F^ FMr-.MFFRINQ LIBRARY aidtneU ItttOTtsitg ffitbrarg Jtlfaca, ?Jew Snrk BOUGHT WITH THE INCOME OF THE SAGE ENDOWMENT FUND THE GIFT OF HENRY W. SAGE 1891 ine aate saows wuou i^» Tcr-ane was taken. To renew this book cooy the call No. and give to the librarian. HOME USE RULES All Books subject to recall Oct 6 i *■ ii "^ T 1^ *" borrowers must regis- _ !t..M..4^ ter in the librarv to borrow OCT.^..1..194B.. ter in the library to borrow books for home use. All books must be re- turned at end of college year for inspection and repairs. Limited books must be returned within the four wsek limit and not renewed. Students must return all books before leaving town. Officers should arrange for the return of books wanted during their absence from town. Volumes of periodicals and of pamphlets are held in the library as much as possible. For special pur- poses they are given out for u limited time. Borrowers should not use their library privileges for the benefit of other persons. Books of special value and gift books, when the giver wishes it, are not allowed to circulate. Readers are asked to re- port all cases of books marked or mutilated. Do not deface books by marks and writing. TN 260.R55E3" """"'">"-"■"'' IIMmmillJiiiiS 3 1924 004 122 689 Cornell University Library The original of tiiis book is in tine Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924004122689 WORKS OF PROF. HEINRICH RIES PUBLISHED BT JOHN WILEY & SONS, Inc. Clays: Their Occurrence, Properties and Uses With Especial Reference to Those of the United States. Second Edition, Revised, xix+554 pages, 6 by 9, 112 figures, 44 plates. Cloth, $5.00 net. Economic Geology Fourth Edition, Rewritten, xx+856 pages, 6 by 9, 291 figures, 75 plates. S5.00 net. By RIES AND LEIGHTON History of the Clay Working Industry in the United States By Prof. Heinrich Ries, and Henry Leighton, Pro- fessor of Economic Geology, University of PittS" burgh, viii+270 pages, 6 by 9, illustrated. Cloth, 12.50 net. By RIES AND WATSON Engineering Geology By Prof. Heinrich Ries, and Thomas L. Watson, Professor of Economic Geology, University of Vir- ginia, and State Geologist of Virginia. xxvi+V22 pages, 6 by 9,1 249 figures in the text, and 104 plates, comprising 175 figures. Cloth, S5.00 net. Elements of Engineering Geology v+365 pages, 5J by 85, 252 figures. Cloth, $3.75 net . ELEMENTS OF ENGINEERING GEOLOGY BY H. RIES, Ph. D. Professor of Geology, Cornell University AND I I < THOMAS L. WATSON, Ph. D. Professor of Geology, University of Virginia, and State Geologist of Virginia NEW YORK JOHN WILEY & SONS, Inc. London: CHAPMAN & HALL, Limited 1921 COPTRIGHT, 1921, BT H. RIES AND THOMAS L. WATSON TECHNICAL COMPOSITION CO. CAMBRIDGE, MASS., U. S. A. PREFACE The importance of geology to engineering is constantly receiving wider and stronger recognition, for it is realized that whatever line of work the engineer is engaged in, whether highway construction, tunnel- ing, quarrying, river and harbor improvement, water supply, mining, etc., he is almost certain to encounter problems of a geologic character. Indeed during the late European War, the importance of engineering geology to mUitary operations came to be widely though tardily recog- nized. There has thus naturally developed an increased demand for instruction in geology as applied to engineering. The aim has been, therefore, to emphasize the practical application of the subjects treated in this volume to engineering work. This volume is more than a condensation and simplification of a larger text "Engineering Geology" published by the authors in 1914, since it has involved complete rewriting of many parts of the larger book and the amplification of other parts. While "Engineering Geology" has met with a very gratifying reception, there are many institutions that desire a smaller volume to meet the requirements of a briefer course. It was to meet this demand that the present book was prepared. Ithaca, N. Y., and Charlottesville, Va. January 1, 1921 CONTENTS CHAP. ^^^^ I. The Important Rock-making Minerals 1 II. Rocks and Their Relations to Engineering Work 25 III. Structural Features and Metamorphism HI IV. Rock-weathering and Soils 154 V. Development, Work and Control op Rivers 174 VI. Underground Water ^^^ VII. Landslides, Land Subsidence and Their Effects 247 VIII Relation op Wave Action and Shore Currents to Coasts and „ 263 Harbors IX. Origin and Relation of Lakes and Swamps to Engineering Work 286 X. Origin, Structure, and Economic Importance of Glacial Deposits 302 XI. Road Foundations and Road Materials 325 XII. Ore Deposits 353 Appendix: Geologic Column 355 Index ELEMENTS OF ENGINEERING GEOLOGY CHAPTER I THE IMPORTANT ROCK-MAKING MINERALS Introduction. — Of the eighty-odd elements known to the chemist, some are extremely rare, others are exceedingly abundant. Only sixteen enter largely into the composition of the outer solid portion of the earth so far as it is accessible to observation. Arranged in their order of abundance the percentages of these elements, as calculated by Professor F. W. Clarke, are: Oxygen 47.33 Titanium 0.46 Silicon 27 .74 Hydrogen 0.22 Aluminum 7 . 85 Carbon . 19 Iron 4 . 50 Phosphorus . 12 Calcium 3.47 Sulphur 0.12 Potassium 2 . 46 Barium . 08 Sodium 2.46 Manganese 0.08 Magnesium 2 . 24 Chlorine . 06 A few of these elements occur in nature uncombined in elemental form, but most of them are found only in combination forming com- pounds called minerals. All rocks, with the exception of the glassy igneous ones, are com- posed of minerals, and since these minerals not only make up the rocks but vary greatly in their resistance to weather, it is necessary that we have a good knowledge of the characters and properties of the im- portant rock-forming ones, in order to be able to identify rocks and to judge correctly their value. The present chapter is devoted first, to an account of the general properties of the common rock-forming minerals that are of use in their megascopic determination, and second, to individual descriptions of the more important rock-forming minerals, with brief reference to their practical importance. ■ 1 2 THE IMPORTANT ROCK-MAKING MINERALS Definition of a mineral. — A mineral may be defined as any natural inorganic ^ substance of definite chemical composition. It is usually a solid, generally crystalline in structure but sometimes amorphous/ and may or may not occur bounded by crystal faces. As a rule external form is not developed in minerals as they occur in rocks, but usually as crystalline grains marked by irregular boundaries, because of inter- ference with one another during growth. Crystalline grains are com- monly referred to as anhedrons, signifying absence of crystal faces. Altogether more than a thousand distinct mineral species are known; but the more common ones number less than thirty. This relatively small number of the large total of known minerals includes those which may be rated as the more abundant ones or as the essential components of rocks, hence they are aptly referred to as rock-forming minerals. Definition of a crystal. — A crystal may be defined as a solid bounded by natural plane surfaces called crystal faces, symmetrically grouped about axes. By axes are meant imaginary lines which connect the centers of opposite faces, edges, or solid angles, and which intersect at some point within the crystal. Such a polyhedral form results when the molecules of a substance of definite chemical composi- tion possess such freedom of movement as to arrange themselves according to mathematical laws, which results in internal crystalhne structure and the outward expression of plane surfaces or faces. Under such conditions the minerals will usually crystallize with out- ward crystal form, such as cubes, octahedrons, prisms, etc. In the formation of rocks conditions sometimes favor definite arrangement of the molecules, and one or more of the minerals may assume out- ward crystal form, as shown in some igneous and metamorphic rocks. The number of crystal forms is large and yet when they are grouped in their relations to crystallographic axes they fall into six systems. The names usually given to the six crystal systems and their axial relations are : Isometric system. Three axes of equal lengths and inter- secting one another at right angles. Tetragonal system. Three axes intersecting at right angles, the two lateral axes being of equal length, while the vertical axis is longer or shorter than the two lateral ones. Hexagonal system. Four axes, the three laterals being of equal length and intersecting at angles of 60°, while the vertical axis is perpendicular to and longer or shorter than the three laterals. Orthorhombic system. 1 Such organic substances as coal, amber, petroleum, asphalt, etc., are frequently included. ' Quite a number of minerals fail to show both crystal form and internal crystal- line structure, when they are said to be amorphous. Opal and Hmonite are good examples. PHYSICAL PROPERTIES OP ROCK-MAKING MINERALS 3 Three axes intersecting at right angles and of unequal length-s. Mono- clinic system. Three axes of unequal lengths, the two lateral ones at right angles to each other, while the vertical axis is oblique to one of the laterals. Triclinic system. Three axes of unequal lengths making oblique intersections with one another. Twin crystals. — Crystals sometimes appear not to be simple or single forms but compound, in which one or more parts regularly ar- ranged are in reverse position with reference to the other part or parts (Dana). This pecuhar grouping is known as twinning, the different parts of such a crystal appearing as if revolved 180° about a line known as the twinning axis. The plane normal to the twin axis is called the twinning plane, and the plane of union of the two parts is called the composition plane. Many minerals frequently exhibit twinning, and in some it serves as an important means in determining them. Feld- spars very often show several kinds of twinning, two of which are of importance in megascopic determinations, namely, Carlsbad and albite (multiple) twins (see Figs. 3 to 5). Multiple twinning is character- istic of the plagioclase or soda-lime feldspars, and affords the surest means of distinguishing them from orthoclasc (see under feldspar group). Carlsbad twinning may be developed in any variety of feldspar, but is generally more frequent in orthoclase than in plagio- clase. General Physical Properties of Rock-making Minerals The more important physical properties of rock-making minerals which are of value in their megascopic determination are hardness, cleavage, luster, streak, color, tenacity, specific gravity, and crystal form. These have not equal weight in determining minerals. The behavior of minerals before the blowpipe and with chemical reagents is an important means of determining them and forms that division of the subject known as determinative mineralogy. Hardness. — Hardness is an important property of minerals and is of great value in their rapid determination. It may be defined as the resistance of a mineral to abrasion or scratching. The hardness of minerals is usually determined by comparing with Mohs's scale, which includes ten minerals arranged in the order of increasing hardness, as follows: 1. Talc; 2. Gypsum; 3. Calcite; 4. Fluorite; 5. Apatite; 6. Feldspar; 7. Quartz; 8. Topaz; 9. Corundum; 10. Diamond. In testing the hardness of a mineral care must be taken to select a fresh fragment, and not mistake a scratch for a mark left by a soft mineral on the surface of a hard one. If an unknown mineral scratches 4 THE IMPORTANT ROCK-MAKING MINERALS and in turn is scratched by a member of the scale, its hardness is the same as that of the scale member. Again if the unknown mineral scratches fluorite its hardness is greater than 4, but if it does not scratch apatite and is scratched by it, its hardness is between 4 and 5, or ap- proximately 4.5. In the absence of a scale, the hardness of a mineral may be approxi- mated by use of the following materials: The finger nail will, scratch gypsum (2), but not calcite (3); a copper coin will just scratch cal- cite (3); and the blade of an ordinary pocket knife will scratch apa- tite (5), but not feldspar (6). Cleavage. — When properly tested most minerals exhibit more or less readiness to part along one or more definite planes. In most minerals possessing crystalline structure the molecules are so arranged that the force of cohesion is less along a particular direction or direc- tions than along others. This property is called cleavage. It is a fairly constant property of minerals and is of great value in deter- mining them. Cleavage always occurs parallel to possible crystal faces, and is so described. Thus we have cubic cleavage (galena), octahedral cleavage (fluorite), rhombohedral cleavage (calcite), pris- matic cleavage (amphibole), basal cleavage (mica). All minerals do not possess cleavage, and comparatively few exhibit it in an eminent degree. Quartz and garnet do not show cleavage, but such minerals as feldspars, amphiboles, pyroxenes, and calcite are distinguished chiefly by their cleavage. The terms perfect, imperfect, good, distinct, indistinct, and easy are used to indicate the manner and ease with which cleavage is obtained. Luster. — The luster of a mineral is the appearance of its surface in reflected light, and is an important aid in the determination of minerals. Two kinds of luster are recognized: Metallic luster, the luster of metals, most sulphides, and some oxides, all of which are opaque or nearly so; nonmetallic luster, the luster of minerals that are transparent on their thin edges, and in general of light color, but not necessarily so. The more common nonmetallic lusters are described as follows: Vitreous, the luster of glass; example quartz. Resinous, the appearance of resin; example sphalerite. Greasy, the appearance of oil; example some sphalerite and quartz. Pearly, the appearance of mother-of-pearl; example talc. Silky, the appearance of silk (satin), due to a fibrous structure; example satin spar and asbestos. Adamantine, the briUiant, shiny luster of the diamond. Dull, as in chalk or kaolin. Streak. — By the streak of a mineral is meant the color of its pow- der. It is frequently one of the most important physical properties PHYSICAL PROPERTIES OF ROCK-MAKING MINERALS 5 to be applied in the determination of minerals, such as hematite and limonite. The color of a mineral in mass may vary greatly from that of its powder (streak, which is frequently fairly constant), and is usually much lighter. The streak may be determined by crushing, filing, or scratching, but the most satisfactory method is to rub the sharp point of a mineral over a piece of white, unglazed porcelain. Small plates, known as streak plates, are made especially for this purpose. Streak is of most value in distinguishing between the dark-colored minerals like the metallic oxides and sulphides, and is of less value in discriminating between the hght-colored silicate and carbonate minerals. Color. — Color is one of the most important properties of minerals, and, when used with proper precaution, it is of great help in their rapid determination. The color of metallic minerals is a constant property; but it may vary greatly in many of the nonmetallic minerals, due to the presence of pigments or impurities, which may be either chemically combined or mechanically admixed. Even the metallic minerals, such as the sulphides (pyrite, marcasite and chalcopyrite) whose color is constant, are susceptible to tarnish (alteration), and a fresh surface should always be examined in noting the color. The color of minerals is dependent upon their chemical composition, in which case it may be natural, or it may be due to some foreign sub- stance distributed through them and acting as a pigment, and their color may then be termed exotic (Pirsson) . Precaution should be used, therefore, in the latter case when color is employed in the determination of minerals. The introduction of the metallic oxides, the commonest one of which is iron, will influence the color, and according to its quantity the mineral wiU ordinarily exhibit some shade of green, brown, or even black. Examples among the silicate minerals are the iron-bearing members of the amphibole, pyroxene, and mica groups. Exotic color may result (1) from the presence of a very small amount of some compound in chemical combination, such as manganese oxide in quartz imparting an amethyst color; or (2) mechanically admixed impurities such as small amounts of hematite in quartz producing the red variety jasper. Tenacity. — Tenacity relates to the behavior of a mineral when an attempt is made to break, hammer, cut, bend, or crush it. The well known terms brittle, malleable, sectile, tough, flexible, elastic, etc., are used in describing minerals. A mineral is brittle when it breaks or powders easily, malleable when it flattens under the hammer, sectile when it can be cut but crumbles when hammered, tough when its resistance to tear apart under a blow or strain is great, flexible when 6 THE IMPORTANT ROCK-MAKING MINERALS it bends and remains bent after the pressure is released, elastic when bent it recovers its original position upon release of pressure. Quartz is brittle; gold, malleable; talc, sectile; chlorite, tough and flexible; and mica, elastic. Specific gravity. — The specific gravity of a mineral is its weight compared with that of an equal volume of water. In a pure mineral of given composition, it is a constant factor, and is an important aid in identification. The specific gravity of most silicate minerals lies between 2.25 and 3.5; of minerals with metallic luster usually between 4.5 and 10; and of natural-occurring metals as high as 23 (iridium). As ordinarily carried out in the laboratory, the determination of the specific gravity of a mineral is made as follows: The fresh mineral is first weighed in air, which value we may call x. It is then immersed in water and weighed again, and the value may be called y. Then x — y equals the loss of weight in water, or the weight of an equal volume of water. We then have G = I G being the specific gravity. X -y The determination of specific gravity may be car- ried out on several different kinds of balances, but one of the most convenient forms is the Jolly balance, shown in Fig. 1. The time required for the whole determination on this balance should not exceed several minutes. Crystal form. — Minerals are usually developed in rocks as crystalline grains without definite shape rather than as distinct crystals. Exceptions to this are the phenocrysts in porphyritic rocks (p. 41), some minerals formed in rocks by replacement (p. 329), and minerals hning cavities in rocks. When minerals exhibit definite shapes crystal form becomes an important aid in their determination. Because of the fact, however, that min- erals composing rocks are more often developed without crystal bound- aries, crystal form is less important as an aid in determining them than other physical properties. Fracture. — When a mineral breaks irregularly without regard to definite direction it is described as fracture. The appearance of a fracture surface is somewhat characteristic and is commonly desig- nated by the following terms: Conchoidal, when the surface presents a somewhat shelly appearance; fibrous or splintery, when the surface Fig. 1. DESCRIPTION OF ROCK-FORMING MINERALS 7 shows fibers or splinters; hackly, when the surface is irregular with sharp edges; uneven, when the surface is rough and irregular. Other physical properties of minor importance but nevertheless useful at times in the determination of minerals are taste, odor, feel or touch, and magnetism. Chemical tests. — Since chemical composition is the most funda- mental property of minerals, chemical tests with dry and wet reagents form the safest and most satisfactory means of identification. The common rock-forming minerals, however, can usuaily be readily and quickly determined by their physical properties, and since the equip- ment of a laboratory is not available in the field, it is essential that a thorough knowledge of the physical properties of minerals be obtained. Tables employing both physical and chemical tests for the deter- mination of minerals are to be found in a number of excellent manuals on determinative mineralogy. Description of Rock-forming Minerals The number of known minerals is large; but only a few are of im- portance as rock-makers. The principal ones from the geological standpoint may be grouped chemically as silicates, oxides, carbonates, sulphates, and sulphides, under which in the order named the individual minerals are treated. Silicates The silicates are the most important rock-forming minerals, since they compose a large part of the earth's crust. They are salts of silicic acids, many being of complex composition. Those of most importance as rock-forming minerals are the feldspar, pyroxene, amphibole, mica, olivine, garnet, tourmaline, and epidote groups. For convenience of treatment the silicates may be divided into A. Anhydrous silicates, and B. Hydrous silicates. A. Anhydrous Silicates Feldspar General properties. — The important rock-making feldspars are silicates of alumina, together with potash, soda, or lime, or their mix- tures. They include (1) the potash feldspars orthoclase and micro- dine (KAlSisOs), (2) the soda feldspar albite (NaAlSisOg), and (3) the lime feldspar anorthite (CaAUSizOg) ; (4) mixtures of 1 and 2, alkalic feldspar, and (5) mixtures of 2 and 3, plagioclase or soda-lime feldspars. Feldspar may be monoclinic (orthoclase) or triclinic (microcline and 8 THE IMPORTANT ROCK-MAKING MINERALS plagioclase) in crystallization, but perfect crystals (Fig. 2) are rarely observed, except when developed as phenocrysts in porphyritic igneous rocks (see Chapter II). Usually the feldspars develop as formless n~7^ Fig. 2. y Fig. 3. Fig. 4. grains. Twinning is common in the feldspars (Figs. 3 to 5) and is an important means of distinguishing between orthoclase and plagioclase varieties. Figure 3 shows an orthoclase twin. The polysynthetic twinning (Figs. 4 and 5) of plagioclase is indicated by fine parallel striations on the cleavage surfaces. All species of feldspar possess good cleavage in two directions, which intersect at 90° in orthoclase and at about 86° in plagioclase. Their hardness is 6; specific gravity 2.55-2.76. Luster vitre- ous; pearly on cleavage faces. Color vari- able, colorless and glassy feldspars being limited to fresh and recent lavas. Ortho- clase is commonly red, while plagioclase is commonly gray or white. Feldspar is often the dominant coloring mineral in granites. Alteration. — Feldspars commonly alter to kaolinite in the belt of weathering, when acted on by water containing carbon dioxide, with the separation of free silica and alkaline carbonates. The lime-bearing species on alteration may yield calcite. Potash feldspar, under con- ditions of dynamic metamorphism (see Chapter III), or by hydro- thermal metamorphism, commonly alters to sericite (p. 9) . The change of feldspar to kaolinite, known as kaolinization, is first noted in the feldspars by loss of luster, the mineral becoming dull and chalky or earthy in appearance. Lime-soda feldspars are more susceptible to alteration than orthoclase, and both are less durable than quartz, with which they are frequently associated, but they are not to be regarded as unsafe in building stones on that account. Pig. S. DESCRIPTION OF ROCK-FORMING MINERALS 9 The changes involved in the alteration of feldspar to kaolinite and muscovite have been expressed chemically as follows: Orthoolase Water Carb. diox. Kaolinite Quartz Potas. carb. 2KAISi308+ 2H2O + CO2 = H4Al2Si20<, + 4 SiOa + K2CO3 Orthoolase Water Carb. diox. Muscovite Quartz Potas. carb. 3 KAlSisOs + H2O + CO2 = H2K(AlSi04)3 + 6 SiOj + K2CO3 Occurrence. — Feldspars are probably more widely distributed than any other group of rock-forming minerals. They occur in most of the igneous rocks, such as granites, syenites, and lavas; in certain sand- stones (arkases) and conglomerates among sedimentary ones; and in gneisses and other metamorphic rocks. Hence feldspar is an im- portant constituent of many building stones. The feldspar of com- merce is obtained from pegmatites (p. 44). Mica General properties. — The micas form a group of silicate minerals of complex composition. For megascopic study they may be divided into (1) light colored micas represented by muscovite (H2KAI3 (8104)3), the potash mica, and (2) dark colored micas, represented by biotite ((K,H)2(Mg,Fe)2(Al,Fe)2(Si04)3), the iron-magnesium mica. They crystallize in the monoclinic system in tabular crystals having flat bases, and often of hexagonal outline. Although crystals are observed in rocks, the micas usually occur in flakes, scales, or shreds, sometimes bent or curved, with shining cleavage faces. All micas are charac- terized by perfect basal cleavage, along which they may split into extremely thin elastic plates or leaves, that are tough and flexible. This property, combined with transparency, toughness, and flexibility, makes the large sheets of muscovite of considerable commercial value. The color of micas varies, dependent chiefly on chemical composition. Muscovite is colorless, white to gray, sometimes greenish to light brown, while biotite is usually brown to black, sometimes dark green. The color of mica frequently exerts an important effect on building and ornamental stones containing it. Hardness 2-3 ; easily scratched with the knife; specific gravity 2.7-3.2. Luster vitreous to pearly or silky. Alteration. — Muscovite is very resistant to weathering processes, but it probably alters ultimately to clay. By the action of heated vapors and water feldspars may be changed into muscovite of silky luster, known as the variety sericite, and this process of formation 1 Kaolin may sometimes be formed in other ways. See Ries " Clays, Their Occur- rence, Properties, and Uses," Wiley & Sons, Inc., New York. 10 THE IMPORTANT ROCK-MAKING MINERALS termed sericitization has occurred in many ore deposits. Biotite, on account of its iron content, alters more readily, the commonest products of alteration being chlorite and iron oxides. The alteration of biotite in some building stones may cause unsightly coloration at times from the liberation of iron oxide. This is frequently observed in natural outcrops of many granites, and in opening a new quarry failure to strip the stone below the depth of oxidation may result in placing an inferior stone (sappy granite) on the market. Occurrence. — The common micas, muscovite and biotite, have wide distribution in igneous and metamorphic rocks and in some sedimen- tary ones, especially sandstones. Muscovite is a common constituent of the more acid igneous rocks, hke granites and their pegmatites, from which latter commercial mica is ordinarily derived, and is an abundant mineral in metamorphic rocks, especially crystalline schists and gneisses. Biotite occurs in many granites, diorites, gabbros, and peridotites, and their fine-grained equivalents, and in crystalline schists and gneisses. The presence of mica in building stones may exert an important influence on their durability and workability. When present in abundance and the shreds have parallel arrangement, the rock may split readily along this direction. In quantity, mica is an undesirable constituent of marble, because it is apt to weather out leaving pitted surfaces, and at times interferes with the production of a good polish. Although some building stones, such as granite, etc., are rarely free from mica, it is not an injurious constituent unless present in large quantity, or segregated into large and small areas through the stones as "knots," rendering the rock unsightly and therefore undesirable for some uses. Pyroxene General properties. — The pyroxenes form an important group of rock-making minerals which, like the amphiboles, are salts of metasihcic acid (H2Si03). The more important rock-making members of the group are: Enstatite (MgSiOg); hyper sthene ((MgFe)Si03); diop- side (CaMg(Si03)2) with little or no ferrous iron; and augite, which is more complex and in addition contains alumina and ferric iron. Pyroxenes are orthorhombic, monoclinic, or triclinic in crystalliza- tion, but members of the triclinic system are of no importance as rock-forming minerals. They all agree in general crystal habit, a prism with an angle of about 87° and 93°; ^ Fig. 6. DESCRIPTION OF ROCK-FORMING MINERALS 11 Fig. 7. usually short, stout, prismatic (Fig. 6), or columnar. A cross section of the prism form is usually octagonal in outline (Fig. 7). (Compare with cross section of amphibole. Fig. 8.) Pyroxenes are also com- monly developed in shapeless grains and masses. All pyroxenes show cleavage developed in two directions parallel to the prism faces, intersecting at an angle of 87° (Fig. 7). The cleavage angle is a fundamental property and serves to distinguish pyroxenes from amphiboles. Hardness 5-6; specific gravity 3.2-3.6. Color varies according to the iron content ; white to gray and pale-green in enstatite and diopside; dark brown to greenish brown in hypers- thene; and various shades of green to black in augite. Luster vitreous to resinous and pearly. Alteration. — Under the action of weathering agents pyroxenes com- monly alter into serpentine and chlorite, accompanied at times by carbonates and iron oxides. Under conditions of dynamic meta- morphism pyroxenes alter into amphiboles. Occurrence. — The pyroxenes are chiefly found in igneous rocks, especially the basic ones, such as basalts, gabbros, and peridotites (see Chapter II). They are less common in metamorphic rocks, being noted in some crystalline limestones and gneisses, but are rare in sedi- mentary rocks. They are not very important in the common building stones, and when present in quantity and of the brittle variety, they may interfere with the production of a smooth pohsh. Amphibole General properties. — The amphibole group of minerals is parallel to the pyroxene group, the two groups having similar chemical com- position and physical properties. Both groups are salts of metasilicic acid (HiSiOs), but the amphiboles differ from the pyroxenes mainly in the prism and cleavage angle which is 125° and 55° instead of 87°. For megascopic purposes the more important varieties of amphibole are: Tremolite (CaMg3(Si03)4); adinolite (Ca(MgFe)3 (8103)4); and hornblende, which is more complex in composition but contains also alumina and ferric iron. Amphiboles may be orthorhombic, monoclinic, or triclinic in crystallization, but only the monoclinic members are of megascopic importance as rock-making minerals. All agree in general habit and in having a prismatic cleavage of 55° and 125°. They gen- erally occur in long and bladed forms (Fig. 8), sometimes fibrous and columnar, and in shapeless grains and masses. The outline cross-section Fig. 8. 12 THE IMPORTANT ROCK-MAKING MINERALS Fig. 9. of the prism form is usually hexagonal (Fig. 9) . Cleavage in two direc- tions parallel to the cleavage faces intersecting at angles of 55° and 125°. The cleavage angle is one of the most distinguishing characteristics of the amphiboles. Compare Fig. 9 showing cleavage of amphibole with Fig. 7 which shows cleavage of pyroxene. Hardness 5-6; specific gravity 2.9-3.5, according to the amount of iron present. Color varies according to the iron content from white or gray in tremolite to light green in actinolite to dark green and black in hornblende. Luster vitreous to pearly on cleavage faces, often silky in fibrous varieties. Alteration. — Under conditions of weathering amphiboles com- monly alter, according to composition, into chlorite or serpentine usually accompanied by carbonates, quartz, and epidote. They may finally break down into carbonates, iron oxides, and quartz. Occurrence. — Amphiboles are important rock-making minerals and occur in a variety of igneous and metamorphic rocks. Tremolite and actinolite are metamorphic minerals, the former occurring chiefly in crystalline limestones and the latter in crystalline schists. They may also occur as products of alteration in igneous rocks. Owing to its tendency to decompose, tremolite is a harmful mineral in crystalline dolomitic limestones. Hornblende occurs both in igneous and meta- morphic rocks, and when derived from pyroxene by metamorphism, it is known as the variety uralite. Garnet Physical properties. — Garnet corresponds to the general formula R3"R2"'(Si04)3,' the most common varieties of importance as rock-form- ing minerals being: Grossularite (Ca3Al2(Si04)3); vyrope (Mgikh- (8104)3); almandite (common garnet, (Fe3Al2(Si04)3); and an- dradite (Ca3Fe2 (8104)3). Garnet crystallizes in the isometric sys- tem, commonly as dodecahedrons (Fig. 10) and trapezohedrons (Fig. 11). It often occurs in rocks as formless grains and granular aggregates of rounded or irregular outline. Hardness 6.5-7.5; specific gravity 3.15-4.3, vary- ing with the composition, common garnet being 4.0. Color varies 1R3" = Ca,Mg,Fe,Mn; R2'" = Al,Fe,Cr, etc. Fig. 10. Fig. 11. DESCRIPTION OF ROCK-FORMING MINERALS 13 according to composition: Grossularite, white to pale shades of pink, yellow, and brown; pyrope, deep red to nearly black; almandite, deep red to brownish-red; and andradite is black. Luster vitreous. Alteration. — Dependent upon composition, garnet may alter to chlorite or serpentine, less often to hornblende. Chlorite is the com- monest alteration product of common garnet, and limonite is a common end product in the alteration of the iron-bearing varieties. Occurrence. — Garnet is found chiefly in metamorphic rocks and, to a less extent, in some igneous ones. Grossularite is chiefly found in crystaUine limestones; pyrope in some basic igneous rocks, especially peridotites; ahnandite in crystalline schists or gneisses, sometimes in pegmatites, and in contact metamorphic zones; while andradite is restricted to some basic igneous rocks and contact-metamorphic deposits. The principal use of garnet is as an abrasive. Olivine General properties. — Olivine (chrysolite) corresponds to the general formula Mg2Si04, in which magnesium may be replaced by more or less ferrous iron. It crystallizes in the orthorhombic system, but distinct crystals are rare, since its common occurrence in rocks is as formless grains and granular masses. Cleavage indistinct; fracture conchoidal. Hardness 6.5-7; specific gravity 3.27-3.37. Color olive to yellow green, but bottle green very common. Luster vitreous. Olivine commonly alters to serpentine and iron oxide. Occurrence. — Olivine is a characteristic mineral of the less siliceous igneous rocks, such as gabbros, peridotites, and basaltic lavas, but it also occurs in metamorphosed magnesian limestones and some schists. Epidote General properties. — Epidote, a basic orthosilicate of calcium and aluminum with variable iron, is monoclinic in crystallization, but crystal form is of little importance, since it commonly occurs in rocks in formless grains and granular aggregates. Cleavage unequally developed in two directions. Hardness 6-7; specific gravity 3.3-3.5. Color usually some shade of green, yellowish green being the most common. Luster vitreous. Occurrence. — Epidote occurs abundantly as a secondary mineral in igneous rocks, derived from the alteration of ferromagnesian minerals and hme-soda feldspars, and commonly accompanies chlorite. It has a similar occurrence in crystalline schists and gneisses. It may be abundant in some limestones altered by contact metamorphism. 14 THE IMPORTANT ROCK-MAKING MINERALS Tourmaline General properties. — Tourmaline is a complex silicate of aluminum, boron, iron, magnesium, and the alkalies. It crystallizes in the rhom- bohedral division of the hexagonal system in short to long prismatic forms (Fig. 12), three-, six-, or nine-sided, the prism faces being often vertically striated. Triangular cross-section of the prism form (Figs. 13 and 14) is especially characteristic of rock-making tourmaline. Fig. 12. Fig. 13. (Compare with pyroxene (Fig. 7) and amphibole (Fig. 9).) Tourma- line also occurs in shapeless grains and masses. It has a hardness of 7-7.5; specific gravity 2.98-3.20. Color variable, but the common rock-making variety is black. Luster vitreous. Tourmahne is very resistant to weather. Occurrence. — Tourmaline is a widely distributed constituent of the crystalline schists and of the more acid igneous rocks, such as granites and their accompanying pegmatites. It also occurs in some gneisses and clay slates, and is a common mineral of contact metamorphic zones. It is sometimes used as a gem mineral. B. Hydrous Silicates Of the hydrous silicates kaolinite, talc, serpentine, and chlorite are the most important rock-making minerals. These are of secondary origin, and may be formed either by weathering or by heated circulat- ing waters or vapors acting on rock masses. They are of most im- portance in sedimentary and metamorphic rocks, and are of no impor- tance in fresh igneous rocks. They occur as constituents in the wall rock of many ore deposits formed by the alteration of original silicate minerals by different geologic processes (see Chapter on Ore Deposits). Kaolinite General properties. — Kaolinite (H4Al2Si209) crystallizes in the monoclinic system as minute scales or plates with sometimes hexagonal outlines, but crystal form is of no value in megascopic determinations. It may occur in clay-like masses, or scattered irregularly through feldspathic rocks. Its color is white, but it is often colored by impuri- DESCRIPTION OF ROCK-FORMING MINERALS 15 ties. Luster usually dull, earthy. Hardness 1-2.5; specific gravity 2.2-2.63. Neither hardness nor gravity is serviceable for practical tests. It usually has an unctuous, greasy feel, and is plastic when wet. Occurrence. — Kaolinite has widespread occurrence, and is a com- mon constituent of clay. It is always a secondary mineral, formed usually by the weathering of aluminous silicate minerals, chiefly feldspars. The chemical equation showing derivation of kaoHnite from orthoclase by weathering ikaolinization) is given under feldspar, page 9. By it rock-masses are decomposed and soils are formed. Extensive deposits often result from the alteration of aluminous rocks and, when not discolored by iron oxide and other impurities, form the sources ot white ware and paper clays. Deposits of clay of variable thickness and extent, showing all degrees of admixture with sand, etc., and discolored by impurities, occur. Masses of sericite are some- times mistaken for kaolinite. Talc General properties. — Talc (H2Mg3(Si03)4) commonly occurs in foliated masses, compact, and fibrous. Its crystal form is doubtful. Two varieties are usually recognized: (1) Foliated talc having Hght green to white color, a pronounced greasy feel, and foliated structure, used in paper manufacture, as a toilet powder, filler in cloth, etc.; and (2) steatite or soapstone, an impure talc of greenish color and mass- ive granular in structure, extensively used for sinks, laundry tubs, etc. Talc has perfect basal cleavage like mica; the laminte though flexible are inelastic. Hardness 1; specific gravity 2.6-2.8. Color white to greenish or gray. Luster pearly; soapy or greasy feel. Occurrence. — Talc is derived by alteration from non-aluminous magnesian silicates, such as olivine, enstatite, tremolite, etc. Its derivation from enstatite may be represented chemically as follows: Enstatite Water Carb. diox. Talc Magnesite 4 MgSiOa + H2O + COj : H2Mg3(Si03)4 + MgCOs It may occur as an alteration product of basic igneous rocks, such as peridotites and pyroxenites, but it is of chief importance in the metamorphic rocks, such as talc schists and soapstone or steatite. (See further under metamorphic rocks.) Serpentine General properties. — Serpentine (H4Mg.3Si209) does not crystallize but usually occurs compact or granular massive and often fibrous, the fibers being flexible and easily separated from each other. Ordi- 16 THE IMPORTANT ROCK-MAKING MINERALS nary serpentine is massive, opaque, and of various shades of green, while chrysotile is the fibrous or asbestiform variety occurring in veins in massive serpentine and, for the most part, is the asbestos of com- merce. Precious serpentine, the massive, translucent, light to dark green variety, is often mixed with calcite or dolomite and shows variegated coloring, when it is called verd antique marble, or ophical- cite. The color of serpentine is usually some shade of green, yellow, or red, and black, but often variegated showing mottling in lighter and darker shades of green. Luster is greasy and wax-like in the massive varieties and silky in the fibrous. It has a greasy feel and is translucent to opaque. Hardness varies, but is usually 4; specific gravity, 2.2-2.8. Occurrence. — Serpentine is a secondary mineral formed as an alteration product from magnesian silicates, such as olivine, pyroxene, and amphibole in igneous and metamorphic rocks. Its derivation from olivine may be shown chemically as follows: Olivine Water Carb. diox. Serpentine Magnesite 2Mg2Si04 + 2H2O + CO2 = HiMgaSizOs + MgCOs. Serpentine is an important constituent of the verd antique marbles (ophicalcite), used as an ornamental stone in decoration and building. (See p. 108.) Chlorite General properties. — Chlorite is a general name for a group of minerals that cannot usually be distinguished from each other by the naked eye. The chlorites are hydrous silicates of aluminum with ferrous iron and magnesium. Like mica, they are monoclinic in crystallization, but distinct crystals are rare and they commonly occur in rocks as flakes, scales and scaly masses. They have perfect basal cleavage, yielding foliae which are tough, but unlike mica are inelastic. Color green of various shades, usually dark green. Luster vitreous to pearly. Hardness 1-2.5; specific gravity 2.65-2.96. Occurrence. — Chlorite is a common secondary mineral in many igneous and metamorphic rocks, or even some sedimentary ones, formed chiefly by the alteration of aluminous ferromagnesian silicates, such as pyroxene, amphibole, mica, etc. The green color of many basic igneous rocks, such as traps and basalts, and of many meta- morphic rocks, such as schists and slates, is due to the presence of chlorite. It is a common product of hydrothermal action along some ore bodies. DESCRIPTION OF ROCK-FORMING MINERALS 17 Oxides The more important roclc-making minerals among the oxides are quartz (Si02), corundum (AI2O3), and the iron ores, including mag- netite (Fe304), ilmenite (FeTiOs), hematite (Fe203), and limonite (2Fe203.3H20). Quartz General properties. — Quartz (Si02) crystallizes in hexagonal prisms capped by pyramidal faces (Figs. 15 and 16), but, except when formed in cavities and as phenocrysts in some igneous rocks (porphy- ries), crystal form is not often observed. Its j. usual occurrence in rocks is as shapeless grains ^i^K and masses. It has no cleavage, which helps to distinguish quartz from feldspar. The color varies widely from white or colorless to almost