.3 Fig. 26. Figs. 21-26.— Biaxial interference figures. The section is cut at right angles to the acute bisectri.\. smaller the axial angle, the nearer together will j^e the loci of the optic axes, until finally the figure approaches the form given by a uniaxial mineral. When the axial angle is large neither of the loci will show, but the biaxial character can Ije determined by the movement of the bars. {b) Sections perpendicular to the obtuse bisectrix. When the axial angle is not too large, these show similar figures to those seen in sections cut a.': right angles to the acute bisectrix. 28 EXAMINATION IN CONVERGENT POLARIZED LIGHT. Generally, however, the angle is large, the axes suffer total reflection from the lower side of the section, and no figure is seen. (r) Sections inclined to the bisectrix. More and more of one eye and less of the other is seen as the section is more and more Fig. 27. Fig. 2S. Fig. 29. Fig. 30. Fig. 31. Fig. 3: Fig. 33 Fig. 34. Fig. Fig 36. Figs. 27-36. — Biaxial interference figures. One oijtio axi.s and the acute bisectrix emerge within the field of the microscope. Fig. 37. Fig. 3S. Fig. 39. Fig. 40. Fig. 41. Fig. 42 Fig. 43. Fig. 44. Fig. 45. Fig. 46. Fios. 37-46. — Biaxial interference figures. The 0|3tic axes and the acute bisectrix emerge beyond tlie field of tlie microscope. inclined. The convex side of the hvijcrl^ola is always toward the acute bisectrix and the arm rotates in a direction opposite to the turning of the stage. (Figs. 27-3G and 37^G.) INTERFERENCE FIGURES. 29 [d) Sections at right angles to an optic axis. These show nearly circular concenti'ic curves crossed by a single dark bar which is straight whenever it is parallel to the planes of vibra- tion of the nicols (Fig. 47). Upon rotating the stage, this bar changes to a slightly curved hyperbola with is convex side toward the acute bisectrix. Sometimes the arm is not sharply defined, especially when the section is not quite at right angles to the optic axis, and the arm may show as a bar, straight on one side and concave on the other (Fig. 48). The straight side is here toward the acute bisectrix. Upon turn- ing the stage the arm rotates in the opposite direction. Fig. 47. — Biaxial interference figure showing the emergence of an optic axis in tlie center of the field of the microscope. Fig. 48. — Biaxial interference figure with the optic axis emerging beyond the field Of the micro- scope. Since light is dispersed in all biaxial crystals, a section can be actually at right angles to an optic axis only for a given color. The dispersion is generally so slight, however, that it may be overlooked, and one will see, in white light, a series of colored rings, whose tints will differ from the pure colors of Newton's scale more and more with increasing dispersion. (e) Sections parallel to tlie plane of the optic axes. Sections cut parallel to the plane of the optic axes (perpendicular to the fi axis) may be recognized in parallel polarized light by the fact that they show the highest interference colors. In convergent light they show figures similar to those shown by uniaxial crystals parallel to the optic axis. The hyperbolffi come in from the sides very rapidly, darken the field, and, upon a very slight rotation, immediately disappear in the direction of the acute bisectrix. When the field is dark the axes a 30 EXAMINATION IN CONVERGENT POLARIZED LIGHT. and c are parallel to the cross-hairs. Becke* has shown how one may determine the position of the acute bisectrix in this section. The method has already been described in reference to uniaxial figures (p. 26). The line uniting the quadrants containing the lower colors is the direction of the acute bi- sectrix, (o) in (— ) and (c) in ( + ) minerals. When the axial angle approaches 90° the color variation becomes indistinct; when 27=90° it disappears. The above may be verified by using the gypsum plate, with red of the first order, as a mineral section. In sections cut parallel to the plane of the optic axis the optical character of the mineral may also be determined by observing whether the acute bisectrix is o or c. Locating the points of emergence of the optic axes, (a) In uniaxial minerals. When the eye of a uniaxial figure is be- yond the field, there is no difficulty in determining its position, as may be seen in Figs. 7 to 18. It is at the junction of the two bars. (6) In biaxial minerals. The appearance of a biaxial figure, when the eyes are beyond the field of the microscope, is given in Figs. 37 to 46. (1) When the bisectrix is within the field, it is to be noted that the hyperbolae come in from the sides upon which the eyes are located and disappear toward them. (2) When the bisectrix is outside the field of the microscope, the arm of only one hyperbola will be seen; the other arm, and consequently the bisectrix, is upon the convex side of the bar. (Figs. 27 to 46, and Fig. 48.) The axial angle. Apparent and true optic angle. When a section of a biaxial mineral, cut normal to the acute bisectrix, is turned to the 45° position between crossed nicols, the dis- tance between the hyperbolae, that is, between the points of emergence of the optic axes, is a measure of the angle between these axes. Owing to the different rates of vibration for differ- * Tschermak's Mineralogische und petrographische Mitteilungen, vol 16, 1897, p. 181. o THE AXIAL ANGLE. 31 ent rays, a section normal to one color will not be normal to the others, except in the orthorhombic system. If, however, the section is cut to the mean color, the result will be approxi- mately correct. In Fig. 49, which is a cross-section through a slide, the angle BOA is one half the true axial angle and is represented by V. Owing to the change in direction of the ray in passing into air, the observed angle, E, is BO' A'. If the axial angle is acute, it is represented by 2£„ or 2y„; if it is obtuse, by 2Eo or 27o. Measurement of the axial angle. There are jpjq 49^ ^xme and numerous methods for measuring the axial apparent axial angles. angle, but most of them are not applicable to the ordinary petrographical microscope, since the objective is too near the slide to permit turning about an axis.* If the shde is turned about the point 0' (Fig. 49), with the hne at right angles to the axial plane as an axis, until the line O'C coincides with the cross-hairs of the telescope of a goniometer, and is then turned until O'A' coincides, the angle through which the section has been turned is the apparent axial angle, 2E. In a thin section under the microscope the angle can be determined by measuring the distance between the points of emergence of the axes with a micrometer eyepiece. The sUde is turned until the hyperbolae are in the 45° position, after which the distance (2d) between them is measured. The distance of the point of emergence of an optic axis from the bisectrix was shown by MaUard to be proportional to the sine of the angle which it makes with the axis of the microscope. Thus sin E=Yr, in which E is half the axial angle in air, d half the distance between the hyperbolae, and C, a constant for each lens combination, which may be determined by using a mica flake whose axial angle is known and substituting in the for- * A summary of methods for measuring the axial angle will shortly be published by Dr. Fred. E. Wright, in the American Journal of Science. 32 EXAMINATION IN CONVERGENT POLARIZED LIGHT. sinE mula. By substituting these values in the equation sin 7= ^, , in which n' is the mean index of refraction of the mineral examined, the true value of the axial angle (27) may be found. If the section examined is not exactly at right angles to the plane of the optic axes, by the use of a tilting stage, it may be tipped until the bisectrix coincides with the axis of the microscope. There are numerous appliances which may be used for this purpose: Klein's Universaldrehapparat, Fedo- row's Universaltisch, or a simple apparatus described by Jag- gar.* The determination of the axial angle may be made graphi- cally by the Mallard-Becke method. With a camera lucida the hyperbolae appearing in the field are transferred to a sheet of paper and, knowing the value of C, the angle may be plotted and measured. Schwarzmann's axial angle scale may be used for the deter- mination of 2E. This is a scale made upon logarithmic principles, and from it the angle can be read direct. It con- sists of two parts, of which the lower is adjusted to the constant of a certain combination of lenses. The distance, D, upon the upper scale, will be found directly above the value of 2E for that distance. A different scale is used for each different lens combination. Dispersion of the Optic Axes. As light passing through a prism is separated into the spec- trum, on account of the difference in wave lengths of different- colored rays, so the value of V and the loci of the hyperbolae in the interference figures will vaiy with different wave lengths of light. This variation produces what is called the dispersion of the optic axes, and is recognized under the microscope by a change in the position of the loci of the axes. To indi- * American Journal of Science, 3, 1897, p. 129. DISPERSION OF THE OPTIC AXES. 33 cate this dispersion, red and violet rays are considered, p is used to indicate red light and u to indicate violet light. Thus /)< y shows that the dispersion of red is less than that of violet. Optical characteristics of the different systems, (a) Ortho- rhombic system. In the orthorhombic system the three vibra- tion axes (o, 6, t) coincide with the crystallographic axes (a, b, c), and any of the former may coincide with any of the latter. These positions being fixed, the color distribution will be sym- metrical with reference to both bisectrices, although the optic angle may differ. The plane of the optic axes always Ues in one of the pinacoids. There may be two cases: p> u, in which the axes are more dispersed for red than for violet, and p< u. Fig. 50 shows a section through an orthorhombic crystal with c=0 a=C Fig. 50. — Section through an ortho- rhombic crystal with p>ij. Fig. 51. — Dispersion in an ortho- rhombic crystal with p> u. (The solid lines and hatchures may be colored blue, the dotted Unes, red.) the optical scheme p>u. Fig. 51 shows its dispersion in the interference figure. The dispersion is always least for that color which occurs within the circle nearest the bisectrix and which touches the concave side of the hyperbolae when the section is in diagonal position. The colors in the two eyes are symmetrical with reference to the bisectrix. (6) Monoclinic system. In the monoclinic system only the 6 axis coincides with an optic axis, and this shows no dispersion. The other two optic axes lie in the (010) plane and are at right angles to each other and to the one coinciding with b. There are three kinds of dispersion. 34 EXAMINATION IN CONVERGENT POLARIZED LIGHT. 1. Inclined. In this the axial plane is parallel to (010) and 6 = 6. The color distribution will be symmetrical to the line joining the loci of the hyper bolse, but not in the other direc- tion. The intensity of the eyes will be different. One eye will be less intense, larger, and oval; the other, smaller, intense, and round. The distribution of color will depend upon the amount of dispersion of the axes. If this is slight the colors will be symmetrical with reference to the bisectrix. (Fig. 52, Fig. 52. — Inclined dispersion in a monoclinic crystal. Gypsum, with pu), like colors will occur upon the same sides of the loci of the axes. The smaller eye, also, will be farther from the cross-bar at the bisectrix, and one hyperbola will be broader than, the other. ^-m Fig. 53. — Inclined dispersion in a monoclinic crystal. Diopside, p>u. (The solid lines and hatchures m^y be colored blue; the dotted lines and dotted areas, red.) 2. Horizontal. Crystallographic h is the obtuse bisectrix, the acute bisectrix is in the clinopinacoid and the plane of the optic axes is perpendicular to (010). The axiar figure will be symmetrical to the (010) plane only, and the figure must be dispersed as in Fig. 54. The colors will be sym- metrical only to the one plane. OPTICAL CHARACTER OF THE CRYSTAL. 35 3. Crossed. Crystallographic b is th? acute bisectrix and will remain unchanged for all colors of light. The obtuse bi- sectrix and the B axis are dispersed. There are no planes, but only a point of symmetry. This point is the locus of the acute bisectrix, which is the axis of symmetry (Fig. 55). Fig. 54. — Horizontal dispersion in a monoclinic crystal. p<. u. Fig. 55. — Crossed dispersion in a monoclinic crystal p> u. (The solid lines and hatchures may be colored blue ; the dotted lines and dotted areas, red.) c. Triclinic system. All of the vibration axes are dispersed in the triclinic system and the distribution of the colors in the mterference figures is unsymmetrical throughout. Determination of the Optical Character of the Crystal in Convergent Polarized Light. In convergent polarized light the character of a mineral can be determined by inserting a mica or gypsum plate, or a quartz or mica wedge, above the interference figure. Uniaxial crystals, (a) Quarter undulation mica plate. Upon inserting a quarter undulation mica plate, with its a direction parallel to the elongation, above the mineral and at 45° to the cross-hairs, the dark cross will separate, for positive minerals, as shown in Fig. 56 a. In negative minerals the position of the dark spots will be reversed, as Fig. 56 &. If the mica plate 36 EXAMINATION IN CONVERGENT POLARIZED LIGHT. has f for its long direction, the phenomenon will be reversed. The separation of the cross in the interference figure is caused by the one fourth wave length {\X) decrease in the rays vibrating in the first and third quadrants, and the increase by |A in Fig. 56.— Determination of the optical character of a uniaxial crystal with the mica plate, (o) Positive. (6) Negative. the others. The direction of the arrow upon the mica plate is usually that of c. The method for testing this direction is given in Part IV. Fig. 57. Fig. 58. Fig 57. — Determination of the optical character of a uniaxial Crystal with the gypsum plate. Quartz ( + ). Fig. 58. — Determination of the optical character of a uniaxial crystal with the gypsum plate when the center of the cross is beyond the field of the microscope. Quartz ( + ). (b) Gypsum plate giving red of the first order. If the double refraction of a mineral is weak, or the section is very thin, the mica plate is unsatisfactory, and a gypsum plate having a OPTICAL CHARAcn^R Or THE ORV.STAL. 37 thickness of 0.0."),") mm. and giving tlie first-order red may be used in a similar manner. Witli tlie o direction parallel to the elongation of the gypsmn plate, positive crystals will be raised to blue in the second and fourth quadrants and reduced to yellow in the first and tliird (Fig. 57). For negative crystals the phenomenon is re\'ersed. This accessory may b:-' used even when the uniaxial cross is be3'ond the field of the microscope, by oljserving the location of the center (Fig. 58). (c) Quartz or mica wedcje. If the expansion or contraction of the rings cannot be recognized with a mi(.'a or gypsum plate, it can often be seen with a quartz or mica wedge. When such a wedge, with its a direction parallel to its lengtli, is pushed with the thin edge foremost between crossed nicols, the colors in the first and tliird quadrants move away from the center. Fig. 50. — Determination of the optical cliaracter of a uniaxial crystal by mean.s of a tjuartz wedge, {u) (Quartz ( + ). (''*) Calcite ( — ). while in the second and fourt-h (p.iadrants they move toward it (Fig. 59). With negative crystals or with c parallel to the long direction of the wedge, the plienomenon is of course re- versed. Biaxial crystals, (a) Mica plate. The phenomenon observed 38 EXAMINATION IN CONVERGENT POLARIZED LIGHT. upon inserting a quarter undulation mica plate above the inter- ference figure of a biaxial mineral rotated until the hyperbolae form a cross, is exactly the sa'ne as that seen in imiaxial min- erals. For positive minerals the two black dots show along the direction of a of the mica plate and for negative minerals along the direction of t. (Fig. 56, a, h.) The mica plate is only of use when the hyperbolae are well defined. (b) Gypsum -plate. Sections which show the emergence of the acute bisectrix but which show no isochromatic curves, when turned upon the stage so that the hyperbolae form a cross, are ~ in parallel position. If a gypsum plate with its a direc- tion to the N.W., S.E., is now inserted, the center of the field will appear red, and, in positive minerals (Fig. 60), the N.E., S.W. quadrants will be colored blue, while the other two "will be yellow. The position of blue and yellow will be reversed in negative minerals or if the gyp- sum plate has its elongation parallel to t. Sections which show the emergence of only one axis may be determined in the same way. The convexity of the hyperbola is always toward the acute bisectrix; in the interference fig- ure shown in Fig. 61 it is, therefore, to the N.W. By rotating the stage so that the acute bisectrix is in this position, and inserting a gypsum plate with its a direction also N.W., S.E., in positive minerals the concave side of the curve will be colored yellow, while in negative minerals it will be colored blue. (Fig. 62.) In sections cut parallel to the plane of the optic axes (pp. 26 and 29), the direction of the acute bisectrix may first be deter- mined, and then, in parallel light, the optical sign of the mineral. Fig. 60. — Determination of the op- tical character of a biaxial crystal in a section cut perpendicular to the acute bisectrix by means of the gypsum plate ( + ). (After Wiilfing.) OPTICAL CHARACTER OP THE CRYSTAL. 39 (c) Mica or quartz wedge. For minerals which are thick, or which are strongly doubly refracting and which show several isochromatic curves, it is better to u^e a quartz or mica wedge FiQ. 61-62. — Determination of the optical character of biaxial crystals in sections showing the emergence of an optic axis by means of the gypsum plate. (After Wulfing.) than a mica or gypsum plate. If, for example, a positive crys- tal (acute bisectrix = c) is placed under the microscope in the diagonal position (Fig. 63), and a quartz or mica wedge, with Figs. 63-64. — Determination of the optical character of a positive biaxial crystal by means of a quartz wedge. tt parallel to its long direction, is inserted with the thin edge forward, the greatest ease of vibration in the slide and plate will be at right angles to each other, and the insertion of the 40 EXAMINATION IN CONVERGENT POLARIZED LIGHT. wedge has acted as a thinning of the plate. On shoving the wedge forward, the rings will become less numerous and will appear to move from the eyes toward the center and then out along the wedge. If the section is turned 90° (Fig. 64) the insertion of the wedge acts as a thickening of the section, the lemniscate curves will apparently move along the wedge to the center and then out toward the eyes. In either case, in a positive mineral, with o as the long direction of the wedge, the colors move out of the field in the long direction of the wedge and into the field at right angles to it. For a negative mineral, or with t as the long direction of the wedge, the reverse movements will take place. (Figs. 65 and 66.) Figs. 65, 66. — Determination of the optical character of ,a negative biaxial crystal by means of a quartz wedge. If instead of turning the figure to the 45° position, it is placed in the parallel position with the hyperbolae bars forming a cross, and one considers the center of a uniaxial interference figure as the emergence of the two optic axes of a biaxial mineral infinitely close together, or looks upon the bisectrix as located in the center of a uniaxial cross, the phenomena indicating the positive or negative character are alike in both cases. For OPTICAL CHARACTER OF THE CRYSTAI^. 41 example, with the quartz wedge inserted with its o direction parallel to the elongation of the wedge, in either the uniaxial cross (Fig. 67) or the biaxial figure in parallel position (Fig. 68) , the colors wiU move away from the center at right angles to Fig. 67. Fig. 68. Fig. 67. — A positive uniaxial interference figure. The arrows indicate the movement taking place upon inserting a quartz wedge above the section. Fig. 68. — A positive biaxial interference figure with the principal sections nearly corresponding to those of the nicols. The arrows indicate the movement taking place upon inserting a quartz wedge above the section. the wedge in negative, and the reverse way in positive min- erals. The optical character of a mineral may be determined Fig. 69. — A biaxial interference figure showing the emergence of an optic axis. The arrow indicates the movement taking place upon inserting a quartz wedge above the section. The mineral is augite. ( + ). with a quartz or mica wedge, even if the bisectrix emerges at the edge or entirely beyond the field of the microscope. 42 EXAMINATION IN CONVERGENT POLARIZED LIGHT. The case is exactly similar to that just described, and is best seen from the following figures which are given for a positive (augite) and a negative mineral (epidote). (Figs. 69-75.) The Figs. 70-74. — Movement of the left bar of Fig. 69 upon rotating the stage to the right, and movement of the colored rings upon inserting a quartz wedge. arms in every case rotate in a direction opposite to that in which the stage is rotated. In these sections, which show. only the emergence of one optic axis, account must carefully be taken of the position of the acute bisectrix, which is upon the convex Fig. 75. Fig. 76. Fig. 75. — A negative biaxial interference figure. The arrows indicate the movement taking place upon inserting a quartz wedge above the section. Epidote. ( — ). Fig. 76. — A positive biaxial interference figure. The arrows indicate the movement taking place upon inserting a quartz wedge above the section. Olivine. (+). side of the hyperbola, otherwise there will be confusion between the left optic axis of the ( + ) mineral and the right axis of the (— ) mineral and vice versa. (Figs. 69 and 75.) MEASUREMENTS UNDER THE MICROSCOPE. 43 When the point of emergence of the optic axis is entirely outside the field, it is sometimes possible to tell the position of the bisectrix by the curve of the bar (Fig. 48) . In this case the optical character can be determined in the same way, as is shown by Fig. 76 for olivine. Optical Anomalies. In certain, minerals the optical behavior is different from what it should be according to the crystalline system. Thus, isomorphous crystals sometimes show double refraction (garnet, leucite, perofskite, etc.), or uniaxial minerals appear biaxial (quartz at times). The discussion of the cause of these anomalies is too long for this book, and the student is referred to Rosenbusch and Wiilfing's Mikroskopische Physiographic, Ii, pp. 356-359, where the subject is fully treated and many references are given. Measurements Under the Microscope. Measurement of enlargement. The magnifying power of different combinations of objectives and oculars may be com- puted from the focal length of the combination, or may be directly measured by comparing the enlarged image of some object with its actual size. A small scale photographed on glass is placed upon the stage, the enlarged image is drawn by means of a camera lucida upon a sheet of paper and measured. The comparison may also be made by looking through the micro- scope with one eye and at a scale placed alongside with the other. Measurement of lengths. A movable stage with micrometer screws working at right angles to each other serves best to meas- ure actual size under the microscope. A micrometer eyepiece, whose divisions have been compared with a known scale for different combinations of lenses, may also be used. Measursment of thicknesses. One may focus upon the top and bottom of a section and read the amount that the objec- 44 MEASUREMENTS. tive has been raised by means of the micrometer screw, apply- ing a correction for refraction. If any mineral in the section is known, the thickness of the section, assuming it to have parallel faces throughout, may also be determined by the order of bi- refringence (cf., p. 19, and the colored plate in Part IV). Measurement of angles. Angles may be measured under the microscope by moving the section until the point of the angle coincides with the cross-hairs of the microscope, bringing the two sides successively parallel to a cross-hair, and reading the angle through which the stage has been rotated. Measurement of the axial angle. This subject has been fully treated above (p. 31). PART II. THE MINERAL GROUPS. Spinel Group. RiiR™204 where R" is Mg, Zn, Fe, and R™ is Al, Fe, Cr, Mn. Spinels: Precious, MgO-AlA- Colorless to pale red. light green, blue. Pleonaste, (Mg, Fe)0-(A1, Fe)A• Green. Hehoynite, FeO•AlA• Green. Gahnite, ZnO•Al,03■ Green. PiCOTITE, (Mg,Fe)0-(Al,Fe,Cr)A- Yellow or brown. Magnetite, FeO-FeA- Opaque, black. Chromite, FeO-CrA- Brown or thin edges. Optical anomalies are common in some varieties: Reactions: The precious varieties are not affected by acids; those that are rich in iron are decomposed by long treatment with HF or H2SO4 or by fusion with alkali carbonates. Occurrence and alteration. Spinels occur as original constituents in igneous rocks or secondary in metamorphic rocks, and are derived, according to Van Hise,* from almandite, biotite, corundum, diaspore, garnet, gibbsite, olivine, pyrope. They alter to mica, serpentine, and talc. Magnetite occurs as an original constituent in igneous rocks and is very common. As a metamorphic mineral it is derived, * C. R. Van Hise: A Treatise on Metamorphism. Monograph XLVII, U. S. Geological Survey. 45 46 THE MINERAL GROUPS. according to Van Hise, from actinolite, ankerite, arfvedsonite, augite, biotite, bronzite, diop&ide, garnet, griinerite, hematite, hornblende, hypersthene, ilmenite, marcasite, oHvine, pyrite, pyrrhotite, sahlite, and siderite. It alters to hematite, limonite, and siderite. Chromite occurs as an orig-nal or as a metamorphic mineral in magnesium-rich igneous rocks, often in connection with ser- pentine. It is, in many cases, derived from olivine. Garnet Group. R3"R2™Si30i2, where R° is Ca, Mg, Fe, Mn, and R™isAl, Fe,Mn, Cr,Ti. Grossular: Ca3Al2(Si04)3. Colorless, light green. No optical anomalies. Is usually fresh; known to alter to calcite, quartz, zoisite. Occurs in marbles, and calcareous schists and gneisses. Almandine: Fe3Al2 (8104)3. Red to brownish. No optical anomalies. Generally unaltered, though known to alter to chlor- ite, hypersthene, and spinel. Occurs in granites, andesites, schists, and gneisses. Melanite: Ca3Fe2 (8104)3. Brown. Optical anomalies not seen on account of the deep color. Generally fresh. Occurs in basic eruptive rocks rich in alkali; nephelite-syenites, phono- lites, tephrites, leucitophyres; also in serpentine. Spessartine. Mn3Al2(8i04)3. Blood-red. Unaltered. Oc- curs in contact rocks, quartzites, and schists, and in lithophysse in rhyolites. Common Garnet : Mixture of the grossular, almandine, and melanite molecules. Red-brown to yellowish-red. Zonal struc- ture and optical anomalies common. As a metamorphic mineral may be derived from vesuvianite. Alters to calcite, chlorite, epidote, hematite, hornblende, hypersthene, iron oxide, limonite, magnesite, magnetite, quartz, and siderite. Occurs in amphib- olites, metamorphosed diabases, gabbros, pyroxenites, and in schists and gneisses. SODALITE GROUP. 47 Pyrope: Mg3Al2 (8104)3. Blood-red. Kelyphite rims com- mon. Alters to chlorite, enstatite, gibbsite, magnesite, quartz, serpentine, spinel, and talc. Occurs in peridotites and basalts. Uwarowite: Ca3Cr2 (8104)3. Deep green, unaltered and with optical anomalies. Occurs In serpentines and is also found in marbles. Separation from other minerals. The strong index of refraction and insolubility In acids, united with isotropism, are characteristic of garnets. 8pinels, only, resemble them, and from these they are separated by the presence of 8102 in garnets. Spinels usually have octagonal outlines, while garnets generally show many-sided polygons. Sodalite Group. SODALITE, Al2(S104)3-(Al, Cl)-Na4. . NosELiTE, Al2(S104)3-AlS04Na-Na4. Hauynite, Al2(8i04)3 • AlSOiNa ■ CaNa2. Sodalite, hauynite, and noselite occur as original constitu- ents of igneous rocks, and are not known in schists and gneisses. Optical anomalies rare. Separation. Sodalite dissolves in HCl, and cubes of NaCl form upon dry- ing the solution. If a few drops of a slightly acid solution of lead acetate are placed upon a thin section of sodalite, thin, flat needles of strongly doubly refracting lead chloride form over it. Upon treating noselite and hauynite with HCl, gelatinous silica separates and they gelatinize in thin sections. Upon drying a solution of noselite much NaCl and a little CaS04 separates; from hauynite much Ca804 separates. Separation from other minerals. The low indices of refraction distinguish the minerals of the sodalite group from other isotropic minerals except glass, fluorite, leucite, and analcite. Form and cleavage sepa- 48 THE MINERAL GROUPS. rate them from glass; cleavage, lack of twinning, easy gelatini- zation and chemical reactions separate them from leucite and fluorite; form from analcite. Individual members of the group are separated by chemical means as given above. Carbonate Group. Calcite and aragonite may be separated by Meigen's method.* This has been modified by Panebianco, t who gives the follow- ing: If calcite is boiled for one minute with a dilute cobalt nitrate solution, it becomes blue, and, after four minutes' boiling, lavender-blue. Aragonite, similarly treated, immediately be- comes lilac and, with continued boiling, violet. The reaction for aragonite is very sensitive. In a mixture of one part of aragonite and nineteen parts of calcite, the color characteristic of the former will be produced. Calcite and dolomite may be separated by observing the ease with which the powdered mineral is acted upon by cold HCl. They may also be separated by Lemberg's method. J A solution is made of Aluminium chloride, dry 4 parts Logwood 6 " Water 60 " Boil half an hour, replacing the evaporating water, and filter. If a few drops of this solution are placed upon a thin sec- tion of calcite and are allowed to stand for from five to ten min- utes, and are then carefully washed off with water, the section will be colored violet; a section of dolomite will be unchanged. Another method of separation is as follows : If 1 gm. of calcite is boiled with 5 cc. of a 10% copper sul- * Meigen: Neues Jahrbuch, Ceutralblatt, 1901, p. 577. t G. Panebianco: Rivista min. crist. ital., vol. 28, 1902, p. 5. J Zeitschrifi der deutschen geologischen Gesellschaft. Berlin, vol. xi, 1888, p. 357. SCAPOLITE GROUP. 49 phate solution, the solution will become blue; with dolomite it will remain unchanged. If less calcite has been used than will replace the copper in the solution, upon adding ammonia to the filtrate the calcite solution will give no reaction, while the dolomite solution will become dark blue. Magnesite is not affected by cold HCl; boiling HCl dissolves it with effervescence. Siderite is soluble in HCl, and Fe is precipitated from a di- lute solution, by adding ammonia. Scapolite Group. Isomorphous mixtures of two molecules, not known to occur pure in nature: Al2(Si308)3Na4(AlCl) =Ma=Marialite molecule. Al6(Si04)6Ca2(CaOCa) = Me = Meionite molecule. DiPYR, A variety of mizzonite. MizzoNiTE, MeiMaa to MeiMag. Wernerite, McgMai to MeiMa2. Meionite, Me. Optical anomalies are rare and show a separation in the inter- ference cross of basal sections. Occurrence and alteration. The scapolites occur in crystalline limestones, often as the result of contact metamorphism in volcanic rocks, in crystalline schists, gneisses, and as an alteration product of feldspars. They alter to biotite, calcite, diaspore, epidote, gibbsite, kaolin, mus- covite, quartz, and talc. Separation. The basic mixtures from Me to Me2Mai are decomposed by acids; Me2Mai to MeiMa2 are but slightly affected, and marialite not at all. 50 THE MINERAL GROUPS. Feldspars, cordierite, and zoisite have lower double refrac- tion, are biaxial, and have different cleavage. Quartz is (+), shows no cleavage, and has lower indices. Prehnite and light-colored micas, with apparent uniaxial character, differ in cleavage. Andalusite has lower double refraction and is biaxial. Olivine Group. MONTICELLITE, MgCaSi04. FORSTERITE, Mg2Si04. Olivine, (Mg, FelaSiO*. Fayalite, Fe2Si04. Occurrence and alteration. Monticellite occurs in the crystalline limestones. It is con- sidered by many as isomorphous with olivine. Forsterite rarely occurs in volcanic rocks. It is a mineral of dynamic and contact metamorphosed marbles, basic schists, and gneisses. Olivine is usually an original mineral and occurs in the basic rocks, and as an accessory in basic schists, gneisses, and marbles. It alters to actinolite, anthophyllite, chromite, hematite, her- cynite, iddingsite, limonite, magnesite, magnetite, opal, quartz, serpentine, sideritej spinel, and tremolite. Fayalite occurs occasionally in volcanic rocks. Separation. Monticellite has a maximum birefringence of 0.017, while in the others it varies from 0.035 to 0.050. It is (— ), while fors- terite and olivine are (+). Fayalite has an index of 1.824 and a value of 2V about. 50°, while monticellite has n= 1.660 and 27 = 37.5°. Forsterite is ( + ). This separates it from all but olivine. Monticellite has lower birefringence, as given above. Occur- rence is different from olivine, and it is generally colorless. Olivine is (+). This separates it from all but forsterite, EPIDOTE GROUP. 51 which is a contact mineral of metamorphosed limestone. Mon- ticellite has lower birefringence, as given above. Fayalite is ( — ). This separates it from all but monticellite, which has much lower birefringence. Fayalite has 2F=50°, while monticellite has 37.50°. Olivine has 27 = 88°, and has lower birefringence; 0.035 being the maximum, while the maxi- mum in fayalite is 0.050. Humite Group. Chondrodite, Mg3[Mg(OH,F)]a(Si04)2. Humite, Mg5[Mg(OH,F)]2(Si04)3- Clinohumite, Mg7[Mg(OH,F)]2(Si04)4. The members of this group occur as masses in magnesian limestones and volcanic rocks bearing carbonates. Humite is orthorhombic; the other two are monoclinic with /? equal to 90°, but with extinctions of 25°-30° in chondrodite, and 7°-15° in clinohumite. Humite may. be mistaken for olivine, which it resembles in its alteration to serpentine. All of this group alter to brucite and serpentine. Epidote Group. ZoisiTE, Ca2(A10H)Al2(Si04)3. PiSTAciTE, Ca2(A10H)(Al,Fe)2(Si04)3. PiEDMONTiTE, Ca2(A10H)(Mn,Al)2(Si04)3- Orthite (AUanite), Ca2(A10H)(Al,Ce,Fe)2(Si04)3. Occurrence and alteration. Zoisite is not known as an original constituent of igneous rocks. It occurs in schists, gneisses, and as an alteration of feldspar in the granitic rocks. It may be derived from corun- dum, diaspore, gibbsite, grossular, and plagioclase, and may alter to calcite, gibbsite, kaolin, and quartz. Pistacite * (green epidote) is rarely, if ever, an original con- * The name Pistacite is used in the following pages for the green epidote, and Epidote as the group term. 52 THE MINERAL GROUPS. stituent of igneous rocks. It occurs secondary in metamorphosed igneous rocks and in schists, genisses, and marbles. It is derived from anorthoclase, augite, biotite, garnet, hornblende, micro- cline, orthoclase, plagioclase, and scapolites; and may alter to calcite, gibbsite, kaolin, limonite, and quartz. Piedmontite replaces epidote, where Mn is an important con- stituent in the rocks. Orthite is a minor constituent of acid and intermediate erup- tive and plutonlc rocks; and occurs in metamorphosed schists, gneisses, and marbles. Separation. Zoisite has a maximum birefringence of 0.009, while the other members of the group have a maximum of 0.032. It usually has abnormal interference colors. Piedmontite has strong characteristic pleochroism in red and violet tones. Orthite, being brown, differs from pistacite in color, is ( + ), and has an extinction of + 36°, c : c, while pistacite has only + 3°. Mica Group. The micas may be divided into two classes: 1. The plane of the optic axes is normal to (010), and 6= t. a. Anomite. a magnesia-jron mica. h. Muscovite. c. Lepidolite. d. Paragonite. The three latter are alkali micas. 2. The plane of the optic axes is parallel to (010), and b=i. e. Biotite. /. Phlogopite. g. Zinnwaldite. These are all magnesia-iron micas. Separation from other minerals. The micas can be mistaken only for other silicates showing perfect basal cleavage. The birefringence, very low in basal sections, (0.000 iji phlogopite, 0.003 in muscovite, and 0.008 in CHLORITE GROUP 53 biotite) combined with the high birefringence in sections show- ing cleavage (0.044 in phlogopite, 0.038 in muscovite, and 0.040 in biotite), is very characteristic. Chlorite has very low double refraction. Brittle micas are softer, and do not spring back into their former position on bending. Margarite has an extinction angle of +6.5°. Talc has a very small value for 2E (6°-20°). It does not become blue with cobalt solution, nor does it give reactions for K or Na. It has orientation like the first group, b=t, which, with its lack of color, separates it from the second group. Its hardness is 1.0. Separation of the micas. The optical orientation separates the two groups. Anomite is separated from h, c, and d, by being strongly pleo- chroic. Muscovite and paragonite do not give .Li reaction, which separates them from lepidolite. From each other they can only be separated chemically, muscovite being the K mica and paragonite the Na mica. In the second group: Zinnwaldite is separated by its Li reaction from all the micas- but lepidolite. From this the orientation separa es it. Biotite has strong pleochroism, while in phlogopite it is weak. The value of 2E is always small in the latter, which is especially characteristic of the crystalline limestones and dolo- mites, often associated with pyroxene, amphibole, and serpen- tine. It has been described by Cross as occurring in madupite, orendite, and wyomingite, and by the writer, in mica peridotite. Chlorite Group. According to Tschermak the members of this group may be- regarded as isomorphous mixtures of the amesite molecule- (H4Mg2AliiSi09) = At, and serpentine (H4Mg3Si209) = Sp. 54 THE MINERAL GROUPS. Pennine, SpjAtj to SpAt. Clinochlore, CRipidolite, Kobell). SpAt to SpjAt^. Pbochlokite, (Chlorite, Kobell; Ripidolite, Rose). SpjAtj to SpjAt,. COHUNDOPHILITE, SpjAt, tO SpAtj. Amesitb, SpAtj to At. Oficurrence and alteration. Chlorite is very abundant as a secondary mineral, although not known as an original mineral. It occurs in altered igneous rocks, and in schists, gneisses, slates, etc. Pennine and clinochlore are derived from biotite. Prochlorite may be derived from almandine, anthophyllite, aragonite, augite, diopside, dolomite, epidote, fluorite, garnet, gypsum, haiiynite, hornblende, noselite, sahlite, scapolites, tremolite, or zoisite. Amesite may be derived from pyrope. Separation from oilier minerals. The low birefringence, peculiar interference colors, and pleo- chroism are characteristic of the chlorite group. The Zeolites. Isometric. ANAI.CITE, NaAl(Si03)2+HjO. Tetragonal. Apophyllite, 4H2Ca(Si03)2+KFl + H20. Bexagonal. Hydronephelite, IINa2Al3(Si04)2+3H20. Chabazite, (Ca, Na2)Alj(Si03)<+6H20. Gmelinitb, (Naj, Ca)Al2(Si03)4+6H20. Orthorhombic. Thomsonite, 2(Na2,Ca)Al2(Si04)2+-5H20. Natrolite, Na2Al2Si30io + 2H2O. MonocHnic. ScoLECiTE, Ca(A10H)2(Si03)3 + 2H20. Epistilbite, H,CaAlJSi03)c+3H02. Heulandite, H4CaAl2(Si03)e+3H02. Stilbite, (Na2, Ca)Al2SieO«+6H20. Phillipsite, 2 (K2, Ca)Al2(SiO J, + 9H2O. Harmotome, H^CK^, Ba) Al,(Si03)5 + 5H2O. Laumontite, H4CaAl2Si40„ + 2H2O. THE ZEOLITES. 55 The zeolites, with the exception of analcite, are always secondary minerals. They occur in almost every variety of erup- tive rocks, especially in the more basic kinds, and also in altered sedimentary rocks. They are derived from many minerals, but especially from plagioclase, leucite, sodalite, etc., and generally occur as cavity fillings or in cracks. Separation. Analcite is isotropic or has weak anomalous double refrac- tion. Apophyllite, hydronephelite, chabazite and gmelinite are iini- axial. Of these hydronephelite contains no Ca. Gmelinite gelatinizes with HCl; the others are decomposed with separa- tion of flocculent silica. Chabazite generally occurs in rhombo- hedral forms, nearly cubical, while gmelinite has an angle of about 68°. Thomsonite and natrolite are biaxial and have parallel ex- tinction. Natrolite has a birefringence of 0.012 (Thomsonite 0.028), 2y=60°-62.5° (Thomsonite 87°). The monoclinic zeolites are separated from the others by being biaxial and. having inclined extinction. The extinction angle w above 9° in all but heulandite. Heulandite, therefore, might be confused with the ortho- rhombic zeolites, thomsonite and natrolite. The elongation of heulandite is (— ), of natrolite ( + ), of thomsonite (±). Thom- sonite has 2E=87°. (Heulandite =0°-55°.) Birefringence: thomsonite =0.028, heulandite =0.007, natrolite =0.012. The other monoclinic zeolites are separated from heulandite by the larger extinction angles. Of the remaining monoclinic zeolites, scoledte, epistilbite, stilbite, and laumontite are ( — ). ScoLBOiTB, c:o = 17°, 2£' = 50°-60° elongation (-). Epistilbite, c:t=-9°, 2£=67°-83°, " ( + ). Stilbite, c:q = 5°-8°, 2E = 52°, " ^-). Laumontite, c:c=+ 20°, 2£=54° ca., " ( + ). 56 THE MINERAL GROUPS. Pyroxene and Amphibole Groups. Pyroxenes. Orthorhombic. Enstatitb, MgSiO, = 3.1-3.3 Bronzite, (Mg,Fe)Si03 Mg:Fe = = 8 :lto3:l 3.2-3.3 Hypersthbne, (Mg,Fe)Si03 Mg:Fe = = 3: : 1 to nearly 1 ;1 3.4-3.5 Monoclinic. I. With little or no aluminium. DiOPSIDE, CaMgSi^O, G = 3.2-3.38 DiALLAGE, * 3.2-3.35 Hedenbergite , CaFeSi.Oe 3.5-3,58 II. Aluminous. AUGITE, Ca(MgFe)SijO„ with (MgFe)(AlFe)2SiO<, = 3.3-3.5 /Egiritb-au- Augite, rich in Eegirite molecule GITE, III. Alkali rich. jEgirite, NaFeSiA (? = 3.5-3 6 ACMITE, NaFeSiA 3.5-3.6 SPODnMBNE, LiAlSijOe 3.1-3.2 Jadbite, NaAlSljO 3.33-3.35 IV. Containing Ca. WOLLASTONITE, , Ca^Si,©!, = 2.8-2.9 Pbctolite, NaHCa^SijOg 2.74-2.88 Se'paration. Pyroxenes differ from amphiboles in having a prismatic angle ■ of 87° (amphiboles 124°), generally less perfect cleavage, and the crystals are usually stouter. The pleochroism, also, is gen- erally weaker, often wanting, and the extinction angle is greater. Monoclinic pyroxenes are separated from the orthorhombic by having inclined extinction. Acmite, segirite, and pectolite have extinction angles usually less than 5°, and may be con- fused with the orthorhombic pyroxenes, but basal sections show the emergence of an axis in convergent light, while the ortho- rhombic pyroxenes show the emergence of a positive bisectrix.. yEgiribe has different pleochroism, much higher double refrac- tion, and (— ) elongation, while all the orthorhombic pyroxenes have ( + ) elongation. * Similar in composition to diopside, but often contains Al, sometimes in considerable amount: should then be classed with augite. PYROXENE AND AMPHIBOLE GROUPS. 57 The mode of occurrence is about the same for all the ortho- rhombic pyroxenes. Enstatite, ( + ) 2B=135° Pleochroism: none. Dispersion: pu Bastite is ( — ), 2^=20° — 90°, pleochroism weak, dispersion p>o, birefringence weak. It is an alteration product of enstatite, and bronzite. Diopside, c:t=-39° ( + ) 2K = 59°- DiaUage. c:t -39° ( + ) 2^ = 59° and less. Hedenbergite c:t -44° ( + ) 2K = 60°.* Augite. c:c -45° to 55° ( + ) 2K = 60°. .iEgirite-augite, c-.O-oy <-S7° ( + ) iEgirite, c:o=-3° to -6° (-) 27 = 62°.. Aomite, c:b=-3° to -6° (-)t Spodumene, c:t=-2:jno -26° ( + ) 2y = 54°-60°.* Jadeite, c: c=-33.5° ( + ) 2V' = 72°. WoUastonite, c:a=+32° ',-) 2 K =69° and less.* PeotoUte, c:o=-5° ( + ) 2F = 60°.t * Birefringence is 0.015 to 0.019. t " "0.050. i " "0.03S. Others have birefringence from 0.015 to 0.029. Pleochroism: Weak or wanting in all but segirite-augite,, segirite, and acmite. Augite is sometimes quite strongly pleochroic in purple tones. Mgirite and acmite have different color and pleochroism. As seen above the chief mode of separating the monoclinic pyroxenes optically is by meansi of the extinction angles, which are characteristic as shown in Fig. 77. Occurrence. Diopside occurs in marbles,, especially in those that are magnesium rich, in the crystalline schists, pyroxene granites, diorites, and lamprophyres. 58 THE MINERAL GROUPS. Diallage occurs in gabbros and related rocks, and in peri- dotites and the serpentines derived from them. Hedenbergite is rather common in some nephelite-syenites and in other basic syenites. Augite is the most common of the pyroxenes in the igneous rocks. It also occurs in metamorphosed sedimentary and igne- ous rocks. Fig. 77. — Maximum extinction angles in the Pyroxene and Amphibole groups. Solid lines indicate extinction angles from c to c; broken lines from c to 0. The extinction angle in an amphibole is generally less than 23°; in a pyroxene it is generally greater. Mgirite, cegirite-augite, and acmite occur in eruptive rocks that are rich in sodium, especially in the nephelite-syenites, phonolites, leucitophyres, and in some granites and syenites. Spodumene occurs in pegmatitic masses in granites, gneisses, and schists; sometimes as an accessory constituent in normal granite and gneiss. PYROXENE AND AMPHIBOLE GROUPS. 59 Jadeite is not a common mineral. It occurs in metamor- phosed hornblende schists and metamorphosed limestones. Wollastonite occurs as a contact mineral in crystalline lime- stones, as a metamorphic mineral in calcareous rocks, — marbles, schists, gneisses, — and in calcareous inclusions in eruptive rocks. Pedolite occurs in cracks and druses within many basic eruptive rocks. Amphiboles. Orthorhombic. Anthophyllite, (Mg,Fe)Si03 = 3.1-3.2 Gedrite, (Mg,Fe)2SiA-MgAl2Si0e 3.1-3.2 Monoclinic. I. Containing little or no aluminium. Themolite, CaMgjSi.Oij = 2.9-3.1 AcTiNOLiTE, Ca(Mg,Fe)3Si40u, 3.0-3.2 OKiJNERiTE, FeSiOj 3 . 713 II. Aluminous. Common hoknblende, Ca(Mg,Fe)3Si40,2 with (Mg,Fe),(Al,Fe).Si,Oi, and Na^AUSijOi^ = 3.05-3.47 Pakgasite. Katophohite, No analyses. Basaltic hornblende, Rich in iron and alkali molecules. Barkevikite. III. AlkaK rich. u. Iron poor, light-blue color. Glaucophane, ■ NaAlSiA-(Fe,Mg)Si03 = 3.10.3-3.113 Oastaldite, Contains more Al and less Fe. 6. Iron rich, dark blue in color. RiEBECKiTE, NazFejSiiOij-FeSiOj = 3.3 Arfvedsonitb, (NajCaFe)4Si40,2 ■ (CaMg)2(AlFe)4Si20i2 3.44-3.45 Separation. Amphiboles differ from pyroxenes in having a prismatic angle of 124° (pyroxenes 87°), generally more perfect cleavage, and the crystals are usually longer, thinner, and sometimes fibrous. The pleochroism is generally stronger and the extinc- tion angle smaller. The orthorhombic amphiboles are separated from the mono- clinic by having parallel extinction; from other minerals all 60 THE MINERAL GROUPS. the amphiboles are separated by their prismatic cleavage o£ 124°. The orthorhombic amphiboles occur in metamorphic schists and gneisses, and as contact minerals. Anthophyllite often occurs as an alteration product of olivine in serpentines and in rocks of the gabbro and peridotite families. It has been de- scribed as an original mineral in a mica-peridotite dike from. the East Indies. Anthophy^'ite, Gedrite, (±) 27^90°, (-) 2F = 57°-79°, Dispersion p^ u. Dispersion p>u. Extinction Angle. Optical Char. Axial Angle. Tremolite, c:c=-16° (-) 2F = 87.5°. Actinolite, c:c=-15° (-) 2F = 80°. Grunerite, c:c= -11° to -15° (-) 2y = large. Common iibl. , c : t = - 12° to - 20° ( =F ) 2 F = 54° to 84°. Pargasite, c:c= -18° to -21° ( + ) 2F = 52°to60°- . Katophorite, c:c=-23°to-60° ( + ) 2F= small. Basaltic libl., c:c= 0°to-12° (-) 2F = 80°- Barkevikite, c:c--14° (-) 2F = 54°. Glaucophane, c:c=- 4°to - 6° (-) 2£' = 85.5° Gastaldite, c:t=— 6° Riebeckite, c : c = — 85° (-) 2E = 7Q°. (±) 2F = unknown. Arfvedsonite, c:t= + 10° to +20° ( + ) 2F = large. Pleochroism. Non-pleochroic. Faint, in green tones. Colorless; light brown. Strong; green and yellow tones. Green and yel- low tones. Red and yellow tones. Strong, green and brown tones. Brown tones. Colorless, yel- lowish green, blue. Ditto. Deep blue, light blue, yellow- ish green. Bide to green- ish. Riebeckite, arfvedsonite, and crossite have very low bire- fringence. This is high in basaltic hornblende and barkevikite, and between 0.016 and 0.026 in the others. Amphiboles are- PYROXENE AND AMPHIBOLE GROUPS. 61 determined chiefly by means of the extinction angles and pleo- chroism, which is colorless to greenish in the first three, in green and brown tones in the next five, and in tones of blue in the last four. Occurrence. TrenioLte, adinolite, and grunerite occur in schists meta- morphosed from carbonate rocks, especially those rich in mag- nesium and iron. In those in which the iron is not abundant, tremolite forms; where there is much ferrous iron, actinolite; and where iron is the chief or only carbonate, griinerite. The hornblendes — common, basaltic, and pargasite — are very common in igneous rocks, metamorphic schists, and gneisses. Katophorite occurs in a tinguaite dike in Norway. Glaucophane occurs in schists, especially those derived from basic rocks which formerly contained much sodium. Riebeckite occurs in igneous rocks rich in iron and sodium, as alkali granites, and in metamorphosed sedimentary and eruptive rocks. Arfredsonite occurs in soda-bearing igneous rocks, as nephe- lite-syenites, phonolites, tinguaites, and pantellerites. THE DETERMINATION OF THE FELDSPARS. Since the classification of a rock often depends upon the character of the feldspar occurring in it, its accurate deter- mination is one of the problems of the petrographer. In the following pages are given the most common and the most help- ful methods employed. In all the members of the feldspar group the general charac- teristics are closely related. They are usually colorless, belong to the monoclinic or triclinic systems (although with close resem- blance in angles, twinning, etc.), have a cleavage of 90° or nearly 90°, a hardness of between 6.0 and 6.5, and a specific gravity of from 3.84 in ceLsian, through 2.55 in orthoclase, to 2.76 in anor- thite. 62 THE DETERMINATION OF THE FELDSPARS. They may be classified as follows: rCELSiAN, BaAljSiA or BaO-ALOj -28102 ,, ,. • J Hyalophane, {BajiKn)ALSLO,, Monoclmic. H Qethoclase, RAlSiaO, or K^O-AlA'CSiO, [_SODA-ORTHOOLASE, (KMNam)Al SijOs MicROCLiNE, KAlSijOs or K^O • Al A • 6SiO, Anorthoclase, (Na7iK7i)AlSi308 Albite, NaAISijOs or Na^O • Al A ■ eSiO^ Oligoclase, g , Albite, Labradorite, Bytownite, Anorthite, CaALSinOs or CaO • ALO3 ■ 2Si02 TricHnic- Cleavage. An of the feldspars have a good cleavage parallel to (001) and another, nearly as good, parallel to (010). The angle between these cleavage lines varies slightly in the diiTerent feldspars. Celsian, 001 A 010 = 90° ^ = 64° 58' =001 A 100 Hyalophane, 001 A 010 = 90' forCeiOr^ /? = 64° 16' =001 A 100 Orthoclase, 001 A 010 = 90° /? = 63° 56' 46" = 001 A 100 MioroeUne, 001 A 010 = 89° 30', circa. ^ = 116° =001A100 Anorthoclase, 001 A 010 = 90° 6'-90° 33' /?=116° 30' =001A100 Albite, 001A010 = 86°24' /9 = 116°29' =001A100 Oligoclase, 001 A 01 = 86° 32' /3=116°22.5' =001A100 Andesine, 001 A 010 = 86° 14' i?=116° 28.5' =001A100 Labradorite, 001 A 010 = 86° 12' /?=116° 3' =001A100 Anorthite, 001 A 010 = 85° 50' ;?=115° 55.5' =001A100 As shown above, the angle between the crystallographic axes a and c varies between 64° and 65° in the monoclinic feldspars; and in the triclinic it remains within 30' of 116°; in either case the actual acute angle is not far from 64°. Chemical Composition. Chemically the feldspars are silicates of aluminium with either potassium, sodium, calcium, or barium, or mixtures of these, and with iron and magnesium present in small amounts. According to the law of Tschermak, the plagioclase feldspars are isomorphous mixtures of pure albite and anorthite mole- CELSIAN. 63 cules. In a similar manner, according to Strandmark, the barium feldspars are isomorphous mixtures of pure celsian and orthoclase molecules. The pure molecules are Celsian, BaAljSijO, = Ce. Orthoclase, KAlSijOg =0r. Albite, NaAlSijOg =Ab. Anorthite, CaALSioO. =An.. Grouping the feldspars- (After Strandmark.) Celsian, Hyalophane, Orthoclase, Anorthoclase, Ce,Or„ to CsiOr^ Ce,Orj to Ce,Orj OrjAbj to OriAb, (After Tschermak.) contains from 16 . 5% to 39 . 5% BaO. " 16.4% to 7.5% BaO. 16.9% K,0. " 7.1% to 10.1% Na,0. Albite series, Ab,An„ to AbjAn, contains from 11 . 79% to 10 . 27% Na^O Oligoclase series, AbjAn, to Ab,Ani Andesine series, AbjAnj to Ab^Anj Labradorite series, AbjAni to AbjAn^ Bytownite series, AbjAnj to AbiAn^ Anorthite series, AbiAn,, to AbjAni 10.01% to 6.91% to 5.73% to 2.82% to 1.55% to 7.63% NajO 6.56% Na^O 3.77% Na^O 1.55% Na^O 0.0 % Na-jO Celsian. Chemically varies from CeiOro to CeiOr2. G = 3.84. Monoclinic. ( + ). Cleavage: (001), (010), good. Occurs in plates, prisms, and grains. Orientation: b = i, a:a=—Q2°. Colorless, non-pleochroic. a =1.584, /?= 1.589, r= 1-594. ^-a = 0.010, 7— /? = 0.005, /?-a = 0.005. Carlsbad and Baveno twins occur. 2V increases with the increase in Ce; above 90° of Ce the optical character is positive, and the axial angle in pure celsian is 94°. Soluble in HCl with gelatinization. 64 THE DETERMINATION OF THE FELDSPARS. Hyalophane. Chemically varies from CeiOr2 to CeiOre. Monoclinic. (-). Cleavage: (001), (010), good. Occurs in tablets, prisms, and grains. Orientation: b=c, a: n= — 6°. Colorless, non-pleochroic. a = 1.537, ^=1.540, r= 1.542. r-a = 0.005, 7—^ = 0.002, /9-a = 0.003. Celsian and hyalophane show neither microcline structure nor polysynthetic twinning. They are, consequently, only like- ly to be confused with orthoclase and anorthoclase. From these they may be distinguished by the higher indices, larger extinction angles, higher specific gravities, and the chemical reactions of the barium feldspars. The different members of the barium feld- spar series may be separated by their different specific gravities and their extinction angles on 010. (See Fig. 88.) Orthoclase. Chemically, KAlSisOg. H=6, G = 2.54. Monoclinic. (-). Cleavage: (001),' (010), good. Occurs in plates, prisms, and grains. Orientation: b=t, a:a= ±5°. Extinction on (001) from (010) cleavage =0°, on (010) from (001) cleavage = ±5°. Colorless, non-pleochroic. a = 1.519, /?= 1.523, r= 1-525. 7— a = 0.006, r-/?=0.002, ^-a = 0.003. Dispersion: p> u, very plain. 27=70°-80°, 2Ka = 120° ca. Twinning. Carlsbad law. The most common form of twin- ning is according to the Carlsbad law, in which the vertical axis is the twinning axis and the composition plane is a plane parallel to the c axis, usually (010). The twins are turned 180° with respect to each other (Fig. 86). In the (001): (100) zone the ORTHOCLASE. 65 extinction is parallel to the (010) cleavage and to the twinning line. When the twinning line shows on the (010) face, it makes an angle of 63° 57' (/?) with the (001) cleavage and of ±21° with the extinction of each individual, (c : 6 = 19°-23°, Fig 85.) In this zone, as the sections depart from (010) and approach (100), the angle of the cleavage with the twinning line naturally increases b=C 010 Fig. 78, Fig. 80. Fig. 81. /ON no no 010 Fig. 82. Fig. 83. Fig. 84. Fig. 78. — The (010) plane of an orthoclase crystal. Fig. 79. — The (001) plane of an orthoclase crystal. Figs. 80-82. — Simple orthoclase crystals. Fig. 83. — Simple orthoclase crystal showing cleavage lines and the position of the twinning plane in a Carlsbad twin. Fig. 84. — A simple orthoclase crystal opened out to show the optical orien- tation. from 63° 57' to 90°. The extinction angle also changes from 21° to 90°, the increase being slight at first, but, as the section ap- proaches the (100) face, the change of extinction angle becomes very rapid. In any section in this zone the twinning line bisects the extinction angle and the cleavage. 66 THE DETERMINATION OF THE FELDSPARS. The (001) : (010) zone of one individual of a Carlsbad twin almost coincides with the (TOl) : (010) of the other (Fig. 86). In all sections in this zone the cleavage cracks of one individual are parallel to the twinning Hne, and the extinction angle from this line varies from 0° on (001) to 3° to 7° (12° in soda-ortho- clase) on (010). In the other individual the cleavage lines are at right angles to each other, and the extinction is parallel to them and to the twinning line. As the section approaches the (010) face, the extinction angle increases until it reaches ± 48° on (010). Baveno law. In Bayeno twins the twinning axis is the line normal to (021), which is the composition plane (Fig. 87). In sections at right angles to this plane the twinning line is diagonal .110 010.1 Fig. 85. Fig. 86. Fig. 87. Fig. 85. — Extinction angle on the (010) face of a Carlsbad twin of orthoclaae. Fig. 86. — A Carlsbad twin of orthoclase. Fig. 87. — A Baveno twin of orthoclase. to the cleavage. The two parts extinguish at the same time and parallel to the cleavage, but the direction of o in one indi- vidual is at right angles to o in the other, and the interference figures lie at right angles to each other. In sections in the (001) : (010) zone the cleavages of the two individuals are parallel to each other and to the twinning line. As the extinc- tion angle in one increases from 0° to 7°, it decreases in the other in the same proportion. Manebacher law. Manebacher twinning is comparatively rare. The (001) plane is the composition plane and the twin- ning axis is a line at right angles to it. ORTHOCLASE. 67 The low indices and low double refraction separate ortho- clase and sanidine from the barium and plagioclase feldspars. As seen in Fig. 88, the indices of refraction are lower than 3 » k s i 1 1 I 1 1 1 §. §„ S„ g„ g. S„ = + 4 Cel&ian-=Ba Al2Si20g / \a>= 100« o / ' K / II ' Celsiai^ /^ =f yp /I o / 3 ?>/ / it- s- ^ ?A /d :> \ / ^ R ■ Hyalophane- / « 1/ ■^ f ^- / si \, / ■ ?\ V ^» Bar'um-bearing- Orthoclase. Ortlioclase,=K Al SU O^ Soda-ricli- -^ / n \ ^ IS k Or-lOOg \ TJ Orthoclase. -^ ;' Soda-Orfhoclas^ 5" ~ g. 2, o I > \ o \ t S \ \' Ab= \ ^\ \ 3 1 °' I / ^ "^ \s \ / \*' J \ /^ ' ■i- \ A ^ lAndeBine vn \ o ' ^ b §•■ l^- Labradorita o * \\ 1 r 1 By townite i h An =100? Anorfliite = Ca Al'z 812 3 1 i ■ > ! ( 3 S 1 3 M J Si I ^ p < { i I < » H + Fig. 88. — Diagram showing the comparative indices of refraction, specific gravities, chemical relations, and extinction angles on (001) faces of the feldspars from celsian to anorthite. in any of the other feldspars, although anorthoclase, albite, and oligoclase-albite have indices lower than Canada bal- sam. From these they may be separated by the extinction 68 THE DETERMINATION OF THE FELDSPARS. on (010) (see Fig. 95), upon which face it is lower than in orthoclase only in other feldspars with an index greater than Canada balsam. The polysynthetic twinning of the plagio- clase feldspars usually distinguishes them, while in the barium feldspars, which are simply twinned like orthoclase, the values of the indices of refraction, double refraction, and specific gravity are entirely different. Sanidine. Chemically, crystal habit, indices and double refraction like orthoclase. Like orthoclase its optical character is ( — ). Orientation : Like orthoclase or sometimes 6 = B,a:o=+5°ca. Dispersion for 6= c, ^> y; ior b = i, p u, 27=71°-84°. Polysynthetic twinning, albite and pericline, almost invari- ably occurs. Twinning according to the pericline law. In this the twin- ning axis is parallel to b, and the composition plane is parallel to the rhombic section. When the twinning is polysynthetic, the striations appear on the (010) face. THE PLAGIOCLASE FELDSPARS. 69 Separated from orthoclase, with which it may be confused when untwinned, by its extinction on (001) and (010) ; generally by its polysynthetic twinning with "grating " structure. The indices of refraction are lower than in the plagioclase and barium feldspars. From anorthoclase it is separated by the extinction angles on (001) and (010) and by the larger axial angle of microcline. Anorthoclase. Chemically, OrgAbg to Or i Abe, H^Q.O, (?= 2.56-2.61. Triclinic. (-). Cleavage: (001), (010), good; 4 ±89°. Occurs in plates, prisms, and grains. Orientation: Extinction on (010) =+4° to +10°; on (001) =+1° to +4°. Colorless, non-pleochroic. a = 1.523, ^=1.528, 7-= 1.529. 7— a = 0.006, r-/? = 0.001, ^-a = 0.005. Dispersion: p>u, 27=43°-53°. Sometimes has polysynthetic twinning, according to the albite and pericline laws, like microcline. Separated from microcline by the extinction angles on (001) and (010) and by the smaller axial angle of anortho- clase. Orthoclase has a larger axial angle. The plagioclase feldspars have higher indices of refraction higher double refraction, and higher specific gravities. The Plagioclase Feldspars. The special characteristics of the different members of the plagioclase group are given in the following pages, and no general descriptions of the individual members are necessary here. For their separation from the preceding feldspars, the distinguish- ing marks have already been given under those feldspars. Fur- ther than that. Fig. 88 graphically shows the relationships. Twinning. Albite law. This is the most common form of twinning in the plagioclase feldspars. The twinning axis is nor- 010 110 70 THE DETERMINATION OF THE FELDSPARS. mal to (010), and the lamellse are consequently parallel to this plane. The twinning is usually polysynthetic, and the striations show on the (001) and the (100) faces. Very often this twinning is combined with the Carlsbad law, as in Fig. 89. When combined with the pericline law, the albite twinning may be recognized upon the (001) face, by the fact that the elonga- tion of the twinning lamellse lies in the direc- tion of the axis of greatest ease of vibration (o): while in pericline twinning this length Fig. 89.-Albite and >. ., ,. J e ^u , ^ ■ Carlsbad twinning IS m the direction of the ( c) axis. combined. ^j^g determination of the plagioclase feld- spars. The following are the most important methods used for the determination of the different members of the plagioclase group : 1. By specific gravities. 2. By the optical character of the mineral. 3. By the relative indices of refraction of the feldspar and some known mineral with which it is in contact. (Becke.) 4. By the index of refraction, according to Schroeder van der Kolk's embedding method. 5. By the extinction angle on cleavage flakes parallel to (001). (Schuster.) 6. By the extinction angle on cleavage flakes parallel to (010). (Schuster.) 7. By the position of emergence of the bisectrix in (010) plates. 8. By the extinction angle in Carlsbad twins upon the (010) face. (Michel-Levy.) 9. By the extinction angle on sections at right angles to both (001) and (010). (Becker and Becke.) 10. By the extinction angle on sections at right angles to either bisectrix. (Fouqu6.) 11. By the extinction angle on sections at right angles to the optic normal (b). (Fedorow.) THE PLAGIOCLASE FELDSPARS. 71 12. By the extinction angle on sections from the (001), (010) zone. 13. By the extinction angles on sections from the zone at right angles to (010). ■ (Michel-Levy.) 14. By the extinction angles on sections from the zone at right angles to (010), when the albite twinning is com- bined with Carlsbad twinning. (Michel-Levy.) 1. By specific gravities. The specific gravity of minerals is an important constant in their determination, and is apphcable to the separation of the feldspars. (Fig. 88.) For the pur- pose of determination, small fragments of the mineral should be separated imder the microscope from the rock powder. Cleavage flakes can very often be obtained from the granular rocks or from the phenocrysts of effusive rocks. The value of the specific gravity is constant so long as the material used is pure and unaltered. Glassy inclusions or alterations to kaolin and zeolites reduce the value, while the inclusion of other min- erals or the alteration to carbonates, mica, or saussurite in- creases it. The following values are after Tschermak: Orthoclase and microcline, sp. gr. 2 . 54-2 . 57 Anorthoclase series, 2 . 58-2 . 61 Albite series, 2.62-2.64 Oligoclase series, 2 . 64-2 . 66 Andesine series, 2 . 66-2 . 69 Labradorite series, 2 . 69-2 . 71 Bytownite series, 2 . 71-2 . 74 Anorthite series, 2 . 74-2 . 76 Day gives the following for artificial plagioclase feldspars: Albite, Ab 2.605 AbsAni 2.649 AbaAni 2.660 AbiAni 2.679 AbiAng 2.710 AbiAns 2.733 Anorthite, An 2 . 765 72 THE DETERMINATION OF THE FELDSPARS. 2. By the optical character of the mineral. The optical char- acter alone does not determine the kind of feldspar, but is of value when taken in connection with the index of refrac- tion, birefringence, and other characteristics. According to the optical character, as determined in convergent polarized light, the two groups are as follows: Negative (— ) character. Positive (+) character. Hyalophane. Celsian. Orthoclase. Albite. Sanidine. Oligoclase-albite. Microcline. Andesine. Anorthoclase. Labradorite. Oligoclase. Bytownite. Anorthite. The values of 2V and the optical characters are shown in Fig. 90. 3. By the relative indices of refraction of the feldspar and some known mineral with which it is in contact. (Becke method.) When the feldspar to be determined lies in contact with a known mineral, as quartz or nephelite, or even in contact with the Canada balsam, its relative index may be found by the movement of the Becke line. (See Part I, pp. 14, 15.) Mean values of the indices are as follows: Celsian, a! = 1.584 /?= 1.589 r= 1.594 Hyalophane, 1.542 1.545 1.547 Orthoclase, 1.519 1.523 1.526 Sanidine, 1.521 1.525 1.525 Anorthoclase, 1.523 1.528 1.529 Albite, 1.529 1.532 1.539 Oligoclase-albite, 1.534 1.538 1.543 Oligoclase, 1.540 1.544 1.547 Andesine, 1.549 1.553 1.555 Labradorite, 1.555 1.558 1.563 Bytownite, 1.661 1,564 1.569 Anorthite, 1.575 1.583 1.588 Quartz, £0=1.544 £ = 1.553 Nephelite, 1.542 1.537 THE PLAGIOCLASE FELDSPARS. 73 o o ANORTH, X ] / 1 z o t— >- CO / / f / / 1— cc o Q ■< cc to -J y / X ^ / y 2 \ \ CO UJ o \ \ ^ \ UJ GO ■<: —J o o cs o T ] / 1 -_J L / / / ALBITE. / ± t3 13 1^ 3 O ^:4 74 THE DETERMIN AXON OF THE FELDSPARS. Canada balsam, n = 1.542+ to 1.55 — , depending upon the de- gree of heating which the balsam has under- gone. It rarely rises higher than 1.545. Select a place in the section where the extinction of the feldspar is parallel to that of the mineral with which it is in contact; its direction of greater ease of vibration will be parallel or at right angles to that of the other mineral. By the position of the Becke line the feldspars are thus divided into six groups, which are given by Rosenbusch as foUov/s: (Quartz: w = 1.544, e=1.553.) Group. Parallel Position. Crossed Position. Feldspars. I. m>a ^>r (0>^ £>a Albite, Ab- AngAni II. (!)'> a =>r lo^r e>a > Ohgoclase, f AbgAnj-AbjAni 1 AbjAni-AbjAni III. aj^a £>r oj<-f e>a IV. V. oia) ( = 1 . 544) of quartz. ( + ) (-) <01igoclase ( — )> Andesine. Labradorite. Bytownite. Anorthite. THE PLAGIOCLASE FELDSPARS. 75 1.680- " 1.670- ~ i Pig. 91. — Comparative mean indices of refraction of the feldspars, quartz, nephelite, and Canada balsam. 1.660- 1.550- ir 1,540- 1.530- 76 THE DETERMINATION OF THE FELDSPARS. Index < E ( = 1 . 553) of quartz. (+) (-) Albite. Oligoclase-albite. Oligoclase. Index > £ ( = 1 . 553) of quartz. ( + ) (-) < Andesine ( + )> Labradorite. Bytownite. Anorthite. Where nephelite occurs in the rocks instead of quartz, that mineral may be used in a similar manner. Potassium feldspars and anorthoclase have all their indices lower than s of nephelite. (Nephelite: w = 1.542, £ = 1.537.) In ParaUel Position In Crossed Position. Albite, Oligoclase-albite, I (i}>a 1 ^^r Oligoclase, ui^r 1 ui\oi Andesine, Labradorite, Bytownite, Anorthito, '(jj- ai- CO \ \ lOy A^ -32° 38' ] -33°54'l -35° 01' -36° J Andesine Labradorite ■ Bytownite Bytownite- a n o r - thite and anor- thite * Tschermak's Mineralogische und " petrographische Mitteilungen, 5. 1882. 80 THE DETERMINATION OF THE FELDSPARS. o o o ANO JTH, / r / // / / >- CO / / i / / i f UJ 1— o o -c cc CQ ej « O O 0 1^ ^4 n t I THE PLAGIOCLASE FELDSPARS. 81 These are graphically represented in Fig. 96, from which it can be seen that there are upon the (001) face both positive (+) and negative ( — ) values up to 5°, making the feldspars, giving lower extinction angles, indeterminable unless the orien- tation of the section is known. Upon the (010) face there will be confusion if the extinction angle is less than 19°. How to recognize the (001) face. These plates cannot be recognized in random thin sections of unknown feldspars, and one is obliged to obtain cleavage flakes. A fragment of the rock is crushed, not ground, in an iron or agate mortar, the powder is passed through a sieve with meshes of 0.5 mm., the dust floated off in water, the dark constituents separated by means of a heavy solution, and the remainder sorted under the microscope. A large part of the feldspar will be found to be in cleavage flakes along (001) and (010). Only flakes of less than 0.5 mm. in thickness and with parallel faces, which may be recognized by their uniform interference tints, are of use. The (001) flakes show albite twinjiing, while those along (010) do not. In the (001) sections the extinction is measured from the twinning lamellse, in the (010) sections it is measured from the (001) cleavage. How to recognize the (010) fcux. Upon the (010) face the albite twinning lamellae are wanting; those according to the pericline law are sometimes seen. The crystal form (Fig. 97) is often shown in out- line or by the zonal growth. The (001) cleavage is usually distinct, and is best seen when the diaphragm below the stage of the microscope is partially closed. In the acid feldspars the elongation is nearly parallel to 0. The extinction is positive or nega- aoi tive, according to its direction from a, as ^i"^,- 97.— A (OIO) , ■ ,^ f 1 I • 1 1 cleavage flake of shown m the figure and as expiamed above. plagioclase. 7. By the position of emergence of the bisectrix in (010) plates. In convergent light the different feldspars show different posi- 82 -THE DETERMINATION OF THE FELDSPARS tions of emergence of the bisectrix in (010) plates. In air they are as follows: In albite the inclination is small and the bisectrix is posi- tive. In oligoclase the bisectrix is nearly normal to thfe face. In andesine the inclination is nearly 20° to the top and left. In labradorite, upon the left face, the axis is not in the field; only part of a bar, part of one system of axial rings, and a small part of the lemniscate curves appear. In bytownite the appearance is the same as in labradorite, except that no lemniscate curves show. In anorthite the axis emerges at the edge of the field. 8. By the extinction angle in Carlsbad twins upon the (010) face (Michel Levy).* The extinction angles of Carlsbad twins showing upon the (010) face are particularly characteristic. The twinning line bisects the extinction angle of the two in- dividuals (w) and also their (001) cleavages, which make an angle of approximately 128° with each other (Fig. 98). Albite twinning does not show upon the (010) face, unless the section '^oiofface'S a ^^ slightly incHned from this direction. In the Carlsbad twin values given below for the angles between of plagioclase. , . . ... • i i i the two extmctions, it is immaterial whether the crystal shows a simple Carlsbad twin or polysjmthetic albite twinning in combination with the Carlsbad twin, pro- vided the direction of the section is not far from (010). Albite, AbgsAns w=166° Oligoclase-albite, Ab86Ani4 = 152° Oligoclase, Ab75An25 = 130.5° Andesine, AbesAusT = 110.5° Labradorite, AbsoAnso =90.5° Bytownite, Ab2sAn75 = 64.5° Anorthite, AboAnioo = 51° These are graphically shown in Fig. 99. * Etude sur la determination des feldpaths, 1894. THE PLAGIOCLASE FELDSPARS. 83 o o o ANORTH. / / o OQ / 1 / t o o oa ■<: / / / / / / / £3 o z / / / UJ CO «c .1 o o « —1 o / / / / ALBITE. o n ^ m ^^ .no -l^rH fi a ^ .^ fS e < < 84 THE DETERMINATION OF THE FELDSPARS. 9. By the extinction angle on sections at right angles to both (001) and (010). (Method used by Becker* and Becke.f) These sections are easily recognizable in microlites and in phenocrysts of eruptive rocks by their nearly quadratic sections, and in the granular rocks by zonal growths with quadratic outlines. Sections at right angles to both (001) and (010) have the division lines between the albite twinning lamellae and the (001) cleavage lines extending at right angles to the section; consequently, when the tube of the microscope is slightly raised or lowered, there will be no lateral displacement of these lines. 010 Figs. 100-101. — Cross-sections at right angles to both (001) and (010) showing ( + ) and ( — ) character of extinction in plagioclase feldspars. When twinning and cleavage lines are wanting in microlites, one needs but remember that the extinction angle from (010) to is always less than 45°, consequently it is the smaller extinction angle. The small cross-sections, Figs. 100 and 101, show the directions of (+) and (— ) extinction; the angles are shown in Fig. 102. If the cross-section does not happen to be exactly at right angles to (001) and (010), it does not greatly matter, for the variation on tilting the section is small near this position. This method is good, because the increase in the extinction angle from albite to anorthite is rapid and uniform. * U. S. Geological Survey. 18th Annual Report, part III, 1898, p. 34. t Tschermak's Mineralogische und petrographische Mitteilungen. VoL 18, 1900, p. 556. THE PLAGIOCLASE FELDSPARS. 85 o g e ■n + o + o I s * < < ANOBTH, \ \ \ o CD \ y \ \ UJ t o o CO \ V \ \ \ \ V CO LlJ Q z •<: \ \ \ \ \ UJ CO o —1 o N \^ k \ \ \ ALBITE. s K ^ PH^^ ts B a V! TS n UJ 25 g i^ t3 ^m op, B^ «j3 O bO -4- + 7 86 THE DETERMINATION OF THE FELDSPARS. 10. By the extinction angle on sections at right angles to either bisectrix. (Fouque.*) Sections cut at right angles to the bisectrices may be easily- recognized by the fact that they show, in parallel light, an inter- ference color intermediate between the maximum and mini- mum tints. In convergent light the interference figure will show as a cross in the center of the field, when the principal sec- tions of the slide and nicols are parallel. This cross will open into two symmetrically located hyperbolae, when the section is rotated into the diagonal position. The section must next be tested by means of a gypsum plate, to see whether it is at right angles to the o or the t axis. If the line connecting the hyperbolse has less ease of vibration than that at right angles to it, that is, if the t direction connects the eyes while the 6 direction is at right angles to it and in the plane of the section, the slide is one which is cut at right angles to tt and vice versa. In sections at right angles to a the extinction angle is meas- ured from the twinning lines (Fig. 103, solid line). In sections at right angles to c, in the basic feldspars, the extinction angle is measured from the trace of the twinning lines or of the (010) cleavage. (Fig. 103, dotted line.) In the acid feldspars the section at right angles to c is very near the (010) face, and therefore shows neither twinning lamellse nor (010) cleavage; the extinction is measured from the (001) cleavage. (Fig. 103, broken line.) The following values are given by Fouque. Sections at right angles to a give good values as high as AbiAni, and are of use when one can determine the positive or negative direction of extinction. The values in sections at right angles to c are good in the basic feldspars. * Contribution h I'dtude des feldspaths des roches volcanique, Bulletin de la Soci^t^ min(5ralogique de France, Vol. XVII, 1894, p. 306. THE PLAGIOCLASE FELDSPARS. 87 ■= 2 o ■o e < < ANOKTH, '■ \ t o 00 \ \ '» O o ■«: QC QQ _l ^^^ \ \ ^ c/j LU o •a: \ \ \ \ \ \ \ \ \ \ Llj o o o \ \ \ \ \ \ \ A '^x =V. N— - ALBITE. \, \ N ^ -S -43 -g ■*^ o .4g . ^ TH '^J P .2.S « g S (H ^ O 0) .^ b ° P.H MH t-( H ^ o u a ™ 2 3 C H m n 0, o 1^ g g M §a-43 .2 '43 O o N M THE DETERMINATION OF THE FELDSPARS. Sections J. to (o). JL to (t). Albite, AbggAns -13° Oligoclase-albite, AbgoAnio — 5° Oligoclase, Ab75An25, +12° Andesine, Ab65An45, +22.5° Labradorite, Ab4oAn6o, +32° Bytownite, Ab25An75 Anorthite, An, +33° + 34° -19° - 3° + 4° + 7.5° + 32° +42° +48° Measured from the (010) cleavage 11. By the extinction angle on sections at right angles to the optic normal (B). (Fedorow.) The interference colors between crossed nicols in sections at right angles to the optic normal are the highest of any in that mineral. In thin sections of feldspar as usually cut for microscopical use, the colors seldom exceed yellow of the first order, and the highest color may be difficult of determination. A very convenient way of making the tints more distinct is to turn the nicols into parallel position. The tints are given on pp. 10 and 20. Extinctions in the acid feldspars from albite to andesine, in sections at right angles to an optic normal, vary only from +2° to —2°; but in the feldspars from andesine to anorthite they change rapidly from —6° to —39° (Fig. 104). 12. By the extinction angle on sections from the (001), (010) zone. (Extinction angles of microlites.) In sections from this zone the (001) and (010) cleavages are parallel. Spherulite rays and the microlites of the effusive rocks are bounded by these cleavages. In these sections the extinction angle increases from 0° to a different maximum in each feldspar. Basic oligo- clase has the lowest maximum, and anorthite has the highest. Since it is impossible to obtain the positive or negative char- acter of the extinction in microlites, albite and andesine cannot be separated; above 20° the method is good. (Fig. 105.) Albite and andesine may be separated by the indices of refraction, and their extinction angles may then be used. THE PLAGIOCLASB FELDSPARS. 89 e O a o + I o e N go o o o ANORTH. / / / i >- CO 1 / 1 / UJ o •< CD 3 I / f / / « / ' / o / / / / LU CO ■•c o o (3 _J o ALBITE. o fa o + o o3 o ho q 'A C5 90 THE DETERMINATION OF THE FELDSPARS. o © o ANORTH^ \ \ \ o >- OQ \ \ \ UJ 1— cc o o -c cc m •<: _i \ \ \ \, \ V ^ CO LU Q ■^ N s. \ LlJ CO -c —J o cs _l o \ / / / / ALBITE. / bDT3 -^ oi O-g IB tSJ Sj3 C3 CQ 2 to C rj (U '^ H -a 3 e o o.S OQ ^ ;:= c ^ o '^ m ^ 60 o ■-So o ^ S § gg5 < < THE PLAGIOCLASE FELDSPARS. 91 13. By the extinction angles on sections from the zone at right angles to (010) or the symmetrical zone. (Statistical method of Michel-Levy.*) Sections in this zone may be recognized as fol- lows: (1) the albite twinning lamellse are separated by very sharp lines, which, since they do not overlap, are not laterally displaced upon raising or lowering the tube of the microscope; (2) the extinction angle from the twinning line of the albite twins of one system is the same to the right as it is for the other to the left; and (3) if a section at right angles or very nearly at right angles to the twinning plane is turned until the twinning lines make an angle of 45° with the principal sections of the crossed nicols, the two systems of twins and the narrow strips where they overlap will become of uniform color, and the section will have the appearance of a simple crystal. The latter is the quickest and most sensitive method of finding sections in this zone. Each section in the zone at right angles to (010) has equal extinction angles, varying from 0° to a maximum, on either side of the twinning line, but only one section gives the maximum extinction, which is different in each species of feldspar, as is also the position of this plane giving maximum extinction. (Fig. 106.) This method is one of the most valuable for the determina- tion of the feldspars, although confusion might arise if two species occur in the same rock, or if there are but few feldspar sections in the slide and none of them gives the maximum extinction. Another confusion arises in the positive and nega- tive character of the extinction angles of albite and oligoclase- albite, on the one hand, and andesine on the other. The number of sections occurring in a slide which give good results by this method is greatly increased by the fact that sec- tions varying 10° or more from the true position may be used. In such sections the extinction angles on either side of the twin- ning lamellse will not be the same, but half of the sum of the * Annales des mines, 1877, pp. 392-471. 92 THE DETERMINATION OF THE FELDSPARS. + o + o I ANORTH. & o 1— >-> CO \ \ UJ H; CC O o -c cc ca •5 \ \ V \ \ \, \ CO UJ o z -a: \, \ \ \ s UJ CO -c o es _i o \ N, S \ \, \ \ \ ALBITE. \ o <-> o ^n P g ^ ■a 1 s a O d §T3 O JS ■:;3 ft m O " si .» a J I ■II :S^ o I 1 fe THE PLAGIOCLASE FELDSPARS. 93 two angles very nearly coincides with the true values found in the zone. The following values are given by Rosenbusch for the maximum extinction angles: AbgsAns -15.2° AbseAnu - 7.7° AbrsAnas +6.8° AbesAnsT +19.3° AbsoAnso +28° AbzsAnrs +43.7° AboAnioo 90° 14. By the extinction angles on sections from the zone at right angles to (010), when the albite twinning is combined with Carls- had twinning. (Michel-Levy.) If the albite twins of the pre- ceding section are combined with Carlsbad twins, the difference in extinction between the two systems will determine the feld- spar. When these twins are in combination, the sections in the zone at right angles to (010) may be recognized by the fact that in the 45° position the albite twinning disappears and the crystal appears as a simple Carlsbad twin. If one turns the stage of the microscope so that the traces of the twin- ning planes are parallel to the cross-hairs of the microscope, the albite and Carlsbad twinning will both disappear and the feld- spar be equally illuminated, except for the fact that the narrow lines, where the albite lamellse overlap, will appear as fine black lines. Michel-Levy* has recently published very useful diagrams for the determination of plagioclase feldspars in this zone, with these twins combined. By observing the extinction angles, without considering the sign, the pole of the section chosen and the percentage of anorthite in the plagioclase may be determined from the diagram (Fig. 107). * Etude sur la determination des feldspaths, Troiseme fascicule, 1904. 94 THE DETERMINATION OF THE FELDSPARS. A section is chosen which gives approximately the maxi- mum extinction in or very near the zone of symmetry, that is, one in which the position of greatest illumination, as explained above, is at 45°. (As in the preceding case, sections inclined as much as 10° from the perpendicular to (010) may be used.) The different values of symmetrical extinction of the two individuals of the Carlsbad twin are taken, and of these the smaller is found in the column at the left (Fig. 107) and the larger upon the curves. The vertical line at the intersection of the two gives the percentage of the anorthite molecule. The broken lines indicate the angles which the sections make with the (100) face. The size of this angle is indicated by the figures which are not followed by the degree (°) mark in the diagram. the lime-soda feldspars in sections from the zone at right angles to (010) Carlsbad twinning. Solid lines, extinction angles; broken lines, poles. To face page 94. .1. -•SOI •01 i- (•OfO) o PART III. EXPLANATION OF THE TABLES. The method of using the following tables needs but little explanation. A brief resume of methods and a few cautions may be helpful. The mineral is Opaque. Observations are made with the analyzer out , color determined by incident light. Isotropic or anisotropic. Observations made in polarized light, nicols crossed. With slightly isotropic minerals it may be necessary to make use of the gypsum plate (p. 12). Basal sections of uniaxial minerals are isotropic but give an inter- ference figure, Extinction parallel or inclined. Angles are measured from the cross-hairs of the microscope. For very small angles a Bertrand ocular (p. 13) may be used. In cases where the extinction angle of a mineral is less than 5°, it will be found under minerals having parallel extinction as well as under minerals having inclined extinction, thus allowing for slight errors in observation. Color. Minerals nearly colorless, or which occur both colorless and colored, are given under both heads. Pleochroism. Observed in plane polarized light. Index greater or less than Canada balsam. Observed by Becke method (p. 14). Maximum birefringence greater or less than quartz. In sec- tions of normal thickness this may be determined at once 95 96 EXPLANATION OF TABLES. by the eye, using polarized light. In sections of unknown thickness containing quartz or any other known mineral, it may be determined by the quartz wedge, mica wedge, or compensator (pp. 18-21). Uniaxial or biaxial character. Determined by obtaining interfer- ence figures (.pp. 24-30). Optical character. Determined with some accessory (pp. 85^3). Value of birefringence. Obtained by comparison with known minerals by the use of some accessory (pp. 18-21). Amann's birefractometer and the Babinet compensator are excellent (p. 21). Index of ref rac tion. Determined by comparing (Becke method) the unknown mineral with one that is known or with the Canada balsam in contact, or by any of the methods given on pages 14 and 15. Care should be exercised in estimating the value of the index of refraction of a mineral, especially if it has medium or high relief, when there is no known mineral in the rock section with which to compare it. A colorless mineral with medium relief appears rougher than a colored one with high relief. The spacing of the indices and the double refraction upon the diagrams is uniform until the very high figures are reached. Since few minerals occur here, and the difference in relief and birefringence is hard to estimate, this is of no importance. The length of the line under each mineral gives the range of its indices of refraction; the position of the line, its maximum birefringence. After narrowing down the characteristics of an unknown mineral, one may find at that point, upon the plate, a group of several from which it can readily be separated by referring to the descriptive tables opposite, which are arranged in the same order as are the minerals upon the plates. When some of the properties of a mineral as, for example, its uniaxial or biaxial, or positive or negative character are indeterminable, it may be necessary to follow out two divisions; but even when it becomes necessary to look under both classes the known EXPLANATION OF TABLES. 97 properties will reduce the possible minerals to a small num- ber. Throughout the tables, the following form has been observed : Name. — Chemical composition, i?= Hardness, G= Specific •gravity. System. Optical character of mineral (+ or — ). Cleavage. Cleavage angle. Habit. Common form of occurrence, twinning, etc. Elongation. The principal zone or plane. Optical character of elongation (+ or — ). Optical orientation. Giving the extinction angles, etc. Color. Pleochroism. Indices of refraction. Birefringence. Dispersion. Axial angle. Chemical and physical properties. Other minerals which are similar and characteristics by which they may be distinguished. NOTICE. Since it is impossible to fold the signatures of every book exactly alike, it is suggested that the student first «cut all of the descriptive pages ofi the lines indicated, and then cut the guide pages as follows, paying no attention to the positions of the printed lines: A. The upper third of page 99, the upper two-thirds of page 105, and the whole of page 139 form the widest guides and should be of the same width. They form the first series of guides. B. The upper halves of pages 107 and 141, and the whole of pages 121 and 371 should be of the same width and form the second series of guides. They are one space narrower than the "A" guides. C. The upper halves of pages 109, 123, 143 and 373, and the whole of pages 117, 129, 259 and 437 form the third series of guides and are one space narrower than "B." D. The upper halves of pages 145, 261, 375 and 439, and the whole of pages 181, 315, 399 and 467 form the fourth series of guides and are one space narrower than "C." E. The upper halves of pages 147, 183, 263, 317, 377, 401, 441 and 469, and the whole of pages 165, 209, 281, 333, 391, 411, 447 and 479 form the fifth series of guides and are one space narrower than " D." F. The upper halves of pages 149, 167, 185, 211, 265, 283, 319 and 335, and the whole of pages 157, 173, 197, 227, 273, 293, 325 and 345 form the sixth series of guides and are one space narrower than "£." Pages 259 and 281 should not be indented, but should have straight edges and be of the width indicated at the top. The guide words should be transferred to the lower halves of these pages. After having cut the guide pages according to these directions, the index slips on the accompanying sheets should be separated on the lines, and pasted in their proper positions on the stubs. C o The mineral is opaque. 99 The mineral is opaque. Yellow by incident light P3rrite. Pyrrhotite. Black by incident light Magnetite. Graphite. Umenite. Thin edges transparent : Red Hematite. Brown Chromite. Picotite. 100 OPAQUE. 101 Pyrite.— FeS^, H = 6.0-6.5, G = 4.9-5.21. Isometric. Cleavage: Not seen in thin sections. Habit: Cubes, octahedrons, and irregular grains. By incident light: Yellowish with metallic luster. Non-magnetic; insoluble in HCl. Magnetite is black by incident light, and, when pulver- ized, is attracted by the magnet. Pyrrhotite.— FenSnn+i, H = 3.5-4.5, = 4.58-4.64. Hexagonal. Prismatic cleavage; not seen microscopically. Habit: Irregular grains. By incident light: Bronze-yellow with metallic luster. Is somewhat attracted by the magnet when pulverized. Solu- ble in HCL Pyrite is not attracted by the magnet and is insoluble in HCl. Magnetite.— FejOi, H = 5.5-6.0, G = 4.9-5.2. Isometric. Cleavage: (111), rarely seen under the microscope. Habit: Octahedrons, cubes, irregular grains, dust. By incident light: Metallic, bluish black. Attracted by magnet; soluble, even in thin sections, in HCl. Pyrite, ilmenite, chromite, and graphite are not attracted by the magnet. Pyrite, chromite, and graphite are insoluble in acids. Ilmenite is soluble with difficulty in acids and gives reac- tion for Ti. Graphite.— Cn , H = 1.0-2.0, = 2.1-2.3. Hexagonal. Cleavage: (0001), perfect. Habit: Hexagonal to rounded plates; scale-Uke and rod-hke aggregates. By incident light: Metallic luster to dull black or steel-gray. Slowly consumed in BB. Insoluble in acids. Black streak. Molybdenite is not combustible, but gives off fumes of SO^ and has a green streak. Ilmenite.— FeTiOj, H = 5.0-6.0, = 4.5-5.0- Hexagonal. Cleavage: Not distinct. Hab:t: Tabular, thin plates, scales. Elongation: (0001), lath-shaped. Madder to clove brown on thin edges. Submetallic luster. a) = high, £ = high. o) — £ = high. OPAQUE. 103 Slowly soluble in hot HCl; with tin the solution becomes violet. Very slightly or not attracted by the magnet. Characteristic alteration to leucoxene. Magnetite is readily soluble in acids and is magnetic. Hematite is red to yellow on thin edges. Titaniferous magnetite; separation may be difficult. Dif- ferent crystaUine system. Hematite.— Fe A, H = 5.5-6.5, G = 4.9-5.3. Hexagonal. ( — ). Cleavage: Not distinct. Habit: Tabular, thin plates, scales. Elongation: (0001), lath-shaped. ( + ). Red, yellowish, yellowish gray. Pleoohroism: = brownish red, E = light yellowish red. ft) = 3.22, £ = 2.94. &j-£ = 0.28. Becomes magnetic in the reducing flame. Very slowly soluble in acids. Not attracted by magnet. Streak, red. Magnetite is magnetic and readily soluble in HCl. Limonite and earthy hematite may be confused. ^ Rutile is optically (-I-). Habit is different. o HO Chromite.— FeCrA. H = 5.5, = 4.32-4.57. ^ Isometric. No cleavage. Habit: Octahedrons, grains. Incident light: Brownish black; thin edges same. Insoluble in acids. Cr reaction. Magnetite is magnetic and soluble in HCl. Hmenite gives Ti reaction. Picotite.— (Mg,Fe)0-(Al,Cr,Fe)A. H = 7.5-8.0, G = 4.80 Isometric. Cleavage: Imperfect. Habit: Grains, rarely octahedrons. Yellow or brown to greenish brown on thin edges. Index about 1.7. Occurs as inclusions in olivine, rarely individuals in basalt, grains in Iherzolite, etc. Chromite is distinguished by hardness specific gravity and chemically. The mineral is isotropic. 105 CO (» M O The mineral is isotropic. o Colorless. ^?'<*""^*"^-^ i>- /^ 107 The mineral is isotropic. Colorless. Index of refraction is less than S that of Canada balsam. ^Jl] y o 109 The mineral is isotropic, colorless, index of refraction is less than that of Canada balsam. (Arranged according to mean indices of refraction.) The mineral forms crystals Fluorite n= 1.433 SodaUte 1.483 Noselite 1.490 Hauynite 1.503 Leucite 1.508 The mineral fills cavities or is amorphous Opal n= 1 .443 Glass 1.490 Cavity filling or attached crys- tals Analcite n= 1.488 110 ISOTROPIC. Ill Fluorite.— CaFj, H = 4.0, G=3.18-3.20. Isometric. Cleavage: Perfect (111). Habit: Seldom shows crystal form; generally cavity and inter- space filling, or irregular or drop-like grains. Colorless to violet or purple. Color often irregularly or zonally distributed. Index:re = 1.4332-1.4340. With HjSO^ gives off hydrofluoric acid which will etch glass; very slightly affected by other acids. Opal and sodalite groups have higher indices and are often slightly anisotropic. Opal has no cleavage. Sodalite group is attacked by weak acids and has (110) cleavage. Other colorless minerals usually have lower specific gravities and higher indices. Sodalite — (SiO J3AU • AlCl • Na, , H = 5.5, G = 2.28-2.34. Isometric. Cleavage: Sometimes fair (110). Habit: Cloudy quadratic, hexagonal, or rounded grains, often sj distorted. '^ Colorless, bluish, greenish, light pink, red, yellowish. "§ Index: «„„ = 1.4827-1.486. • ^ Anomalous double refraction is rare. Gelatimzes with HCl; crystals of NaCl form on drying. Gela- tinizes even in thin sections with acetic acid. Isotropic grains (except glass, fluorite, leucite, and analcite) have higher indices. Glass has no cleavage and no crystal form. Leucite, fluorite, and analcite have different cleavage, gelatinize with greater difficulty, have twinning and different chemical reactions. Haiiynite, noselite, and lazurite are separated by chemical tests, cleavage, and crystal form. tfoseUte.— (SiOi)3Al2-AlS04Na-Na„ H = 5.5, G=2.27-2.S0. Isometric. Cleavage: Usually shows right-angled cleavage in cross-sections parallel to (100) and (110). Sections parallel to (111) show three series of lines cutting each other at 60°. Habit: Usually occurs as crystals, crystal fragments,or rounded grains. Colorless, blue, gray, brownish, red, yellow, and green. The color is often irregularly distributed or zonal. Index: 71^0 = 1.495. ISOTROPIC. 113 Often has opaque borders, and is often altered. Gelatinizes easily with HCl, and few gypsum crystals and many NaCl crystals form. Sometimes shows optical anomalies. Haiiynite gives many gypsum crystals in drying gelatine from HCl. Sodalite has less perfect cleavage, and seldom has borders. Hauynite.— (SiOJsAlj-AlSO.Na-CaNa,, H = 5.5, = 2.27-2.50. Isometric. Cleavage: Cross-sections parallel to (100) and (110) show two systems at right angles. Habit: Usually occurs in crystals (110), crystal fragments, or rounded grains. Colorless, blue, gray, brownish, red, yellow, and green. Color often irregularly distributed or zonal. Index: «,«, = 1.496-1.504. Sometimes shows optical anomalies either about inclusions or as a dark cross. Hauynite usually contains inclusions at the center or around periphery. It is often altered. Gela- tinizes easily with HCl, and gypsum crystals result from the drying gelatine. Noselite gives few gypsum crystals. Sodalite has less perfect cleavage, and seldom has borders. Leucite.— KAlSi^Oe, H = 5.5-6.0, = 2.45-2.5. Isometric. Cleavage: Not noticeable; irregular cracks. Habit: Six- or eight-sided to rounded grains. Very often con- tains inclusions in regular zones. Colorless to gray. (u = 1.508, £ = 1.509. e — iu = .O01 and less. Optical anomalies; shows polysynthetic twinning. HCl attacks leucite sUghtly in thin sections, but the powdered mineral is dissolved with a separation of silica. All minerals but analcite differ from leucite in having higher indices and higher double refraction. The twinning and crystal form of leucite also distinguishes it. Analcite and sodalite groups may be confused with isotropic leucite. Either may be distinguished by the greater resistance of leucite to HCl and by its micro-chemical reactions, giving isometric potassium fluosilicate crys- tals, while analcite gives hexagonal sodium fluosilicate crystals. Opal.— SiOj-t-aq., H = 5.5-6.5, = 1.9-2.3. Amorphous. Cleavage: Wanting. ISOTROPIC. 115 Habit: Irregular grains, vein fiUing, druses; pseudomorphs after feldspar, augite, or other silicates. Transparent, colorless. Index: Low, varying with aq. contents. Mean: 1.36-1.458. Birefringence: Low. Anomalous double refraction in irregular patches and spots is not uncommon. Analcite.— NaAlSijOe+HA H = 5.5, G = 2.15-2.18. Isometric. ( + ). Cleavage: Mediima (100). Habit: Druses or cavity filling, cloudy grains (211); pseudo- morphs after leucite, nephelite, or sodalite. Colorless, white. Index: n = 1.486-1.488. Birefringence: Anomalous weak double refraction with a separa- tion into fields, occurs. Becomes cloudy on heating strongly; soluble even in thin sections in acids. Sodalite group differs in cleavage and chemically. Leucite is attacked by HCl with difficulty. Analcite takes stain readily. Treating leucite with hydrofluosificic acid ^ produces isometric potassium fluosilicate crystals, while analcite so treated gives hexagonal crystals of sodivim S fluosilicate. Garnets have much higher indices. ^ The mineral is isotropic. Colorless. Index of refraction is greater than that of Canada balsam. O < n < o o 117 The mineral is isotropic, colorless, index of refraction is greater than that of Canada balsam. (Arranged according to mean indices of refraction.) Octahedrons: generally quadratic sections. Spinel n= 1.716 Periclase 1.736 Polygonal sections and rounded grains. Grossular (garnet) .... n= 1.750 Spessartine (garnet) 1 . 811 118 ISOTROPIC. 119 Spinel.— MgO-AlA, H = 7.5-8.0, = 3.6-3.7. Isometric. Cleavage: Wanting. Habit: Generally in sharply defined quadratic (111) crystals. Colorless to faint red, green, or blue. Index: w = 1.712-1.720. Not affected by acids. Never altered. Periclase is gray to yellow and has good cleavage. Periclase.— MgO, H = 6.0, = 3.642-3.674 Isometric. Cleavage: Good (100). Habit: Grains, crystals (111). White, gray, dark gray, yeUow to brownish yellow. Index: ran a =1.736. Dissolves in hot concentrated HCl; is colored brown in thin sections when moistened with silver nitrate. Spinel, see above. Grossular.— CajAljSiaOij, H = 7.0-7.5, 0=3.42-3.72 . Isometric. Cleavage: Strong, irregular fractures. Habit: (110), quadratic, hexagonal, or octagonal outlines, or irregular rounded grains. Colorless to weak yellowish. Index: n„a = 1.744-1.757. Birefringence: Optical anomalies. Insoluble in acids. Zonal structure common. Periclase and spinel: Separate by specific gravity, then test for Si02. Periclase has a good cleavage. Spinel is generally in (111) crystals. Spessartine.— MnsAl^SijOij , H = 7.0-7.5, = 4.0-4.3. Isometric. Cleavage: Strong, irregiTlar fractures. Habit: (110) grains; quadratic, etc., outlines. Blood-red, yellowish red, red-brown, colorless. Index: n7io= 1.8105. Birefringence: Optical anomalies. Insoluble in acids. Spinel, periclase, see above. . The mineral is isotropic. Colored. P o ■-) o u 121 I -9) u The mineral is isotropic. ^ Colored. Index of refraction is less than that g of Canada balsam. 5 123 The miaeral is isotropic, colored, index of refraction is less than that of Canada balsam. (Arranged according to mean indices of refraction.) The mineral forms crystals. . .Fluorite n= 1 .433 Sodalite 1.483 A/flloselite 1.490 Haiiynite 1 . 503 Leucite 1 . 508 Amorphous filling Glass n= 1 .490 124 ISOTROPIC. 125 Fluorite.— CaF^, H = 4.0, G = 3. 18-3.20. Isometric. Cleavage: Perfect (111). Habit: Seldom shows crystal form; generally cavity and inter- space filling, or irregular or drop-like grains. Colorless to violet or purple. Color often irregularly or zonally distributed. Index: n= 1.4332-1.4340. With HjSO, gives off hydrofluoric acid which will etch glass; very slightly affected by other acids. Opal and sodalite groups have higher indices and are often very shghtly anisotropic. Opal has no cleavage. Sodalite group is attacked by weak acids and has (110) cleavage. Other colorless minerals usually have lower specific gravities and higher indices. Sodalite.— (SiOJaAlj-AICl-Na,, H = 5.5, = 2.28-2.34. Isometric. Cleavage: Sometimes fair (110). Habit: Cloudy quadratic, hexagonal, or rounded grains, often distorted. Colorless, bluish, greenish, light pink, red, yellowish. Index: 7i„a = 1-4827-1.486. Anomalous double refraction is rare. Gelatinizes with HCl; crystals of NaCl form on drying. Gela- tinizes even in thin sections with acetic acid. Isotropic grains (except glass, fluorite, leucite, and anal- cite) have higher indices. Glass has no cleavage and no crystal form. Leucite, fluorite. and analcite have different cleavage, gela- tinize with greater difficulty, have twinning and differ- ent chemical reactions. Haijynite, noselite, and lazurite are separated by chemical tests, cleavage, and crystal form. HoseUte.— (SiOJjAlj-AlSO.Na-Na. H = 5.5, G = 2.27-2.50. Isometric. Cleavage: Usually shows right-angled cleavage in cross-sections parallel to (100) or (110). Sections parallel to (111) show three series of lines cutting each other at 60°. Habit: Usually occurs in crystals, crystal fragments, or rounded grains. Colorless, blue, gray, red, yellow, or green. The color is often irregularly distributed or zonal. ISOTROPIC. 127 Index: nma = 1.495. Often has opaque borders and is often altered. Gelatinizes easily with HCl, and few gypsum crystals and many NaCl crystals form. Sometimes shows optical anomalies. Haiiynite gives many gypsum crystals in drying gelatine from HCl. Sodalite has less perfect cleavage, and seldom has borders. Hauynit . (SiO,)3Al2 • AlSO.Na • CaNaj , H = 5.5, G = 2.27-2.50. Isometric. Cleavage: Cross-sections parallel to (100) or (110) show two systems at right angles. Habit: Usually occurs in crystals (110), crystal fragments, or rounded grains. Colorless, blue, gray, brownish, red, yellow, and green. Color often irregularly distributed or zonal. Index: n„<,= 1.496-1.504. Sometimes shows optical anomahes either about inclusions or as a dark cross. Haiiynite usually contains inclusions at the center or around periphery. It is often altered. Gelatinizes easily with HCl, and gypsum crystals result from the drying gelatine. Noselite gives few gypsum crystals. Sodalite has less perfect cleavage and seldom has borders. Leucite.— KAlSijO„ , H = 5.5-6.0, G = 2.45-2.5. Isometric. Cleavage: Not noticeable, irregular cracks. Habit: Six- or eight-sided to rounded grains. Very often con- tains inclusions in regular zones. Colorless to gray. w = 1.508, £ = 1.509. Birefringence: Very low, =001. Optical anomalies; shows poly- synthetic twinning. HCl attacks leucite slightly in thin sections, but the powdered mineral is dissolved with a separation of silica. All minerals but analcite differ from leucite in having higher indices and higher double refraction. The twiiming and crystal form of leucite also distinguish it. Analcite and sodalite group may be confused with isotropic leucite. Either may be distinguished by the greater re- sistance of leucite to HCl and by its micro-chemical reactions, giving isometric potassium fiuosilicate crys- tals, while analcite gives hexagonal sodium fluosiUcate crystals. The mineral is isotropic. Colored. Index of refraction is greater than that of Canada balsam. <§ i-i < n < I U O 129 The mineral is isotropic, colored, index of refraction is greater than that of Canada balsam. (Arranged according to mean indices of refraction.) Generally octahedrons or quadratic sections. Spinel n= 1.718 Periclase 1.736 Hercynite (spinel) . . 1 . 749 Picotite (spinel) .... 1 . 7 ca. Gahnite (spinel) .... 1 . 765 Pleonaste (spinel) . . . high Pyrrhite high Beckelite high Chromite 2.097 Perofskite 2.38 Rounded, quadratic, hexagonal, octahedral, etc., or irregular grains. Usually strong, irregular fractures. Generally com- pletely isotropic or faint optical anomalies. Often show zonal structure. Grossular (garnet) . , Pyrope Almandine Spessartine Uwarowite Melanite Tiet) . . . n= 1.744 1.744 1.810 1.811 1.838 1.856 130 ISOTROPIC. 131 Spinel— MgO-AlA, H = 7.5-8.0, = 3.6-3.7. Isometric. Cleavage: Wanting. Habit: Generally in sharply defined quadratic (111) crystals. Colorless to faint red, green, or blue. Index: n = 1.712-1.720. Not affected by acids. Never altered. Periclase is gray to yellow and has good cleavage. Periclase— MgO. H = 6.0, = 3.642-3.674. Isometric. Cleavage: Oood (100). Habit: Orains, crystals (111). White, gray, dark gray, yellow to brownish yellow. Index: Kno = 1.736. Dissolves in hot concentrated HCl; is colored brown in thin sections when moistened with silver nitrate. Spinel, see above. Hercynite.— FeOAlA. H = 7.5-8.0, = 3.91-3.95. Isometric. Habit: Grains, crystals (111). Dark green. Index: ?i = 1.749. Insoluble in acids. Pleonaste is only to be separated by chemical means. Picotite.— (Mg,Fe)(Al,Cr,Fe)A. H = 7.5-8.0, = 4.08. Isometric. Cleavage: Wanting. Habit: Orains, rarely crystals (111). Yellow, brown, greenish brown. Index: n = 1.7 + . Reaction for chromium. Chromite is distinguished chemically, by specific gravity, and by hardness. Gahnite.— ZnAlA, H = 7.5-8.0, = 4.0-4.6. Isometric. Cleavage: (111), indistinct. Habit: (111) grains. Green, greenish black. Index: n = 1.765. Reaction for Zn. Occurs in crystalline schists. Pleonaste is distinguished chemically. ISOTROPIC. 133 Pleonaste.— (Mg,Fe)(AI,Fe)A, H = 8.0, G = 3.82. Isometric. Cleavage: (111), imperfect. Habit: Generally in sharply defined (111) quadratic crystals. Green. By incident light or when opaque it is black without metallic luster. Index: High. Insoluble in acids, except concentrated HjSOj. Common in gneisses. Often included in garnets and cor- dierite. Gahnite is distinguished chemically. Pyrrhite — Calcium niobate or tantalate, H=5.5, G = 4.1-4.3. Isometric. Cleavage: Not observed. Habit: Crystals (111). Orange-yellow to red. Index: High. Insoluble in HCl. Spinels are much harder. Beckelite.— Ca3(Ce,La,Di),Si30,s, H = 5.0, G = ca. 4.15. Isometric. Cleavage: (100) distinct. Habit: Grains (111), (110). Light yellow. Index: High. Birefringence: Large crystals show optical anomalies. Chromite.— FeCrA. H = 5.5, = 4.32-4.57. Isometric. Cleavage: Wanting. Habit: Irregular grains, crystals (111). Opaque. By incident light, brownish black; same on thin edges. Index: w = 2.0965. Insoluble in acids. Chromium reaction with soda. Peiofskite.— CaTiO^, H = 5.5, G = 4.02-4.04 Isometric. Cleavage: In large individuals distinct (100). Not seen in thin sections. Habit: Octahedrons (111); sharp, sometimes rounded, cubes. Grayish white, violet-gray, brownish to red-brown, rarely green- ish gray. Index: nna=2.38. ISOTROPIC. 135 Anomalous double refraction in large individuals; often not seen in thin sections. Insoluble in HCl, not magnetic, Ti reaction. Ilmenite and hematite resemble perofskite when opaque, but are soluble in HCl and show different crystal out- lines or are irregular. Chromite, picotite, and melanite may be separated by combining specific gravity, solubility in HCl, and attrac- tion by the magnet. In the separated material perof- skite will show the presence of Ti, melanite wiU show SiOa and chromite Cr. Grossular.— CasAljSijOij, H = 7.0-7.5, = 3.42-3.72. Isometric. Cleavage: Wanting; shows irregular cracks. Habit: (HO), quadratic, hexagonal, or octagonal outlines or irregular rounded grains. Colorless to weak yellowish. Index: »„« = 1.744-1.757. Birefringence: Optical anomalies. Insoluble in acids. Zonal structure common. Spinel and periclase are separated by their specific gravi- ties ; then tested for SiOz. Periclase has a good cleavage. Spinel is generally in (111) crystals. Pyrope.— MgjAljSijO.j, H = 7.0-7.5, G = 3.7-3.8. Isometric. Cleavage: Wanting; shows irregular cracks. Habit: Rounded grains, never crystals. Red to blood-red. Index: 71 = 1.741-1.750. Kelyphite rims common. Occurs only in peridotites and their derivatives. Spinels are separated by their specific gravities and the absence of SiOj. Almandine.— FejAljSiaOij, H = 7.0-7.5, G = 4.1-4.3. Isometric. Cleavage: Wanting; shows irregular fractures. Habit: (110), (211), and grains. Quadratic, hexagonal, etc., outUnes. Red, in various tones, to brownish red. Index: ?iuo = 1.808-1.811. Optical anomalies appear not to occur. Insoluble in acids. Zonal structure common. Spinels are separated by their specific gravities and the absence of SiO,. ISOTROPIC. 137 Spessartine.— MnsAljSiaOia, H = 7.0-7.5, G = 4.0-4.3. Isometric. Cleavage: Wanting; shows irregular cracks. Habit: (110), grains; quadratic, etc., outlines. Blood-red, yellowish red, red-brown, colorless. Index: ?iuii = 1.8105. Birefringence: Optical anomaHes. Insoluble in acids. Spinels and periclase are separated by their specific gravi- ties and the absence of SiOj. Periclase has good cleavage. Spinels generally occur in (111) crystals. Uwarowite.— CaaCr^SijOij, H = 7.5, G = 3.51. Isometric. Cleavage: "Wanting; irregular cracks. Habit: (110), grains. Deep green. Index: re = 1.838. Usually shows optical anomalies. Alteration unknown. Confined to Cr rich serpentines, granu- lar limestones, and dolomites. Spinels are separated by their color, specific gravities, and S the absence of SiO^. Melanite.— CajFeaSijOis, H = 7.0-7.5, G = 3.8-4.1. ^ Isometric. Cleavage: Wanting; shows irregular cracks. Habit: Grains and crystals (110), (211). Various tones of brown, sometimes green. Index: ?iiia = 1.8566. Optical anomalies, owing to deep color, hardly observable. Zonal structure very common. Alterations wanting. Occurs in phonohtes, nephelites, tephrites, etc. Spinels are separated by their specific gravities, the absence of SiO„ by crystal form, and want of zonal structure. The mineral is anisotropic. u o Pi H O m g < 139 The mineral is anisotropic. Parallel extinction. S O c;3 141 i The mineral is anisotropic. 8 Parallel extinction. Colorless. O^U-xio ' /■ '-'I 143 CO The mineral is anisotropic. Parallel extinction. Colorless. g Index of refraction is less than that of Canada balsam. •t*» a AAA. /.i_6..*tc ^, /f / I 145 SI H O V w o The mineral is anisotropic. g Parallel extinction. S Colorless. g Index of refraction is less than m that of Canada balsam. Maximum birefringence is less than that of quartz. ^i U^H^K i^\' Uv ^1 u. 2F = 70°-80°, 2£na = 120° ca. Insoluble in HCl. Plagioclase, except albite, and other colorless minerals differ generally in having higher indices and double refraction. Nephelite gives uniaxial figure and is acted upon by HCl. Scapolites, with low indices, have higher double refraction. Quartz and cordierite have no cleavage, and have indices higher than Canada balsam. Soda-orthoclase has a higher extinction angle, o : tt = 10°-12°. g (See also special methods for the separation of the feld- spars in Part II.) S Sanidine.— KAlSijOs, H = 6.0-6.5, = 2.54-2.56. .„ Monoclinic. ( — ). ^ Cleavage: (001), (010), good. Habit: Plates, prisms, grains. Carlsbad twins frequent. Elongation: (001): (010), laths. (110):(1T0), plates. Orientation: 6= C, a: 0= +5° ca.; or 6 = 1, a: 0= +5° ca. Colorless. Pleochroism: None. a: = 1.519, ^ = 1.523, r=l-525. r-a = 0.006, }-- i8=0.002, ^-of = 0.004. Dispersion for 6=c, p>u; for 6 = 6, pu. 2F = 90°-98°. (See special methods for the determination of the feldspars in Part II.) Microcline.— KAlSi.Os, H = 6.0, G = 2.54. Triclinic. ( — )- Cleavage: (001), (010), good. (001): (010) = 89° 30'. Habit: Plates, prisms, grains. Polysynthetic twinning, albite and pericline laws. Elongation; (001): (010) laths. (110): (110) plates. Orientation: A (,(,,= + 16°, ^„i„=+5°- Colorless. Pleochroism: None. a = 1.519, /?=1.523, r=l-526. J— a = 0.007, r-i9=0.003, ^-a: = 0.004. Dispersion: p>D. 2F = 71°-84°. Insoluble in acids. Other minerals are separated by the cleavage, low indices, and low double refraction of microcline, which has indices always lower than Canada balsam, lime-soda feldspars, or nephelite. Only to be confused with ortho- c5 clase or anorthoclase. Orthoclase has not the "grating" texture of microcline.' Optical behavior on (001) and (010) faces is different. Anorthoclase has smaller axial angle and different orien- tation on (001) and (010). Has no "grating" texture. Cordierite.— MgjAl^SijO,,, H = 7.0-7.5, = 2.59-2.66. Orthorhombic. ( — ). Cleavage: (010), poor. Habit: Grains, short prisms. Trillings and polysynthetic twins occur. Elongation: (110):(lT0), short laths. (-). Orientation; b=c, c=n. Colorless, bluish, yellowish. Pleochroism : Wanting or weak with S > t > n. a = 1.535, /?= 1.540, 7- = 1-544. r-a = 0.009, r-/5=0.004, ^-a = 0.005. Dispersion: Weak, pl b ANISOTROPIC !». PARALLEL EXTINCTION. U COLORLESS, INDEX OF Tio Mineral b NEGATIVE (-> Tho Minei^ U POSITIVE (+^, 1.55 ISO 1.45 1.40 1.35 In- oreii't 1.35 1.40 1.45 1 50 1.55 The Indox o( Refraclioii u LOW. The Indei of Refraciian ■• LOW. I .010 .015 I [ydronci Chalce heUte, • lony. _ .020 025 Cancn ute. , .030 ','/ — , .035 .040 .045 t 1 ' -050 OSS 0«0 .065 t [ 070 075 ■ 080 1 . .085 .090 .095 .100 120 .140 160 .200 .250 '^■ ! ANISOTROPIC. 169 Cancrinite.— H„Na8CaAl6(SiOJs(COj2, H = 5.0-6.0, = 2.42-2.5. Hexagonal. ( — ). Cleavage: (lOTO), good. Habit: Prisms, needles, grains. Elongation: (1010), narrow laths, ( — ). Colorless, yellow, bluish, reddish. Pleochroism : None. w= 1.524, £ = 1.496. w- £ = 0.028. Soluble in HCl with slow but continuous evolution of CO.y, the solution gelatinizes on cooling. Nephelite has much lower double refraction. Rhombic carbonates differ in cleavage and have much higher double refraction. Other minerals are separated by the chemical behavior of cancrinite; the evolution of CO, may be seen in thin sections. Cancrinite also becomes cloudy on heating. ANISOTROPIC. 171 Chalcedony.— SiOj+aq., H = 6.0-7.0, = 2.59-2.64. Orthorhombic. (+). Cleavage; None. Habit: Thread-like aggregates, spherulites, concretionary masse.s, etc. Elongation: (110):(II0), thread-like. (-). Colorles.'i, yellowish, brownish. Pleoohroism: Wanting. q: = 1.533, ,.?= 1.536, r= 1-544. r-a:=o.oii, r-/?=o.oo8, /3-«=o.oo3. 2F = 10°-40°. Chemically like quartz. Zeolites are not thread-like and gelatinize easily with acids. Pseudochalcedony is optically ( — ) and has smaller double refraction. Hydronephelite.— Na2HAl,Si30,24-3aq., H = 4.5-6.0, = 2.26 He.^agonal (probably). (-I-). Cleavage: (lOTO), poor. Habit: Rod-like, leafy or rod-like granular aggregates. Elongation: (1010), short laths, (-f-). Colorless, often cloudy. § Pleochroism : AVanting. Index: n = 1.49 ca. c2 £-w = 0.012. Soluble in HCl with formation of jelly. The mineral is anisotropic. Parallel extinction Colorless. Index of refraction is less than that of Canada balsam. Maximum birefringence is greater than that of quartz. Biaxial. s i4 COLORLESS, INDEX OF REFRACTION < Cenada b>Ifu Qu.rte. BIAXIAL. The Miner J i. NEGATIVE (-). The Miner.1 it POSITIVE (+). s § :; § s In- in o in o u) n t -^ ui Ui mis'g BIREFR -.--„- The Index of Refraction i. LOW. The Index of Refraction ii LOW. 1 ^Cordierite. .010 .015 1 Chalc Natrolite.— Serp •dony.— :ntine. • .025 030 Tho Tisonite. .035 .040 .045 .055 .070 .075 .080 .055 .090 .09S .100 .120 .140 .160 .200 .250 ANISOTROPIC. 175 Cordierite.— Mg^Al^SijO,,, H = 7.0-7.5, = 2.59-2.66. Orthorhombic. ( — ). Cleavage: (010), poor. Habit: Grains, short prisms. Trillings and polysynthetic twins occur. Elongation: (110);(1T0), short laths, (~). Orientation; b=t, c=fl. Colorless, bluish, yellowish. Pleochroism : Wanting or weak with 6 > c> n. a = 1.535, ^=1.540, }-=l-544. J— a = 0.00f), )--/? = 0.004,/? -a = 0.005. Dispersion : Weak, p u. 2E = m°-5Q°. Soluble in boiling HCl with separation of gelatinous silica. Pennine is (±), has lower birefringence, and is pleochroic. Chemical test for Al is often the only sure separation. ANISOTROPIC. 179 Thomsonite.— (CaNa,)2Al4Si40,e + 5 aq., H = 5.0-5.5, G = 2.3-2.4. Orthorhombic . ( + ) . Cleavage: (010), (100), good. Habit: Plates, leaves. Elongation: (110):(1T0), lath-shaped. (±). Orientation : a= a, 6 = (. Colorless. Pleochroism: Wanting. a = 1.497, /?=1.503, r = l-525. r-a: = 0.028, r-/?=0.022, /3-a = 0.006. Dispersion : p Canada balsam. MAXIMUM BIREFRINGENCE < Quartz, UNIAXIAL The Mineral U NEGATIVE (-). The Mineral is POSITIVE (+). < Increase in Index of Refraction. > oooom o u) o U3 lnoaln^< t^ w to >a In- in o in o inoo oa tn w (0 r« r-oonom NC^^ « - „ ^ creas g 8IREFR. t-i •-■ i- — ««^ N N VenH. Hiidi- Medium. Hot Marked. Not Marked. Medium. H [*' taH. . , , 001 002 • Eudialite. Garnet. — h * • Perofskite. _ ,. .003 004 Apatite. - . e. Nephelite. — - ,005 " Vesuvianite." '" - Gehlenite. .006 ■ 007 008 009 - • Quarti - 010 " * Garnet and perofskite are isometric but often have anomalous double refraction. ANISOTROPIC. 187 Eucolite.—Ca, Fe, Mn, Zr, Na silicate. H = 5.5-6.5, G = 3.05. Hexagonal. ( — ). Cleavage: (0001) distinct. Habit: Grains. Elongation: (0001), short lath-shaped. ( + ). Reddish, yellowish, brownish, colorless. Pleochroism: 0>E. Weak or wanting. «= 1.621, e = 1.618. w-E = 0.003. Often anomalous biaxial character with 2£ as high as 50°. Gelatinizes with HCl, which diluted gives reaction for Zr. Garnets have much higher indices and are isotropic unless with anomalous double refraction. Tourmaline has stronger double refraction. Eudialyte is optically ( + ), elongation is (— ), and indices and birefringence are lower. Apatite.— CaF-Ca,(P04)3 and CaCl.CaXPOJs, H = 5.0, G = 3.10-3.22. Hexagonal. ( — ). Cleavage: (0001), (lOTO), poor; long prisms often show cross fractures. Habit: Long prisms, rounded grains, granular aggregates. g Elongation: (lOTO) : (OlTO), lath-shaped. (-). Colorless, violet, brownish, reddish, gray. ^ Absorption: E>0 ia colored crystals. ^ w = 1.638, £ = 1.634. «>- £ = 0.004. Anomalous biaxial character in large individuals only. Easily soluble in H^SOj, yellow precipitate with ammonium molybdate. Easily soluble in HNO3. Vesuvianite, melilite, gehlenite, zoisite, and tourmaline are either not soluble or have different crystal form or color. Tourmaline has 0>E, while colored apatite has E>0. Melilite.— Ca, Mg, Fe, Al silicate, H = 5.0, = 2.9-3.1. Tetragonal. (T). Cleavage: (001): (110), poor; only basal cleavage generally seen in thin sections, and this occurs as a single cleft exactly in the center of the lath-shaped section. Habit: Plates, short prisms. Elongation: (001), laths. (±). Colorless, yellowish. Pleochroism: None in colorless melilite; when yellow .E = dark yellow, = light yellow. 01 = 1.634, £ = 1.629. w— £ = 0.005; often unusual indigo-blue interference colors. o ANISOTROPIC. 189 Gelatinizes easily with HCl. "Peg'' structure is characteristic. Vesuvlanite and zoisite are insoluble in acids, j Gehlenite has not the unusual interference acids, k^ ce QjflK Nephelite.— NaAlSi04, H = 5.5-6.0, . = 2.55-2.61. Hexagonal. ( — ). Cleavage: (0001), (1010), distinct. Habit: Short prisms, square sections, and grains. Elongation: (1010), rectangular, quadratic. ( — ). Orientation: c=a. Colorless. Pleochroism: Wanting. (u = 1.539-1.542, £ = 1.532-1.542. o)- £ = 0.005. Sometimes has anomalous small 2E. Gelatinizes easily with HCl and is readily stained. Sodalite minerals have lower indices and no double refrac- tion. Apatite, gehlenite, and melilite have higher indices. ,S Scapolites have higher indices and higher double refraction. Different cleavage. Sanidine and orthoclase have lower double refraction and ^^ are unaffected by HCl. They are also biaxial. (;;> Vesuvlanite.— Al2(SiOj5Cao(A10H), H = 6.5, G = 3.35-3.47. Tetragonal. ( — , rarely +). Cleavage: (110), (100), poor. Habit: Short prisms, grains, duU masses. Elongation: (110):(lI0), prismatic. (¥). Colorless, yellowish, greenish, reddish, brownish, blue. Pleochroism: Weak to unnoticeable. w = 1.705-1.732, £ = 1.701-1.726. w— £ = 0.006-0.001, often abnormal colors. Anomalous biaxial character. Insoluble in acids unless fused to glass. Occurs in metamorphic rocks. Zoisite has the high indices and weak double refraction found in vesuvlanite. The optic angle of zoisite often falls as low as 0°, and vesuvlanite has often anomalous double refraction with very variable axial angle. Zoisite, however, has good pinacoidal cleavage. Gehlenite gelatinizes easily with HCl. Andalusite always has a larger axial angle. ANISOTROPIC. 191 Gehlenite.— Ca3(A10)2Si,Os, H = 5.6-6.0, = 2.9-3.1. Tetragonal. ( — ). Cleavage: (001), (110), poor. ^ Habit: Thick plates, cubic and rounded grains, cloudyTOasses. Colorless. Pleochroism: None. w = 1.663, £ = 1.658. w- £ = 0.006. Gelatinizes easily with HCl. Apatite has cross-parting and different crystal form. Vesuvianite has lower double refraction and is insoluble in acids. Melilite has lower double refraction and different micro- structure. Corundum.— Al A, H = 9.0, = 3.9-4.10. Hexagonal. ( — ). Cleavage: A poor parting (lOTl), (0001). Habit: Plates, grains, prisms. Twins (lOTl) occur. Elongation: (0001), lath-shaped. { + )■ 5J Colorless, blue, lightered. Pleochroism: = blue, red; E = sea-green, yeUow, or greenish yel- ^ low; seen only in deeply-colored corundum. '-^ ft) =1.769, £ = 1.760. [3 oj- £ = 0.009. Insoluble in acids or fused soda. Other minerals which resemble it are separated by the high indices, negative character, uniaxial figure, etc., of corundum. O ANISOTROPIC. 193 Garnet.— Ca, Mg, Fe, Mn, Al, Cr silicate, H = 7.0-7.5, = 3.4-4.3. Isometric. Cleavage: Wanting; shows strong, irregular cracks. Habit: (110), quadratic, hexagonal, or octagonal outlines, or irregular rounded grains. Colorless, red, brown, green, yellow. Index: 1.74-1.86. Anomalous double refraction common in most varieties. Zonal structure common. Insoluble in acids. Perofskite has different crystal form. Perofskite.— CaTiOs, H = 5.5, = 4.02-4.04. Isometric. Cleavage in large individuals, (100), distinct. Not seen in thin sections. Habit: Octahedrons, (111); sharp, sometimes rounded cubes. Grayish white, violet-gray, brownish to red-brown, rarely green- ish gray. Index: n„o = 2.38. Anomalous double refraction in large individuals; often not seen under the microscope. g Other anistropic minerals show higher double refraction, "^ interference figures, etc. "§ Eudialyte.— Ca, Fe, Mn, Zr, Na sihcate, H = 5.5-6.5, G = 2.92. Hexagonal. (. + ). Cleavage: (0001) distinct. Habit: Rhombs, thick plates. Elongation: 0001), short lath-shaped. ( — ). Reddish, yellowish, brownish, colorless. Pleochroism: 0> E, weak or wanting. w = 1.606-1.611, £ = 1.610-1.613. £-£0 = 0.002. Often anomalous biaxial character, 2E up to 50°. Gelatinizes with HCl, which, when diluted, gives reaction for Zr. Garnets have much higher indices and are isotropic unless with anomalous double refraction. Tourmaline has stronger double refraction. Eucolite is optically ( — ), elongation is (-I-), and indices and double refraction are higher. Quartz.— SiO^, H = 7.0, G = 2.65. Hexagonal. ( + ). Cleavage: Irregular cracks. ANISOTROPIC. 195 Habit: Dihexahedrons, grains; irregular, allotriomorphic. Colorless. Pleochroism: None. «« = 1.544, £=1.553. £-tu = 0.009. 2£ = 12°-18°, rarely to 24°. Extinction often undulatory; optical anomalies common. Unaffected by acids. Other minerals: The low indices and double refraction of quartz combined with its lack of color, lack of cleavage, freshness, insolubility in acids, etc. separate it from most of the other minerals. Cordierite is biaxial, ( — ), and when treated with HF gives characteristic prismatic crystals of magnesium fluosili- cate. o The mineral is anisotropic. Parallel extinction. Colorless. Index of refraction is greater than that of Canada balsam. Maximum birefringence is less than that of quartz. Biaxial. s o < X < 197 198 ANISOTROPIC. The Mineral is ANISOTROPIC, ha* PARALLEL EXTINCTION, U COLORLESS. INDEX OF REFRACTION The Mineral it NEGATIVE {-). The Miaeral U POSITIVE (+j. ■ — IncreaBe in Index of Reaction. - ooootn o >no(ncoi> i« iS s s In- S g 3 g RgggS NM^^r* - « - ^ oreasg BIREFR. •-■•-•«-■ M ,-i«p«NN VeryH. High. Msdlum. Nol Mvked. Not Marked. Medium. High. VerrH. ' .001 .002 ' . .003 .004 . - Oligoc lase. .006 .007 .008 ■ Andesine. - . — CUnozoisite. ordierite. - — .009 , Bronzite. .010 — Topaz. Andalusite. ^ 1 ANISOTROPIC 199 Lepidolite.— AKSiOJjAljKLiH + Al(Si308)3K3Li3(AlF J3 , H = 2.5-4.0, G = 2.8-2.9. Monoclinio. ( — ). Cleavage: (001), good. Habit: Leaves, leafy aggregates. Elongation: At right angles to (001), lath-shaped. ( + ) Orientation : 6 = c, c : = + 0° to + 2° or rarely 6 = b,c:o=+0°to +2°. Colorless, reddish. Pleochroism: Weak or none. a = ( ), /?=1.598, r=.l-605. r-/3 = 0.007. Dispersioi;: Weak, p^u. 2E = 32°-84°. One of the mica group. Gives Li flame in BB. Attacked but not completely decom- posed by acids. After fusion gelatinizes with acid. Muscovite and paragonite do not give Li flame. Oligoclase.— AbeAn, to Ab^An,, H = 6.0-6.5, = 2.64-2.66. Triclinic. ( — ). Cleavage: (001), (010), good. (001): (010) = 86° 32'. Habit: Plates, prisms, grains. Elongation: (001): (010), (110): (110). Orientation: ^|,oi= + l°i ^010= +4-5°. Colorless. Pleochroism: Wanting. a: = 1.540, /3 = 1.544, 7- = l-547. r-a: = 0.007, 7-/5=0.003, /?-a = 0.004 Dispersion: p> u. 2F = 90°-98°. Insoluble in acids. Other feldspars. See Part II. Cordierite.— Mg^AliSisO.i, H = 7.0-7.5, = 2.59-2.66. Orthorhombic. ( — ). Cleavage: (010), poor. Habit: Grains, short prisms. Trillings and poly synthetic twins occur. Elongation: (110), (ITO), short laths. (-). Orientation: t'=c, c=a. Colorless, bluish, yellowish. Pleochroism : Wanting or weak with B > t> a = 1.535, /?=1.540, r = l-544. r-a = 0.009, r-j8=0.004, j8-a = 0.005 Dispersion: Weak, p u. 2F = 0°-60°. Insoluble in acids. Abnormal interference colors. ANISOTROPIC. 207 Pistacite, with weak double refraction, is similar when the axial plane is at right angles to the cleavage. Vesuvianite has poor cleavage. Melilite gelatinizes with acids. Topaz.— Al2Si04(F,OH)2, H = 8.0, = 3.533-3.574. Orthorhombic. ( + ). Cleavage: (001), good. Habit: Short prisms, grains, rods. Elongation: (001). (-).' Orientation: a=a, c=c. Colorless. Pleochroism : None in thin sections, a = 1.619, /3= 1.622, r = 1-628. r-ff=o.oio, r-/S=o.oo6, ^-a:=o.oo3. Dispersion: p> u. 2E = 7r-129°. Occurs in schists, gneisses, and in cavities of volcanic rocks. Insoluble in acids. Quartz has lower indices and no cleavage, basal sections give uniaxial interference figures. Andalusite is ( — ), has good (110) cleavage, and different si orientation. o Sillimanite and prismatine have different orientation. i^ The mineral is anisotropic. Parallel extinction. Colorless. Index of refraction is greater than that of Canada balsam. Maximum birefringence is greater than that of quartz. w o W o w 04 209 The mineral is anisotropic. < < t3 Parallel extinction. Colorless. Index of refraction is greater than that of Canada balsam. Maximum birefringence is greater than that of quartz. Uniaxial. (^.4 V-C^f /^. 1^^7 211 212 ANISOTROPIC. The Mineral it ANISOTROPIC, haa PARALLEL EXTINCTION, ■■ COLORLESS, INDEX OF The Mineral it NEGATIVE (-). The Mineral i» POSITIVE (+). < Inereaae in Index of Refraction. ooooio o in o in inooMts. t* to « m In-, S S g g Rgggg c^e^^l-lm^ ,1 _ ,« ^ eiREFR ^ ^ »- ^ i^mmNN VenH. High. Medium. Not Mirked. Not Marked. Medium. m- VenH. 1 • Corun9um. 010 Quartz. Dipyr. . . Mosandrite. ■ • Johns rupite. Mi zonite.— ■ 015 .020 — Br ^lunite.- jcite. .025 .030 .040 .045 .050 ,eucoxene. - OSS Zircoi . ■• .065 .070 • Ana tase. .075 .080 .085 .090 .095 .100 .120 .140 .160 .200 .250 Ma -alcUe.. Hussak 'Cassiter te. ;pe^.— - Siderite. ANISOTROPIC. 213 Corundum.— AI,03, H = 9.0, G = 3.9-4. 10. Hexagonal. ( — ). Cleavage: A poor parting, (1011), (0001). Habit: Plates, grains, prisms. Twins (lOTl) occur. Elongation: (0001), lath-shaped. ( + ). Colorless, blue, light red. Pleochroism: = blue, red; E = sea-green, yellow, or greenish yellow. Only seen in deeply colored corundum. w = 1.769, E = 1.760. w- £ = 0.009. Insoluble in acids or fused soda. Other minerals which resemble it are separated by the high indices, negative character, uniaxial figure, etc., of corundum. Dipyr. — Aluminium, sodium, calcium silicate, H = 5.0-6.0, G=±2.6. Tetragonal. ( — ). Cleavage: (100), good; (110), poor. Habit: Rods, prisms, grains. Elongation: (110"): (110), lath-shaped. ( — ). Colorless. ;:? Pleochroism: None. co = 1.555, £=1.542. 6;- £ = 0.013. Optical anomalies showing opening of interference cross, rare. ^ Dipyr occurs in contact metamorphosed limestones and schists. It is a scapolite. Feldspar, cordierite, and zoisite have lower double refraction, are biaxial, and have different cleavage. Quartz is (-1- ), has no cleavage, and has lower double refrac- tion. Prehenite and light-colored micas with apparent uniaxial character are characterized by their cleavage. Andalusite has lower double refraction and is biaxial. Mizzonite. — Like dipyr chemically, etc. Tetragonal. ( — ). Cleavage: (100), good; (110), poor. Habit: Rods, prisms, grains. Elongation: (110):(lT0), lath-shaped. (-). Colorless. Pleochroism: None. w = 1.558, £ = 1.543. w- £ = 0.015. Optical anomalies rare, showing an opening of the interfer- ence cross. It is never primary in eruptive rocks, but occurs e ANISOTROPIC. 215 as an alteration product of feldspars. Occurs in crystalline schists and gneisses. Other, minerals. See Dipyr above. This is also a scapo- lite. Meionite. — Like dipyr chemically, etc., G = 2.735. Tetragonal. ( — ). Cleavage: (100), good; (110), poor. Habit: Rods, prisms, grains. Elongation: (110):(1T0), lath-shaped. (-). Colorless. Pleochroism: Wanting. a) = 1.595, £ = 1.560. oi— £ = 0.035. Varies, sometimes 0.019. Optical anomalies rare, showing opening of interference cross. Never occurs primary in eruptive rocks, but as an alteration product of feldspars. Occurs in crystalline schists and gneisses. Other minerals. See Dipyr above. This is also a seapo- lite. Phlogopite.—Fe poor mica, H = 2.5-3.0, G = 2.85. Monoclinic. ( — ). "^ Cleavage: (001), good. ^ Habit: Leaves, leafy aggregates. ^ Elongation: Right angles to (001), lath-shaped. ( + ). Orientation: b = 6, c:o = 0° to +7°. Colorless, yellowish, brownish, greenish. Pleochroism: Weak with t>S>o. a: = 1.562, /3=1.606, r=l-606. r-a = 0M4, r-^ = 0.0, ^-a: = 0.044. Dispersion: Weak with pE. w = 1.658, £ = 1.486. w- £ = 0.172. g Often incloses rhombs of dolomite and magnesite. Readily "^ soluble in cold HCl. "3 Aragonite is biaxial and has different cleavage. See also ^ special tests in Part II. Dolomite twinning (0221). Special tests, see Part II. Dolomite.— MgCaCjOs, H = 3.5-4.0, = 2.85-2.95. Hexagonal. ( — ). Cleavage: (1011), good. (1011) :(T101) = 73° 45'. Habit: Rhombohedrons, granular aggregates. Twins occasion- ally occur, (0221). Colorless, gray, yellowish, brownish. Absorption : 0> E, strong. w = 1.682, £ = 1.503. o)- £ = 0.179. Soluble with difficulty in cold acid; easily in hot. Calcite has lower indices, is soluble in acetic acid, and has (01T2), twinning. Chemical differences between calcite, t dolomite, and aragonite, see Part II. Magnesite.— MgCOs, H = 3.5-4.5, = 2.9-3.1. Hexagonal. ( — ). Cleavage: (1011), good. (10Tl):(T101) = 72° 40'. Habit: Rhombohedrons, granular aggregates. ANISOTROPIC. 219 Colorless, yellowish, grayish, brownish. Absorption : 0> E. ftJii(i = 1.717, £na=1.515. w- £ = 0.202. ' Cold HCl does not affect magnesite. Dolomite and calcite are acted upon more readily by acids. Siderite.— FeCOj, H = 3.S-4.0, = 3.936-3.938. Hexagonal. ( — ). Cleavage: (lOTl), good. (10T1):(T101) = 73°. Habit: Rhombohedrons, granular aggregates. Colorless, yellowish,' brownish. Absorption: Strong 0>E. w = 1.872, £ = 1.634. w- £ = 0.238. Soluble in HCl. Other carbonates separated by chemical means. o 8 ANISOTROPIC. 221 Quartz.— SiO^; H = 7.0, G = 2.65. Hexagonal. ( + )• Cleavage: Irregular cracks. Habit: Dihexahedrons, grains. Colorless. Pleochroism: None. M = 1.544, £ = 1.553. £-w = 0.009. 2B = 12°-18°, rarely to 24°. Extinction often undulatory; optical anomalies common. Unaffected by acids. Other minerals: The low indices and double refraction of quartz combined with its lack of color, lack of cleavage, freshness, insolubility in acids, etc., separate it from most of the other minerals. Cordierite is biaxial, ( — ), and when treated with HF gives characteristic prismatic crystals of magnesium fluosihcate. Mosandrite. ( Ti, Zr, Th, Ce, Y, Al, Fe, Mn, Ca, Mg, Na, K, F silicate, Johnstrupite. ) H = 4.0 and more, = 3.10-3.29 (Johnst.), 2.93-3.07. (Mosand.) Monochnic. ( + ). Cleavage: (100), good. g Habit: Tablets, (100), elongated on c. Twinning (100) frequent. Elongation: (100): (010), lath-shaped. (-). S Orientation: 6 = 6, c;n = 3°. Grayish yellow to colorless. Pleochroism: Not seen in thin sections. « = 1.646, ^=1.649, r = l-658. r-a = 0.0l2, r-/5 = O.O09, ^-a = 0.003. Dispersion: Strong, p> u. 2V = 7CP. 2£;„a=128°37' Soluble in HCl with separation of SiOjl the dark-red solution becomes yellowish on heating and gives off CI. Rinkite has strong pidolite. .040 • Pecto ite. . . ... ._ PWog ipite. .045 dritc. tfonazitr. L OSS 060 • .070 .075 .080 " .085 .090 .095 . 00 . 20 . 40 . 60 . 00 .250 ' Ti^nit * ANISOTROPIC. 229 Cordierlte.— Mg^Al^SisOis, H = 7.0-7.5, = 2.59-2.66. Orthorhombic. ( — )• Cleavage: (010), poor. Habit: Grains, short prisms. Trillings and polysynthetic twins occur. Elongation: (110) :(1T0), short laths. (-). Orientation: 6=t, c=n. Colorless, bluish, yellowish. Pleochroism : Wanting or weak with B > t > a. a = 1.535, /3=1.540, r=l-544. r-Q: = 0.009, r-^ = 0.004, /?-a = 0.005. Dispersion: Weak with pKu. 2£; = 63°-150°. When treated with HF gives characteristic prismatic crystals of magnesium fluosilicate. Quartz has higher interference colors, is uniaxial, and ( + ) . Chemically different, reactions as above. Dumortierite.— 3(Al8Si,0,8VAlB30„-2HA H = 7.0, = 3.22-3.36. Orthorhombic. ( — ). Cleavage: (100), good. Habit: Prisms, needles. Elongation: (110): (ITO), long lath-shaped. (-). Orientation: a=t, c=a. "3 Blue, brownish, greenish, colorless. ^ Pleochroism: a = blue; 6 = yellowish, reddish violet, greenish; t = colorless. Strong. q: = 1.678, /3=1.686, 7- = 1-689. J— q: = 0.011, r-/5=0.003, /9-a = 0.008. Dispersion : p > u. 2F = 30°. Insoluble in acids, even HF. Blue amphibole is monochnic. Serendibite is triclinic, extinction 40° ca Sapphirine is monoclinic with c : c = 8°-9° Grandidierite has different habit, a=a, 6 = c, and strong blue-green to colorless pleochroism. "* Andalusite.— AljSiO^, H = 7.5, = 3.10-3.20. Orthorhombic. ( — ) . Cleavage: (110), good. (110): (ITO) = 89° Habit: Prisms, grains. Elongation: (110) :(1T0), short laths. (-). Orientation: a=(, c=a. Colorless, reddish. Pleochroism : a = rose, B = ( = colorless to light green. ANISOTROPIC. 231 01 = 1.632, ^=1.638, }-= 1-643. r- a = 0.011, r-/5 = 0.005, ^- a = 0.006. 2F = 84°-8S°- Not attacked by HF or other acids. Sillimanite is ( + ), has basal cleavage, and has higher double refraction. Topaz has perfect basal cleavage, is ( + ). Scapolites have higher double refraction in prisms, and are uniaxial. Zoisite has (010) and (100) cleavage, and is ( + ); 2F = 0°-60°. Thulite is ( + ); 2F = 0°-40°. Orthoclase has lower indices of refraction. Prismatine has 2E=e5.5°. Cornerupine has 2E = 14°-32°. Antigorite.— H4(Mg,Fe)3SiA. H = 2.5, G = 2.622. Orthorhombic. ( — ) . Habit: Lamellae, leaves, scales. Elongation: At right angles to (100), lath-shaped. ( + ). Orientation: u=o, b=t. Greenish, colorless, yellowish. Non-pleochroic. § a: = l.S60, /3=1.570, r=l-571. r-a=o.oii, r-/5=o.ooi, /9-a=o.oio. cS Dispersion: p>u. 2£=16°-98°. In boiling HCl silica separates. Pennine is pleochroic, the double refraction is lower and the indices are higher. Ordinary serpentine is ( + ). Bastite. — Serpentine pseudomorph after an orthorhombic pyroxene, H = 3.5-4.0, G = 2.6-2.8. Orthorhombic. ( — ). Cleavage: (100), good. Habit: Leaves. Elongation: (110), (ITO), lath-shaped. (-I-). Orientation: a=o, c=c. Greenish, yellowish, colorless. Pleochroism: Weak or wanting. Index: 1.5-1.6. Birefringence: Weak. Dispersion: p>u. 2.E = 20°-90°. Gelatinizes more or less easily with acids. Other pyroxenes differ in orientation. See Part II. ANISOTROPIC. 233 Hypersthene.— (Fe, Mg)SiO, , H = 5.0-6.0, G = 3.40-3.50. Orthorhombic. ( — ) . Cleavage: (010), distinct ; (100), good; (110), good. (110):(ll0) = 91° 40'. Habit: Prismatic, often massive. Elongation: (110):(lI0), lath-shaped. ( + ). Orientation : b= a, c = c. Colorless, yello-nish, reddish. Pleochroi.sm : c = greenish, n = reddish yellow, 6 = reddish brown. Strong. a = 1.692, /?=1.702, ;- = 1.705. r-a = 0.013, r-/?=0.003, /?-q: = 0.010. Dispersion : p> u. 2E = 85° ca. Partially decomposed by HCl. Bronzite is ( + ) and has weaker pleochroism. Monoclinic pyroxenes have inclined extinction and higher double refraction. Basal sections showing cleavage give the emergence of an axis, while orthorhombic pyroxenes show the emergence of a positive bisectrix. See Part II also. .Slgirite has different pleochroism, greens to yellowish greens, higher double refraction, and has ( — ) elongation. Cornerupine.— MgAljSiOo ca., H = 6.5, G = 3.273. Orthorhombic. ( — ). Cleavage: (110) distinct. (110):(110) = 81°- Habit: Prismatic. Elongation: (110): (110), lath-shaped. Orientation : 6 = c, c= a. Colorless in transmitted light. Non-pleochroic. a = 1.669, /?= 1.681, r = l-682. r-a = 0.013, r-^-O.OOl, ^-q: = 0.012. Dispersion: Not apparent. 2^ = 32° 30'. (Also 26°, 19°, 14°.) Insoluble in acids. Prismatine is yellowish, slightly pleochroic, G = 3.341, and 2E = 65.5°; otherwise they are alike. Sillimanite has good (100) cleavage and higher double refraction. Topaz has good basal cleavage. Andalusite generally has larger value for 2E. ANISOTROPIC. 235 Monticellite.— MgCaSiOj , H = 5.0-6.0 = 3.12-3 275. Orthorhombic. ( — ) . Habit: Isometric, grains. Orientation : a = t, b=a. Colorless. a = 1.651, ^=1.662, 7-=l-668. r-a = 0.017, r- ,8 = 0.006, ^-a=0.011. Dispersion: Distinct, p> u. 2)/ = 37° 31'. Soluble in HCl. A member of the olivine group. Olivine and forsterite have much larger axial angles, are ( + ), and have higher birefringence. Fayalite has 2F = 50° ca., is usually colored, has higher birefringence and higher indices of refraction. Gedrite.— (Mg, Fe)Si03 and (Mg, Fe)ALSiOe, H = 5.5-6.0, = 3.1-3.2. Orthorhombic. ( — ). Cleavage: (110), good; (100), distinct; (010), poor. (110) : (110) = 55° 12' Habit: Rods, leaves, grains. Elongation: (110):(lT0), lath-shaped. ( + ). Orientation : a= a, c = c. Colorless, yellowish, brownish, greenish. Pleochroism: c = yellowish, brownish; B = clove-brown, reddish; (t = yellowish, greenish, colorless. a= 1.623, /?= 1.636, r= 1-644. J— a = 0.021, 7— /? = 0.008, /3-q: = 0.013. Dispersion : p>u. 2F = 79°-57° Not affected by acids. A member of the orthorhombic amphibole group. Other amphiboles. See Part II. Pistacite.— (Epidote), Al(SiO,)3(Al,Fe),Ca-CaOH, H = 6.5, = 3.3-3.5. Monoclinic. ( — ). Cleavage: (001), good; (100), distinct. (001) : (100) = 64° 37'- Habit: Prisms, and rods elongated along 6, grains; twinning (100). Elongation: (001 ): (100), lath-shaped. (±). Orientation : 6 = 1), c:a=4-3°- Green, yellow, brownish, colorless. Pleochroism: (I = colorless to yellowish or greenish; 6 = yellowish to yellowish gray, lavender; t = green, light yellowish brown. 01 = 1.714, /9=1.741, r=l-746. r-a = 0.032, r-|3 = 0.005, j8-a = 0.027. Dispersion: p>ij. 2F = 74°-90°- ANISOTROPIC. 237 Not affected by HCl. Pistacite is the Fe-rich epidote; clinozoisite, the Fe-poor or Fe-free. Pyroxene has (110) cleavage; in sections parallel to the long direction the plane of the optic axes is parallel to the cleavage; in epidote at right angles to it. Clinozoisite has lower double refraction, is ( + ), and has strong p u. 2E = 70° ca. Insoluble in acids. One of the mica group. Muscovite and lepidolite are different chemically. Talc has 2.B = 6°-20°. ANISOTROPIC. 239 Muscovite.— AlCSiOJsKH^Al,, H = 2.0-2.5, = 2.8-2.9. Monoclinic. ( — ). Cleavage: (001), good. Habit: Leaves, leafy aggregates. Elonga,tion: Perpendicular to (001), lath-shaped. ( + ). Orientation: 6=c, f:n=+0°to +2°. Colorless, greenish, yellowish. Pleochroism: Weak or wanting. a: = 1.563, ^=1.598, r = l-601. 7--q: = 0.038, 7-/3 = 0.003, /3-q: = 0.035. Dispersion: Weak, p> u. 2£;=60°-70°. Insoluble in HCl and HjSO^. One of the Mica group. Micas can only be confused with other minerals that have complete basal cleavage. The very low birefringence in basal sections and high birefringence in sections show- ing cleavage is very characteristic of the micas. Chlorites have weak double refraction. Talc has 2£; = 6°-20°. Biotite has 2E generally much smaller, also h = 'b. Lepidolite and paragonite may be separated by flame test for Li and treatment with HF, by which muscovite will give many crystals of KjSiFj and few NaSiFj. See also § Part II. Lepidolite.— AKSiOJjAl^KLiH -I- Al(Si30,)3K3Li3(AlF,)3 , H = 2.5-4.0. O G = 2.8-2.9. Monoclinic. ( — ). Cleavage: (001), good. Habit: Leaves, leafy aggregates. Elongation: At right angles to (001), lath-shaped. (-1-).. Orientation: b= r, c: o= -1-0° to -1-2°; or rarely t = b, c: o= -1-0° to -1-2°. Colorless, reddish. Pleochroism: Weak or none /?= 1.598, r = 1-605. r-/3 = 0.007. Dispersion: Weak, fi^u. 2E=32°-84°. Insoluble in acids. Muscovite and paragonite do not give Li flame. See also under muscovite. Datoiite.— HCaBSiOj, H = 5.0-5.5, G = 2.9-3.0. Monoclinic. ( — ). No distinct cleavage. Habit: Grains, rods. Orientation: 6 = b, c:o=-l-l° to H- '^ ANISOTROPIC. 241 Colorless. a = 1.625, /3 = 1.653, }-=l-669. r-a=O.OU, r-/? = 0.016, /9-a = 0.028. Dispersion: Weak, p>u. 2r = 74° Gelatinizes with HCl. Gives reaction for boron. Pistacite has cleavage, no B reaction, and has lower bire- fringence. Phlogopite. — A magnesium mica, near biotite, but containing little Fe, H = 2.5-3.0, = 2.78-2.85. Monoclinic. ( — ). Cleavage: (001), good. Habit: Leaves, leafy aggregates. Elongation: At right angles to (001), lath-shaped. (-1-). Orientation: b = i, c -.0 = 0° to +7° Colorless, yellowish, brownish, greenish. Pleochroism : Weak, with c > b > c. « = 1.562, /?= 1.606, r= 1-606. r-a = 0.044, r-/5 = 0.0, /?-a = 0.044. Dispersion: Weak, p u. 2F = 50° ca. An iron rich olivine. Gelatinizes quite easily with HCl. Olivine has 2F = 88°, is (-I-), and has lower birefringence. Forsterite is (-I-), has 2F = 86°, lower indices, and different occurrence. Monticellite has 2F = 37.5°, has lower indices and lower double refraction. See also Part II, olivine group. ANISOTROPIC. 243 Talc— H,Mg3Sip,2, H = 1.0, G = 2.7-2.8. Orthorhombic. ( — ). Cleavage: (001), good. Habit: Leaves, scales. Elongation: At right angles to (001), lath-shaped. ( + "). Orientation: b=c, c=a. Colorless, greenish, bluish. Non-pleochroic. a: = 1.539, /?=1.589, }- = l-589. r- a =0.050, r-/5=o.ooo, ^-0=0.050. Dispersion: Distinct, p> u. 2E = 6°-20°. Hardly affected by acids. Brucite is ( + ), uniaxial, and has lower double refraction. Muscovite cannot be separated optically when it has a small value for 2E; it must be separated by testing for the alkalies or with cobalt solution. Aiagonite.— CaCOs, H = 3.5-4.0, G =2.93-2.95. Orthorhombic. ( — ). Cleavage: Not recognizable under the microscope. Habit: Rods, grains. Elongation: (110):(lT0), lath-shaped. (-). Orientation : 6 = c, c= a. Colorless. Co Non-pleochroic. a = 1.530, /3= 1.682, 7- = 1.686. r-a = 0.156, r-/5 = 0.004, /3-a = 0.152. Dispersion: Weak, pij. 2£; = 71°-129°. Occurs in schists, gneisses, and in cavities of volcanic rocks. Insoluble in acids. Quartz has lower indices and no cleavage, basal sections give uniaxial interference figures. Andalusite is ( — ), has good (110) cleavage, and different orientation. Sillimanite and prismatine have different orientation. Enstatite.— (Mg,Fe)2Si20o, H = 5.0-6.0, G = 3.1. Orthorhombic. (+). Cleavage: (110), good; (010), distinct; (100), good. (110):(ll0) = 91° 40'. , Habit: Prismatic, cloudy masses. Elongation: (110):(lI0), lath-shaped. ( + ). Orientation : 6=0, c = t. Colorless. Pleochroism: Wanting. a: = 1.656, /?=1.659, r = l-665. }-- a = 0.009, r-/? = 0.006, /3-q: = 0.003. Dispersion: p u. 2F = 0°-60°. Insoli'ble in acids. Abnv-x-mal interference colors. Pistacite, with weak double refraction, is similar when the axial plane is at right angles to the cleavage. Vesuvianite has poor cleavage. Melilite gelatinizes with acids. Mosandrite. ) Ti, Zr, Th, Ce, Y, Al, Fe, Mn, Ca, Mg, Na, K, F silicate, Johnstrupite. ) H = 4.0 and more, = 3.10-3.29 (Johnst.), 2.93-3.07 (Mosan). Monoclinic. ( + ). Cleavage: (100), good. Habit: Tablets (100), elongated on t. Twinning (100) fre- quent. Elongation: (100) : (010), lath-shaped. (-). Orientation. 6 = 6, c:n = 3°. Grayish yellow to colorless. ANISOTROPIC. 249 Pleochroism: Not seen in thin sections. a = 1.646, /3= 1.649, r = 1-658. r-a = 0.012, )--/5 = 0.009, /3-a = 0.003. Dispersion: Strong, |0>o. 2F = 70° 2E„a = 128°37'- Soluble in HCl with separation of iiiO.y, the dark-red solution on heating gives off CI and becomes yellowish. Einkite has strong p u. 2F = 31°-42°. Insoluble in acids. Andalusite is ( — ) and has ( — ) elongation, double refrac- tion is lower, and relation of axial plane to cleavage is different. Scapolites are ( — ), have ( — ) elongation, and are uniaxial. Zoisite has weaker double refraction and different orien- tation. ANISOTROPIC. . 251 Anthophyllite.— (Mg,Fe)Si03, H = 5.5-6.0, = 3.1-3.2. Orthorhombic. (±"t. Cleavage: (110), good; (100), distinct; (010), poor. (110):(1I0) = 54° 23'. Habit; Rods, leaves, grains. Elongation: (110) :(1T0), lath-shaped. ( + ). Orientation: a=a, c=t. Colorless, yellowish, brownish, greenish. Pleochroism: c = yellowish, brownish; S = clove-brown, reddish; = yellowish, greenish, colorless. a = 1.633, /?=1.642, r=l-657. r-Qi = 0.024, 7--/5 = 0.015, .a-a = 0.009. Dispersion: p u. 2F^90° Not noticeably affected by acids. A member of the amphibole group. Amphiboles and pyroxenes, see Part II. Manganese rich Piedmontite. See piedmontite above. Prehnite.— H^Ca-Al^SijO,,, H = 6.0-6.5, G = 2.80-2.95. Orthorhombic. ( -I- ) . Cleavage: (001), good. Habit: Tablets, leafy aggregates. Elongation: (001), lath-shaped. ( — ). Orientation : a=a, c = c. Colorless. Non-pleochroic. a: = 1.616, ^ = 1.626, j- = l-649. 7— q: = 0.033, r-/?=0.023, /3-a: = 0.010. Dispersion: Weak, p^u. 2F = 69°. Optical anomalies not rare. Attacked with difficulty by acids. Thomsonite has lower indices and is more easily attacked by acids. Andalusite, topaz, cordierite and zoisite have lower double refraction. Datolite gives reaction for B., and has higher double refraction. Forsterite.— Mg^SiO, , H = 7, = 3.21-3.32. Orthorhombic . ( + ) . Cleavage: (010), (001), distinct. Habit: Isometric, short prisms, grains. Orientation: a=t, b=tt. Colorless. Index: ^=1.659. Birefringence: High. ANISOTROTIC. 253 Disperaioh: Weak, p u, and different occurrence. Piedmontite has characteristic pleochroism. Humite.— Mg5[Mg(F,OH)],(SiOj3, H = 6.5, G = 3.1-3. Orthorhombic. ( + ) . Cleavage: (001), distinct. Habit: Grains. Twins. Often lamellar. Orientat on: a=o, 6=t. Colorless, yellow, yellowish brown, reddish. Pleochroism: Weak in yellow and colorless tones. ^=1.643. r = 0.035. Disperson: Weak, |0>u. 2F = 68° Gelatinizes quite easily with HCl. Occurs as a not especially common constituent of crystalline dolomites and limestones. Olivine resembles colorless humite, but has the plane of the optic axes at right angles to the cleavage, while in humite it is parallel to the cleavage. Olivine.— (Mg,Fe)Si04, H = 6.5-7.0, = 3.27-3.45. Orthorhombic. ( + ) . Cleavage: (010), (001), distinct; (100), poor. Generally shows heavy, irregular cracks. Habit: Isometric, short prisms, grains. Twins (Oil) and (012), rare. Orientation : a = t, & = a. Greenish, yellowish, reddish, colorless. Pleochroism: Wanting or weak in yeUow and red tones. a = 1.654, /?=1.670, r=l-689. r- a = 0.035, r- 18= 0-019, j8- a =0.016. Dispersion: Distinct, p u, contains no P., extinction 4-39°, weak pleochroism, and much higher birefringence. ANISOTROPIC. 257 Titanite.— CaSiTiOa , H = 5.0-5.5, = 3.4-3.56. Monoclinic. ( + ). Cleavage: (110), distinct. (110):(1I0) = 46° 8'. Habit: Prisms, rhombs, grains, and rods. Orientation: b-i, c:c=+39°- Colorless, yellowish, reddish, brownish. Pleochroism: Weak, c>6>n. q: = 1.913, ^=1.921, r = 2.054. r- (1 = 0.141, 7—^ = 0.133, /3-a = 0.008 Dispersion : Very strong, p> u. 2£'„a = 45°-68°. Very slightly affected by HCl. Monazite has lower birefringence, weak dispersion, and low extinction. Brookite has parallel extinction, 2F = 0°-23°, and great difference in birefringence in two directions; )-— « = 0.158, while/?- a = 0.003. Other minerals: The very strong double refraction and indices, and the very strong dispersion, separate titanite from other minerals. Rutile, xenotime, hussakite, and cassiterlte give no reaction for Ca. o Brookite.— TiO., H = 5.5-6.0, = 3.87-4.01. Orthorhombic. ( + ). (2 Cleavage: (010), good, though not always seen microscopically. Habit: Tabular, plates. Elongation: (100), narrow laths. (T). Orientation: a=c, 6=0, for red; a=t, c=o, for green. Axial p ane for red and yellow lies in (001), for green and blue in (010). Q! = 2.583, ^=2.586, r = 2.741. r-a=0.158, )--/5 = 0.156, /3-a: = 0.003. Dispersion : p> u. 2F = 0°-23°. Insoluble in acids. Gives Ti reaction. Cassiterlte and rutile have different habit, and brookite has very different strength of double refraction in (100) and (010) sections. w o The mineral is anisotropic. " Parallel extinction. Colored. o 259 o W o w h-l The mineral is anisotropic. ^ Parallel extinction. a Colored. Non-pleochroic. Cj 261 N H Pi w u The mineral is anisotropic. g Parallel extinction. S Colored. w Non-pleochroic. M Maximum birefringence is less than that of quartz. ^-L/^^-^Lo tiC^K, ft4.an^^,-p, ti^i L ^ o 263 The mineral is anisotropic. E, rarely 00, in colored crystals. w = 1.638, £ = 1.634. M- £ = 0.004. Anomalous biaxial character in large individuals only. Easily soluble in HjSO,, yellow precipitate with ammonium molybdate. Easily soluble in HNO . Vesuvianite, melilite, gehlenite, zoisite, and tourmaline are not soluble or have different crystal form or color. Tourmaline has 0>E, while colored apatite has E>0. Melilite.— Ca,Mg,Fe,Al siljcate, H = 5.0, G = 2.9-3.1. Tetragonal. ' (T). Cleavage: (001): (110), poor; only basal cleavage generally seen in thin sections, and this occurs as a single cleft exactly in the center of the lath-shaped section. Habit: Plates, short prisms. Elongation: (001), laths. (±). Colorless, yellowish. Pleochroism: None when colorless; for yellow mehlite, E = da.Tk yellow, = light yellow. ANISOTROPIC. 269 w = 1.634, £=1.629. tti— £ = 0.005; often unusual indigo-blue interference colors. Gelatinizes easily with HCl. "Peg" structure and central cleav- age cracit are characteristic. Vesuvlanite and zoisite are insoluble in acids. Gehlenite has not the unusual interference colors. Vesuvlanite.— Al2(Si04),Ca„(A10H), H = 6.5, = 3.35-3.47. Tetragonal. ( — , rarely -I-). Cleavage: (110), (100), poor. Habit: Short prisms, grains, dull masses Elongation: (110):(lT0), prismatic. (T). Colorless, yellowish, greenish, reddish, brownish, blue. Pleochroism: Weak to unnoticeable. w = 1.705-1.732, £ = 1.701-1.726. w— £ = 0.006-0.001, often abnormal interference colors. Anoma- lous biaxial character. Insoluble in acids unless fused to glass. Zoisite has the high indices and weak double refraction found in vesuvianite. The optic angle of zoisite often falls as low as 0°, and vesuvianite has often anomalous double refraction with very variable axial angle. Zoisite, however, has very good pinacoidal cleavage. § Gehlenite gelatinizes easily with HCl. Andalusite always has a larger axial angle. ANISOTROPIC. 271 Eudialyte.—Ca, Fe, Mn, Zr, Na silicate, H = 5.5-6.5, G = 2.92. Hexagonal. ( + ). Cleavage: (0001), distinct. Habit: Rhombs, thick plates. Elongation; (0001), short lath-shaped. ( — ). Reddish, yellowish, brownish, colorless. Pleochroism : 0> E, weak or wanting. (0 = 1.606-1.611, e = 1.610-1.613. £-w = 0.002. Often anomalous biaxial character, 2E up to 50° Gelatinizes with HCl, which, when diluted, gives reaction for Zr. Garnets have much higher indices and are isotropic unless anomalous. Tourmaline has stronger double refraction. Eucolite is optically ( — ), elongation is ( + ), and indices and double refraction are both higher. Garnets. — Isometric. Belong here only on account of the anomalous double refraction. The crystal form and high indices of refraction make the dstermination easy. See garnets under isotropic minerals. The mineral is anisotropic. Parallel extinction. Colored. Non-pleochroic. Maximum birefringence is less than that of quartz. Biaxial. =3 < 273 274 ANISOTROPIC. The Mineral U ANISOTROPIC, h.. PARAliEL EXTINCTION, ii COLORED, NON-PLEOCHROIC, | MAXIMUM BIREFRINGENCE < Qu.rti. BIAXIAL The Mineral i> NEGATIVE (-). The Mineral i. POSITIVE (+). , ooooin g S g s In in o ifl o inoooo u) (0 (0 i» r-voioin NN--..,'^ - - - - crees'g BIREFR. « ^ « " r-««NN VorjH. High, Medium. Not Marked NolMirked. Medium. High. VenH. 001 - 002 ,003 " 004 005 ■ ' - - 006 007 - - 008 — Clinozoisite.-| :ordierite. — - 009 _ Zoisite._ - 010 - ANISOTROPIC. 275 LepidoUte.— Al(Si04)3Al2KLiH + Al(Si30s)3K3Li,(AlF2)3, H = 2.5-4.0, G = 2.8-2.9. Monoclinic. ( — ). Cleavage: (001), good. Habit: Leaves, leafy aggregates. Elongation: At right angles to (001), lath-shaped. ( + ). Orientation: b=i, c:a = 0° to +2°; or rarely b = B, c:a = 0° to + 2°. Colorless, reddish. Pleochroism: Weak or none. a = ( ), /?= 1.598, r= 1-605. r-/9 = 0.007. Dispersion; Weak, p^u. 2E = 32°-84°. Insoluble in acids. Muscovite and paragonite do not give Li flame. See also under muscovite and Part IL Cordierite.— Mg2Al,Si,0,8, H = 7.0-7.5, = 2.59-2.66. Orthorhombic. ( — ). Cleavage: (010), poor. Habit: Grains, short prisms. Trillings and polysynthetic twins occur. o Elongation: (110), (ITO), short laths. (-). ■§ Orientation : 6 = c, e = n. O Colorless, bluish, yellowish. Pleochroism : Wanting or weak with B > c> n. a = 1.535, /9=1.540, r=l-544. r-a = 0.009, ?--,8 = 0.004, ^-a: = 0.005. Dispersion: Weak, pu. 2y=0°-60°. Insoluble in acids. Abnormal interference colors. Pistacite, with weak double refraction, is similar when the axial plane is at right angles to the cleavage Vesuvianite has poor cleavage. Melilite gelatinizes with acids. Thulite.— Manganese zoisite, H = 6.0-6.6, G = 3.124. Orthorhombic. ( + ). Cleavage: (100), distinct. Habit: Rods, leaves. Elongation: (110) :(1I0), short lath-shaped. (±). Orientation: a=c, c=o. Red, yellow. Pleochroism: o = nearly colorless, B = rose, t = yellowish. Index: n = 1.702. Birefringence: Like zoisite, 0.009 maximum. Dispersion: p Qu«u, UNIAXIAL Tie Mineral it NEGATIVE (-), The Mineral ia POSITIVE (+). 2.50 2,00 1.90 1.80 1.75 1 70 1.65 1.60 t 55 In- crBas'g BIREFfl ui o in o ifloooe in u> (D r« r-cogioin - - - - -• -mNN VerfH. Hi h. Medium. NolMirkad. Not Marked. MBdlum. High., Ver^H. .010 • .015 .020 025 .030 035 045 .050 Lrcucoxene. Xenotim Zirco e. n. ■" 055 060 .065 .075 080 OSS 090 095 100 120 .140 .160 .200 .250 . Calcite. Hussaki ite." ' Cassite Ma jnesite.^ P Jtile.- ANISOTROPIC. 285 Melinophane.— NaCa^Be^SijOioF, H = 5.0-5.5, = 3.00-3.018. Tetragonal. .( — ). Cleavage: (001), poor. Habit: Tabular, leafy aggregates. Elongation: (001), lath-shaped. ( + ). Yellow. Pleochroism: Only in thick sections, = honey-yellow to brownish yellow; jE' = weak greenish yellow. ft) = 1.613, e = 1.593. 6;- £ = 0.020. Kormally a sharp interference figure; sometimes this separates with 2E as high as 36°. Insoluble in acids. Calcite.— CaCOj, H = 3.0, = 2.714. Hexagonal. ( — ). Cleavage: (lOIl), good. (lOTl): (Tl01) = 74° 55'. Habit: Grains, granular aggregates, threads, rounded aggregates. Twinning (0112) common. Colorless, gray, yellowish, brownish. Absorption: 0>E. •§ a; = 1.658, £ = 1.486. '^ w- £ = 0.172. o Often incloses rhombs of dolomite and magnesite. Readily "§ soluble in cold HCl. ^ Aragonite is biaxial and has different cleavage. See also special tests in Part II. Dolomite has twinning (0221). Special tests, see Part II Dolomite.— MgCaCjO,, H = 3.5-4.0, = 2.85-2.95. Hexagonal. ( — ). Cleavage: (lOTl), good. (10I1):(I101) = 73° 45'. Habit: Rhombohedrons, granular aggregates. Twins (0221) occa- sionally occur. Colorless, gray, yellowish, brownish. Absorption : 0> E, strong. w = 1.682, £ = 1.503. w-£ = 0.179. Soluble with difficulty in cold acid; easily in hot. Calcite has lower indices and is soluble in acetic acid, twin- ning (01T2). Chemical differences between calcite, dolo- mite, and aragonite, see Part II. ANISOTROPIC. 287 Magnesite.— MgCOj, H = 3.5-4.5, 0=2.9-3.1. Hexagonal. ( — ). Cleavage: (1011), good. (1011): (1101) = 72° 40'. Habit : Rhombbhedrons, granular aggregates. Colorless, yellowish, grayish, brownish. Absorption: 0>E. Wn(i=1.717, £Ka=1.515. w- 6 = 0.202. Cold HCl does not act upon magnesite. Dolomite and calcite are acted upon more readily by acids. See also special tests in Part II. a ANISOTROPIC. 289 Leucozene. — An alteration product of ilmenite. Habit: Granular; fibrous at right angles to the ilmenite crystal. Yellowish to nearly opaque. By incident hght it is white, yel- lowish, or brownish. Index: High. Birefringence: Strong, when transparent enough to be observed. Other minerals differ in the mode of occurrence, leucoxene being associated with ilmenite. Xenotime.— YPO^, H = 4.0-5.0, = 4.45-4.59. Tetragonal. ( + ). Cleavage: (110) good. Habit: Pyramids, prisms. Elongation: (110):(lT0), prismatic. ( + ). Colorless, yellowish, light brown. w = high, e = high, but lower than zircon. Birefringence: High. Optical properties very similar to zircon. H, enotime, which is sulphur free, may be decomposed hussakite, which name would then be unnecessary, since xenotime has priority. Monazite is biaxial. Titanite is harder, has different chemical composition, and is biaxial. g Anatase is ( — ), is harder, and is different chemically. Cassiterite and rutile have higher double refraction, are P harder, and differ chemically. Zircon is harder, has higher indices of refraction, and differs chemically. Zircon.— ZrSiO,, H = 7.5, = 4.2-4.86. Tetragonal. ( + ). Cleavage: (110), good; (100), poor. Habit: Prisms, rarely pyramids, grains. Elongation: (110) :(1T0), prismatic. ( + ). Colorless, seldom yellowish to reddish. Pleoehroism: "Weak, not generally noticeable in thin sections. ftj = 1.931, £=1.993. £-w = 0.0443-0.0618. Insoluble in acids, except the powder in hot concentrated sulphuric acid. Cassiterite has higher double refraction and different chemical, reactions. Xenotime is softer and has lower indices of refraction. Hussakite.— SP^Os-SOs- 3R A > where R is Y,Er,Ga, H = 5.0, = 4.587. Tetragonal. (+). Cleavage: (110), good. Habit: Pyramids, prisms, grains. o ANISOTROPIC. 291 Elongation: (110) :(1I0), prismatic. ( + ). Colorless, yellowish, brownish. ft> = 1.717-1.724, £=1.8113-1.8196. £-« = 0.0948. Insoluble in acids. The SO, free xenotime may be decomposed hussakite; if so the name xenotime has priority. Zircon has lower double refraction and contains neither PA norY. Cassiterite has higher indices and contains neither PO5 nor Y. Cassiterite.— SnOj, H = 6.0-7.0, = 6.8-7.1. Tetragonal. ( + ). Cleavage: (110), poor; (100), distinct. Habit: Grains, prisms, rods, pyramids, twins. Elongation: (110):(lT0), prismatic. ( + ). Yellowish, brownish, colorless. Pleochroism: Very weak, generally not observable in thin sec- tions. 0) = 1.997, £ = 2.093. £-w = 0.096. Optical anomalies, rare and weak. S Insoluble in acids. Rutile has higher double refraction and better cleavage. ^ Anatase is ( — ). Brookite and pseudobrookite are bia.xial. Perofskite and zircon and all the above differ chemically and in having lower specific gravities than cassiterite. RutUe.— TiO,, H = 6.0-6.5, = 4.2-4.3. Tetragonal. ( + ). Cleavage: (110), (100), good to fair. Habit : Prisms, grains, genieulated and heart-shaped twins, sagenite webs. Elongation: (110) :(1T0), prismatic. (. + )■ Yellow, fox-red, violet. Pleochroism: Seldom noticeable, = yellow to brownish, E — brownish yellow to greenish yellow. ^ = 2.616, £ = 2.903. e-w = 0.2871. Insoluble in acids. Cassiterite and zircon have lower double refraction and different chemical reactions. Anatase, brookite, and pseudobrookite differ in crystal form and cleavage and have lower birefringence. Perofskite contains Ca and is generally isotropic. The mineral is anisotropic. Parallel extinction. Colored. Won-pleochroic. Maximum birefringence is greater than that of quartz. Biaxial. ^' ;:§ s o I 1^ < t-t < 293 294 ANISOTROPIC. The MineroJ u ANISOTROPIC, hsi PARALLEL EXTINCTION, i> COLORED, NON-PLEOCHROIC, MAXIMUM BIREFRINGENCE > Qu.rtz, BIAXIAL. The Miner.1 {• NEGATIVE (-). The Mineral i. POSITIVE (+). 1 .. :.. I...I... ...t !?„(».<»:«» -N ooooin o in o us irto± 025 030 - Anthophyllite. ^ ^■ M Pa uscovite. ragontte. ■ 040 Phlc- .045 Monazite. ' Diaspore 060 065 .070 075 .090 .095 .100 .120 .140 .160 .200 .250 Bro jkite.- Pseudobrooktte. ANISOTROPIC. 295 Delessite— H,„(Mg,Fe)4(Al,Fe).Si,023, H = 2.5, G = 2.5-3.0. Monoclinic. ( — ). Cleavage: (001), good. Habit: Scales, spherulitic aggregates. Elongation: At right angles to (001), lath-shaped. ( + ). Orientation : & = B, c : o = very small to 0°. Green, yellow, brown. Pleochroism : a = yellowish to colorless, c and 6 = green. Index: Similar to clinochlore. (a = 1.585, /9 = 1.586, r=l-596.) Birefringence: Similar to clinochlore. (;-— a: = 0.011', )- — /?=0.010, ^-a = 0.001.) Gelatinizes easily with acids. Clinochlore is ( + ). Other minerals: Its resemblance to chlorite separates delessite from other minerals. Antigorite.— H4(Mg,Fe)3SiA, H = 2.5, G = 2.622. Orthorhombic. ( — ). Habit: Lamellse, leaves, scales. Elongation: At right angles to (100), lath-shaped. { + ). Orientation: a=B, b=c. Greenish, color^ess, j'ellowish. Non-pleochroic. a: = 1.560, /?=1.570, r = l-571. § 7— q: = 0.011, r-/5=0.001, ^-a = 0.010. Dispersion: p> u. ^ 2£ = 16°-98°. In boiling HCl silica separates. Pennine is pleochroic, the double refraction is lower, and the indices are higher. Ordinary serpentine is (-I-). Bastite. — Serpentine pseudomorph after an orthorhombic pyroxene, H = 3.6-4.0, = 2.6-2.8. Orthorhombic. ( — ). Cleavage: (100), good. Habit : Leaves. Elongation: (110), (ITO), lath-shaped. ( + ). Orientation : a=a, c=c. Greenish, yellowish, colorless. Pleochroism: Weak or wanting. Index: 1.5-1.6. Birefringence: Weak. Dispersion: p>u. 2E = 20°-90°. Gelatinizes more or less easily with acids. Other pyroxenes differ in orientation. See Part II. ANISOTROPIC. 297 Hypersthene.— (Fe,Mg)Si03, H = 5.0-6.0, G = 3.40-3.50. Orthorhombic. ( — ) . Cleavage: (010), distinct; (100), good; (110), good. (110):(lT0) = 91° 40'. Habit: Prismatic, often massive. Elongation: (110): (110), lath-shaped. ( + ). Orientation: 6=n, c=c. Colorless, yellowish, reddish. Pleochroism: c = greenish, n = reddish yellow, S = reddish brown. Strong, a = 1.692, /3 = 1.702, r = 1-705. r- a = 0.013, r-|3=0.003, ^-a = 0.010. Dispersion: p> u. 2E = 85° ca. Partially decomposed by HCl. Bronzite is ( + ) and has weaker pleochroism. Monoclinic pyroxenes have inclined extinction and higher double refraction. Basal sections showing cleavage give the emergence of an axis, while orthorhombic pyroxenes show the emergence of a positive bisectrix. See also Part II. .Sgirite has different pleochroism, green to yellowish green, has higher double refraction, and has (— ) elonga- tion. Prismatine.— MgAljSiOo ca., H = 6.5, G = 3.341. Orthorhombic. ( — ). Cleavage: (110), distinct, (110) :(1I0)=81° 31'. Habit: Prismatic. Elongation: (110): (110), lath-shaped. Orientation : 6 = c, c = a. Yellowish. Pleochroism: Very weak. a: = 1.669, /3=1.681, r=l-682. r-a = 0.013, r-/9 = 0.001, ^-a = 0.012. Dispersion: Weak, |0>t/. 2£ = 65.5°. Topaz has good (001) cleavage. Andalusite has a larger axial angle and is pleochroic. Celrite.— (Mg, Fe)Si03and(Mg, Fe)Al2SiOe, H = 5.5-6.0, G = 3.1-3.2. Orthorhombic. ( — ). Cleavage: (110), good; (100), distinct; (010), poor. (110):(1T0) = 55° 12'. Habit: Rods, leaves, grains. Elongation: (110):(lT0), lath-shaped. (, + ). Orientation : a = o, c = t. ANISOTROPIC. 299 Colorless, yellowish, brownish, greenish. Pleochroism: t = yellowish, greenish; 6 = clove-brown, reddish; 0= yellowish, greenish, colorless. ■ a = 1.623, /9= 1.636, r= 1-644. r-a = 0.021, r-/5 = 0.008, /3-c( = 0.013. Dispersion : p>ij. 2F = 79°-57°. Not affected by acids. One of the orthorhombic pyroxenes. Other amphiboles, see Part II. Muscovite.— AlCSiOJaKHjAlj, H = 2.0-2.5, 0=2.8-2.9. MonocUnic. (— ). Cleavage: (001), good. Habit: Leaves, leafy aggregates. Elongation: Perpendicular to (001), lath-shaped, (-f). Orientation: 6=c, c:o=-l-0° to -1-2°. Colorless, greenish, yellowish. Pleochroism: Weak or wanting. Q! = 1.563, ^=1.598, r = l-601. r- a = 0.038, r-^ = 0.003, /3- a = 0.035. Dispersion: Weak, |0>u. ^ 2£ = 60°-70°. => Insoluble in HCl and H.SO,. "§ One of the mica group. ^ Micas can only be confused with other minerals having complete basal cleavage. The birefringence, very low in basal sections and high in sections showing cleavage, is very characteristic of them. Chlorites have weak double refraction. Talc has 2£=6°-20°. Biotite has 2E generally much smaller, also b = B. Lepidolite and paragonite may be separated by flame test for Li and treatment with HF, by which muscovite will give many crystals of KjSiFg and few Na^SiFg. Paragonite.— AKSiOJaAljNaH,, H = 2.0-2.5, 0=2.8-2.9. Monoclinic. ( — )• Cleavage: (001), good. Habit: Leaves, leafy aggregates. Elongation: Perpendicular to (001), lath-shaped. (-1-). Orientation: 6= t, c: a = 0° to 4-2°. Colorless, greenish, yellowish. Pleochroism: Weak or none. Indices: Very similar to muscovite, see above. Birefringence: Like muscovite, see above. ANISOTROPIC. 301 Dispersion: Weak, p>u. 2E = 70° ca. Insoluble in acids. One of the mica group. Muscovite and lepidolite are different chemically. Talc has 2E = 6°-20°. Phlogopite. — A magnesium mica, near biotite, but containing little Fe, H=2.5-.3.0, = 2.78-2.85. Monoclinic. ( — ). Cleavage: (001), good. Habit: Leaves, leafy aggregates. Elongation: At right angles to (001), lath-shaped. ( + ). Orientation: 6 = 6, c:ii = 0° to +7° Colorless, yellowish, brownish, greenish. Pleochroism : Weak with t> 6 > tt. a = 1.562, /?=1.606, r = l-606. j--a = 0.044, 7--|i3 = 0.0, /3-a = 0.044. Dispersion : Weak, p u. 2F=S0° ca. An iron-rich olivine. Gelatinizes quite easily with HCl. Olivine has 2F = 88°, is (-I-), and has lower birefringence. Forsterite is (^-), has 2F = 86°, lower indices, and is of different occurrence. Monticellite has 2F = 37.5°, lower indices, and lower double refraction. ANISOTROPIC. 303 Talc— H^MgsSi^Ojo, H = 1.0, = 2.7-2.8. Orthorhombic. ( — ). Cleavage: (001), good. Habit: Leaves, scales. Elongation: At right angles to (001), lath-shaped. ( + ). Orientation : b= c, c= a. Colorless, greenish, bluish. Non-pleochroic. a = 1.539, ^ = 1.589, r=l-589. r-a = 0.050, r-^ = 0.000, /?-ffi = 0.050. Dispersion : Distinct, p>u. 2£ = 6°-20°. Hardly affected by acids. Brucite is ( + ), uniaxial, and has lower double refraction. Muscovite cannot be separated optically when it has a small value for 2E; it must be separated by testing for the alkalies or with cobalt solution. S o <5 ANISOTROPIC. 305 Bronzite.— (Mg,Fe),Si20o , H = 5.0-6.0, G = 3.29. Orthorhombic. ( + )- Cleavage: (010), distinct; (100), good; (110), good. (110):(lT0) = 91° 40'. Habit: Prismatic, cloudy masses. Geniculated and star-shaped twins occur rarely. Elongation: (110):(1T0), lath-shaped. ( + ). Orientation : b= a, c=t. Colorless, yellowish, reddish. Pleochroism: Weak; t = greenish, a = reddish yellow, b = reddish brown. a: = 1.665, /3 = 1.669, r=l-674. r- a = 0.009, r-i8= 0.005, /?-a: = 0.004. Dispersion: p u. 2F = 0°-60°. Insoluble in acids. Abnormal interference colors. Pistacite, with weak double refraction, is similar when the axial plane is at right angles to the cleavage. Vesuvianite has poor cleavage. Melilite gelatinizes with acids. Thulite.— Manganese zoisite, H = 6.0 6.5, G = 3.124. Orthorhombic. ( H- ) . Cleavage: (100), distinct. Habit: Rods, leaves. Elongation: (110): (ITO), short lath-shaped. (±). Orientation : a = t, c = o. Red, yellow. ANISOTROPIC. 307 Pleochroism: a = nearly colorless, B = rose, t= yellowish. Index: to= 1.702. Birefringence: Like zoisite, 0.009 max. Dispersion: pu. 2F = 70°. 2Ena = l28°2,T. ^ Soluble in HCl with separation of SiOj) the dark-red solution on heating gives off CI, and becomes yellowish. Rinkite has strong po, 2£=16°-50°. Soluble with separation of gelatinous silica in boihng HCl or H,SO,. Pennine, when optically ( — ), separated by optical char- acter; when (-I-), by chemical test for AI2O3. It has lower birefringence and is pleochroic. ANISOTROPIC. 309 Sillimanite.— Al,SiO», H = 6.0-7.0, G = 3.23-3.248. Orthorhombic. ( + ) . Cleavage: (100), good. Habit: Prisms, threads. Elongation: (110):(lI0), long laths. ( + ). Orientation: 6=0, c=t. Colorless, yellowish, cloudy. Pleoohroism: Not seen in thin sections. a = i.660, ^ = 1.661, r = l-682. r-a = 0.022, r-/5 = 0.021, ^-q: = 0.001. Dispersion : Strong, p>u. 2T' = 31°-42° Insoluble in acids. Andalusite is ( — ), and has ( — ) elongation, double refrac- tion is lower, and relation of axial plane to cleavage is different. Scapolites are ( — ), have (— ) elongation, and are uniaxial. Zoisite has weaker double refraction and has different orientation. Anthophyllite.— (Mg,Fe)Si03, H = 5.5-6.0, G = 3.1-3.2. Orthorhombic. (±). Cleavage: (110), good; (100), distinct'; (010), poor. (110):(ll0 | = 54° 23'. Habit: Rods, leaves, grains. ' (;j Elongation: (110): (110), lath-shaped. (-I-). Orientation: a= a, c=c. Colorless, yellowish, brownish, greenish. Pleoohroism: t = yellowish, brownish; Ii = clove-brown, red- dish; = yellowish, greenish, colorless. ff = 1.633, /? = 1.642, r = l-657. 7— a = 0.024, r-/? = 0.015, j3-a = 0.009. Dispersion : p u. 2V^ 90°. Not noticeably affected by acids. A member of the amphibole group. Amphiboles and pyroxenes, see Part II. Humite.— Mg5[Mg(F,OH)]j(SiO,)3, H = 6.5, = 3.1-3.2. Orthorhombic. ( + ). Cleavage: (001), distinct. Habit: Grains. Twins. Often lamellar. Orientation: a=a, b=(. Colorless, yellow, yellowish brown, reddish. Pleoohroism: Weak in yellow and colorless tones. /?= 1.643. r- a = 0.035. ANISOTROPIC. 311 Dispersion: Weak, p> u. 2F = 68°. Gelatinizes quite easily with HCl. Occurs as a not especially common constituent of crystal- line dolomites and limestones. Olivine resembles colorless humite, but has the plane of the optic axes at right angles to the cleavage, while in humite this is parallel to the cleavage. Olivine.— (Mg,Fe)giO,, H = 6.5-7.0, = 3.27-3.45. Orthorhombic. ( + ). Cleavage: ((010), (001), distinct; (100), poor. Generally shows heavy irregular cracks. Habit: Isometric, short prisms, grains. Twins (Oil) and (012) rare. Orientation : a = t, b= a. Greenish, yellowish, reddish, colorless. Pleochroism: Wanting or weak in yellow and red tones. a = 1.654, /?=1.670, r=1.689. r- a = 0.035, r-/'=0.019, ;?-a = 0.016. Dispersion: Distinct, p u, contains no P., ex- tinction + 39°, weak pleochroism, and much higher birefringence. Brookite.— TiOj, H = 5.5-6.0, G = 3.87-4.01. Orthorhombie. ( + ). Cleavage: (010), good, though not always seen microscopically. Habit: Tabular, plates. Elongation: (100), narrow laths. (=F). Orientation: a=t, 6=n, for red; a=t, c=o, for green. Axial plane for red and yellow lies in (001); for green and blue in (010). a: = 2.583, /3 = 2.586, r=2.741. r-a = 0.158, r-/?=0.156, jJ- a=-0.Q03. Dispersion : p> u. 2y = 0°-23°. Insoluble in acids. Gives Ti reaction. Cassiterite and rutile have different habit, and brookite has very different strength of double refraction in (100) and g (010) sections. Pseudobrookite.— TiOj and Fe^Oa, H = 6.0, G = 4.39-4.93. Orthorhombie. ( + ). S Cleavage: (010), distinct. Habit: Plates, always idiomorphic. Elongation: (100), narrow laths. (±) Orientation: a=t, b=a. Fox-red, brownish. Pleochroism: Weak, ti>a=t i9 = high._ J-— a = high. Dispersion: pu. ■tr^ The mineral is anisotropic. Parallel extinction. Colored. Pleochroic. g o P< W u o M >-) a, 315 Pleochroic. Maximum birefringence is less than that of quartz. fhuZUA^ t/iA^L f4.U^iZ. j, J^^^ N Pi < The mineral is anisotropic. o Parallel extinction. p2 Colored. w n •s o cS 317 The mineral is anisotropic. i= Parallel extinction. Colored. Pleochroic. Maximum birefringence is less than that of quartz. Uniaxial. ..tA<{ :4 • "^ X'^ 3 319 320 ANISOTROPIC. The Mineral U ANISOTROPIC hu PARALLEL EXTINCTION, u COLORED, FLEOCMROIC, | MAXIMUM BIREFRINGENCE < Qiurtz, UNIAXIAl. The Mineral U NEGATIVE (-). Tke Mineral ii POSITIVE (+). ■flooteot* r* Q> w lo fn- g g 3 S KSSSS NNt^HM ,. ^ ^ ^ creas'g - - - - MM»i0, in colored crystals. oj = 1.638, £ = 1.634. w- £ = 0.004. Anomalous biaxial character in large individuals only. Easily soluble in HjSOj, yellow precipitate with ammonium molybdate. Easily soluble in HNO3. Vesuvianite, melilite, gehlenite, zoisite, and tourmaline are not soluble or have different crystal form or color. Tourmaline has 0> E, while colored apatite has E>0. Melilite.— Ca,Mg,Fe,Al, silicate, H = 5.0, G = 2.9-3.1. Tetragonal. (T). Cleavage: (001): (110), poor; only basal cleavage generally seen in thin sections, and this occurs as a single cleft exactly in the center of the lath-shaped section. Habit: Plates, short prisms. E:ongation: (001), laths. (±). Colorless, yellowish. G Pleochroism: None when colorless; for yellow melilite £?=dark yellow, 0= light yellow. aj = 1.634, £ = 1.629. a)— £ = 0.005; often unusual indigo-blue interference colors. Gelatinizes easily with HCl. "Peg" structure and cleavage crack in center are characteristic. Vesuvianite and zoisite are insoluble in acids. f»e>ilenite has not the unusual interference colors. Vesuvianite.— Al2(SiO,)jCa8(A10H), H = 6.5, G = 3.35-3.47. Tetragonal. ( — , rarely +). Cleavage: (110), (100), poor. Habit: Short prisms, grains, dull masses. Elongation: (110):(lT0), prismatic. (T). Colorless, yellowish, greenish, reddish, brownish, blue. Pleochroism: Weak to unnoticeable. w = 1.705-1.733, £ = 1.701-1.726. ftj— £=0.006-0.001, often abnormal interference colors. Anomalous biaxial character. Insoluble in acids imless fused to glass. Zoisite has the high indices and weak double refraction found in vesuvianite. The optic angle of zoisite often ANISOTROPIC. 323 falls as low as 0°, and vesuvianite has often anomalous double refraction with very variable axial angle. Zoisite, however, has very good pinacoidal cleavage. Gehlenite gelatinizes easily with HCl. Andalusite always has a larger axial angle. Corundum.— AlA . H = 9.0, ' G = 3.9^.10. Hexagonal. ( — ). C;ieavage: There is a parting along (1011). Habit: Plates, grains, prisms. Twinning lamellse (10' 1) occur, and parting is along these planes. Elongation: (0001), lath-shaped. ( + ). Colorless, blue, hght red. Pleochroism: 0= blue, red; .B = green, yellow, or greenish yel- low. Only seen in deeply colored corundum. *^ = 1.769, £ = 1.760. NEGATIVE (-). The Mineral i. POSITIVE (+). 2.50 2.00 1.90 1 80 1 75 1 70 1.65 1.60 1 55 In- 1.55 I 60 1 65 1 70 1.80 1.90 2.00 2 50 1'7l- , "' Eh. Medium. Nol Mirked. Not Marked. Medium.' w. VoriH. 001 002 .003 004 005 006 007 008 009 010 ' - ^huringi e. - Peni ine.t - - Pennine. ± ■ Sa aphirine.- ■ • ■ - ordierite ■ ■ i^nstatite Bronzite. - • ■ ANISOTROPIC. 327 Thuringite.— H,sFes(AlFe)8Si60„, H=2.5, = 3.15-3.19. Monoclinic. ( — ). Cleavage: (001), good. Habit: Scales, irregular or concentric scaly aggregates. Elongation: At right angles to (001), lath-shaped. ( + ). Orientation: c:n = 0°. Green, yellow. . Pleochroism: o = yellow; b=c = green. Index similar to pennine, 1.583. Birefringence similar to pennine, 0.002. 2£ = 0°-25°ca. A lepto-chlorite. Gelatinizes easily with HCl. Sapphirine.— MgsAll^SijOj, , H = 7.5, G = 3.486. Monoclinic. ( — ). Cleavage: Not seen. Habit: Tablets, rods, grains. Elongation: (110) :(1T0), lath-shaped. (-I-). Orientation : b = B, c:t=-l-8°to-l-9° Bluish, greenish, colorless. Pleochroism: o = colorless; B=f = blue; or o = light greenish blue; 6 = dark blue-green; t = yellowish green. a = 1.706, ^=1.709, 7-=l-711. g r- a = 0.005, r- ^ = 0.002, ^- a = 0.003 "= Dispersion: Distinct, p t> a. q: = 1.535 /3 = 1.540, r=l-544. r- a = 0.009, 7--,9=0.004, /?-a = 0.005. Dispersion: Weak, p u, or p Qujifc WlAXIAl. The Mineral i> NEGATIVE (-). The Mineral !• POSITIVE (+). < ■ — Increase in Index of Refraction. > 2 50 2.00 1.90 1 80 1 75 ,1.70 1.65 1 60 1 SS In- oreas'g I 55 1 60 1,65 1,70 V7S 1.80 1.90 2.00 2.50 •eriH. Hieh, Medium. NotMirkf , Nol Marked Medium, Nleh. VerrH. • Corund 1 m. 010 .015 .020 ,025 ■1 030 Tourmali .040 .045 .050 ,055 - 060 ,065 .070 •Ana ase^ 075 .080 ;:alcite. 085 ,81? ,100 .120 ,140 .160 200 .250 —Mai • Hematite. I R 1 utile.- ANISOTROPIC. 337 Corundum.— AlA. H = 9.0, G = 3.9-4. 10. Plexagonal. ( — ). Cleavage: There is a parting along (lOTl). Habit: Plates, grains, prisms. Twinning lamellae (lOTl) occur and parting is along these planes. Elongation: (0001), lath-shaped. ( + ). Colorless, blue, light red. Pleochroism: = blue, red; i?=green, yellow or greenish yel- low; only seen in deeply colored corundum. w = l,769, £ = 1.760. 6;- £ = 0.009. Insoluble in acids or fused soda. Other minerals which resemble it are distinguished by the high indices, negative character, uniaxial figure, etc., of corundum. Tourmaline.— Na,Al,B,Mg,Fe silicate, H = 7.0, G = 3.0-3.24. Hexagonal. ( — ). Cleavage: Not seen in thin sections. Habit: Prisms, grains, rod-like aggregates. Elongation: (lOTO), prismatic. ( — ). g Orientation: c=a. Brown, reddish, greenish, yellow, violet, blue. Pleochroism : Strong, 0> E. w= 1.632-1.685, £=1.612-1.652. c3 w- £ = 0.017-0.034. Often anomalous biaxial character. Insoluble in HF and other acids. Other minerals: The strong pleochroism, 0>E, the form, lack of cleavage, and insolubility in acids separate tourmaline from all other minerals. Biotite.— Mg.Fe.Al silicate, H = 2.5-3.0, = 2.8-3.2. Monoclinic, often appears uniaxial. ( — ). Cleavage: (001), good. Habit: Leaves, leafy aggregates. Elongation: At right angles to (001), lath-shaped, (-t-). Orientation: b = i, c: = 0° to -1-7°. Brown, green, yellow. Pleochroism: Strong, t^S>a; e and 6 = deep brown to red- brown, deep green to black; o = light yellow to red, light green. a: = 1.557, ^ = 1.589, r = l-597. r- a = 0.040, r- /?=0.008, /3-a = 0.032. Dispersion: Weak, p^u. 2E=0° to 72°. Soluble in HjSOj and the Fe-rich biotites in HCl with separa- tion of SiO, scales. ANISOTROPIC. 339 Minerals other than micas, see Part II. Alkali micas have different color, pleochroism, larger 2F, different position of the plane of the optic axes, and are different chemically. Anomite has different position of the plane of the optic axes. Lithionite, when dark in color, may be distinguished by the Li reaction. Anatase.— TiO„ H = 5.5-6.0, = 3.82-3.95. Tetragonal. ( — ). Cleavage: (001), (111), good. (111):(11T) = 43° 24'. Habit: Pyramids, tabular. Colorless, blue, also yellowish or greenish. Pleochroism: = deep blue, orange yellow; E=light blue, light yellow. &) = 2.562, £ = 2.489. £0- £=0.0732. Optical anomalies, interference cross opens; colorless and yel- low anatase is usually normal, blue portion seldom fully ex- tinguishes, g Rutile, brookite, pseudobrookite, cassiterite, and zircon differ in being ( -H ) and by having different cleavage and form. Perofskite has different form, and the anomalous inter- ^ ference colors are lower than those in anatase. Calcite.— CaCOj, H = 3.0, G = 2.714. Hexagonal. ( — ). Cleavage: (lOll), good. (10T1):(1T01) = 74° 55'. Habit: Grains, granular aggregates, threads, rounded aggregates. Colorless, gray, yellowish, brownish. Absorption: 0>E. w = 1.658, £=1.486. w- £=0.172. Twinning (0112) common. Often incloses rhombs of dolomite and magnesite. Readily soluble in cold HCl. Aragonite is biaxial and has different cleavage. See also special tests in Part ll. Dolomite has (0221) twinning. Special tests, see Part II. Dolomite.— MgCaCjOj , H = 3.5-4.0, = 2.85-2.95. Hexagonal. ( — ). Cleavage: (lOIl), good. Habit: Rhombohedrons, granular aggregates. Twins (0221) oc- casionally occur. ANISOTROPIC. 341 Colorless, gray, yellowish, brownish. Absorption: 0>E, strong. cu = 1.682, £=1.503. ai- £ = 0.179. Soluble with difficulty in cold acid; easily in hot. Calcite has lower indices and is soluble in acetic acid, twin- ning (0112). Chemical differences between calcite, dolomite, and aragonite, see Part II. Magnesite.— MgCOj , H = 3.5-4.5, = 2.9-3.1. Hexagonal. ( — ). Cleavage: (lOTl), good. (10Tl):(Tl01) = 72° 40'. Habit: Rhombohedrons, granular aggregates. Colorless, yellowish, grayish, brownish. Absorption: 0>E. 0)1111 = 1.717, £ma = 1.515. oj- £ = 0.202. Cold HCl does not act upon magnesite. Dolomite and calcite are acted upon more readily by acids. See also special tests in Part II, Siderite.— FeCOs, H = 3.5-4.0, = 3.936-3.938. Hexagonal. ( — ). g Cleavage: (lOIl), good. (lOIl) :(I101) = 73°. ° Habit: Rhombohedrons, granular aggregates. S Colorless, yellowish, brownish. Absorption: Strong, 0>E. w = 1.872, £=1.634. 0)- £ = 0.238. Soluble in HCl. Other carbonates are separated by chemical means. Hematite.— Fe A . H = 5.5-6.5, = 4.9-5.3. Hexagonal. ( — ). Cleavage: Parting, (0001) and (lOTl). Habit: Tabular, thin plates, scales, earthy masses, and granular aggregates. Elongation: (0001), lath-shaped. (-I-). Red, yellowish, yellowish gray. Pleochroism: = brownish red, £ = light yellowish red. 01 = 3.22, £ = 2.94. a)-£ = 0.28. Very slowly soluble in HCl. Magnetite is magnetic and soluble in HCl. Limonite may be confused with hematite. ANISOTROPIC. 343 Rutile.— TiO^, H = 6.0-6.5, G = 4.2-4.3. Tetragonal. ( + ). Cleavage: (110), (100), good to fair. Habit: Prisms, grains, geniculated and heart-shaped twins, sage- nite webs. Elongation: (110): (ITO), prismatic. ( + ). Yellow, fox-red, violet. Pleochroism: Seldom noticeable; = yeUowish to brown- ish, B = brownish yellow to greenish yellow. w = 2.616, £ = 2.903. Quartz. BIAXIAL. The Minral is NEGATIVE (-). The Mineral is POSITIVE (+). , 2 50 2.00 1.90 1.80 1.75 1.70 1.65 1.60 1.55 In- oreas'g BIREFR. 1.S5 1.60 1.6B 1.70 1.7B 1.80 1.90 2.00 2 50 VenH. m. HBdiam. Not Marked. NolMirked. Medium. m- VarrH. L Cordierite.— .010 Bronzitc. -^ I ,._ Dumortierit - Delessite. — Clinochlore. j | — Hypers thene. 015 .020 Gla ucophane <— — Gedrite. • Carpholite. _ ± .025 Anthoph rlUte. — ' Pistac te. P edmonti e. .030 Mn. Piedmonti ) Biotite Anomite nwaldite .040 Zin .045 .050 Fay-, alite. _ Aegirite Acmite. .055 ^strophy llite. F^ lingsite. .060 065 070 .075 — ■'Basa He horr blende. .085 .090 .095 .100 .120 .140 160 .200 .250 Titani Bro e.— jkite. - Pseudobro >kite. ANISOTROPIC. 347 Cordierite.— MgjAliSisOis, H = 7.0-7.5, = 2.59-2.66. Orthorhombic. ( — ). Cleavage: (010), poor. Habit: Grains, short prisms. Trillings and polysynthetic twins occur. Elongation: (110), (110), short laths. (-). Orientation: b=t, c=tt. Colorless, bluish, yellowish. Pleochroism: Wanting or weak with 'b> c> n. a = 1.535, /3= 1.540, r= 1-544. r-« = 0.009, r-|S = 0.004, /?-a = 0.005. Dispersion: Weak, p Orientation: a = t, c= a. "§ Blue, brownish, greenish, colorless. ^ Pleochroism: (l = blue; 6 = yellowish, reddish violet, greenish; t = colorless. a = 1.678, /?= 1.686, r= 1-689. 7--q: = 0.011, y-/?=0.003, /3-a = 0.008. Dispersion : p> u. 27 = 30°. Insoluble in HF and other acids. Blue amphibole is monoclinic. Serendibite is triclinic, extinction 40° ca. Sapphirine is monoclinic with c:t = 8°-9°. Grandidierite has different habit, a=n, b=c, and strong blue-green to colorless pleochroism. Andalusite.— AljSiO.,, H = 7.5, G = 3.10-3.20. Orthorhombic. ( — ). Cleavage: (110), good. (110): (1T0) = 89°. Habit: Prisms, grains. Elongation: (110):(lT0), lath-shaped. (-). Orientation: a=c, c= a. Colorless, reddish. Pleochroism: o = rose; B = c = colorless to light green. ANISOTROPIC. 349 a = 1.632, /?=1.638, r=l-643. r-a = 0.011, r-/5 = 0.005, /3-a: = 0.006. 2F = 84°-85°- Not attacked by HF or other acids. Sillimanite is ( + ), has basal cleavage, and has higher double refraction. Topaz has perfect basal cleavage and is ( + ). Scapolites have higher double refraction in prisms and are uniaxial. Zoisite has (010) and (100) cleavage, and is ( + ); 2F=0°- 60°. Thulite is ( + ), 2F=0°-40°. Orthoclase has lower indices of refraction. Prismatine has 2E = 65.5°. Cornerupine has 2E = U°-Z2°. Delessite.— H,„(Mg, Fe)4(Al, Fe),Sii023, H = 2.5, G = 2.5-3.0. Monoclinic. ( — ). Cleavage: (001), good. « Habit: Scales, spherulitic aggregates. Elongation: At right angles to (001), lath-shaped. ( + ). Orientation: 6 = 6, c:p = very small to 0°. Green, yellow, brown. § Pleochroism : a = yellowish to colorless, c and 6 = green. Inde.x: Similar to clinochlore, in which a = 1.585, /? = 1.586, (;;) r= 1.596. Birefringence: Similar to clinochlore, in which ;-— ar = 0.011, r-^=0.010, j3- a^O.OOl. Gelatinizes easily with acids. Other minerals: Its resemblance to chlorite separates delessite from other minerals. Clinochlore is ( + ). Hypersthene.— (Fe,Mg)Si03, H = 5.0-6.0, G = 3.40-3.50. Orthorhombic. ( — ). Cleavage: (010), distinct; (100), good; (110), good. (110):(lT0) = 91° 40'. Habit: Prismatic, often massive. Elongation: (110):(lIO), lath-shaped. ( + ). Orientation: 6=0, c=c. Colorless, yellowish, reddish. Pleochroism: t = greenish, o = reddish yellow, b = reddish brown. Strong, a = 1.692, /?=- 1.702, r = 1-705. r-a = 0.013, r-/3=0.003, /3-a=0.010. Dispersion: p> u. 2S = 85° ca. ANISOTROPIC. 351 Partially decomposed by HCl. Bronzite is ( + ) and has weaker pleochroism. Monoclinic pyroxenes have inclined extinction and higher double refraction. Basal sections showing cleavage give the emergence of an axis, while orthorhombic pyrox- enes show the emergence of a positive bisectrix. See also Part II. .ffigirite has different pleochroism, green to yellowish green, has higher double refraction, and has ( — ) elongation. Glaucophane— NaAl(SiO,),(Fe, Mg)Si03, H = 6.0-6.5, G = .3.103-3.113. Monoclinic. ( — ). Cleavage: (110), good; (100), distinct; (010), poor. (110):(1T0) = 55° 16'. Habit: Short prisms, grains. Twinning (100). Elongation: (110):(1T0), lath-shaped. ( + ). Orientation: 6 = S, c:c=-4°to-6° Blue, violet, yellowish green, almost colorless. Pleochroism: a = nearly colorless to yellowish green. 6 = red- dish to bluish violet, c = blue. a: = 1.621, ^=1.638, r=l-639. r-a = 0.018, r-/5 = 0.001, /3-a = 0.017. Dispersion: Strong, p u. 2F = 79°-57°. Not affected by acids. One of the orthorhombic pyroxenes. Other amphiboles. See Part II. ANISOTROPIC. 353 Carpholite— H^MnAl^SijOio, H = 5.0-5.5, G = 2.935. Monoclinic. ( — ). Cleavage : Unnoticeable. Habit: Rods, fibres. Elongation: (110):(lT0), narrow laths. ( + ). Orientation : b=a, c : c = 4° ca. Colorless, greenish yellow. Pleoohroism: Distinct, [ = colorless, n = 6 = yellowish green. Index:« = 1.627. 1— a = 0.022. 2F = 60°. Insoluble in acids. Rosenbuschite is soluble in acids, elongation ( — ), 2F = 90°, cleavage (001), good. Sillimanite is ( + ), non-pleochroic, good (100) cleavage. Pistacite.— (Epidote), Al(Si04)3(Al, Fe)2Ca-CaOH, H = 6.5, = 3.3-3.5. Monochnic. ( — ). Cleavage: (001), good; (100), distinct. (001) : (100) = 64° 37'- Habit: Prisms and rods elongated along b, grains; twins (100). Elongation: (001): (100), lath-shaped. (±). Orientation: 6 = B, c:n=-l-3°. Green, yellow, brownish, colorless. Pleochroism: o = colorless to yellowish or greenish, B = yellowish to yellowish gray, lavender; c = green, light yellowish brown. q: = 1.714, /?=1.741, r = l-746. r-af = 0.032, r-/5 = 0.005, /3-a = 0.027. .0^6 --.a**. Dispersion: p> u. 2y = 74°-90° Not affected by HCl. Pistacite is the Fe-rich epidote, clinozoisite the Fe-poor or free. Pyroxene has (110) cleavage; in sections parallel to the long direction the plane of the optic axes is parallel to the cleavage; in epidote at right angles to it. Clinozoisite has lower double refraction, is ( + ), has strong o u. 2E = 50°. Insoluble in acids. Sapphirine has no cleavage, is monoclinic with 8° extinction, and has 0.005 for maximum birefringence. Biotite.— Mg, Fe, Al silicate, H = 2.5-3.0, 0=2.8-3.2. Monochnic, often appears uniaxial. ( — ). Cleavage: (001), good. Habit: Leaves, leafy aggregates. Elongation: At right angles to (001), lath-shaped. ' (-I-). Orientation: 6 = 6, c: = 0° to +7°. Brown, green, yellow. Pleochroism: Strong, c^Ii> a; t and 6 = deep brown to red-brown, deep green to black; i:=light yellow to red, light green. « = 1.557, /?= 1.589, r= 1-597. r-a:=0.040, r-/8=0.008, ;9-q: = 0.032. Dispersion: Weak, p^u. 2E=0° to 72°. Soluble in HjSOj and the Fe-rich biotites in HCl with separa- tion of SiO, scales. Minerals other than micas, see Part II. Alkali micas have different color, pleochroism, larger 2V, different position of the plane of the optic axes, and are ■ different chemically. Anomite has different position of the plane of the optic axes. Lithionite, when dark in color, may be separated by the Li reaction. Anomite. — Under anomite, following Rosenbusch, are included those rock- forming biotites which have the plane of the optic axes at right angles to the symmetry plane. See Part XL H = 2.5-3.0, = 2.8-3.2. ANISOTROPIC. 357 Monoclinic. ( — ). Cleavage: Good, (001). Habit: Leaves, leafy aggregates. Elongation: At right angles to (001), lath-shaped. ( + ). Orientation: 6=c, c:o = 0° to +4°. Brown, green, yellow. Pleochroism: Very strong, like biotite. t>S>a. Indices: Like biotite. Birefringence: Like biotite. Dispersion: Weak, p^u. 2£; = 10°-68°. Biotite has fe = B. Other micas, see biotite. Other minerals than micas, see Part II. Zinnwaldite.— K,Li,Fe,Al sihcate, H = 2.5-3.0, = 2.82-3.20. Monoclinic. ( — ). Cleavage: (001), good. Habit: Leaves, leafy aggregates. Elongation: At right angles to (001), lath-shaped. { + ). Orientation: 6 = 6, c: = 0° to -1-7°. Brown, red, yellow, rarely green. -S Pleochroism: Strong, c>'6>o; c and n = dark brown, brownish g gray; fi = yellowish brown or reddish, nearly colorless. Indices are similar to biotite. Birefringence is similar to biotite. ^ Dispersion: Weak, p>u. 2£=10°-60°. Lepidolite has different position of the plane of the optic axes. Other micas, sec biotite. * Other minerals than mica, see Part II. Phlogopite. — A magnesium mica, near biotite, but containing little Fe, H = 2.5-3.0, = 2.78-2.85. Monoclinic. ( — ). Cleavage: (001), good. Habit: Leaves, leafy aggregates. Elongation: At right angles to (001), lath-shaped, (-f-). Orientation: 6 = 6, c:il = 0° to +1°. Colorless, yellowish, brownish, greenish. Pleochroism : Weak with c > 6 > a. q: = 1.562, /9 = 1.606, j-=1-606. }--a = 0.044, r- ,5=0.0, /3-a: = 0.044. Dispersion: Weak, pn. 2F = 50° ca. An iron-rich olivine. Gelatinizes quite easily with HCl. Olivine has 2T = 88°, is ( + ), and has lower birefringence. Forsterite is ( + ), has 2F = 86°, has lower indices, and differ- ent occurrence. Monticellite has 2T = 37.5°, has lower indices and lower double refraction. I See also Olivine group. Part II. o .ffigirite.— NaFeSi^O^, H = 6.0-6.5, = 3.5-3.6. ^ Monoclinic. ( — ). Cleavage: (110), good. (110) : (1T0) = 92° 49'. Habit: Thin prisms, needles. Crystals bluntly terminated. Elongation: (110): (ITO), lath-shaped. (-). Orientation: 6 = 6, c: B = 3° to 6°. Green, yellow, brown. Pleochroism: B = deep green; S = lighter green to yellowish green; c = yellow to brownish. a = 1.763, /?=1.799, r=l-813. r- a = 0.050, 7--|8=0.014, /3-a: = 0.036. Dispersion: p>'j. Strong dispersion of bisectrices; c:ap u around a. A rare pyroxene. .ffigirite has different pleochroism. Other pyroxenes: Like a;girite. See also Part II. .^girite occurs in bluntly terminated crystals. Iddingsite. — Fe, Ca, Mg, Na silicate. Perhaps antigorite intergrown with hematite, H = 2.S, G = 2.839. System: Biaxial. ( — ). Cleavage: (100), good. Habit: Pseudomorphs after olivine. Orientation: a=tt, c=t. Red-brown, green. Pleochroism: n = brown, red, yellow, also yellowish green. B = c = greenish, yellowish. Index: Not marked. Birefringence: Strong. 2£ = small to very small. •§ Other minerals: Iddingsite may be distinguished by its ^ habit, cleavage, and ( — ) character. "^ Hematite shows no cleavage and is uniaxial. "3 Basaltic Hornblende.— Ca,Mg,Fe,Al silicate, H = 5.5-6.0, = 3.05-3.47. Monoclinic. ( — )• Cleavage: (110), good; (100), distinct; (010), poor. (110):(ll0) = 55.5°. Habit: Short prisms, grains. Elongation: (110): (110), lath-shaped. ( + ). Orientation: 6 = i, c:t = 0° to -12°. Brown. Pleochroism: Strong in brown and yellow tones with t>i>o, also in green ■ and brown tones with ii = green, Ii and c = brown. a = 1.680, /9=1.725, 7- = l-752. r-a = 0.072, r-/?=0.027, /?-q:-0.045. 2y = 80°. But slightly acted upon by HCl. An amphibole. Other amphiboles and pyroxenes, see Part II. Common hornblende has c:t=— 12°to —20°, sometimes ( + ) character, lower double refraction, and some- times lower 2F. Mica generally had a peculiar " bird's-eye maple " appearance. O ANISOTROPIC. 363 Bronzite.— (Mg,Fe)2SiO„, H = 5.0-6.0, G = 3.29. Orthorhombic. ( + ). Cleavage: (010), distinct; (100), good; (110), good. (110):(lT0) = 91° 40'. Habit: Prismatic, cloudy masses. Geniculated and star-shaped twins occur rarely. Elongation: (110) : (ITO), lath-shaped. (-1-). Orientation : 6=0, c = c. Colorless, yellowish, reddish. Pleochroism: Weak: c = greenish; o = reddish yellow; 6 = red- dish brown. a: = 1.665, /3 = 1.669, r=l-674. r-a = 0.009, r-j^-O-OOS, /3-a = 0.004. Dispersion: pu. 2F = 89°. Insoluble in cold HF or other acids. Bronzite has larger 2V, weak pleochroism. = 3.65-3.8. = 2.71. Clinochlore.— Mg, Fe, Al silicate, H = 2.0-2.5, Monoclinic. { + )■ Cleavage: (001), good. Habit: Leaves, scales, leafy aggregates. Twinning common. Elongation: At right angles to (001), lath-shaped. ( — ). Orientation: 6 = 6, c:c=-2°to -9°. Green. Pleochroism: a and B = green; t = yellow. a = 1.585, /3 = 1.586, r = l-596. r-Q: = 0.011, r-/' = 0.010, /3-a = 0.001. Dispersion : p u, p u. 2i; = 160°ca. Insoluble in HCl. Micas have smaller axial angles and lower indices. Brittle micas have lower double refraction, smaller axial angle around the bisectrix emerging on cleavage flakes, and are ( — ). Titanite.— CaSiTiOs, H = 5.0-5.5, = 3.4-3.56. Monoclinic. (-I-). Cleavage: (110), distinct. (110):(1M)= 46° 8'. Habit: Prisms, rhombs, grains, and rods. Orientation: 6 = B, c: c= +39°. Colorless, yellowish, reddish, brownish. Pleochroism : Weak, c> b > b. a = 1.913, /?=1.921, r = 2.054. § r-a: = 0.14l, r- /? = 0.133, /9-a = 0.008. "^ Dispersion: Very strong, p> u. O 2E„a = 45°-68°. Very slightly affected by HCl. Monazite has lower birefringence, weak dispersion, and low extinction. Brookite has parallel extinction, 2F = 0°-23°, and great difference in birefringence in two directions, y— a = 0.158, while /?-a = 0.003. Rutile, xenotime, hussakite, and cassiterite give no reac- tion for Ca. Other minerals: The very strong double refraction and indices, and the very strong dispersion, separate titanite from other minerals. Brookite.— TiO„ H = 5.5-6.0, = 3.87-4.01. Orthorhombic. ( + ). Cleavage: (010), good, though not always seen microscopically. Habit: Tabular, plates. Elongation: (100), narrow laths. (T). Orientation: o=t, 6=0, for red; a=t, c=o, for green. Axial plane for red and yellow lies in (001); for green and blue, in (010). ANISOTROPIC. 369 a = 2.583, /3 = 2.586, r = 2.741. r-a = 0.158, r-/5 = 0.156, /?-« = 0.003. Dispersion: p> u. 2F = 0° to 23° Insoluble in acids. Gives Ti reaction. Cassiterite and rutlle tiave different habit, and brookite has very different strength of double refraction in the (100) and (010) sections. Pseudobrookite.— TiOj,andFeA, H = 6.0, = 4.39-4.98. Orthorhombic. ( + ). Cleavage: (010), distinct. Habit: Plates, always idiomorphic. Elongation: (100), narrow laths. (±). Orientation : a = (, 6 = o. Fox-red, brownish. Pleochroism : Weak, S > o = t. /? = high. ;-— a: = high. Dispersion: p u. The mineral is anisotropic. Inclined extinction. o O O X w o o 371 CO s o The mineral is anisotropic. 8 Inclined extinction. Colorless. QMjb^uL(L ^, ^37 ^' / <5 373 The mineral is anisotropic. Inclined extinction ^ jr.'Jctc f/ifXAA. u-aJ^^^M^ ^ . i ^'cj < CO LOW. 1 .001 ■ .002 ■ ■ .004 1 ■ 005 .006 .007 .008 Anorthocla.,^ Orthoclase.ASa . Stilbite. iidine._ 1 ■ • Kaolir crocUne. . .009 goclase-albite.— - •Epistilbite. . 1 1 .010 Gypsum. -«Albite. ANISOTROPIC. 379 Hyalophane.—K.Ba feldspar, H = 6.0, = 2.771-2.832. Monoclinic. ( — ). Cleavage: (001), (010), good. Habit: Tablets, prisms, grains. Elongation: (001): (010), laths; (110):(ll0), plates. Orientation : fc=c, a:a=— 6°. Colorless. a=-1.537, /?= 1.540, r= 1-542. y- a: = 0.005, r-/' = 0.002, /3-a = 0.003. 2F = 74°-78.5°. Insoluble in HCl; with increase in Ba, gelatinizes. Other feldspars, see Part II. Anorthoclase.— K,Na feldspar, H = 6.0, = 2.56-2.61. Triclinic-. (— )• Cleavage: (001), (010), good. (001) : (010) =88.5° to 89.5°. Habit: Plates, prisms, grains. Elongation: (001): (010), laths; (110):(lT0), plates. Orientation: see Part II. Cdlorless. ff = 1.523, /?=1.528, r=l-529. 7--a = 0.006, r-/5 = 0.001, /3-a: = 0.005. Dispersion: p> u. S 2F = 32° to 53°. Unacted upon by acids. Other feldspars, see Part II. OrthoiJase.— KAlSijOs, H = 6.0-6.5, = 2.54-2.56. Monoclinic. ( — ). Cleavage: (001), (010), good. (001): (010) = 90°. Habit: Plates, prisms, grainsi Elongation: (001) : (010), laths, (-); (110) :(1T0), plates. (±). Orientation : 6 = t, a : o = ± 5° ca. Colorless. a = 1.519, ^ = 1.523, r=l-525. r -a = 0.006, r-/? = 0-002, /?-a = 0.004. Dispersion: Very plain, p>u. 2y=70°-80°, 2E«a = 120° ca. Insoluble in acids except HF. Wephelite is uniaxial and gelatinizes with HCl. Other feldspars, see Part II. Soda-orthoclase has a: a = 10°-12°. Sanidine.— KAlSijO, , H = 6.0-6.5, = 2.54-2.56. Monoclinic. ( — ). Cleavage: (001), (010), good. Habit: Plates, prisms, grains. Carlsbad twins frequent. ANISOTROPIC. 381 Elongation: (001): (010), laths; (110): (110), plates. Orientation : 6 = c, a ; a = + 5° ca. ; or 6 = b, a : n = + 5° ca. Colorless. a = 1.519, /?=1.523, r=l-525. r-« = 0.006, r-/' = 0.002, /3-a = 0.003. Dispersion for b=t, p> u; for 6 = 6, p u. 2F = 90°-98° Insoluble in hot HCl. Other feldspars, see Part II. MicrocUne.— KAlSi^O,, H = 6.0, G = 2.54. Triclinic. ( — )• Cleavage: (001), (010), good. (001) :(0T0) =89.5°. Habit: Plates, prisms, grains. Polysynthetic twinning. Elongation: (001): (010), laths; (110):(lT0), plates. Orientation: See Part II. Colorless. ANISOTROPIC. 383 a = l.S19, /3=1.523, r=l-S26. r-o: = 0.007, r-i3 = 0.003, /?-a = 0.004. Dispersion: p>u. 2F = 71°-84°. Insoluble in acids, except HF. Orthoclase has different extinction on (010) and (001), no microcline texture. Anorthoclase has different extinction on (010) and (001), smaller axial angle and different indices. Other feldspars, see Part II. Nephelite has higher indices and is soluble in HC'l. Other minerals separated by cleavage, indices, and double refraction. Scolecite.— CaAl2Sl,0,t, + 3HA H = 5.0-5.5, G = 2.28. Monoclinic. ( — ). Cleavage: (110), distinct. (110) :1T0) = 88° 37V'. Habit: Prisms, needles. Elongation: (110) :(1I0), narrow laths. ( — ). Orientation: h=t, c:a = 17° Colorless. g Pleochroism: None. /?= 1.502. r-/? = 0.007. Dispersion: Strong, p Quarb. UNIAXIAU The Mineral u NEGATIVE (-). The Mineral ia POSITIVE 1+). lA o in e in at ui V V n In- S S 5 g S oreiig BIREFR The Index of Refraclion is LOW. The Index of Refraction is LOW. 1 1 010 1 ^ Laumontite. Chalcedony. ■* Serpentine.. 1 O20 Hydra [illite.— - ' 050 OSS .060 065 070 — .07S 095 100 120 .140 160 200 .250 ANISOTROPIC. 393 EpistUbite.— H^CaAljSioO.s+SHjO, H = 4.0-4.5, G = 2.25. Monoclinic. ( — ). Cleavage: (010), good. Habit: Prisms, leaves. Elongation: (110):(110), laths. ( + ). Orientation : 6 = B, c : c = — 9°. Colorless. Pleochroism: None. (8=1.51. r-a = 0.010, 7—^8 = 0.008, ;J-a:=0.002. Dispersion: Distinct, p< u. 2£=67°-83°. A zeolite. Imperfectly soluble without gelatinization in concentrated HCl. Gives strong Ca and Al reactions and weak, K and Na. Feldspars are insoluble in HCl and generally have lower double refraction. Different habit. Laumontite.— H^CaAljSi,0,4 + 2H20, H = 3.5-4.0, G = 2.30. Monoclinic. ( — ). Cleavage: (010), good; (100), poor. Habit: Small prisms. Elongation: (110):(lT0), lath-shaped. ( + ). Orientation: b = b, c:t=+20°. Colorless. p"* Non-pleochroic. a = 1.513, /? = 1.524, r= 1-525. r-a = 0.012, r-/3=0.001, /9-a=0.011. Dispersion : Strong, p u. 2E=16°-5Q°. Soluble with separation of gelatinous silica in boiling HCl or H,SO,. ANISOTROPIC. 397 Pennine, when bptioally ( — ), separated by optical char- acter; when ( + ), by chemical test for AI2O3. It lias lower birefringence. HydrargUlite.— A1(0H)3, H = 2.5-3.0, = 2.34-2.42. Monoclinic. ( + ). Cleavage: (001), good. Habit: Small hexagonal plates, leaves. Elongation: (001), narrow laths. ( — ). Orientation: b = h, c:t=H-21°. Colorless. Non-pleochroic. o: = /9= 1.535, r= 1-558. r- a = 0.023. Dispersion : Strong, p>u. 2E = 0°-40°. Soluble in concentrated sulphuric acid. Kaolin has weaker double refraction. Muscovite is ( — ). Calcite has parallel extinction and is ( — ). Brucite has parallel extinction and different chemical reactions. o The mineral is anisotropic. Inclined extinction. Colorless. Index of refraction is greater than that of Canada balsam. s o 3 COLORLESS, INDEX OF MAXIMUM BIREFRINGENCE < Quarti. The Minanl U NEGATIVE (-). The Uineral is POSITIVE (+). e in ndax oF Ref oooein lOOOlCQt^ 1.70 1.65 1.60 1.55 h- WBls'g BIREFR. 1.55 1.60 1.65 1.70 inc ?ooo oototn ->iF-NN V.i,H. High. Medium. Not Marked. Not Marked. MadHim. High. VenfH. .001 ' - 002 ■ ■ .003 — Phillipsite. — — Rinki ■ .004 .005 ■ ■ - .006 - • ■Andesine.. jidolite.< Bytow "- .007 .008 .009 .010 - ligoclase mite. — ■ ~ Labradorite. — Clinozoisitc- ■ — Axin te. - ■ ANISOTROPIC 403 Sapphirine.— MgjAl,,Si20„,, H = 7.5, G = 3.486. Monoclinic. ( — ). Cleavage: Not seen in thin sections. Habit: Tablets, rods, grains. Elongation: (110): (110), lath-shaped. ( + ). Orientation: 6 = B, c:c=+8° to +9°. Bluish, greenish, colorless. Pleochroism: n = colorless, B=t = blue; or D = hght greenish blue ; li = dark blue-green ; t = yellowish green. a: = 1.706, /5=1.709, r = l"ll. 7— a = 0.005, r-|9=0.002, ^- a = 0.003. Dispersion: Distinct, pu. 2y = 90°-98°. Insoluble in hot HCl. Other feldspars, see Part II. Bytownite.— Ab,An3 to Ab,An„, H = 6.0, = 2.71-2.74. Triclinic. ( — ). Cleavage: (001), (010), good. Habit: Plates, prisms, grains. Elongation: (001): (010), laths; (110):(lT0), plates. Orientation: See Part II. Colorless. a = 1.561, ,8=1.564, 7-= 1-569. r-a = 0.008, r-/'=0.005, /?-q: = 0.003. Dispersion: p>u. 2F = 76°-80°. Decomposed by HCl with separation of gelatinous siUca. Other feldspars, see Part II. Axinite.— Ca;Al,Bj(SiOJ,, H = 6.5-7.0, = 3.27-3.29. Triclinic. ( — ). Cleavage: (010), distinct. Habit: Grains, tablets. ? Orientation: Plane of the optic axes makes an angle of 24° 40' with the edge (111): (111), and of 40° with (111):(1T0). Colorless, yellowish, brownish, violet. Pleochroism: In thin sections weak or not noticeable. Q! = 1.685, /?=1.692, r=l-6.95. ?--a = 0.010, r-/?=0.003, '^-a = 0.007. Dispersion: Distinct, p b > D. (1 = 1.665, ^=1.668, r=( )• ^- a = 0.003. Dispersion: Strong, pij. Andesine.— AbjAn^ to Ab^An,, H = 6.0, G =2.66-2.69. Triclinia. (-I-). Cleavage: (001): (010), good. (001) ; (010) = 86° 14'. Habit: Plates, prisms, grains. Elongation: (001): (010), laths; (110):(ll0), plates. Orientation: See Part II. Colorless. a = 1.549, ;3 = 1.553, r = l-556. r-Q: = 0.006, 7—^ = 0.002, ^-a = 0.004. Dispersion: p Cinada tnlsani, MAXIMUM BIREFRINGENCE > Quarb. The Mineral ii NEGATIVE (-). The Mineral is POSITIVE {+). < Increase in Index of Refraction. > NN,-.^M ws Weilliiiii. HotWirted, In oraas'g BIREFR. Wol Wamaa. WeJIlim, :m m Antigoritc. — ' I Anofthite. -^ -Wollastonite. - Carpholite.* Rosenbuschite. Actinolite." Lazulite. ■■ Muscovite. Paragonite. _£tl^gopite. ,085 .090 095 ,100 ,120 140 160 .200 250 - •— Celaian.. r JiAosandrite. • -Johnstnipite. ^— Spodumene. Jadeite. • - Augite.- Diopside. Diallage.. Mn. Piedmontite. ^^— I ■-" Olivine.- -MonaziJSi ANISOTROPIC. 413 Axinite.— C^Al^B^CSiO,)^, H = 6.5-7.0, = 3.27-3.29. Trielinic. (-). Cleavage: (010), distinct. Habit: Grains, tablets. Orientation: Plane of the optic axes makes an angle of 24° 40' with the edge (111):(1T1), and of 40° with (111);(1I0). Colorless, yellowish, brownish, violet. Pleochroism: In thin sections weak or not noticeable. a = 1.685, /?= 1.692, r= 1-695. r-a = 0.0]0, r-/5 = 0.003, /?-a = 0.007. Dispersion: Distinct, pu. 2£=16°-98° In boiling HCl siHca separates. Pennine is pleochroic, tlie double refraction is lower, and the indices are higher. Ordinary serpentine is ( + ). Disthene.— Al^SiO^ , H = 4.0-7.0, G = 3.56-3.67. Trielinic. ( — ). Cleavage: (100) good, (010), distinct; (100): (010) = 74°. Habit: Prisms, tablets. Elongation: (100) : (010), lath-shaped. (-I-). Orientation: o = nearly at right angles to (100), t = inclined 30° on (100) against edge (100): (010). Colorless, blue. Pleochroism: Weak, between blue and colorless. a = 1.717, ^=1.722, r=l-729. }-- a = 0.012, 7--/? = 0.007, (3- a =0.005. ANISOTROPIC. 415 Dispersion: p> u, 2F = 82°. Insoluble in acids. Does not occur in eruptive rocks, but in crystalline schists and gneisses. Sillimanite and andalusite are orthorhombic, have parallel extinction, and different cleavage. Sillimanite has higher double refraction and smaller 2F(31° to 42°). Topaz has basal cleavage, and is orthorhombic. Zoisite and colorless epidote have different position of axial plane in relation to cleavage. Serendibite and dumortierite have different pleochroism. Blue amphibole has different cleavage and lower indices of refraction. Anorthite.— CaAljSi^Oj, H = 6.0, = 2,74-2.76. Triclinic. ( — ). Cleavage: (001), (010), good. Habit: Plates, prisms, grains. Elongation: (001): (010), laths; (110):(1T0), plates. Orientation: See Part II. Colorless. a = 1.575, ^=1.584, r= 1-588. r- a = 0.013, 7--^ = 0.004 /?-a = 0.009. Dispersion : p> u, 2F = 77°-80°. Decomposed by HCl with separation of gelatinous silica. Other feldspars, see Part II. WoUastonite.— Ca Si^Oj , H = 4.5-5.0, G = 2.8-2.9. Monoclinic. ( — ). Cleavage: (100), good; (001), distinct. (100) : (001) =84.5°. Habit: Tablets rods along b. Twinning on (100). Elongation: (001): (100), lath-shaped to tabular. (±). Orientation: 6 = i, c: n= +32° 12'. Colorless. Pleochroism: None. a = 1.619, /!=1.632, r=1634. r-Q: = 0.015, r-i8 = 0.002, /3-a = 0.013. Dispersion: Distinct, p>u. 2F = 40° to 69° Gela'.inizes wi h HCl. Pectolite and tremolite differ in not having the plane of the optic axes at right angles to the elongation, which is very characteristic of sections in the ortho-diagonal zone of wollastonite. Epidote, when colorless, is separated by not gelatinizing with acids, higher indices, and higher double refraction. ANISOTROPIC. 417 Carpholite.— H,MnAl2Si,0,„, H = 5.0-5.5, G = 2.935. Monocl nic. ( — ). Cleavage: Unnoticeable. Habit: Rods, fibres. Elongation: (110) :(1T0), narrow laths. ( + ). Orientation : b=c, c : t = 4° ca. Colorless, greenish yellow. Pleochroism : Distinct, t = colorless, = 6 = yellowish green. Index :?i = 1.627 J— ff = 0.022. 2F = 60°. Insoluble in acids. Rosenbuschite is soluble in acids, elongation ( — ). 2F = 90°, cleavage (001), good. Sillimanite is ( + ), non-pleochroie, good (100) cleavage. Rosenbuschite.— 2Na,Zr02F2 • eCaSiO^ • TiSiOj • TiO, , H = 5.0-6.0, G = 3.30- 3.315. Monoclinic. (±). Cleavage: (001), good; (100), distinct. (001) : (100) = 78°. Habit: Rods along b. <» Elongation: Narrow laths. ( — ). -S Orientation : i = a, c:c= + 13°. Colorless, light greenish yellow. Pleochroism : Not marked, c > B > o. 's Index :n = 1.65 ci. ^ r-a=0.026ca 2F = near 90°. Easily attacked by HCl. Sillimanite and carpholite are not affected by HCl and have ( + ) elongation WoUastonite has (±) elongation, lower double refraction, and higher extinction angle. Pectolite has ( + ) elongation. Tremolite.— CaMg3Si,Oi2, H = 5.0-6.0, = 2.9-3.1. Monoclinic. ( — ). Cleavage: (110), good; (100), distinct; (010), poor. (110):(lT0) = 55° 49'. Habit: Rods, leaves, grain.s. (100) twinning occurs. Elongation: (110):(ll0), lath-shaped. ( + ). Orientation: 6 = B, c: t= — 16°. Colorless. Non-pleochroic. a = 1.604, /?= 1.618, r= 1-630. r-« = 0.026, r-/?=0.012, /3-a = 0.014. Dispersion : Weak, pKu. 2F = 81° to 87.5°. ANISOTROPIC. 419 Hardly affected by HCl. Wollastonite gelatinizes with HCl and has trace of the plane of the optic axes at right angles to the cleavage, elongation (±). Pyroxenes, see Part II. Actinolite is pale green and pleochroic. Actinolite.— (FeMgXSi^Oij, H = 5.0-6.0, G = 3.0-3,2. Monoelinic. ( — ). Cleavage: (110), good; (100), distinct; (010), poor. (110):(lT0) = 55° 49'. Habit: Rods, leaves, grains. (100) twinning occurs. Elongation: (110):(ll0), lath-shaped. ( + ). Orientation: b = i, c:c= — 15°. Green. Pleochroism: Hardly noticeable in thin sections. t = green; 6 and o = yellowish green, a = 1.607, /3 = 1.6247r= 1-634. r-a=0.027, y--.5 = 0.010, ^-a = 0.017. Dispersion: Weak, p u. 2y = 74°-90° Not affected by HCl. Pistacite is the Fe-rich epidote; clinozoisite the Fe-poor or free epidote. Pyroxene has (110) cleavage; in sections parallel to the long direction the trace of the optic axes is parallel to the cleavage; in epidote at right angles to it. Clinozoisite has lower double refraction, is ( + ), and has strong pu. 2^=60°-70°. Insoluble in HCl or H^O,. One of the mica group. Micas can only be confused with other minerals which have perfect basal cleavage. The birefringence, very low in basal sections and high in sections showing cleavage, is very characteristic. Chlorites have weak double refraction. Talc has 2E = %°-2Q°. Biotite has 2E generally much smaller than muscovite; also b = i. Lepidolite and paragonite may be separated from mus- covite by the Li flame test. See also Part II. Paragonite.— Al(SiO,).,AlJ^aH,, H = 2,0-2.5, = 2.8-2.9 Monoelinic. ( — ). Cleavage; (001), good. Habit: Leaves, leafy aggregates. Elongation: Perpendicular to (001), lath-shaped. { + ). Orientation; b=i, c:o = 0° to -1-2° Colorless, greenish, yellowish. Pleoohroism: Weak or none. Indices: Very similar to muscovite (see above). Birefringence: Like muscovite (see above). ! Dispersion : Weak, p> u 2E = 70° ca. Insoluble in acids. One of the micas. Muscovite and lepidolite are different chemically. Talc has 2.E = 6°-20°. Datolite.— HCaBSiO,, H = 6.0-5.5, G = 2.9-3.0. Monoelinic. ( — ). No distinct cleavage. Habit Grains, rods. Orientation; 6 = J, c: a= -H° to -1-4°. Colorless. a =1.625, /3 = 1.653, ?-= 1-669. r-a: = 0.044, 7--^=0.016, ^-a = 0.028. Dispersion: Weak, p> u. 2F = 74°. Gelatinizes with HCl. B reaction. Pistacite has cleavage, no reaction for B, and has lower birefringence. ANISOTROPIC. 425 Phlogopite. — A magnesium mica, near biotite, but containing little Fe, H = 2.5-3.0, G = 2.78-2.85. Monoclinic. ( — ). Cleavage: (001), good. Habit: Leaves, leafy aggregates. Elongation: At right angles to (001), lath-shaped. ( + ). Orientation: 6 = 6, c:o = 0° to +7°- Colorless, yellowish, brownish, greenish. Pleochroism : Weak, with c > 6 > o. a = 1.562, ^ = 1.606, r=l-606. r-a^o.ou, r-/?=o.o, /3-q:=o.044. Dispersion: Weak, p? Elongation: (110):(lT0), lath-shaped. (-I-). Orientation: b = b, c:c=-ll° to -15°. Colorless, brownish. Pleochroism: c = light brown; b = o = colorless. /9=1.73. }-- a=0 056. 2F = large. Pyroxenes differ in cleavage and have higher extinction angles. ANISOTROPIC. 427 Celsian.— BaAIjSi^Og, H = 6.0, G = 3.384. Monoclinic. ( + ). Cleavage: (001), (010), good. Habit: Plates, prisms, grains. Carlsbad twinning common. Elongation: (001): (010), lath-shaped; (110):(lT0), plates. Orientation : b = b, a : o = — 62°. Colorless. Non-pleochroic. a = 1.584, ^ = 1.589, r = l-594. 7— a=0.010, 7— /S=0.005, /3-q: = 0.005. 2F = 86°. Gelatinizes with HCl. Orthoclase and sanidine have lower indices of refraction, smaller extinction angles, lower specific gravitie.'', and no Ba. Plagioclase feldspars show polysynthetic twinning, celsian never. Microcline shows ' ' grating structure " which never occurs in celsian. See also Part II. Ottrelite.— H„(Fe,Mn)Al^iO, , H = 6.0-7.0, = 3.53-3 55. Monoclinic. ( + ). § Cleavage: (001), good. Habit: Leaves, plates. Tschermak twinning frequent. {;_-, Elongation: At right angles to (001), lath-shaped. ( — ). Orientation: b= o, c; [=0° to 20° Blue, green, colorless. Pleochroism: c = yellowish green, colorless; B = blue; o = olive- green. /3=1.74. ?— -o: = 0.010. Dispersion: Strong, p> u. Strong dispersion, c:Cp>c:e„. 2 .E= large, very variable. Hour-glass structure common. Insoluble in HCl. Almost e.xelusively confined to the phyllitic schists. Other minerals: The high indices in combination with the low double refraction, the strong dispersion of the bisec- trices and axes, and the peculiar pleochroism, separate the ottrelite group from all other minerals showing good (001) cleavage only. Mosandrite. I Ti, Zr, Th, Ce, Y, Fe, Mn, Ca, Mg, Na, K, F silicate, H = 4.0 Johnstrupite. / and more, = 3.10-3.29 (Johns.), 2.93-3.07 (Mosan.). Monoclinic. ( + ) Cleavage: (100),. good. ANISOTROPIC. 429 Habit: Tablets (100), elongated on l. Twinning (100) frequent. Elongation: (100); (010), lath-shaped. (-). Orientation: 6 = 6, c:o = 3°. Grayish yellow to colorless. Pleochroism: Not seen in thin sections. q: = 1.646, ^=1.649, r= 1-658. r-a = 0.012, j--/S = 0.009, /3-a = 0.003. Dispersion: Strong, p>u. 2^ = 70°. 2£;„tt = 128°37'. Soluble in HCl with separation of SiOz) the dark-red solution on heating, gives oflF CI, and becomes yellowish. Rinkite has strong p-6.0, = 3.2-3.6. Monoclinic ( -I- ) Cleavage: (110), good. (110):(1T0) = 93°. Habit: Short prisms, grains, (100) twinning. Elongation- (110):(lI0), lath-shaped. Orientation: 6 = 4, c: c= -45° to -55° Green, brown, reddish, violet, rarely yellow, nearly colorless. Pleochroism Usually weak, varies. a = 1.698, /?=1.704, r = l-723. r-a = 0.025, r-|8=0-019, /3-q: = O.O06. Dispersion: Weak, p>u. 2F = 60° ca. Distinct dispersion of bisectrices. Unacted upon by acids. Othe- pyroxenes, see Part II, Olivine has different orientation of inter.'erence figure. ANISOTROPIC. 431 Jadeite.— NaAlSi,0„, H = 6. 5-7.0, G = 3.33-3.35. Monoclinic. ( + ). Cleavage: (110), good. (110): (1T0) = 93°. Habit; Short rod-like. Elongation: (110):(lT0), lath-shaped. Orientation: 6 = B, c:c=— 33.5°- Colorless, light greenish. Pleochroism: Wanting in thin sections. /?= 1.654. r -a = 029. Disperson: po. 2F=59°. Unacted upon by acids. All pyroxenes are characterized by right-angled cleavage on basal sections. Zonal structure is common with increasing green color outward and corresponding increase in the ex- tinction angle. Other pyroxenes, see Part II. Olivine has different orientation of interference figure. Diallage. — Composition near diopside, but often containing Al, H = 4.0, = 3.2-3.35. Monochnic. ( + )■ Cleavage: (110), (100), good. (110):(lI0) = 92° 50'. Habit: Grains, short prisms, (100) twins frequent. Elongation: (110):(lT0), lath-shaped. Orientation: 6 = b, c: t= — 39°. Colorless, greenish, brownish. Pleochroism: Weak, B = yellowish, o=c = greenish Indices: Like diopside. Birefringence: Like diopside. Dispersion: Weak, p> u. ANISOTROPIC. 433 2F = 59° and less. Unacted upon by acids. The complete (100) cleavage is most characteristic. Diallage is confined in occurrence to the gabbros, peridotites, and pyroxenites. Other pyroxenes, see Part II. Olivine has different orientation of interference figure. Piedmontite. — Magnesia rich. See among the negative minerals above. Olivine.— (Mg,Fe)Si04, H = 6.5-7.0, = 3.27-3.45. Orthorhombic. ( + ). h. (— } / 2 ' /T i' tr_ "Avj't.t . Cleavage: (010), (001), distinct; (100), poor. Generally shows heavy irregular cracks. Habit: Isometric, short prisms, grains; twins (Oil) and (012) rare. Orientation : a = c, 6 = o. Greenish, yellowish, reddish, colorless. Pleochroism: Wanting or weak in yeUow and red tones. a: = 1.654, /3=1.670, )- = l-689. r-a = 0.035, r-i9 = 0.019, l3-a = 0.0lG. Dispersion: Distinct, p < u. 2F = 88° ca. Gelatinizes slowly in HCl. Diopslde has two equally good cleavages to which the ex- tinction is inclined; olivine has two very unequal cleav- ages to which the extinction is parallel in the principal zone. Diopside does not gelatinize with HCl. Fayalite has 2V' = 50°, has higher birefringence, and is ( — ). Forsterite has lower indices and occurs as a contact mineral in metamorphosed limestones. Monticellite is (-) and has 2F = 37.5°. Augite and diallage, when in sections showing parallel ex- tinction, have different orientation of interference figures. Pectolite— NaHCajSijOg, H = 4.5-5.0, = 2.74-2.88. Monoclinic. ( + ). Cleavage: (100), good; (001), distinct. (100) : (001) = 84.5°- Habit: Tablets ; rods along b. Elongation: Laths, tabular, (-f-). Orientation: 6= t, c: B= — 5°. Colorless Non-pleochroic. Index: ra = 1.61 ca. y— a = 0.038 2^=60°. Soluble in HCl. 1 WoUastonite is ( — ), has (±) elongation, lower double and higher indices of refraction, and c:a= 4-32° ANISOTROPIC. 435 Monazite— (Ce,La,Di)PO,, H = 5.5, = 4.9-5.3. Monoolinic. ( + ). Cleavage: (100), (010), distinct. Habit: Tablets, often elongated on 6. Elongation: (001) : (100), short laths. (-). Orientation. 6= o, c: ( = 2° to 6° Colorless, yellowish. Non-pleochroic in thin sections. a = 1.796, /3=1.797, r=l-841. r-a = 0.045, 7--i5 = 0.044, /3-a = 0.001. Dispersion: Weak, /) u and contains no P, extinction + 39°, weak pleochroism, and much higher birefringence. Titanite.— CaSiTiO,, H = 5.0-5.5, G = 3.4-3.56. Monoclinic. ( + ). Cleavage: (110), distinct. (110):(ir0) =46° 8'. Habit: Prisms, rhombs, grains, rods. Orientation :6 = B,c:t=+39°. S Colorless, yellowish, reddish, brownish. Pleochroism: Weak, c>b>a. >?? a = 1.913, /3=1.921, 7-=2.054. r-a = 0.147, r-^=0.133, /?-a = 0.008. Dispersion: Very strong, p> u. 2Ena = 4:5°-Q8°. Very slightly affected by HCl. Monazite has lower birefringence, weak dispersion, and low extinction angle. Brookite has parallel extinction, 2V = 0°-23°, and gi-eat difference in the birefringence in the two directions; 7— a = 0.158, while /?-a = 0.003. Rutile, xenotime, hussakite, and cassiterite give no Ca reac- tions. The mineral is anisotropic. Inclined extinction. Colored. o o o 437 The mineral is anisotropic. o o o Inclined extinction. fe Colored. Non-pleochroic. \jtM'tiiA«-^,-^. ^ij, '0' o 3 439 The mineral is anisotropic. Inclined extinction. Colored. Non-pleochroic. Maximum birefringence is less than that of quartz. <2u&^ Ifx^^-i^ a^L^^Jt -j.H^l N H < O M M M o o 441 442 ANISOTROPIC. The Mineral ■• ANISOTROPIC, has INCUNED EXTINCTION, >. COLORED, NON-PLEOCHROIC, MAXIMUM BIREFRINGENCE < Quartz. The Mineral i, NEGATIVE (-). The Mineral ia POSITIVE (+). < OOOOIfl knomoor* o in o 10 r- (D u. 2B=16°-98°. In boiling HCl silica separates. Pennine is pleochroic, has lower double and higher indices of refraction. Ordinary serpentine is (-I-). ANISOTROPIC. 445 Kelyphite. — Alteration product of pyrope. Becke believes it to be composed of picotite and hornblende. Habit: Rims around pyrope, radial fibres. Birefringence is weak. Glauconite. — A hydrous silicate of K, Fe, and Al, with Mg and Ca, H = 2.0, 0=2.2-2.4 Biaxial. Rounded bodies, small spheres, irregular, and aggregates. No cleavage. Yellowish green, bluish green, dark green. Pleoohroism: None, or green and yellow. Index: Higher than quartz. Birefringence: Low to medium. 2£; = to 40°. Slowly soluble in boiling HCl, leaving a residue of SiO^ in the form of spherules. Resembles chlorite. Clinozoisite.— Al(SiO,)3(Al,Fe)2Ca-CaOH, H = 6.5, = 3.3-3.5. Monoclinic. ( + ). Cleavage: (001), good; (100), distinct. (001): (100) = 64° 37'. Habit: Prisms and rods elongated on b grains. (100) twinning. Elongation: (001): (100), lath-shaped. (±). Orientation: b = i, c:(l=— 3° Colorless, reddish. Pleoohroism: Weak. a = 1.716, ^=1.718, r = l-724. r- a = 0.008, r- ,8 = 0.006, ^- a = 0.002. Dispersion: Strong, p Qiurtx. ' The Minenl i> NEGATIVE (-). Th. Mineral U POSITIVE (+). e in Index of Refraction. ooooin 1,70 1 65 1.60 l.SS In orais'g BIREFR. 1,S5 1 60 1.65 1.70 1.75 oooo eocnom «.-iNN WeriH. HI Sll- Medium. Not Maiked. Nol Mirted. MBdlum. High, WIT .010 — D sthene. Antig jrite. — •s Mosandrite.- serpentine. 1 - Johnstrupite. Spodutnene.- - Hedenbergite. «- 025 030 .035 040 045 .050 Rosenb jschite, Utinolit 1. Wdhlei:te. Jadeite. « '■' — Diallage. Diopside. Ml scovite. Paragonite. Wonazlte. 055 Vstrophy — .065 .070 .075 oao .090 .095 ,100 ,120 ,140 ,160 Tiunii s.— .iSo - * ANISOTROPIC. 449 Axinite— Ca,Al,B2(SiO,)f, H = 6.5-7.0, G = 3.27-3.29. Triclinic. ( — )- Cleavage: (010), distinct. Habit: Grains, tablets. Orientation: Plane of the optic axes makes an angle of 24° 40' with the edge (111), (111): and of 40° with (111):(1I0). Colorless, yellowish, brownish, violet.. Pleochroism: Weak or not noticeable in thin sections. 01 = 1.685, /3 = 1.692, }-=l-605. 7— q: = 0.010, r-/5 = 0.003, /?-a = 0.007. Dispersion : Distinct, pu. 2S=16°-98°. In boiling HCl silica separates. Pennine is pleochroic, has lower double and higher indices of refraction. Ordinary serpentine is ( + ). Disthene.— AljSiOs , H = 4.0-7.0, 0=3.56-3.67. Triclinic. ( — ). Cleavage: (100), good; (010), distinct. (100) : (010) = 74°. Habit : Prisms, tablets. Elongation: (100) : (010), lath-shaped. ( + ). Orientation: = nearly at right angles to (100), ( = inclined 30° on (100) against edge (100): (010). Colorless, blue. Pleochroism: Weak, between blue and colorless. a = 1.717, /? = 1.722, r = l-729. r-a = 0.012, ?-- /' = 0.007, /3-a = 0.005. Dispersion: p>o. 2F = 82°. ANISOTROPIC. 451 Insoluble in acids. Does not occur in eruptive rocks, but in crystalline schists and gneisses. Sillimanite and andalusite are orthorhombie, have parallel extinction, and different cleavage. Sillimanite has higher double refraction and smaller 2V (31°-42°). Topaz has basal cleavage, and is orthorhombie. Zoisite and colorless epidote have different position of axial plane in relation to cleavage. Serendibite and duinortierite have different pleochroism. Blue amphibole has different cleavage and lower indices of refraction. Rosenbuschite.— 2NajZr02F2 ■ eCaSiOj • TiSiO^ • TiOj, H = 5.0-6.0, G = 3.30- 3.315. Monochnie. ( ± ?). Cleavage: (001), good; (100), distinct. (001) : (100) = 78°. Habit: Rods along 6. Elongation: Narrow laths. ( — ). Orientation: b=o, c:t= + 13°- Colorless, light greenish yellow. Pleochroism : Not marked, t > i > o S Index: M = 1.65ca. 7" -a = 0.026 ca. ^ 2y = near 90°. Easily attacked by HCl. Sillimanite and carpholite are not affected by HCl and have ( + ) elongation. WoUastonite has (it) elongation, lower double refraction, and higher extinction angle. Pectolite has ( + ) elongation. Actinolite.— (FeMg).Si40,2, H = 5.0-6.0, G = 3.0-3.2. Monoclinic. ( — ). Cleavage: (100), good; (100), distinct; (010), poor. (110):(lI0) = 55° 49'. Habit: Rods, leaves, grains. Twinning (100) occur.?. Elongation: (1101:(lT0), lath-shaped. ( + ). Orientation : b = B, c : c = — 15°. Green. Pleochroism: Hardly noticeable in thin sections. t = greeH; i and a = yellowish green. a: = 1.607, /3 = 1.624, 7-=1.634. r- a = 0.027, r- ,5 = 0.010, j8- a = 0.017. Dispersion: Weak, p>u. 2F = 80°. ANISOTROPIC. 453 But slightly acted upon by HCl. Pistacite has trace of the plane of the optic axes at right angles to the cleavage, and (±) elongation. Pyroxenes differ in cleavage and have higher extinction angle. See Part II. Muscovite.— Al(SiO,)3KH2Al,, H = 2.0-2.5, G = 2.8-2.9. Monoclinic. ( — ). Cleavage: (001)^ good. Habit: Leaves, leafy aggregates. Elongation: At right angles to (001), lath-shaped. ( + ). Orientation: 6 = t, c: 0= 0° to 2°. Colorless, greenish, yellowish. Pleochroism: Weak or wanting. a: = 1.563, /? = 1.598, j-=l-601. r-a^O.038, r-/? = 0.003, /3-r = 0.035. Dispersion: Weak, |0>u. 2£ = 60°-70° Insoluble in HCl or H^SO,. One of the mica group. Micas can only be confused with other minerals wliich have (» perfect basal cleavage. The birefringence, very low in ;_§ basal sections and high in sections showing cleavage, is g very characteristic for the micas. ^ Chlorites have weak double refraction. ^ Talc has 2E = 6°-20°. Biotite has 2E generally much smaller than muscovite; also 6 = 6. Lepidolite and paragonite may be separated from muscovite by the Li flame test. Paragonite.— AKSiOJsAljNaHj, H =2.0-2.5 = 2.8-2.9. Monochnic. ( — ). Cleavage: (001), good. Habit: Leaves, leafy aggregates. Elongation; Perpendicular to (001), lath-shaped. (. + )■ Orientation: 6=(, c: = 0° to -1-2° Colorless, greenish, yellowish. Pleochroism: Weak or none. Indices: Very similar to muscovite (see above). Birefringence: Like muscovite (see above). Dispersion: Weak, p> u. 2£;=70° ca. Insoluble in acids One of the micas. Muscovite and lepidolite are different chemically. Talc has 2B = 6°-20°. See also Part II. O ANISOTROPIC. 455 Phlogoplte.— A magnesium mica, near biotite, but containing little Fe, H = 2.5-3.0, G = 2.78-2.85. Monoclinic. ( — ). Cleavage; (001), good. Habit. Leaves, leafy aggregates. Elongation- At right angles to (001), lath-shaped. ( + ). Orientation: b = 6, c: = 0° to +7°. Colorless, yellowish, brownish, greenish. Pleochroism: Weak with t > i > n. a = 1.562, /3 = 1.606, }- = l-606. r-a = 0.044, r-^ = 0.0, /3-ff=0.044. Dispersion: Weak, p>u. 2£= small to 0°. Completely decomposed by H^SO,, leaving the siHca in thin flakes. One of the mica group. Other micas: Phlogopite has small to very small 2V, and has symmetrical position of the plane of the optic axes. Other minerals, see Part II. a ANISOTROPIC. 457 Mosandrite. \ Ti,Zr,Th,Cc,Y,Fe;Mn,Ca,Mg,Na,K,F silicate, H = 4.0 and more, Johnstrupite. i G = 3.10-3.29 (Johns.), 2.93-3.07 (Mosan.). Monoclinic ( + ) Cleavage: (100), good. Habit: Tablets (100), elongated on c; twinning (100) frequent. Elongation: (100): (010), lath-shaped. (-). Orientation: 6 = 6, c;o = 3°. Grayish-yellow to colorless. Pleochroism: Not seen in thin sections. a = 1.646, /3 = 1.649, r=l-658. r-cc = 0.012, r-Z'-O.OOQ, jS-a: = 0.003. Dispersion : Strong, p>ij. 2y = 70°. 2E„a = 128°37'- Soluble in HCl with separation of SiOj; the dark-red solu- tion on heating gives off CI and becomes yellowish. Rinkite has strong pu. 2£;=16°-50° Soluble with separation of gelatinous silica in boiling HCl or HjSO,. Pennine, when optically ( — ), separated by optical char- acter; when (-I-), by chemical test for AlA- It has lower birefringence. Spodumene.— LiAlSijO,, H = 6.5-7.0, = 3.1-3.2. Monoclinic. (-I-). Cleavage: (110), good; (010), (100), distinct. (110):(lT0) = o. 2F = 59°52'. Unacted upon by acids. Other pyroxenes, see Part II. Augite.— CaMgSi^Oo with (Mg,Fe)(Al,Fe),SiO„ , H = 5.0-6.0 G = 3.2-3.6 Monoclinic. ( + ). § Cleavage: (110), good. (110) : (110) = 93° Habit: Short prisms, grains, (100) twinning. (^ Elongation: (110): (110), lath-shaped. Orientation: 6 = 6, c:c=-45° to -55°. Green, brown, reddish, violet, rarely yellow, nearly colorless. Pleochroism: Usually weak, varies. a: = 1.698, ,8=1.704, ^ = 1.723. j--a = 0.025, r-/?=0.019, /?-a = 0.006. Dispersion : ^Aeak, p > u. Distinct dispersion of bisectrices. 2U = 60° ca. Unacted upon by acids. Other pyroxenes, see Part II. Olivine has different orientation of interference figure. Wohlerite.— Si.oZrsNbjO.jFejCaioNas, H = 5.5-6.0, = 3.41-3.442. Monoclinic. (-H). Cleavage: (010), poor. Habit: Tablets, grains, twinning widespread. Elongation: (100), lath-shaped, tabular. Orientation: &= a, c: (= -t-44°. "i'ellow. Pleochroism: Weak, t>6=a. a = 1.700, ^=1.716, 7"= 1-726. r-a = 0.026, r-iC^O.OlO, a-r = 0.016. ANISOTROPIC. 461 Dispersion: Weak, pu. 2y = 59° and less Unacted upon by acids. The complete (100) cleavage is most characteristic. Diallage is confined in occurrence to the gabbros, peridotites, and pyroxenites. Other pyroxenes, see Part II. Olivine has different orientation of interference figure. Diopside.— CaMgSijO. , H = 5.0-6.0, = 3.34. Monoclinic. {. + )■ Cleavage: (110), good. (110) :(1T0) = 92° 50'. Habit: Prismatic, grains, (100) twinning. Elongation: (110) : (110), lath-shaped. ANISOTROPIC. 463 Orientation: ?) = 6, c: t= -39°. Colorless, greenish. Pleochroisin: Weak or wanting. a- 1.671, /?= 1.678, r=l-VOO. r-a = 0.02d, r-/?=Q.022, /?-« = 0.007. Dispersion : Weak, p> u. 2F = 59° Unacted upon by acids. All pyroxenes are characterized by right-angled cleavage in basal sections. Zonal structure is common with increasing green color outward, and corresponding in- crease In the extinction angle. Other pyroxenes, see Part II. Olivine has different orientation of interference figure. Monazite.— (Ce, La, Di)PO^ , H = 5.5, G = 4. 9-5.3. Monoclinic. (-I-). Cleavage: (100), (010), distinct. Habit: Tabk'ts, often elongated on b. Elongation: (001): (100), short laths. (-). Orientation: 6= n, c: t = 2° to 6°. Colorless, yellowish. Non-pleochroic in thin sections. a = 1.796, /3=1.797, r=1.841. r- a = 0.045, r-^ = 0.044, /?-a = 0.001. (^ Dispersion: Weak, p<'j. 2E = 21° to 36° Soluble with difficulty in HCi, leaving a white residue. Olivine has 2F = 88° ca., and contains no P. Titanite has strong dispersion p> u, and contains no P, extinction 4-39°, weak pleoehrosim, and much higher birefringence. Astrophyllite.—Ti, Fe, Mn, K silicate. H = 3.0-4.0, G = 3.3-3.4. Orthorhombic. (-I-). Cleavage: (100), good. Habit: Plates, laths along b, leaves, rosettes. 'Elongation; At right angles to (001), lath-shaped. (±). Orientation : /i = a,b= t. Yellow. Pleochroism: a = yellow to red; B = orange; c = citron-yellow. a = 1.678, /?= 1.70.3, r = 1-733. r-a^O.055, r- (8 = 0.030, /?-a = 0.025. Dispersion : p> u. 2E = eii. 1G0°. Insoluble in HCI. Micas have smaller axial angle and lower indices. ANISOTROPIC. 465 Brittle Micas have lower double refraction, smaller axial angle around the bisectrix emerging on cleavage flakes, and are ( — ). Titanite.— CaSiTiO, , H = 5.0-5.5, G = 3.4-3.56. Monoclinic. ( + ). Cleavage: (110), distinct. (110): (110) = 46° 8'. Habit: Prisms, rhombs, grains, rods. Orientation: b = I), c: c= +39°. Colorless, yellowish, reddish, brownish. I'leochroism: Weak, c>6>n. « = 1.913, /3 = 1.921, r = 2.054. 7--a=0.147, r-« = 0.133, ;S-a = 0.008. Dispersion' Very strong, p>u. Very slightly affected by HCl. Monazite has lower birefringence, weak dispersion, and low extinction angle. Brookite has parallel extinction, 21=0° to 23°, and great difference in birefringence in two directions; ;■— a = 0.158 while ;S-a = 0.003. Rutile, xenotime, hussakite, and cassiterite give no Ca reactions. S o 6 The mineral is anisotropic. Inclined extinction. Colored. Pleochroic. s o o » o o 467 N H < O O The mineral is anisotropic. g Inclined extinction. 3 Colored. w Pleochroic. « Maximum birefringence is less than that of quartz. 469 470 ANISOTROPIC. Th. Min«r.l U ANISOTROPIC, h>. INCLINED EXTINCTION, i. COLORED. ■• PLEOCHROIC, MAXIMUM BIREFRINGENCE < Quartz. The Mineral U NEGATIVE (-). Tie Mineral i> POSITIVE (+). 2 50 2 00 1 90 1 80 1 75 1 70 1.60 I.S5 In neas'g 1.55 1 60 1.65 1 70 1.75 1.80 1.90 2.00 2 50 VeriH. m h. Medium. Not Mirted. Mol Mirtgd. Msdlum. HiBh. VenH. I 1 1 001 002 ' . ■ huringit e. - Pennine. - - Pennine.-* 003 004 - ^.rfvedsoni Dphirine.- e. , 1 - Riebechite. 006 . • Aenigmatite. 007 008 - 009 010 ~ Dclessite —Antig orite. - Clinochlore. ANISOTROPIC. 471 Thuringite.— Higreg(AlFe)8Si„0„ , H = 2.5, = 3.15-3.19. Monoclinic. ( — ). Cleavage: (001), good. Habit. Scales, irregular or concentric scaly aggregates. Elongation: At right angles to (001), lath-shaped. ( + ). Orientation : c : n = 0° Green, yellow. Pleochroism: n = yellow; 6 = c = green. Index similar to pennine, 1.583. Birefringence similar to pennine, 0.002. 2£' = 0°-25°ca. A lepto-chlorite. Gelatinizes easily with HCl. Arfvedsonite.— NajFe,Si,0,., , 11 = 5.5-6.0, G = .3.44-3.46. Monocline. (T). Cleavage: (110), good; (100), distinct; (010), poor. (110);(lT0) = 56° 5'. Habit: Short prisms, grains. Elongation: (110):(lI0), lath-shaped. (-). Orientation: 6 = 6, r:o=-70°to -80°. Blue, brownish green, grayish green, grayish violet. Pleochroism : Changeable, with n > li > t. Index: High. Birefringence: Very low. Dispersion: Strong, of bisectrices; c;np u. 2£=16° to 98°. Silica separates in boiling HCl. Pennine is pleochroic, has lower double and higher indices of refraction. Ordinary serpentine is (-I-). ANISOTROPIC. 475 Pennine. — A magnesium aluminium silicate. H = 2.0-2.5, G = 2.73. Monoclinic. (±). Cleavage: (001), good. Habit: Leaves, scales, leafy aggregates. Elongation: At right angles to (001), lath-shaped. (±). Orientation : b = b, c : c or o = 0°. Green. Pleoohroism: S and n = green; t = yellowish. « = 1.582, /? = ( ), r= 1-584. 7"- a = 0.002. Dispersion: p> 'j, or p<'j. 2E = 0° to 61°- Partially decomposed by HCl. Often abnormal interference colors. Micas ha\-e higher double refraction and different pleo- chroism. Brittle micas have different pleochroism. Other minerals do not have the scale-like character. Serpentine has higher birefringence and lower indices. The separation from antigorite may be difficult and is sometimes only possible chemically. Serpentine is usu- ally much less pleochroic. See also Part II. § Rinkite. — An iron-bearing titanosilicate of Ca, Ce, and Na. H = 5.0, = 3.46-3.50. • ^ Monoclinic. { + )■ Cleavage: (100), good. Habit: Tablets, (100); (100) twinning frequent. Elongation: (100): (010), lath-shaped. (T). Orientation: 6= tl. c:6 = 7° ca. Yellowish, colorless. Pleochroism : c> 6 > a. a = 1.665, /3 = 1.668, r = ( )■ ^-01 = 0.003. Dispersion : Strong, pu. Kiebeckite — NajFejSiiO,^, H = 5.5-6.0, G>3.33. Monoclinic. (±). Cleavage: (110), good; (100), distinct; (010), poor. (110): (110) = 56° 0'. Habit: Short prisms, grain«. Elongation: (110):(1T0), lath-shaped. (-). Orientation : b = b, c:c=— 85°. Blue, yellowish, green. ANISOTROPIC. 477 Pleochroism: n = deep blue; 6 = Iighter blue; ( = yellowish green. a>1.687, r u. Fuses easily and colors the flame an intense yellow. Other amphiboles, see Part II. See also arfvedsonite, above. .(Enigmatite.— Na,Fe„AlFe(Si, Ti),As, H = 5.5, = 3.80-3.86. Triclinic. ( + ). Cleavage: 110), (lIO), good. 4.66° Elongation: (110): (110), lath-sliaped (±). Orientation: Extinction from cleavage on (100) = about 3° to 6°; on (010), about 44°; on (110) and (110) about 30° and 37°. Brownish red. Pleochroism: = light red-brown; 1) = chestnut brown; c = brownish black. Index: High, about like hornblende, r- a = 0.006. Dispersion : p<'j. 2E = m° 03.. Somewhat acted upon by HCl. Other minerals. The very low double and high index of refraction, the almost complete absorption of all but the red rays, and the good cleavage, are combined in no other minerals. Clinochlore. — Hydrous magnesium, iron, aluminium silicate. H = 2.0-2.5, = 2.71. Monoolinic. ( + ). Cleavage: (001), good. Habit: Leaves, scales, leafy aggregates. Twinning widespread. Elongation; At right angles to (001), lath-shaped. ( — ). Orientation: b = B, c: t= -2° to -9° Green. Pleochroism: and b = green; t = yellow. af=1.58.'5, 3=1.586, r = l-596. r-a=o.oii, r-/5=o.oio, /?-ff=o.ooi. Dispersion: p Quuii. The Mioeral it NEGATIVE {-). The Mineral i> POSITIVE (+). 1 5;IS 1.75 1.70 1.65 1 60 1 55 In- oreasg BIREFR. 1.55 1 60 1.65 1.70 III 1.90 2.00 2.50 VsaH. Hish. Medium. Not Marked. Not Marked. Madlum. 1 HM. 1 Van H.I .010 015 .020 .025 .030 035 040 04S .050 .055 .060 065 070 075 .080 .085 .090 .095 .100 .120 .140 .160 lis l,..J Antigorite. -* — Clinochl TC. Co. imon hor Gl nblende/- aucophan =._ ^ Spo dumene. Carpholite.- ^— Augit -Aegirite- Rosenl uschite. I Actinolite. ' Diopside.i Diallaee..'"-= auglte. Pistac te. P edmontite. Ch jndrodit )rthitc, ne. I «n. PCT Titanoliv — Laaven i Biotite J— Zir Phi nwaldtte _ Aegirite Acmite. Astrophy •Grii lerite. B isaltic hornblende. ivikite. TiUnit '. • ANISOTROPIC. 481 Antigorite.— H.(Mg,Fe),SiA, H = 2.5, G = 2.622. Orthorhombic. ( — ). Habit: Lamellse, leaves, scales. Elongation: At right angles to (100), lath-shaped. ( + ). Orientation: o=n, b=t. Greenish, colorless, yellowish. Non-pleochroic. a = 1.560, ^ = 1.570, r = l-571. 7--« = 0.011, r-/?=0.001, j8-a = 0.010. Dispersion: p> u. 2£ = 16° to 98°. Silica separates in boiling HCl. Pennine is pleochroic, has lower double and higher indices of refraction Ordinary serpentine is ( + ). Disthene.— Al^SiO,, H = 4.0-7.0, = 3.56-3.67. Triclinic. ( — ). Cleavage: (100), good ; (010), distinct. (100): (010) = 74°. Habit: Prisms, tablets. Elongation: (100): (010), lath-shaped. (-I-). Orientation: p = nearly at right angles to (100); c = incllned 30° on (100) against the (100): (010) edge. Colorless, blue. S Pleochroism: Weak, between blue and colorless. a = 1.717, j8 = 1.722, r=l-729. r-a = 0.012, r-i8 = 0.007, j8-a = 0.005. Dispersion: p>u. 2F = 82°- Does not occur in eruptive rocks, but in crystalline schists and gneisses. Insoluble in acids. Sillimanite and andalusite are orthorhombic, have parallel extinction, and different cleavage. Topaz has basal cleavage and is orthorhombic. Zolsite and colorless epidote have different position of axial plane in relation to cleavage. Serendibite and dumortierite have different pleochroism. Blue amphibole has different cleavage and lower indices of refraction. Bastite. — Serpentine pseudomorph after an orthorhombic pyroxene. H = 3.5-4.0,0 = 2.6-2.8. Orthorhombic. ( — ). Cleavage: (100), good. Habit: Leaves. ANISOTROPIC. 483 Elongation: (110), (110), lath-shaped. ( + ). Orientation: a=o, c=c. Greenish, yellowish, colorless. Pleochroism : Weak or wanting. Index: 1.5-1.6. Birefringence: Weak. Dispersion: p> u. 2£ = 20°-90°. Gelatinizes more or less easily with acids. Other pyroxenes differ in orientation. See Part II. Hornblende, Common.— Oa, Mg, Fe silicate. H = 5.0-6.0, G = 2.9-3.1. Monoclinic. (-F). Cleavage: (110), good; (100), distinct; (010), poor. (110);(lI0) = 55.5°. Habit: Prisms, rods, grains. Elongation: (110):(lT0), lath-shaped. { + ). Orientation: 6 = 6, c:t=-12° to -20°. Green, rarely brown. Pleochroism: In green and yellow tones with c>6>a. (1 = 1.640, /5 = 1.643, r = l-656. r-a: = 0.016, r-,8 = 0.013, ,5-a: = 0.003. | 2F = 54°to84°. Insoluble in acids. ^ Arfvedsonite has ( — ) elongation. Other amphiboles, see Part II. Glaucophane.— NaAl(Si03)2(Fe, Mg)SiO,, H = 6.0-6.5, = 3.103-3.113. Monoclinic. ( — ). Cleavage: (110), good; (100), distinct; (010), poor. (110):(1I0) = 55° 16'. Habit: Short prisms, grains. (100) twinning. Elongation: (110):(lI0), lath-shajjed. ( + ). Orientation: b = J), c: (= - 4° to -6°. Blue, violet, yellowish green, almost colorless. Pleochroism: o = nearly colorless to yellowish green, B = reddish to bluish violet, c = blue. «; = 1.621, /3= 1.6.38, r = 1-639. r- Q! = 0.018, r-^ = 0.001, ^-a=0.017. Dispersion: Strong. 2E = 85.5°, 2F„a = 43° 58'. One of the amphiboles. But slightly affected by acids. Other minerals which resemble it differ in cleavage, pleo- chroism, and have ( — ) elongation. ANISOTROPIC. 485 Carpholite.— HjMnAljSijO,,,, H = 5.0-5.5, G = 2.935. Monoolinic. ( — ). Cleavage : Unnoticeable. Habit: Rods, fibres. Elongation: (110): (llO), narrow laths. ( + ). Orientation: b=a, c:t = 4° ca. Colorless, greenish yellow. Pleochroism: Distinct: t = colorless; o = 6=yellowish green. Index: « = 1.627. r- a = 0.022. 21=60°. Insoluble in acids. Rosenbuschite is soluble in acids, has ( — ) elongation, 2F = 90°, and cleavage (001), good. Sillimanite is ( + ), non-pleochroic, cleavage (100) good. Rosenbuschite.— 2Na2Zr02F2 ■ OCaSiOj ■ TiSiOj • TiOs, H = 5.0-6.0, G = 3.30- 3.315. Monoclinic. (±?). Cleavage: (001), good; (100), distinct. (001):(100) = 78° Habit: Rods along b. Elongation: Narrow laths. ( — ). Orientation: b= a, c:t= + 13°. , Colorless, light greenish yellow. Pleochroism : Not marked, c > B > a. Index: n= 1.65 ca. f- a = 0.026 ca. 2F = near 90°. Easily attacked by HCl. Sillimanite and carpholite are not affected by HCl and have ( + ) elongation. WoUastonite has (±) elongation, lower double refraction and higher extinction angle. Pectolite has ( + ) elongation. Actinolite.— Ca(Fe,Mg)3Si,0,2, H = 5.0-6.0, G = 3.0-3.2. Monoclinic. ( — ). Cleavage: (110), good; (100), distinct; (010), poor. (110): (110) = 55° 49'. Habit: Rods, leaves, grains. Twinning (100) occurs. FJlongation: (110) : (110), lath-shaped. ( + ). Orientation: 6 = B, c:c= — 15°. Green. Pleochroism: Hardly noticeable in thin sections. c = green; b and a = yellowish green, a = 1.607, ^ = 1.624, r = 1-634. r-a: = 0.027, r-|8 = 0.010, ^-a = 0.017. Dispersion: Weak, p< u. ANISOTROPIC. 487 2F = 80° to 81.5° But slightly acted upon by HCl. Pistacite has trace of the plane of the optic axes at right angles to the cleavage, and (i) elongation. Pyroxenes differ in cleavage and have higher extinction angles. See Part II. Pistacite.— (Epidote). Al(SiOj)3(Al,Fe)2CaCaOH, H = 6.5, = 3.3-3.5. Monoclinic. ( — ). Cleavage: (001), good; (100), distinct; (001); (100) = 64° 37' Habit: Prisms, and rods elongated along b, grains. Twinning (100) occurs. Elongation: (001) :(100), lath-shaped. (±). Orientation: 6 = 6, c:o=+3°. Green, yellow, brownish, colorless. Pleochroism : o = colorless to yellowish or greenish, i = yellowish to yellowish gray, lavender; c = green, light yellowish green. a = 1.714, ;S= 1.741, r= 1-746. 7— a = 0.032, 7— .5 = 0.005, ^-a = 0.027. Dispersion: p>u. 2F = 74°-90°. Not affected by HCl. Pistacite is the Fe rich epidote; clinozoisite, the Fe poor or free epidote. Pyroxene has (110) cleavage; in sections parallel to the long direction, the trace of the optic axes is parallel to the cleavage; in epidote at right angles. Clinozoisite has lower double refraction, is ( + ), and has strong pu. 2.e=70° ca. Insoluble in acids. One of the micas. Muscovite and lepidolite are different chemically. Talc has 2£; = 6° to 20°. See Part II. Lavenite.— (Si03)2(Mn,Ca,Fe)(ZrOr)Na, H = 6.0, = 3.51-3.547. Monoclinic. ( — ). Cleavage: (100), distinct. Habit: Prisms, grains. Elongation: (110) :(1 TO), short laths. (±). Orientation: 6 = 6, c:o=-l-20°. Yellow. ANISOTROPIC. 491 Pleochroism: c = orange to red-brown; 6 = yellowish; « = light yel- low. Index: n = 1.750. Birefringence: 0.040. Dispersion: Indistinct. 2F = 80°. Wbhlerite usually occurs as tablets, has lower birefringence, weaker pleochroism, and polysyntbetic twinning. Biotite.— Mg,Fe,Al silicate. H = 2.5-3.0, = 2.8-3.2. Monoclinic. Often appears uniaxial. ( — ). Cleavage: (001), good. Habit: leaves, leafy aggregates. Elongation: At right angles to (001), lath-shaped, (-t-). Orientation: 6 = 6, c:o=-|-0° to -f7°. Brown, green, yellow. Pleochroism : Strong, c ^ 6 > n. t and fi = deep-brown to red- brown, deep-green to black; a = light yellow to red, light green, a = 1.557, ^=1.589, r = 1-597. r-a: = 0.040, 7--/?==0.008, /?-« = 0.032. Dispersion: Weak, ^^o. 2£; = 0° to 72°. Soluble in HjSO, and the Fe rich in HCI with separation of SiOa scales. Minerals other than micas, see Part II. Alkali micas have different color, pleochroism, larger 2V , different position of plane of the optic axes, and are different chemically. Anomite has different position of the plane of the optic axes. Lithionite, when dark in color, may be separated by the Li reaction. Anomite.. — Under anomite, following Rosenbusch, are included those rock- forming biotites which have the plane of the optic axes at right angles to the symmetry plane. See Part II. H = 2.5-3.0, = 2.8-3.2. MonooUnic. ( — ). Cleavage: Good, (001). Habit: Leaves, leafy aggregates. Elongation: At right angles to (001), lath-shaped. (-I-). Orientation: &= c, c: = 0° to 4-4°. Brown, green, yellow. Pleochroism: Very .strong, like biotite. c>b>fl. Indices: Like biotite. Birefringence: Like biotite. ANISOTROPIC. 493 Dispersion: Weak, p^u. L!i? = 10°-68° Biotite Las 6 = 6. Other micas, see biotite. Other minerals than micas, see Part II. Zinnwaldite.— K,Li,Fe,Al silicate. H = 2.5-3.0, = 2.82-3.20. Monoclinic. ( — ). Cleavage: (001), good. Habit: Leaves, leafy aggregates. Elongation: At right angles to (001), lath-shaped. ( + ). Orientation: 6 = i, c: 0=0° to + 7°- Brown, red, yellow, rarely green. Pleoohroism: Strong, t>li>o. t and o = dark-brown, brown- ish gray; b = yellowish brown or reddish, nearly colorless. Indices: Similar to biotite. Birefringence: Similar to biotite. Dispersion: Weak, p> u. 2^ = 10°-60°- Lepidolite has different position of the plane of the optic axes. Other micas, see biotite. Other minerals than mica, see Part II. Phlogopite. — A magnesium mica, near biotite, but containing httle Fe. 's H = 2.5-3.0, = 2.78-2.85. O Monoclinic. ( — ). Cleavage: (001), good. Habit: Leaves, leafy aggregates. Elongation- At right angles to (001), lath-shaped. (-I-). Orientation: 6 = B, c: tt = 0° to +7°. Colorless, yellowish, brownish, greenish. Pleochroism: Weak with c>6>0. ,« = 1.562, ^ = 1.606, r=l-606. r-a = 0.044, r-|8 = 0.0, /3-a = 0.04'l. Dispersion: Weak, pij. Strong dispersion of bisectrices: c:Opu around n. A rare pyroxene. .^girite has different pleochroism. Other pyroxenes. IJke aegirite. See also Part II. .ffigirite has bluntly terminated crystals. Grunerite.— FeSiOj, H = 5.0-6.0, = 3.71. Monoclinic. ( — )• Cleavage: (110), good; (100), distinct; (010), poor. (110):(lT0) = 55° 49'. Habit: Rods, leaves, grains. Elongation: (110);(lT0), lath-shaped. ( + ). Orientation: 6 = 6, c:c=-ll° to -15°. Colorless, brownish. Pleochroism: c = light brown; b = o = colorless. /3=1.73. 7— a=0.056. 2y = large. Pyroxenes difl'er in cleavage and have higher extinction angles. Basaltic hornblende.— Ca,Mg,Fe,Al silicate. H = 5.5-6.0, = 3.05-3.47. Monoclinic. ( — ) . Cleavage: (110), good; (100), distinct; (010), poor. (110):(lT0) = 55.5°. ANISOTROPIC. 497 Habit: Short prisms, grains. Elongation: (110): (110), lath-shaped. ( + )- Orientation : 6 = d, c : c = 0° to - 12°. Brown. Pleochroism : Strong in brown and yellow tones with t > i > a, also in green and brown tones with p = green, 6 and (= brown. « = 1.680, j8=1.725, ?- = l-752. r-a = 0.072, r-/5 = 0.027, ^-a = 0.045. 2F = 80°. But slightly acted upon by HCl. An amphibole. Other amphiboles and pyroxenes, see Part II. Common hornblende has c:t= — 12° to —20°, sometimes ( + ) character, lower double refraction, and sometimes lower 2V. Mica generally has a peculiar ' ' bird's eye maple " appear- ance. Barkevikite.— A variety of hornblende, H = 5.0-6.0, G = 3.428. Monoclinic. ( — ). Cleavage: Like hornblende. Habit: Like basaltic hornblende. Orientation: 6 = 6, c:t = 14°. Blown. Absorption: c>6>a. (^ Index: Like basaltic hornblende. Birefringence: Like basaltic hornblende. Dispersion: Distinct p< u. 2£ = 54° ca. A sharp separation between barkevikite and basaltic horn- blende is not possible. Use the name for the R2O3 and TiO, rich basaltic hornblendes with considerable alkali. ANISOTROPIC. 499 Ottrelite.— H,(Fe,Mn,)Al,SiO,, H = 6.0-7.0, G = 3.53-3.56. Monoclinic. ( + ). Cleavage: (001), good. Habit: Leaves plates. Tsehermak twinning frequent. Elongation: At right angles to (001), lath-shaped. ( — ). Orientation: 6= n, c: [ = 0° to 20°. Blue, green, colorless. Pleochroism: ( = yellowish green, colorless; Ii = hlue; tt = olive- grcen. ;9=1.74. r- a = 0.010. Dispersion: Strong, p>u. Strong dispersion c:tp>c:t„. 2E = large, very variable. Hour-glass structure common. Insoluble in HCl. Almost exclusively confined to the phyllitic schists. Other minerals. The liigh indices in combination with the low double refraction, the strong dispersion of the bisec- trices and axes, and the peculiar pleochroism, separate the ottrelite group from all other minerals showing good (001) cleavage only. CUnochlore.^ — Hydrous magnesium, iron, aluminium silicate. H = 2.0-2.5, § G = 2.71. MonocUnic. (-I-). ^ Cleavage: (001), good. Habit: Leaves, scales, leafy aggregates. Twinning widespread. Elongation: At right angles to (001), lath-shaped. ( — ), Orientation: & = B, c:t=-2°to -9°. Green. Pleochroism: a and b = green; c = yellow, a = 1.585, ^ = 1.586, j- = 1-596. 7—01 = 0.011, 7— 15=0.010, /?-a=0.001. Dispersion: pu. 2^ = 60° ca. Distinct dispersion of the bisectrices. Unacted upon by acids. Other pyroxenes, see Part II. Olivine has different orientation of interference figure. Diopside.— CaMgSijO. , H = 5.0-6.0, G = 3.34. Monoclinic. (-I-). Cleavage: (110), good. (110): (110) = 92° 50'. Habit: Prismatic, grains; (100) twinning. AXISOTROnC. 503 Elongation: (110) .(ITO), lath-shaped. Oiontation: 6 = b, c: c= -39°. Colorless, greenish. Pleochroism: Weak or wanting. a = 1.671, /?= 1.678, r =1-700. r-a = 0.029, r-i9 = 0.022, /?-a = 0.007. Dispersion: Weak, p> u. 2F = 59°. Unacted upon by acids. All pyro-xenes are characterized by right-angled cleavage on basal sections. Zonal structure is common with increasing green color outward and corresponding increase in the extinction angle. Other pyroxenes, see Part II. Olivine has different orientation of interference figure. Diallage. — Composition near diopside, but often contains Al. H"4.0, G = 3.2-3.35. Monochnic. (-I-). Cleavage: (110), (100), good. (110):(lT0) = 92° 50'. Habit: Grains, short prisms; (100) twins frequent. Elongation: (110):(lT0), lath-shaped. Orientation: 6 = 6, c:t=— 39° § Colorless, greenish, brownish. Pleochroism: Weak, b = yellowish; o = t = greenish. (j Indices: Ijke diopside. Birefringence: Like diopside. Dispersion: Weak, /)>u. 2F = 59° and less. Unacted upon by acids. The complete (100) cleavage is most characteristic. Diallage is confined in occurrence to the gabbros, peridotites, and pyrox- enites. Other pyroxenes, see Part II. Olivine has different orientation of interference figure. .Sgirite-augite. — An augite rich in Ecgirite molecule. H = 6.0-6.5, G = 3.46. Monoclinic. (4-) probably. Cleavage: (110), good. 4-87°. Habit: Short prisms, (100) tablets. Elongation: (110):(lT0), lath-shaped. Orientation : 6 = B,c:t>-55°<-87° Green, yellow. Pleochroism : o = grass-green ; b = light-green; t= yellow to brownish. a = 1.680, /? = 1.687, r = l-709. r-a = 0.029, )--;9=0.022, ^-q:-0.007. ANISOTROPIC. 505 Dispersion: Strong of bisectrices. jEgirite has larger angle of extinction, c : c. Augite has smaller extinction angle, c : c. Chondrodite.— Mgj(Mg(F,OH) ),(Si04), , H = 6.0-6.5, = 3.1-3.2. Monoclinic. ( + ). Cleavage: (001), distinct. Habit: Grains, twinning lamellae (001). Orientation: b=c, a: = 25° to 30°. Yellow, brownish, reddish. Pleochroism : n = yellow ; 6 = t = yellowish white. ff = 1.607, /3 = 1.619, r= 1-639. r-a: = 0.032, 7— /' = 0.020, /?- a:==0.012. Dispersion: Weak, p^u. 2F = 79.5°. Gelatinizes easily with acids. Occurs in crystalline limestones and dolomites. Orthite.— Ce epidote. Al(Si0,)3(Al,Fe,Ce),(CaFe)-Ca0H. H = 5.5-6.0, = 3.5-4.2. Monoclinic. (Sometimes definitely +, sometimes probably — .) Cleavage: (001), distinct. Habit: Prisms and rods along 6 or c, grains. Elongation: (001) : (100), lath-shaped. (±). Orientation : 6 = b, c:n=+36° ca. Brown. Pleochroism: Distinct when anisotropic. t>b>0. a>1.78, ^=1.682. 7-— a = 0.032, to 0.0. (The birefringence sinks to zero when the mineral is altered to a gum-like substance, common in many of the Ce minerals. 2V varies. Generally attacked by HCl. Also called allanite. Characterized by its high indices, and varying strength of double refraction, brown color, distinct pleochroism, and lack of good cleavage. Chondrodite has extinction a:o = 25° to 30°. Pistacite is ( — ) and has c : n = 4- 3°. Manganese rich piedmontite. — See Piedmontite among negative minerals above. Titanolivine.— Ti, Mg, Fe sihcate. H = 6.0, = 3.20-3.27. Monoclinic. ( + )• Cleavage: Wanting. Habit: Grains. Lamellar twiiming with (6) inclined 20° to the trace of the twinning plane symmetrically in each half. ANISOTROPIC. 507 Orientation: 6= t. Yellow, red. Pleochroism: o = red; B = c = light-yellow. a = 1.669, ^ = 1.678, r= 1-702. r-ff =0.033, r-. 5 = 0.024, ^- a = 0.009. Dispersion: Strong, p^u. 2F = 62° to 63°. The transition between olivine and titan-olivine is gradual, though sometimes it is sharp and there are particles of one in the other. Astrophyllite.— Ti,Fe,Mn,Ksilicate. H = 3.0-4.0, G = 3.3-3.4. Orthorhombic . ( + ) . Cleavage: (100), good. Habit: Plates, laths along b, leaves, rosettes. Elongation: At right angles to (001), lath-shaped. (±). Orientation : a = n, b—c. Yellow. Pleochroism: p = yellow to red; b = orange; t = citron-yellow, a = 1.678, /?= 1.703, )-= 1-733. J— a = 0.055, 7—/?= 0.030, ,9- a =0.025. Dispersion: p> u. 2E = 160° ca. Insoluble in HCl. Micas have smaller axial angle and lower indices. ^ Brittle micas have lower double refraction, smaller axial ^ angle around the bisectrix emerging on cleavage flakes, and are ( — ). Titanite.— CaSiTiO^, H = 5.0-5.5, G = 3.4-3.56. Monoclinic. (-I-). Cleavage: (110), distinct. (110):(lI0) = 46° 8'. Habit: Prisms, rhombs, grains, rods. Orientation : 6 = 6, c : t = -f 39°. Colorless, yellowish, reddish, brownish. Pleochroism : Weak, O 6 > o. a = 1.913,/?= 1.921, r=2.054. }-- a = 0.147, r-i8 = 0.133, /?-a=0.p08. Dispersion: Very strong. p>u. 2£„a = 45°-68°. Very slightly affected by HCl. Monazite has lower birefringence, weaker dispersion, and low extinction angle. Breokite has parallel extinction, 2K = 0°-23°, and a great difference in the birefringence in two directions, j-— « = 0.158, while ^-a = 0.003. Rutile, xenotime, hussakite, and cassiterite give no reaction for Ca. PART IV. 510 TABLE OF MEAN INDICES OF REFRACTION. TABLE OF MEAN INDICES OF REFRACTION. 1. Hematite 3 .08 2. Rutile 2.711 3. Brookite 2.637 4. Anatase 2.537 5. Perofskite 2.38 6. Chromite 2.097 7. Cassiterite 2.029 8. Zircon 1 .952 9. Titanite 1 .938 10. Melanite 1 . 873 11. Fayalite 1 .864 12. Uwarowite 1 .838 13. Spessartite 1 .811 14. Monazite 1.811 15. Ahnandine 1 .810 16 Siderite 1.796 17. ^girite 1.792 18. Corundum 1 .766 19. Gahnite 1.765 20. Epidote 1 .751 21. Lavenite . 1 .750 22. Hercynite 1 .749 23. Grossular 1 . 747 24. Pyrope 1 .745 25. Ottrelite 1 .741 26. Staurolite 1 .741 27. Hedenbergite 1.740 28. GriSnerite 1 .73 29. Clinozoisite 1.725 30. Vesuvianite 1 .723 31. Disthene 1 .723 32. Diaspore 1 .723 33. Basaltic hornblende 1 .719 34. Spinel 1 .718 35. WShlerite 1.714 36. Sapphirine 1 .709 37. Au^te 1.708' 38. Astrophyllite 1 .705 39. Zoisite 1.702 40. Barkevikite 1 .701 41. ^girite-augite 1 .692 42. Axinite 1 .691 43. Riebeckite 1 .687 44. Dumortierite 1 . 684 45. Titanolivine 1 .683 46. Diopside 1 .683 47 Orthite 1 .682 48. Comerupine 1 .677 49. Lawsomte 1 . 673 60. Olivine 1 .671 51. Hypersthene 1.670 52. Clinohumite 1 .670 53 Bronzite 1 .669 54. Rinkite 1 .668 65. Sillimanite 1 .667 56. Gehlenite 1 .661 67. Enstatite 1 .660 58. Monticellite 1 .660 59. Jadeite 1 .654 60. Mosandrite 1 .651 61. Tourmaline 1 .650 62. Kosenbuachite 1 . 65 63. Datolite 1 .649 64. Magnesite 1 .649 65. Black to green hornblende. ... 1 .646 66. Anthophyllite 1 . 644 67. Andalusite 1 .638 68. Apatite 1 .637 69. Gedrite 1.634 70. Glaucophane 1 .633 71. Aragonite 1 .633 72. MeliUte 1 .632 73. Prehnite 1 .630 74. WoUastonite 1 .628 75. Carpholite 1 .627 76. Lazulite 1 .625 77. Pargasite (hornblende) 1.623 78. Topaz 1.623 79. Dolomite 1.623 80. Actinolite 1 .622 81. Chondrodite 1.622 82. Eucolite 1 .620 83. Tremolite 1.617 84. Pectohte 1 .61 85. Eudialite 1 .609 86. Melinophane 1.606 87. Calcite 1.601 88. Phlogopite 1 .591 89. Clinochlore 1.589 90. Celsian 1 .589 91. Anhydrite 1.587 92. Muscovite 1 .587 93. Alunite 1 .585 94. Meionite 1 .583 95. Anorthite 1 .682 96. Pennine 1 .582 97. Biotite 1 .681 98. Talc 1.572 99. Bnicite 1 .667 100. Bytownite 1 .565 101. Labradorite 1.659 102. Mizzonite 1 .553 103. Andesine 1 .562 104. Dipyr 1 .651 105. Bastite 1 .650 106. Quaitz 1 .550 107. Oligoclase 1 .644 108. Hydrargillite 1 .542 109. Ciordierite 1.540 110. Nephelite 1.540 111. Canada balsam 1 .54 112. Kaolin 1.54 113. Albite 1.5.33 114. Anorthoclase 1 . 527 115. Gypsum 1 .524 116. Miorocline 1.523 117. Orthoclase 1 .622 118. Laumontite 1 .521 119. Cancrinite 1.615 120. Epistilbite 1 .51 121. Thomsonite 1 .608 122. Leucite 1 .508 126. Scolecite 1 .602 124. Heulandite 1 .501 125. Hailynite 1 .500 126. Stilbite 1 .497 127. NoseHte 1 .495 128. Hydronephelite 1 .49 129. Analcite 1 .487 130. Sodalite 1 .483 131. Natrolite 1.482 132. Tridymite 1 .477 133. Opal 1 .441 134. Fluorite 1 .433 TABLE OF MAXIMUM BIREFRINGENCES. 511 TABLE OF MAXIMUM BIREFRINGENCES. 1. Rutile . 287 2. Micaceous hematite . 28 3. Siderite . 238 4. Magnesite . 202 5. Dolomite 0. 179 6. Calcite 0.172 7. Brookite . 158 8. Aragonite . 156 9. Titanite 0.145 10. Cassiterite . 096 11. Anatase 0.073 12. Basaltic hornblende . 072 13. Zircon . 062 14. Grunerite 0.056 15. Astrophyllite 0.055 16. Fayalite 0.050 17. iEgirite 0.050 18. Talc 0.050 19. Diaspore 0.048 20. Monazite 0.045 21. Anhydrite 0.044 22. DatoUte 0.044 23. Phlogopite 0.044 24. Biotite 0.040 25. Lavenite 0.040 26. Muscovite 0.038 27. Pectolite 0.038 28. Lazulite 0.036 29. Olivine 0.035 30. Humite . 035 31. Meionite 0.035 32. Prehnite 0.033 33. Titanolivine 0.033 34. Pistacite 0.032 35. Chondrodite 0.032 36. Orthite 0.032 37. Diopside 0.029 38. Jadeite 0.029 39. jEgirit«-augite 0.029 40. Cancrinite 0.028 41. Thomsonite 0.028 42. Actinohte 0.027 43. Tremolite 0.026 44. Wohlerite 0.026 45. Rosenbuschite . 026 46. Tourmaline 0.025 47. Augite 0.025 48. AnthophylUte 0.024 49. Hydrargillite . 023 50. Carpholite. . . 0.022 51. SiUimanite 0.022 52. Brucite 0.021 53. Gedrite 0.021 54. Barkevikite 0.021 55. Ahmite 0.020 56. Melinophane . 020 57. Pargasite 0.020 58. Hedenbergite 0.019 59 Lawsonite 0.019 60. Glaucophane 0.018 61. Monticelhte 0.017 62. Spodumene 0.016 63. Common hornblende. . . . 0.016 64. Mizzonite 0.015 65. WoUastonite 0.015 66. Anorthite 0.013 67. Dipyr 0.013 68. Hypersthene 0.013 69. Cornerupine 0.013 70. Natrolite 0.012 71. Disthene 0.012 72. Johnstrupite 0.012 73. Mosandrite 0.012 74. Hydronephelite 0.012 75. Laumontite . 012 76. Andalusite 0.011 77. Clinochlore 0.011 78. Dumortierite 0.011 79. Gypsum . 010 80. Axinite 0.010 81. Staurolite 0.010 82. Ottrelite 0.010 83. Epistilbite 0.010 84. Albite 0.010 85. Corundum 0.009 86. Quartz . 009 87. Enstatite . 009 88. Bronzite 0.009 89. Cordierite 0.009 90. Topaz . 009 91. Zoisite 0.009 92. Kaolin . 008 93. Clinozoisite . 008 94. Scolecite . 007 95. Heulandite 0.007 96. Orthoclase 0.006 97. , Gehlenite 0.006 98, jEnigmatitc . 006 99. Stilbite . 006 100. Sapphirine . 005 101. MeUlite 0.005 102. Nephelite . 005 103. Riebeckite . 004 104. Apatite . 004 105. Eucolite . 003 106. Phillipsite 0.003 107. Eudialite . 002 108. Tridymite . 002 109. Vesuvianite . 002 1 10. Pennine . 002 111. Leucite 0.001 512 NEWTON'S COLOR SCALE. NEWTON'S COLOR SCALE. (Modified from Quincke.) No. Difference in Wave Lengtli. Phase Difference for Na Light. Interference Colors Between Crossed Nicols. Interference Colors Between Parallel Nicols. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 iO 41 42 43 44 45 46 000000 mm. 000040 000097 000158 000218 000234 000259 000267 000275 000281 000306 000332 000430 .000505 .000536 .000551 .000565 .000575 . 000589 . 000664 000728 000747 000826 000843 000866 000910 000948 000998 OOUOl 0.001128 0.001151 0.001258 0.001334 0.001376 0.001426 0.001495 0.001534 0.001621 0.001652 0.001682 0.001711 0.001744 0.001811 0.001927 0.002007 0.002048 Black Iron gray Lavender gray. . . . Grayish blue Clearer gray Greenish white. . . . Almost pure white . Yellowish white. . . Pale straw-yellow. . Straw yellow Light yellow Bright yellow Brownish yellow. . . Reddish orange. . . Red Deep red Purple Violet Indigo Sky blue Greenish blue Green Lighter green Yellowish green. . . Greenish yellow. . . Pure yellow Orange Bright orange red. . Dark violet red. . . . Light bluish violet. Indigo Greenish blue Sea-green Brilliant green. . . . Greenish yellow. . . Flesh color Carmine Dull purple Violet gray Grayish blue Dull sea-green Bluish green Light green Light greenish gray, Whitisli gray Flesh red Bright white White Yellowish white Brownish white Brownish yellow Brown Light red Carmine Dark reddish brown Deep violet Indigo Blue Grayish blue Bluish green Pale green Yellowish green Lighter green Greenish yellow Golden yellow Orange Brownish orange Light carmine Purplish red Violet purple Violet Indigo. Dark blue Greenish blue Green Yellowish green Impure yeSow Flesh colored Brownish red Violet Violet Sea-green Green Dull sea-green Yellowish green Greenish yellow Yellowish gray Lilac Carmine Grayish red Bluish gray Green TABLE OF SPECIFIC GRAVITIES. 513 TABLE OF SPECIFIC GRAVITIES. Cassiterite 6 . 65 Hematite 6 . 20 Magnetite 5.10 Monazite 5.10 Pyiite 5.05 Pseudobrookite 4.98 Ilmenite 4 . 65 Zircon 4.53 Xenotime 4.52 Chromite 4 . 45 Rutile 4.25 Fayalite 4.14 Piootite 4 . 08 Perofsiute 4 . 03 Melanite 3 .95 Corundum 3 . 95 Siderite 3 . 94 Brookite 3.94 Anatase 3 . 88 Orthite 3.85 Pleonaste 3.82 jEnigmatite 3.81 Pyrope 3.75 Stauro'ite 3.72 Griinerite 3.71 Periclase 3 . 65 Disthene 3.62 Spinel 3.60 Grossular 3 . 56 Topaz 3.55 Hedenbergite 3 . 54 Lavenite 3 . 53 jEgirite 3 . 53 Uwarowite 3.51 Sapphirine 3 . 49 Rinkite 3.48 Titanite 3 .48 jEgirite-augite 3 46 Arfvetfeonite 3.46 Hypersthene 3 .45 Wolilerite 3 .43 Barkevikite 3 . 43 Titan-augite 3 .41 Vesuvianite 3.41 Diaspore 3.40 Piedmontite 3 .40 Pistacite (Epidote) 3 .39 Augite 3 . 36 Olivine 3.36 Astrophyllite 3.35 Prismatine 3 .34 Jadeite 3.34 Zoiaite 3.31 Rosenbuschite 3.31 Diopside ^ ' "^R Bronzite 3 . 29 Dumortierite ^ ' ^o Axinite 3 . 28 Diallage o'n^ Comerupine ^ Z Forsterite 3 . 26 Saiimanite 3.24 Johnstnipite o ' r n Monticellite 3 . .<;0 Fluorite 3.19 Thuringite 3 17 Apatite 3.16 Andalusite 3.15 Anthophyllite 3.15 Humite 3.15 Chondrodite 3.15 Clinohumite ^ 3.15 Enstatite 3.15 Crossite 3.15 Spodumen , 3 . 15 Tourmaline 3.12 Hornblende 3.12 Magnesite 3.10 Glaucophane 3 . 10 Lawsonite 3 .09 Lazuli te 3 . 06 Eueolite 3 . 05 Actinolite 3 . 02 Melinophane 3 .01 Biotite 3 . 00 Gehlenite 3.00 Tremolite 3 . 00 Melilite 3.00 Mosandrite 2 . 98 Datolite 2 . 95 Anhydrite 2.95 Aragonite 2 . 94 Carpholite 2 .94 Eudialyte 2 . 92 Polomite 2.90 Delessite 2 . 89 WoUastonite. . . . : 2 . 88 Prehnite 2 . 87 Muscovite 2 . 85 Lepidolite 2 . 85 Paragonite 2.84 Phlogopite 2.81 Pectolite 2.81 Anorthite 2 . 76 Talc 2.75 Meionite 2 . 73 Bytownite 2.73 Pennine 2 . 73 Calcife 2.71 Clinochlore 2.71 Bastite 2 . 70 Labradorite 2 . 70 Alunite 2 . 70 Andesine 2 . 67 guartz 2.65 ordierite 2 . 63 Albite 2.62 Kaolin 2.62 Mizzonite 2.61 Anorthoclase 2 . 59 Nephelite 2.58 Dipyr 2.57 Serpentine 2 . 57 Orthoclase 2 . 55 Microcline 2 . 55 Leucite 2 . 47 Brucite 2.39 Haiiyite 2.38 Sodalite 2.38 Hydrargillite 2 . 38 Thomsonite 2.35 Gypsum 2.32 Tridymite 2 . 30 Laumontite 2.30 Glauconite 2 . 30 Scolecite 2 . 28 Hydronephelite 2 . 26 Epistilbite 2 . 25 Natrolite 2 . 21 Opal 2 . 21 Heulandite 2 . 20 Phillipsite 2.20 Hyalite 2.17 Analcite 2. 16 Stilbite 2.16 Hydromagnesite ; 2 , 1ft 514 SEPARATION BY HEAVY SOLUTIONS. Separation by Heavy Solutions. The rock, whose minerals are to be separated, is reduced to small size by crushing, not grinding, in a mortar, sifted through a sieve with meshes of 0.5 mm., and washed to free it from dust. Thoulet's* solution is a solution of potassium mercuric iodide, having a maximum density of 3.196. It is made by dissolving mercuric iodide and potassium iodide in cold water in the proportion KI:Hgl2 = 1:1.24, and evaporated on a water- bath until a crystalline coat forms on the surface, or until a crystal of fluorite floats upon it. Upon cooling, the contraction raises the density to 3.196. It is afterwards filtered, and should be perfectly transparent and of a yellowish-green color. This solution can be diluted with water without decompo- sition, and may be reduced to a specific gravity of 1.0 and again increased by evaporation on the water-bath. An excess of Hgl2 causes a yellow hydrous double salt to separate in acicular crystals. KI in excess separates in the form of cubes and does no harm to the solution. Thoulet's solution is decomposed by metallic iron, and all such should be previously removed by the magnet. It is extremely poisonous, and great care should be taken in using it. Klein's f solution is made of cadmium borotungstate (2H20-2CdO-B203-9WoO + 16 aq.) dissolved in ten times its weight of water. It is a light-yellow, innoxious solution, with a specific gravity of 3.28 at 15° C, and is miscible with water without decomposition. It is decomposed by metallic iron, zinc, and lead, and by carbonates. Braun'sJ solution. Methylene iodide (CH2I2), a strongly refracting yellow fluid. It is miscible with benzene and does not attack metallic substances. It has a specific gravity of *Neues Jahrbuch, B. Band I., 1881, p. 179. tComp. Rend., 1881, 93, p. 318; Bull. Soc. Min. Fr., 1881, 4, p. 149. JNeues Jarhbuch II., 1886, p. 72. BRAUN'S SOLUTION. 515 3.3375 at 10° C, 3.3243 at 16° C, and 3.3155 at 20° C. Its index of refraction at 16° is 1.74092. To determine the exact specific gravity of the diluted solu- tion, a direct test may be made or a series of indicators may be used. The following were used by V. Goldschmidt : * 1. Sulphur Girgenti G=2.070 2. Hyalite Waltsch 2.160 3. Opal Scheiba 2.212 4. Natrolite Brevig 2.246 5. Pitchstone Meissen 2.284 6. Obsidian Lipari 2 . 362 7. Pearlite Hungary 2 . 397 8. Leucite Vesuvius 2 .465 9. Adularia St. Gotthard 2.570 10. Elffiolite Brevig 2.617 11. Quartz Middleville 2.650' 12. Labradorite Labrador 2 . 689' 13. Calcite Rabenstein 2.715- 14. Dolomite Muhrwinkel 2 .733 15. Dolomite .Rauris 2.868 16. Prehnite Kilpatrick 2.916 17. Aragonite Bilin 2.933 18. Actinolite Zillerthal 3.020 19. Andalusite Bodenmais 3.125 20. Apatite Ehrenfriedersdorf 3. 180 *Neues Jahrbuch, B. Band I, 1881, p. 215; Verhandlung K. K. Reichs- anstalt, 1883, p. 68. 516 VIBRATION DIRECTIONS IN ACCESSORIES. Vibration Directions in Accessories. Determination of the plane of vibration of the lower nicol. The absorption of biotite is greatest, consequently it is darkest, when its cleavage direction is parallel to the plane of vibration of the polarizer. Tourmaline extinguishes vibrations at right angles to the optic axis, that is, it absorbs the ordinary ray, and only the light rays vibrating parallel to crystallographic c emerge. It is therefore dark when the elongation (c) is at right angles to the vibration-plane of the polarizer. Determination of the direction of t in the one-quarter undulation mica-plate. Examine the interference figure pro- duced by the mica-plate, using it as a mineral section. The axis of least ease of vibration c is the line joining the loci of the hyperbolae, o is at right angles to this line. Determination of the t direction in the g5rpsum-plate (Red of the First Order). Examine the interference figure, using the gypsum-plate as a mineral section. The line joining the quadrants showing the lowest color (yellow) is the c direc- tion. The explanation is given on page 26. Determination of the c direction in a quartz or mica-wedge. See "Determination of the Relative Values of Two Vibration Directions" (p. 22). Use the wedge as a mineral section and use a mica-plate, in which the c direction has been detei^ mined, as an accessory. CRYSTAL SYSTEMS OF MINERALS. 517 MINERALS ARRANGED ACCORDING TO CRYSTALLINE SYSTEMS Isometric. Almandine. Analcite. Beckelite. Chromite. Fluorite. Gahnite. Garnets. Grossular. Haiiynite. Hercynite. Hessonite. Lazulite. Leucite. Magnetite. Melanite. Noselite Periclase. Perofskite. Piootite. Pleonaste. Pyrite. Socialite. Spessartite. Spinel. Pyrope. Uwarowite. Tetragonal. Anatase. Apopliyllite. Cassiterite. Dipyr. Hussakite. Melilite. Meliphanite. Rutile. Scapolite. Vesuvianite. Xenotime. Zircon. Hexagonal. Alunite. Apatite. Brucite. Calcite. Cancrinite. Chabazite. Corundum. Dolomite. Eucolite. Eudialyte. Gmelinite. Graphite. Hematite. Hydronephelite? Ilmenite. Magnesite. Nephelite. Pyrrhotite. Quartz. Siderite. Tourmaline. Tridymite. Orthorhombic. Andalusite. Anhydrite. Anthophyllite. Aragonite. Astrophyllite. Bronzite. Brookite. Cordierite. Cornerupine. Diaspore. Dumortierite. Enstatite. Fayalite. Forsterite. Goethite. Humite. Hypersthene. Lawsonite. Montieellite. Natrolite. Olivine. Prehnite. Pseudobrookite. Serpentine? SiUimanite. StauroUte. Talc. Thomsonite. Thulite. Topaz. Zoisite. MONOCLINIC. Acmite. Aotinolite. jEgirite. jEglrite-augite. jEgirite - hedenber- gite. Anomite. Arfvedsonite. Augite. Baddeleyite. Barkevikite. Biotite. Carpholite. Celsian. Chondrodite. Clinochlore. Clinohumite. Clinozoisite. Crocidolite. Crossite. Cummingtonite. Datolite. Delessite. Diallage. Diopside. Epistilbite. Gastaldite. Glaucophane. Griinerite. Gypsum. Harmotome. Hastingsite. Hedenbergite. Heulandite. Hornblende, Com- mon. Hornblende, Basal- tic. Hyalophane. HydrargiUite. Jadeite. Johnstrupite. KaoKn. Katophorite. LamprophyUite. Laumontite. Lavenite. Lazulite. Lepidolite. Manganpectolite. Monazite. Mosandrite. Muscovite. Nontronite. Omphacite. MONOCLINIC. Orthite. Orthoclase. Ottrelite. Paragonite. Pargasite. Pectolite. Pennine. PhiUipsite. Phlogopite. Piedmontite. Pistacite. Riebeckite. Rinkite. Rosenbuschite. Salite. Sanidine. Sapphirine. Sciolecite. Spodumene. Stilbite. Thurinj:it3. Titanaugite. Titanlte. TitanoUvine. Tremolito. Uralite. Urbanite. Wohlerite. Wollastonite. Zinnwaldite. Triclinic. .(Enigmatite. Albite. Andesine. Andesine-labrado- rite. Anorthite. Axinite. Bytownite. Bytownite- anorthite. Cyanite. Hainite. Labradorite. Labrador! te- bytownite. Microcline. OUgoclase. Oligoclase-albite. Oligoclase - ande- 518 OCCURRENCE OF MINERALS. MINERALS WHICH OCCUR IN NEEDLE-LIKE CRYSTALS. Actinolite. Cancrinite. Hydronephelite. Tremolite. ^girite. ^atolite. Natrolite. Topaz. .Apatite. Dumortierite. Sillimanite. Tourmaline. •Aragonite. Hydromagnesite. Stilbite. WoUastonite. MINERALS WHICH OCCUR IN FIBROUS AGGREGATES. Chalcedony. Datolite. Gypsum. MINERALS WHICH OCCUR IN RADIATING GROUPS OF FIBRES Hydrargillite. Sericite. Kaolin. Serpentine. Natrolite. Sillimanite. Prehuite. Talc. Chlorite. Natrolite. Thomsonite. Chalcedony. Pectolite. Other zeolites. Delessite. Stilbite. MINERALS WHICH OCCUR AS CAVITY FILLINGS. Carbonates. Chalcedony. Zeolites. ALTERATION PRODUCTS WHICH OCCUR IN MINUTE SHREDS. Kaolin. Talc. White mica. (Paragonite, sericite.) LIST OF THE ROCK-FORMING MINERALS. 519 a o to Vo o o° " I?, iS II II A^ " ?^ " IS Bo w* «u** uc « ^ it V^ « ^ I -= I I S B c l-H o M o o (4 w w H (i( O H O H w -3! O^QOOCO 00 Ol (N C0XU30 05 I . I I I O I oooooodo 00 00 Tjl ,-1 t^ Tf Tlr-l 11 II II II II II II II C O G3 coo (N lOINO w OOOOO ooodo COrHCO-^OCOCDTjI^H-^CO OOOOOOOOOOOi-lOO 3 ■ a >4 (M as CT> -B,co CD t>- CD oCiO i-HOOGOCOCOLOOOOOOOfN-^CDCOCO -rlO O CO ^-Ct^ lO o o. o _; _: c c fi o o o o oxx 2_;°d_;^29 !2o9o „-o69 MID o3 -e ;^ aj CD O CB Q G> O 03 mmffl 520 LIST OF THE ROCK-FORMING MINERALS. ° II s l?> I CO «J « u o u — lO I I II II I l-H o M o o Pi H w H o H o H W M <1 o O o C^ (M CM CI I -6q (M Ol o o O o O O lOlM ^ ?« ^ '-' CO I '^ I I o o ■ o o o O (N CDO CO -^ 00 CO I> 00 CD i-H !M !N (N f COQ0 ^ O O i-H o c^ o ■* oooooo o doo ooo o S2 1— l03t~^t^>0T— liCJ>-OiCiOO OOOO-DCOcOOr-itNtMGOCO i-li— IGSr-li-Hr-l.— li-(-iOCOI>- --HtM ,-H,— (i— (i-Hi-H.— I o 2 2 .-; o 2 6 6 6 o o . o o o c o ; fi o - o SS-: 13.^ j3j2^ J j=aa=: o mcqpqfqpqoooooooooooooocDOooci LIST OF THE ROCK-FORMING MINERALS. 521 o I < o 3 o M o o P5 H W H O H 1-^ O H W i-I 3 * o* 1 i^ o o o o ' *"* CO ? 1 1 s c ^ _Q) II " !» II II OJ II II « II ^ -iiogjca o e 1 t: ei, 1 .o J5 1 e 1 1 1 ^ 1 o" II g =r^- II -- - II w II II II « II II T.^ II II r. II II rt II r. r.r. -r. u ;j e u o u S Sou uuuGu o ^ ■ o ■ o - 7 ; ^ o o 00 t:^ • "O ^ II OO 11 ^ • (N 11 lOiO II ^ l-r o o o til o S3 s a g bS s loo 1 ^ - 1 a, T ■M o o o o^o °^%%° §= o?- o'o So o "5 03 CM O O O 00 lOGO COt> CO0 00 t->0 O'O 5j>0 CO II II II 11 11 II II II II II II II II II ^11 II t^ l:^^ ^-^ K)6qBi3;^ t- KJN 63t^ est- 6q (N (M (N 0\0\ (MIM(NC^ (M (M(M (Mo<6<6 W odd Jd dddiJ o o OO^ OO ddo oo CO CO CO CO ■* O O Ci ^ CO O »C 00 ■* rH COCOO ■* CO O c ^ C-l 00 (M (N 00 CD rH c<» O IC CO lO CD ^ CO CO CO CO lO CO Cq rH O o-S t^ CDt^ CO lO O lOCOCOOO ^COt^cOCOCO CDCOt^t^lOlO lO si 1—1 I— 1 s ■j-j o 'y 6 o 6 ki E U 6 o ■ §£ 6 d cj c c c c c E >i V O'C QJ t- tn o v aX. oXoo-CS o o o o o o o X o ShSoo SKWo .i£o.issoH c^a^ssggg^s ^ a3 1 'C QJ 'S 'Sii +3 H :^ 'o g g c3 T3 3 o o 01 1 ?2 HH 2: OJ 1 0. 0. i t c c B 6 g^-^i: .2.9.1-0 § c QQPOqt si t.^^c3:y^0c: cs E .l»-a-aj.|B|a.| Wf£ HHEL,fa(i,SOCCOOOOOOOWKW 1 522 LIST OF THE ROCK-FORMING MINERALS. I a I O [^ o o w fa o H o- b .CM N to =3 1 r^ § li' o' m -t> o -* fe ^ O II =° ■ II CO o tN o o --< ^ ■s O 1 iS;i 88" i& II II II II J- 11 II ^ B 11 11 11 11 11 =- 11 II °"" 11 ?! - II 11 u a u u s II o s u Cou^e«u -o «ow* ouc3«« IN o ^ o O ?l< ooo O 00 00 ■^ 1 o o o 1 o' 'I'OOO o' o CO OU5 00CO O l> I>-,_^t^ lOOOCOr-tcO 11 II II 11 11 11 11 II §rs 11 II II 11 II 1! 1^ t^t^^N N ^ IN (M (N (N (M (N (N CO O (N > T-H <-! 'P OS X 00 lO (N O 05 CO I> T-H IM C .-1 X ^ (M O fe O W5t-i T^ »-< CO OO O ) OO gOOOOOOOO(N 0(N do oooooogoo .a §1 o 05 .-1 U5 OJ « O (N 0- o Tt< ■ Ol t-^ 1-H O CO lO T-H 00 05 '^ 00 Tf O "* --< -^ -^ "* CD Oilv '^ . IO«* ■ in CK (NIO t>(N o o ■* t^ot^^cot^cDioioio-^fX) .^ ':d»o -looict-cococoioo ^S ^c^^rtw^^^rtrt^ M i d Sec (3:5 d c£ y.-£ y ooo_-ooo°ooj -a s « c o c a a-S c c B x S (3 ^ O OJ oooot;oo-Co1: C oog-cooo-gooooo sw ^SS^OSSOffiO K SSSHSSSOgS^SW^ , ^ C ■ 3 1 x 6 St . • GJ "S cs Of z a '5 t- Q g a J 1 K ) E J a - 1 < c s: 1 ^-2 1— ■1 1 C 1 1 1 a a 'a a IS 1 a Is 3 LIST OF THE ROCK-FORMING MINERALS. 523 I O o o o M H O H 02 o(N oo o o to o o I>COCD 00t»00.S t^ cilt~'^=°0 mOOit^ II II II II II II II II II bii ^11 II II II l^ii II II (N (N (N M OJO>0 -^(Xr-IC^C^l^t^CO C^ .-HTtH-^ 3^ t-co tN^CDiOOOOO^O TtiCOI>GOCJ^iy:C).-^CO OO-^OiCOCO 00 CD rH ^ lO OC CD CD lO •'t lO TI^ lO lO CD CD "O I> IC CD O ":> CO »^ »0 t^ I^ 5 e tH 'c 6 ° 6 6 ° ■T. 'Z godbbbCb _ d d 6 6 c c c c t/2 Jg 6 C ■ ^ . ■S w e f o °„ T r?- II II 7 II o "ill 1 ?-2 1 00--I " U 1 "00 u-o u a. ■a o II II II II II II II U II II II = a II a II o- a- u- "."?. " II " " ° II II " II f^ II II II a a UOUUU*OUrO fO UhOu-oOa as o 1 00 o' OJ to "=*! ' — 'CO -a i ^ 1^ o o >> (M c^ 00 "ST T— 1 - c3 o o o o o oa 33 CO O O CO O o t^ o i op a CO CD uo CO CD t^ O 1 III 1 1-1 '^ o o ooooo o ooolol 03^ ooo C^OlOiOcO O '^GiCMOb-O to 00 II II (^ II II CO (M CD LC 1-H (N U2 OC LO O 00 O 11 II II II II 11 II II II II |i II l[ 11 II 11 II II II II II II 11 II II II 11 (NCS (N (M (M iM W (M (M (M tN C^ (M (M (N CM CM 1 -H 1 H- 1 + + 1 + + + 1 +-H-H go ■a • + + + H-4- ++ 1 II + 1 + ++ 1 1 ++ §o i| fO ^ O Tfi iro CO b- Ol CO ^ I> CO 00 (N CDOCOOOOOS "■9, OOO (M GOCN OO O ^CO(N rH rH O iC (M O ><'3 O.M) ooooc-jooooocao oooooo o5 doood d odd odd d) d) d) <6) <6 di o „ lO Or^CO i-iCOfMOlC^l Ot^COi-H 0Ct~-'-HxNO0(N "•fl, '^ LO 00 CD lO i-i 00 (N O O -^ O CD 00 »H r-i CD "* Oi b- O O CO ^ I> iOcOCOCOI>cO»OI>iOiOI>cO"^00 t^cot-^-^ioiot^ S5 -« T— 1 f— Ir-H^Hi— l(Mi-Hi— Ii— IrHj— li— (i— ii— (i— 1 1— li— li— Ir-li— li— li— 1 1 £ iis a « l/i 6 6 6 '. ci ci d H 6 6 6 6 o^ \ o ' f- £ o 2 o S S 9 ^ o "K "G oojojooo-Soooo-gcD'Soo oo'Co-C'K'K hSOCDh^S ^WMgggHSsasowo^^^is ►^^aoaooo 6 a "i 53 S a. P § a ^ a. 0- 'k -i 0) c c a. g "a a 1 a 1 A- c I a iP- ->> t 1 1 'A c a 1 Q. 1 c "C c c c a C 1 - Q. T 1 o '1 02 03 a 'c J cc Is 02 c 1 E c H ^ I LIST OF THE ROCK-FORMING MINERALS. 525 < O \^ o w o o w H o IB co^^^ o t^ CI 1 ^ 1 1 .- to C^ c3 o o o o o 1 J- lOOOO t~ MOO o :;3 -tji ic lo 00 I>rt Ttt O m II II II II II II II II SbgUN ^ NBqN t- J (N O) IM (N (N (M (M (M M^; 1 1 4- 1 If f-H + -H go la ^ •|i ++++ 1 1 4 H-+ 1 1 + + go i| ^ loco O t^ CO C^ CO CD o lo c:^ •^r^ CO O --I C<1 c O CJ TT^ --H lO o x^ .Sp.-^ o o o o c oooo oo S3 'A6>(66,6,6><=. dxzid'dadi C >4 00 CO CO o t-^ r- 00 CO -+ --H 00 Cl c^ CO 00 CI UO ^ l> CO (M -< 00 (M uo O C3 o y^ CD o -^ 00 t^ t^ lO CO c: I> S3 c d d P ." d d d d 6 K (3 d c-S H a X S Ij C C C i-g >) O O O If Oi O Q o -g o o o-g ■£ u^ gggoMSW ^HSSgHO m ci OJ a 3 ■Xj O ii3 a, s CO $ a s 02 Q. s a -^ c H ::3 ^ += oj -^ c n -is t: -t- 1 1 c 1 a •c Ht3 c: > 1 1 J: ;0 c c c 5 GENERAL INDEX. PAGE 5 a 8 Absorption 23 Acute axiai angle 8 Acute bisectrix 9 Albite twinning 69 Almond oil, Index of refraction of 17 Alphabetical list of minerals 519 Amann's birefractometer 21 Amorphous substances 3 Amphibole group 59 Amyl alcohol, Index of refraction of 17 Angle, Axial 8, 30 Angle, Measurement of the axial 31 Angle, Schwarzmann's scale for measuring the axial 32 Angles, Measurement of 44 Angle, To observe the extinction 12 Anise oil, Index of refraction of 16 Anisotropic media 4 Anisotropic plates at right angles to the optic axis in parallel polarized light 11 Anisotropic plates in parallel polarized light 11 Anisotropic plates, Superimposed 12 Anomalies, Optical 43 Apparent axial angle 8, 30 Aragonite and calcite. Method for separating 48 Axes of elasticity 5 Axes of isotropy 6 Axes of vibration 5 Axes, Optic 8 Axial angle 8, 30 Axial angle. Measurement of 31 Axial angle scale, Schwarzmann's 32 Axis of elasticity 5 Axis of isotropy 6 Axis of vibration 5 527 528 GENERAL INDEX. PAGE b 5 iS 8 Babinet compensator 21 Baveno twinning 66 Becker-Becke method for the determination of the feldspars 84 Becke's method applied to the determination of the feldspars 72 Beoke's method for the determination of the position of the acute bisectrix in sections parallel to the plane of the optic axes in biaxial crystals 30 Becke's method for the determination of the position of the acute bisectrix in sections parallel to the plane of the optic axes in uniaxial crystals 26 Becke's method for the determination of the relative indices of refraction 14 Beechnut oil, Index of refraction of 17 Benzene, Index of refraction of 17 Bertrand ocular 13 Biaxial crystals 4, 7 Binormals 9 Birefraetometer, Amann's 21 Birefringence 18 Birefringence, Determination by Amann's birefraetometer 21 Birefringence, Determination by Babinet compensator 21 Birefringence, Determination by Fedorow's mica compensator 20 Birefringence, Determination by parallel nicols 21 Birefringence, Determination by the gypsum plate 19 Birefringence, Determination by the mica plate 19 Birefringence, Determination by the mica wedge 20 Birefringence, Determination by the Michel-L^vy color plate 19 Birefringence, Determination by the Michel-Levy comparateur 21 Birefringence Determination by the quartz wedge 19 Birefringence, Determination of the order of 18 Birefringences, Table of maximum 511 Bisectrix 8 Bisectrix, Acute 9 Bisectrix, Obtuse 9 Bisulphide of carbon, Index of refraction of 16 Braun's solution 514 Bromoform, Index of refraction of 16 ( 5 Calcite and aragonite. Method for separating 48 Calcite and dolomite. Method for separating 48 Canada balsam. Index of refraction of 17, 74 Carbonate group 48 Carbon bisulphide, Index of refraction of 16 GENERAL INDEX. 529 PAGE Carbon tetrachloride, Index of refraction of 17 Carlsbad twinning 64 Cassia oil, Index of refraction of 16 Cavity fillings, Minerals which occur as 518 Cedar oil, Index of refraction of 17 Chlorite group 53 Chromatic interference 11 Cinnamon oil, Index of refraction of 16 Clove oil. Index of refraction of 16 Color scale 9, 10, 512 Comparateur, Michel-Levy's 21 Compensator Babinet 21 Compensator, Fedorow 20 Convergent polarized light. Use of 24 Critical angle 4 Cross, C. W 53 Crossed dispersion 35 Crystalline systems of minerals 517 Day, Arthur 71 Determination of feldspars 61 Dichroism 23 Dispersion in monoclinic crystals 33 Dispersion in orthorhombic crystals 33 Dispersion in triclinic crystals 35 Dispersion of the optic axes 32 Dolomite and calcite. Method for separating 48 Double refraction 5 Double refraction. Measure of 7 E 8 £ 6 Elasticity axes 5 Elongation, Determination of character of 22 Elongation, Determination of character of when the mineral shows a wedge shaped edge 23 Emergence of the bisectrix in (010) plates of feldspars 81 Enlargement, Measure of 43 Epidote group 51 Ethylene bromide. Index of refraction of 16 Examination in convergent polarized hght 24 Examination in parallel polarized light 11 Extinction 12 Extinction angles in Carlsbad twins of the feldspars 82 Extinction angles on sections at right angles to either bisectrix in the feldspars 86 530 GENERAL INDEX. PAGE Extinction angles on sections at right angles to the optic normals in the feldspars 88 Extinction angles on sections at right angles to (001) and (010) in the feldspars 84 Extinction angles on sections from the (001), (010) zone of feldspars. 88 Extinction angles on sections in the zone at right angles to (010) in the feldspars 91 Extinction angles on sections showing both Carlsbad and albite twin- ning in the feldspars 93 Extinction angles on (001) and (010) cleavage flakes of the feldspars 78 Extinction, Determination of the angle of 12 Extraordinary ray 6 Fedorow's method for the determination of the feldspars 88 Fedorow's mica compensator 20 Fedorow's mica wedge 20 Fedorow's mica wedge. Determination of the orientation in 516 Fedorow's Universaltisch 32 Feldspar group 61 Fibrous aggregates of minerals 518 Fouqu6's method for the determination of the feldspars 86 r 8 Garnet group 46 Glycerine, Index of refraction of 17 Goldschmidt's specific gravity indicators 515 Gypsum plate, Determination of the orientation in 516 Gypsum plate, used to observe the character of the elongation 22 Gypsum plate, used to observe the extinction angles 12 Gypsum plate, used to observe the optical character of biaxial crystals 38 Gypsum plate, used to observe the optical character of uniaxial crystals 36 Gypsum plate, used to observe the order of birefringence 19 H 8 Heavy solutions. Separation by 514 Horizontal dispersion 34 Humite group 51 Inclined dispersion 34 Index of refraction 4, 8 Indicators, Specific gravity 515 Indices of refraction. Determination of by v. d. Kolk's method. ... 15 Indices of refraction, Determination of relative 14 Indices of refraction of fluids 16 Indices of refraction. Table of mean 510 GENERAL INDEX. 531 PAGE Intensity of light 3, 11 Interference colors 5, 8 Interference figure, Biaxial, in sections at right angles to an optic axis 29 Interference figure. Biaxial, in sections inclined to the acute bisectrix 28 Interference figure, Biaxial, in sections parallel to the plane of the optic axes 29 Interference figure. Biaxial, in sections perpendicular to the acute bisectrix 26 Interference figure, Biaxial, in sections perpendicular to the obtuse bisectrix 27 Interference figure, Uniaxial, in sections normal to the optic axis. ... 24 Interference figure, Uniaxial, in sections obhque to the optic axis. ... 24 Interference figure, Uniaxial, in sections parallel to the optic axis. ... 26 lodomethylene. Index of refraction of 16 Isometric crystals 4 Isometric plates in parallel polarized light 11 Isotropic media 3 Isotropy, Axis of 6 Jaggar's tilting stage 32 Klein's quartz plate, used to observe extinction angles 13 Klein's solution 16, 514 Klein's solution used for determining indices of refraction 16 Klein's solution used for determining the feldspars 76 Klein's Universaldrehapparat 32 Lemburg's method for separating calcite from dolomite 48 Length, Measurement of 43 Light 3 Light, Blue 3 Light, Dispersion of 32 Light, Intensity of 3, 11 Light, Polarized 3 Light, Red 3 Light, Transmission of 3 Light, Violet 3 Mallard-Beoke method for measuring the axial angle 32 Mallard's formula 31 Manebach twinning 66 Maschke's index of refraction fluids 16 Masohke's mixtures of cassia and almond oils 17 Maximum elasticity axis 5 Maximum vibration axis 5 532 GENERAL INDEX. PAGE Measurement of angles 44 Measurement of axial angles 31 Measurement of enlargement 43 Measurement of length 43 Measurement of thickness 43 Meigen's method for separating calcite from aragonite 48 Mica compensator used to determine the order of birefringence'. 20 Mica group 52 Mica plate, Determination of orientation in 516 Mica plate used to determine direction of elongation 22 Mica plate used to determine optical character of a biaxial crystal. . . 37 Mica plate used to determine optical character of a uniaxial crystal. . 35 Mica plate used to determine order of birefringence 19 Mica wedge, Determination of orientation in 516 Mica wedge used to determine direction of elongation 22 Mica wedge used to determine optical character of a biaxial crystal. . 39 Mica wedge used to determine optical character of a, uniaxial crystal. 37 Mica wedge used to determine order of birefringence 20 Michel-Levy's application of v. d. Kolk's method 16 Michel-Levy's color plate, Use of 19 Michel-Levy's comparateur 21 Michel-Levy's methods for the determination of the feldspars . . 82, 88, 93 Michel-Levy's use of Klein's solution for the determination of the feldspars 76 Minimum elasticity axis 5 Minimum vibration axis 5 Monobrombenzene, Index of refraction of 16 Monobromuaphthalene, Index of refraction of 16 Monoehlorbenzene, Index of refraction of 17 Monochlornaphthalene, Index of refraction of 17 Monochromatic light 6 Monoiodobenzene, Index of refraction of 16 Needle-like crystals 518 Negative biaxial crystals 9 Negative elongation 22 Negative uniaxial crystals 7 Newton's color scale 9, 10, 512 Nicol prism, Vibration directions in lower , 516 Nitrobenzene, Index of refraction of 16 Normal, Optic 9 ft) 6 Obtuse axial angle 8 Obtuse bisectrix 9 Olivine group 50 GENERAL INDEX. 533 PAGE Optical anomalies 43 Optical character of a biaxial crystal 37 Optical character of a biaxial crystal determined with a gypsum plate 38- Optical character of a biaxial crystal determined with a mica plate. . 37 Optical character of a biaxial crystal determined with a mica wedge. . 39 Optical character of a biaxial crystal determined with a quartz wedge 30 Optical character of a crystal, Determination of 35 Optical character of a uniaxial crystal 35 Optical character of a uniaxial crystal determined with a gypsum plate 36 Optical character of a uniaxial crystal determined with a mica plate. 35 Optical character of a uniaxial crystal determined with a mica wedge 37 Optical character of a uniaxial crystal determined with a quartz wedge 37 Optical character of the feldspars appHed to their determination. . 72 Optic angle 30 Optic axes 8 Optic axes, Dispersion of 32 Optic axes, Locating the points of emergence of 30' Optic axes, Plane of 9i Optic binormals 9' Optic normal 9- Optic principal section 6, T Order of colors 9' Ordinary ray 6 Orientation 14 Panebianco's modification of Meigen's method for separating calcite from aragonite 48 Parallel nicols. Use of 21 Parallel polarized light. Use of 11 PericUne twinning 68 Phosphorous, Index of refraction of 1& Plagioclase group 69 Plane of the optic axes 9 Pleochroism 2S Points of emergence of the optic axes. Locating the 30' Polarized Kght J Positive biaxial crystals Qi Positive bisectrix g» Positive elongation 22 Positive uniaxial crystals 7 Principal optic section 6, 7 Principal optic section in biaxial crystals S Principal optic sections in uniaxial crystals 7 Pyroxene group 5S 534 GENERAL INDEX. Quartz wedge, Determination of orientation in 516 Quartz wedge used to determine direction of elongation 22 Quartz wedge used to determine optical character of biaxial crystals. 39 Quartz wedge used to determine optical character of uniaxial crystals 37 Quartz wedge used to determine order of birefringence 19 Quincke, G 9, 10, 512 p 33 Radiating fibres. Minerals which occur in 518 Ray, Extraordinary 6 Ray, Ordinary 6 Red of first order 9 Refraction, Double 5 Refraction, Index of 4 Relative values of indices of refraction 14 Relative values of two vibration directions 22 Rosenbusch, H 43, 74, 77, 80, 87, 90 Scapolite group 49 Schroeder van der Kolk's method applied to the determination of the feldspars 76 Schroeder van der Kolk's method for determining indices of refraction 15 Schuster, Max, Method for the determination of the feldspars 78 Schwarzmann's axial angle scale 32 Sensitive tint 9 Shreds, Minerals which occur in 518 Sodahte group 47 Specific gravities appUed to the determination of the feldspa;rs 71 Specific gravities. Table of 513 Specific gravity indicators 515 Specific gravity methods for the determination of the feldspars 71 Spinel group 45 Strandmark, J. E 63 Sulphur, Index of refraction of 16 Superimposed anisotropic plates in parallel polarized light 12 Table of birefringences. Use of Michel-Levy's 19 Thicknesses, Measurement of 43 Thoulet's solution 514 True axial angle 8, 30 Tschermak, G 53, 63, 71 Twinning, Albite 69 Twinning, Baveno 66 Twinning, Carlsbad 64 Twinning, Manebach 66 Twinning, Pericline 68 GENERAL INDEX. 535 PAGE 33 Uniaxial crystals 4, 6 Universaldrehapparat, Klein's 32 Universaltisch, Fedorow's 32 V 8 Van der Kolk's method for determining indices of refraction 15 Van Hise, C. R 45, 46 Vibration axes 5 Vibration directions, Determination of 22 Vibration directions in accessories 516 Vibration plane of lower nicol 516 Violet of first order 9 Water, Index of refraction of 17 Wave-surface in biaxial crystals 4, 7 Wave-surface in isotropic media 3 Wave-surface in uniaxial crystals 4, 6 Wright, F. E 31 Wright, F. E., Application of v. d. Kolk's method for the determina- tion of the feldspars 78 Wulfing, E. A 38,. 39, 43 Xylene, Index of refraction of 17 Zeojite group 54 MINERAL INDEX. (Figures in bold face type refer to Part II.) Acmite 56, 359, 495, Actinolite 59, 419, 451, 485 Aegirite 56, 359, 493 Aegirite-augite 56, 503 Aenigmatite 477 Albite 69, 389 Allanite (See Orthite) 5 1 , 505 Almandine 46, 133 Alunite 221 Amesite 54 Amphiboles, monoclinic 59 Amphiboles, ortliohombic 59 Analcite 54, 115 Anatase 215, 339 Andalusite 201, 229, 347 Andesine 69, 203, 407 Anhydrite 255 Anomite 52, 355, 491 Anorthite 69, 415 Anorthoclase 69, 379 Anthophyllite 59, 251, 309, 365 Antigorite 231, 295, 413, 443, 449, 473, 481 Apatite 187, 267, 321 ApophyUite 54 Aragonite 48, 243 Arfvedsonite 59, 471 Astrophyllite 365, 463, 507 Augite 56, 429, 459, 501 Automolite (See Gahnite) 45, 131 Axinite 405, 413, 449 Barkevikite 59, 497 Basaltic hornblende 59, 361, 495 Bastite 57, 231, 295, 481 537 538 MINERAL INDEX. Beckelite 131 Biotite 52, 337, 355, 491 Bronzite 56, 205, 247, 277, 305, 331, 363 Brookite 257, 313, 367 Brueite 223 Bytownite 69, 405 Calcite 48, 217, 285, 339 Cancrinite 169 CarphoHte 353, 417, 485 Cassiterite 225, 291 Celsian 62, 63, 427 Chabazite 54, 151 Chalcedony 155, 171, 177, 395 Chiastolite (See Andalusite) 201, 229, 347 Chlorite Group 53 Chlorite (See Pennine and Clinochlore.) 54 Chondrodite 5 1 , 505 Chromite 45, 103, 133 Chrysotile (See Olivine) 50, 253, 311, 365, 433 Chrysolite (See Serpentine) 177, 307, 395, 457 Chnochlore 54, 363, 445, 477, 499 Clinohumite 5 1 Clinozoisite 203, 277, 409, 445 Common hornblende 59, 483 Cordierite 161, 175, 199, 229, 273, 327, 347 Cornerupine 233 CorundophiUte 54 Corundum 191, 213, 323, 337 Cossyrite (See Aenigmatite) 477 Crocidolite 501 Cyanite (See Disthene) 413, 449, 481 Datoiite 239, 423 Delessite 295, 349, 473 Desmine (See Stilbite) 54, 381 DiaUage 56, 431, 461, 503 Dichroite (See Cordierite) 161, 175, 199, 229, 273, 327, 347 Diopside 56, 431, 461, 501 Dipyr 49, 213 Disthene 413, 449, 481 Dolomite 48, 217, 285, 339 Dumortierite 229, 347 Bnstatite 56, 203, 245, 329 Epidote, Green (See Pistacite) 51, 235, 353, 419, 487 MINERAL INDEX 539 Epidote group 5 1 Epistilbite 54, 385, 393 EucoHte 187, 267 Eudialyte 193, 271 FayaKte SO, 241, 301, 359 Fibrolite (See Sillimanite) 249, 309 Fluorite 111, 125 Forsterite SO, 251 Gahnite 45, 131 Garnet, Common 46 Garnets (See also the different varieties) 46, 193, 271 Gastaldite 59 Gedrite 59 235, 297, 351 Gehlenite 191 Gibbsite (See Hydrargillite) 397 Glauconite 445 Glaucophane 59, 351, 483 Gmelinite 54 Grandidierite 355 Graphite 101 Grossular 46, 119, 135 Griinerite 59, 425, 495 Gypsum 389, 395 Harmotome 54, 387 Haiiyne (See Haiiynite) 47, 113, 127 Hauynite 47, 113, 127 Hedenbergite 56, 459 Hematite 103, 341 Hercynite 45, 129 Heulandite 54, 163, 387 Hornblende, basaltic 59, 361, 495 Hornblende, common 59, 483 Humite 51, 253, 309 Hussakite 223, 289 Hyalophane 64, 379 Hydrargillite 397 Hydronephelite 54, 171 Hypersthene 56, 233, 297, 349 Iddingsite 361 Idocrase (See Vesuvianite) 189, 269, 321 Ilmenite 101 lolite (See Cordierite) 161, 175, 199, 229, 273, 327, 347 540 MINERAL INDEX. Jadeite 56, 431, 461 Johnstrupite 221, 247, 307, 427, 457 Kaolin 383, 443 Kaolinite (See Kaolin) 383, 443 Katophorite 59 Kelyphite 445 Labradorite 69, 409 Laumontite 54, 393 Lavenite 489 Lawsonite 249 Lazuli te 421, 489 Lepidolite 52, 199, 239, 275, 403, 443 Leucite 113, 127, 153 Leucoxene 223, 289 Magnesite 49, 217, 287, 341 Magnetite 45, 101 Manganese rich piedmontite (See Piedmontite, manganese rich) 251, 365, 433, 505 Meionite 49, 215 Melanite 46, 137 Melilite 187, 267, 321 Melinophane 285 Menaccanite (See Ilmenite) 101 Mica (See different varieties) 52 Microcline 68, 161, 381 Mizzonite 49, 213 Monazite 255, 311, 435, 463 Monticellite 50, 235 Mosandrite 221, 247, 307, 427, 457 Muscovite 52, 239, 299, 421, 453 Natrolite 54, 177 Nepheline (See Nephelite) 151, 189 Nephelite 151, 189 Nosean (See Noselite) 47, 111, 125 NoseHte 47, 111, 125 Octahedrite (See Anatase) 215, 339 Oligoclase 69 ,159, 199, 381, 403 Oligoclase-albite 69, 389 Olivine 50, 253, 311, 365, 433 Opal 113, 153 Orthite 51, 505 MINERAL INDEX. 541 Orthoclase 64, 159, 379 Ottrelite 427, 499 Owenite (See Thuringite) 327, 471 I'aragonite 52, 237, 299, 423, 453, 489 ]?argasite 59 Pectolite 56, 255, 433 Pennine 54, 329, 475 Periclase 119, 131 Peridot (See Olivine) 50, 253, 311, 365, 433 Perofskite 137, 193 Phillipsite 54, 387, 407 Phlogopite 52, 215, 241, 301, 357, 425, 455, 483 Picotite 45, 103, 131 Piedmontite, 51, 237, 353, 421, 487 Piedmontite, Manganese rich 251, 365, 433, 505 Pistacite 51, 235, 353, 419, 487 Plagioclase 69 Pleonaste 45, 133 Prehnite 251 Prismatine 297 Prochlorite 54 Pseudobrookite 313, 369 Pyrite 101 Pyrope 47, 135 Pyroxenes, Monoclinic 56 Pyroxenes, Orthohombic 56 Pyrrhite 133 Pyrrhotite 101 Quartz 193, 221 Riebeckite 59, 475 Rinkite 407, 475 RipidoHte 54 Rosenbuschite 417, 451, 485 Rutile 291, 343 ganidine 68, 159, 379 Sapphirine 327, 403, 471 Scapolite (See also Wernerite) 49 Scolecite 54, 383 Serpentine (Fibrous variety, Chrysotile) 177, 307, 395, 457 Serpentine (Leafy variety, Antigorite) 231, 295, 413, 443, 449, 473, 481 . Siderite 49, 219, 341 Sillimanite 249. 309 542 MINERAL INDEX. Sodalite 47, 111, 125 Spessartine 46, 119, 137 Sphene (See Titanite) 257, 367, 435, 465, 507 Spinel, Precious 45, 119, 131 Spodumene 56, 429, 457, 499 Staurolite 363 Stilbite 54, 381 Talc 243, 303 Thomsonite 54, 179 Thulite 279, 305, 331 Thuringite 327, 471 Titanite 257, 367, 435, 465, 507 Titanolivine 505 Topaz 207, 245 Tourmaline 337 Tremolite 59, 417 Tridymite 153 Triphane (See Spodumene) 56, 429, 457, 499 Uwarowite 47, 137 Vesuvianite 189, 269, 321 Wernerite 49 Wollastonite 56, 415 Wohlerite 459 Xenotime 289 Zinnwaldite 52, 357, 493 Zircon 223, 289 Zoisite 51, 205, 247, 277, 305 (The manuscript of the tables of Part III was completed early in 1905; the final copy of the complete book, March 1, 1906; and the lettering of the diagrams, February 28, 1907.) Fold out 5 'O IE f g s QC CO § d 1 < o E £ o .E 9 o 32 s 1 1- 00 (M O d & — si i 1 •If 1 5 ^ ^ >^ ^'^ ,^ ^ ^^ ^ --^' ^ .-^^ ,-^^ ^.^^ ^-^ ^ --^ ^ •^ ^-^ -^ ^..^^ „^^ ^^ ^ ^ ^- ^ ^ ^;^ ^ ^^^ '^^^ -^ -^ -^^ --^ -^^ -^ ^ ^^ ;;l.^ " ^-^ ^-^ --^ -^ -^ --^ ^;;>' ;;1.--^ ;;>-^ ^^--- ►.--' -^ <:::^ ';^ ^^ ,_.-' "^ . .^^ ^--^ ___^ --^ --^' --^ _,^- '^ ^^ --^ _--^ "^ .^^ -^ ._., — ^^ ^ ^^ ---' ^^^ _^ ^-^ ___^ -— ^ "^ -- — _.--' -^ . .'^ __— —- ^ u — - ■ "^ _— - —^ — ' ^____^ — —^ - — " -"^ ^______ _- -—^ " ■ — - — — — — — ^ — - — - ^ - — _ ■ — . ■ 1600 > 1700 1800 1900 2000 1 4th. Orde n 0.034 0.035 Meionite 0.036 Otivine Lazullte 0.037 Epidote Pectolite 0.038 Gilbertite Titanolivine 0.039 Thaumasite 0.040 Meroxens Lavenite Muscovites: 0.042 Datolite Phlogopite— 0.044 0.045 Anhydrite Diaspora Monazite— 0.048 Fayalite 0.049 0.050 Talc Aegirite Astroptiyllite— 0.064 0.055 Orunerite— 0.056 0.060 Zircon— 0.062 0.065 0.070 Basaltic iiornblsnde Anatase 0.073 0.080 0.072 0.090 Husaakite— 0.095,Cassiterite 0.096 0.100 0.110 0.120 0.140 Sphene- 0.141 0.160 Aragonite=:0.ie5< 0.180 Calcite= 0.172. Rutile= 0.287 Titanite =0.158 Dolomife 0.179 Retardation in millionths of mm. LitlLAiist.TE.ARirikc,leyrig. JTg 4^ :^ tfC ^ ^ Deferar in vicum vendentem thus et odores, Et piper, et quidquid chartis amicitur ineptis. Hor. Ep. 2, 1. 269.