UC-NRLF dob IA; FOR THK 5COPICAL DETERM >CK-FORMING MINERAT S AND ROCKS ALBERT JOHANNSEN -BERKELEY LIBRARY UNIVERSITY OF CALIFORNIA EARTH SCIENCE* I I9RARY ESSENTIALS FOR THE MICROSCOPICAL DETERMINATION OF ROCK-FORMING MINERALS AND ROCKS THE UNIVERSITY OF CHICAGO PRESS CHICAGO, ILLINOIS THE BAKER 4 TAYLOR COMPANY MEW YORK THE CAMBRIDGE UNIVERSITY PRESS LONDOH THE MARUZEN-KABUSHIKI-KAISHA TOKYO, OSAKA, KYOTO, rUKUOKA, 8ENDA1 THE MISSION BOOK COMPANY SHANGHAI ESSENTIALS FOR THE MICROSCOPICAL DETERMINATION OF ROCK-FORMING MINERALS AND ROCKS IN THIN SECTIONS BY ALBERT JOHANNSEN, PH.D. PROFESSOR OF PETROLOGY THE UNIVERSITY OF CHICAGO THE UNIVERSITY OF CHICAGO PRESS CHICAGO, ILLINOIS ' 1 * EARTH COPYRIGHT 1922 Bv THE UNIVERSITY OF CHICAGO All Rights Reserved Published June 1922 Composed and Printed By The University of Chicago Press Chicago. Illinois. U.S.A. PREFACE This laboratory manual contains practically all of the data originally published in the writer's l>, iirininiition of Rock-I-'anninii Mineral*, and in addition gives modes of occurrence and many more points of separation between similar minerals. Only a few very rare species, such as johnstrupite, mosaiitlrite, laavenite. etc., have been omitted, hut by uniting the tables which contained minerals having birefringences greater or less than quartz, and refractive indices greater or less than Canada balsam, much repetition has been avoided, and the number of pages has Ijcen materially reduced. t M-thorhonibic minerals. Have In-en united with the other biaxial minerals, since sections which cut all of the crystallographic axes in this system show inclined extinction. The maximum extinction angles, of course, are given in the descriptions. The index of Canada balsam is shown on the dia- grams at l..">:57, the mean of that found in good sections. The separation lines between the various plagioclasc feldspars have been changed from those given in the former book to 5, 27$, 50, 72J, and 95 per cent anorthite. Alhite and anorthite have been limited to a variation of only 5 per cent since these names are also applied to the pure end members, and compound names such as oligoclase-albite, labradorite-bytownite, etc., have been omitted. The section on the determination of the feldspars has been but little reduced, but that on optical met hods has been condensed as much as possible since this data is given elsewhere. Manual, through- out the text, refers to the author's Manual of Petrographic Methods, 2d edition, New York, 1918. The alphabetical list of minerals has been much extended, and is now placed at the back of the book in such a position that reference to it may be made without turning pages. Finally, a short section on the determination of rocks has been added, so that when minerals and their percentages have been determined, the rock name also may be known. The writer has intentionally used words such as "sometimes," "often," and "occasionally" for "in some specimens," "in many localities," "here and there," and so on, believing that the use of adverbs of time for adverbs of place, especially in a book such as this, which is intended for laboratory use and which therefore should Ix: as brief as possible, is justified by their use in this manner by many of the writers of the classics. Not only is this usage customary in English, but in foreign languages as well. ALBERT JOHANNSEN UNIVERSITY or CHICAGO April 18, 1922 3374 KEY TO THE TABLES Opaque . /*~1 !,, Uniaxial 4 Odorless < Biaxial 8 Anisotropic f Uniaxial . . . . 14 Colored . } Biaxial .... . 16 Pleochroic . . ( Uniaxial . . . . 20 vi THE MINERAL IS OPAQUE [Magnetite Black l>y incident light ................................................................... {Graphite (llmenite [ Red ......................... Hematite Transparent on thin edges .................................... < Brown ....................... Chromite I Greenish-brown ............... Picotite Pyrite occurs in crystals and irregular grains as a primary accessory in igneous rocks, and abun- dantly in mt'tamorphic rocks and sediments. It is non-magnetic, and insoluble in HC1. By incident light the color is lighter than that of pyrrhotite. 1'ijrrhotite occurs rarely in igneous rocks, abundantly with certain ore deposits. Pyrrhotite has a red-bronze to yellow-bronze color; p y r i t e is brass-yellow. Pyrrhotite is magnetic. Magnetite occurs in the form of octahedrons, cubes, or irregular grains as a common accessory in all kinds of igneous rocks, most abundantly in those that are basic. It also occurs in metamorphic rocks and sediments. As a secondary mineral it is found in igneous rocks, in some cases in dustlike or dendritic forms. It is magnetic. Graphite occurs in irregular flakes or scalelike aggregates, rarely in hexagonal plates, as a con- stituent of metamorphic rocks, schists, quartzites, marbles, rarely in pegmatites or other igneous rocks. May not be separable from magnetite when the latter occurs in irregular grains. Ilmenite is an accessory mineral in igneous rocks, especially in diorites, gabbros, diabases, etc., also in metamorphic rocks. It occurs in hexagonal plates or grains, which in many cases are altered over the entire surface, or along lines intersecting at 60, to a white decomposition product, usually tit unite, called leucoxene. Titaniferou's magnetite also may alter to leucoxene. Ilmenite rarely shows deep-brown thin edges. In some cases magnetite, graphite, and ilmenite may not lie separable under the microscope. I Hematite is found in rocks of all kinds, either as small hexagonal crystals, rare in igneous rocks, as pseudomorphs after magnetite,' as rims around magnetite, as an alteration product from various ferromagnesian minerals, and as stains in cleavage cracks. It occurs in immense deposits among sediments. Magnetite is black by incident light, hematite is red. L i m o n i t e is usually yellow although it may be red, in which case it may be confused with hematite. In such cases it is customary to speak of the material as red-, brown-, or yellow iron oxide, and let it go at that. Chromite is a mineral of peridotites and serpentines. It is black by incident light, sometimes brownish black on thin edges. In many cases it may not be possible to separate chromite from magne- tite except by the reaction for chromium. Picotite, the chrome spinel, occurs as an accessory in peridotites and other basic igneous rocks, in serpentines derived from peridotites, and rarely as crystals in basalts. It is greenish brown to yellowish brown on thin edges, and is isotropic. 1 C THE MICROSCOPICAL DETERMINATION THE MINERAL IS ISOTROPIC Occur as crystals Colorless Refractive index is less than Canada balsam (1.537) Fill cavities or are amorphous Fluorite n = 1.433 Sodalite 1 . 483 Noselite 1.490 Hauynite. ... 1.503 Leucite 1.508 /Opal 1.443 \Glass 1.490 Refractive index is greater than Canada balsam Cavity or interspace filling or at- /Fluorite 1.433 tached crystals \Analcite 1 .488 Form octahedrons, usually show /Spinel 1.716 quadratic sections \Periclase. . . . 1.736 Usually in polygonal sections or/Grossular. . . . rounded grains \Spessartite. . . 1.750 1.811 Refractive index balsam Colored is less than Canada! (Occur as crystals [Fills s cavities or is amorphous Fluorite .... Sodalite .... Noselite. . . . Hauynite. . . Leucite Glass . . 433 483 490 503 1.508 1.490 Refractive index is greater than Canada balsam Form octahedrons, quadratic sections usually Spinel Periclase . . Hercynite . Picotite . . . show/Gahnite. . . Pleonaste. . Pyrrhite. . . Beckelite. . Perofskite . 1.718 1.736 1.749 = 1.7 1.765 High High High 2.38 Rounded, quadratic, hexagonal, oc- tahedral, etc., or in irregular grains. Usually with strong fractures Grossularite., Pyrope Almandite. . . Spessartite. . Uwarowite. . . Melanite . . .744 744 .810 1.811 .838 .856 Fluorite, when colored, is readily separated from all other minerals of low index. The color, in many cases, is irregularly distributed. Cleavage (111) is perfect. The mineral rarely forms crystals, but generally occurs as cavity or interspace filling. It never shows anomalous double refraction. S o d a J i t e and analcite, with which it may be confused when colorless, differ in cleavage, have anomalous double refraction in many cases, and occur in the alkalic rocks, while fluorite is especially confined to acid granites and pegmatites, frequently associated with topaz, tourmaline, and tin, where it was formed by pneumatolytic action. Sodalite, a mineral of the nephelite- and other alkalic syenites, has a fair (110) cleavage while analcite has fair (100) cleavage. The two cannot be separated by optical means, but chemically the presence of chlorine indicates sodalite (Manual, p. 563). Optical anomalies are common in both. Noselite, hauynite, and leucite always occur as crystals and are confined to igneous rocks, where they are primary minerals. Leucite occurs in six- or eight-sided to rounded grains, in many cases with characteristic radial or tangential inclusions in regular zones. Small leucites are generally iso- tropic, but larger grains are almost invariably polysynthetically twinned in a pattern resembling microcline, though of much lower birefringence (Manual, p. 510). Noselite and hauynite cannot be separated under the microscope by optical tests, but may be separated microchemically (Manual, p. 563). OF RorK-FoRMlM! MlNKRALS AND ROCKS Analrite is found filling veins and cavil ic> in lui.-alis. diabase*, and other basic rooks, and as interspace tilling in certain liasaltic rocks, where it has been considered a primary mineral. It can U- .-eparated from sodalite only by microchemical means (Manual, p. 564). Anomalous doulilr refraction is common. Opal has no cleavage and may show anomalous double refraction. It occurs in cavities and veins in acid igneous rocks as a deposit from magmatic waters, and as nodules in limestones, -hales, sandstone-, -ilicitied wood. etc. (flux* has no cleavage. It does not occur in the plutonie rocks except as inclusions in feldspars, etc.. luit is common in the acid extrusives. Its uniform distribution through the slide separates it from opal. Its lack of cleavage separates it from sodalite and a n a 1 c i t e . Spinel occurs as an accessory in peridotites and other basic igneous rocks, in serpentines derived from peridotites, in granular limestones, gneisses, etc. It is pale red, green, or blue, and has poor (111) cleavage. 1'iricliixf is gray to yellow, and has good (100) cleavage. Both periclase and spinel occur in quadratic (111) crystals. is dark green, picotite is yellow-brown to greenish brown, gahniie is greenish black, is green, pyrrhite is orange-yellow to red, beckelite is pale yellow, and perofxkite is grayish white, brownish to red-brown, rarely greenish gray. Hercynite, gahnite, and pleonaste are separable only chemically, the others by color. The garnets all show lack of cleavage, high relief, and isotropism. UniKxtihir is colorless to pale yellow. It may show anomalous double refraction. Zonal struc- ture is common. Occurs in metamorphic calcareous rocks, as a contact mineral, or in crystalline schists. Spessarlite is blood-red, yellowish red, or red-brown to colorless. Anomalous double refraction i- common. It occurs in granites, a rhyolite (C'olorado), and quartzites. Pyrope is red to blood-red. Kelyphite rims are common. The mineral occurs only in peridotites, dunites, and their derivatives. Almandite is red to red-brown, zonal structure is common, but anomalous double refraction does not occur. The mineral is found in igneous rocks which have been dynamo-metamorphosed, and in mica-schists and other metamorphic rocks. Uwarounte is deep green and usually shows anomalous double refraction. It is confined to chromium-rich serpentines, granular limestones, and dolomites. Alterations are unknown. Melanite occurs in various tones of dark brown, less commonly in green. Anomalies are hardly observable owing to the deep color. Zonal structure is common, but alterations are wanting. The mineral is found in various igneous rocks, especially phonolites, nephelinites, leucitites, tephrites, etc., also in contact metamorphosed rocks. ESSENTIALS FOR THE MICROSCOPICAL DETERMINATION The Mineral is AN The Mineral is NEGATIVE ( SOTROPIC, COLORI -) 1 eate in Index of Refrac .ESS, UNIAXIAL. he Mineral it POSITIVE ( + ). ' - Sooo w o w o m o w OO^x r- r, to to in 5 in v CNCN ~ M ^ J _ In cress'g BIREFH. 9 ' 1 M o ui o ""=022 v m J 10 s to r~ r-ooffiOw - 1 _ - - -_-NN j VerrH, Hgh. Medium. Not Marked. u LOW. 1 LOW.' Not Marked. Medium. High. Verj H. Euco Apatite lite. . . ! . f ephelite Tridyr Leucite nite.* - Eudii lite. Vesuv anite --Corund Melilit Quartz. - Quartz Dipyr. - 010 Hydro 1C :>helite Alunite. rucite. B Cancr nite. Vleionite. .040 Phlo ;opjte. - .045 050 Zircon - Am tase. 075 080 .090 .095 .100 .120 140 Cassiteri e. - Mag c Cal( ite. .160 .200 .250 erite. OF RoCK-FoUMIXlite occurs in various nephelite-syenites. It lias very weak (O>E) plooohroism or none. Cleavage (0001) is distinct. Anomalous 21 ta 50. K u (1 i a 1 y t e is optically positive and has negative elongation. T o p a z is biaxial. Apatite has poor cleavage, long crystals shu\\ parting, and elongation is negative. M e 1 i ! i i e has characteristic abrormal^erlin blue interference color and basal cleavago^J Apatite has characteristic basal parting in long prisms. It is easily soluble in HjSO 4 and the solution gives a itfow precipitate with ammonium molybdatc (Manual, p.. 565). Apa- tite is a very common accessory in the form of small prisms in most igneous rocks. In large crys- tals it occurs in pegmatites, some lamprophyres, It is also found in crystalline schists, lime- si ones, argillites, etc. Sillimanite has higher double refraction and positive elongation. 'Hi' is a feldspathoid and does not occur in qovte-beftring rocks. It usually shows abnormal Berlin blue interference colors. The (001) and (110) cleavages are poor; only the basal cleavage is generally seen in thin sections, and this occurs as a single cleft along the middle of the lath-shaped section. Peg structure, due to inclusions growing inward from basal sections, is characteristic. It gelatinizes easily with HC1 i ual, p. 564). Vesuvianite and /. o i s i t e , both of which may give the abnor- mal blue interference color, are insoluble in acids. Vesuvianite has higher relief, and usually occurs as a contact mineral in limestone. Zoisite is biaxial and occurs as a secondary mineral. Nephelite occurs in grains in soda-rich plutonic rocks, and in grains and quadratic sections in extrusives, but it is never found in the same rock with primary quartz. It has rather distinct (0001), (1010) cleavages. Anoma- lous biaxial character with small optic angle may occur. It gelatinizes easily with HC1 (Manual, p. 564). Nephelite resembles q u a r t z in appearance, but is negative. Cordierite is biaxial. Scapolites occur in metamorphic rocks, gneisses, crystalline schists, granular lime- stones, etc., but are rarely primary in igneous rocks. The Mineral is Positive Leucite is isometric at 433 C., but below that temj>erature is doubly refracting (Manual, p. 510). Characteristic radial or tangential inclusions, twinning, and crystal form separate it from all other minerals. It is found only in igneous rocks which are high in potash and low in silica, never in sediments or as a metamorphic mineral. It is fairly common among extrusives but is very rare among plutonites. Tridymite is characterized by low refractive indices and by its occurrence in tabular, hexagonal, or rounded crystals, or in rosette-like aggregates, or in overlapping plates so small that edges appear like rectangular cleavage lines. Anomalous optic angle, 2E, may be as high as 70. The mineral occurs in cavities in silicic extrusive rocks. It resembles no other minerals. Eudialyte is optically positive and has nega- tive elongation, while e u c o 1 i t e is negative and has positive elongation. Anomalous 2E may be as high as 50. It is commonly associated with nephelite. See under eucolite. Quartz usually shows no cleavage It may be separated from nephelite by the cleav- age and negative character of the latter. Cor- dierite is biaxial and negative, with 2V from 40 to 84, but quartz may show an anomalous optic angle, 2E, in some cases as high as 25. When cordierite is treated with HF it gives characteristic prismatic crystals of magnesium fluosilicate. Scapolites are negative, show cleavage, and have higher double refraction. Hydronephelile, a rare mineral in igneous rocks, has a poor (lOlO) cleavage and occurs in rodlike, leafy, or granular aggregates. It is separated from nephelite by its positive character and lower indices ; from quartz by lower indices, from thomsonite by its lower birefringence. It is soluble in HC1 with the formation of jelly. Alunite occurs in veins in certain extrusive rocks and is formed by the action of SOj upon them. It has a good (0001) cleavage, which dis- tinguishes it from quartz, as does also its higher birefringence. ESSENTIALS FOR THE MICROSCOPICAL DETERMINATION The Mineral is Negative Vesuvianite has poor (110), (100) cleavages. It usually occurs as a contact mineral derived from limestone, but has also been found in ancient ejected blocks among the dolomite masses of Vesuvius and Monte Somma. In some cases it shows abnormal Berlin blue interference colors, or biaxial character. It is insoluble in acids unless first fused. Z o i s i t e has better cleav- age and a different mode of occurrence. Corundum occurs as a primary mineral in alumina-rich igneous rocks, both acid and basic, such as pegmatites, syenites, anorthosites, and dunites. It is rare as a contact mineral, but occurs in granular limestones and dolomites, gneisses, mica-schists, etc. The pleochroisrn, O = blue, red, E = sea-green, yellow, or greenish yellow, is seen only in deeply colored specimens. It has poor parting (1011), (0001). The high relief separates it from similar minerals except vesuvianite from which it is separated by its hardness, higher double refraction, and by chemical means. Dipyr (mizzonite), wernerite (common scapo- lite), and meionite are scapolites, and may be considered as isomorphous mixtures of the two molecules marialite (Ma) (SNa^O-SAWVlSSKV 2NaCl) and meionite (Me) (4CaO-Al 2 O 3 -6Si0 2 ). eo to MajMei is called dipyr (mizzonite), to MaiMe 2 wernerite, and MaiMe 2 to oo meionite. Where cleavage is not shown, scapolites resemble quartz, but their birefringence is greater and they are negative. The mode of occurrence and birefringence sepa- rate them from nephelite, as does also the cleavage, which appears right-angled in sections giving an interference figure. They occur in metamorphic rocks, gneisses, schists, in contact metamorphosed limestones, and as secondary minerals in calcium-rich basic rocks. Optical anomalies showing the opening of the inter- ference cross are rare. They readily alter to mica, etc. Cancrinite, a secondary mineral after nephe- lite, by some considered in part primary, resembles muscovite in its high interference colors, but its indices of refraction are less than Canada balsam. No other common mineral has such high colors and low indices except t h o m - s o n i t e and hydrargillite, but both of these are biaxial and positive. For chemical separation of cancrinite from nephelite and hydronephelite see elsewhere (Manual, pp. 564-65). The Mineral is Positive Brucite is a secondary mineral found in serpentines and magnesian limestones. It usually occurs in foliated or fibrous masses, sometimes spherical, or in plates. Anomalous biaxial char- acter is not rare. Muscovite and talc are negative and have positive elongation. Hydromagnesite has lower birefringence and effervesces with HC1 while brucite is soluble without effervescence. Gypsum is very simi- lar in appearance, but its indices are lower than balsam, and it has inclined extinction. Zircon has weak, seldom noticeable pleo- chroism. It occurs in small characteristic crys- tals which are shorter and stouter than those of apatite, and which have brilliant interfer- ence colors. In larger grains, the interference colors are pale and the mineral is brownish. Zircon is especially common in acidic and in sodic igneous rocks, but is also found in schists and gneisses, and as a residual mineral in the decom- position products of igneous rocks. Cassiterite may be pleochroic in weak brown- ish tones. Cleavage (110) is poor, (100) dis- tinct. Geniculated twins are common. It occurs as a pneumatolytic mineral in acid dikes and quartz veins, and as a rare primary mineral in some igneous rocks. R u t i 1 e has better cleav- age and is not so brown, anatase is negative, brookite is biaxial, and perofskite is isotropic. Or HIM K-l-'nllMIM. MlNKKM.x AMI KiM'KS The Mineral is Negative Mi iiinitr. Sec under ilipyr. above. 1'hltHjitpitf is generally at least faintly colored with weak yellowish pleochroism. The "bird's- cyc maple" appearance separates it from all minerals except other micas and talc. Its imiaxial interference figure separates it from all luit talc and I) i o t i t e . The former occur- only in basic rocks and is perfectly colorless or faintly green, the latter is usually strongly plco- chroic. Bleached biotite may l>e color- less and impossible to separate from phlogopite. occurs in pyramids and tablets, and is found in igneous rocks in some granitc- pegmatitos. It usually has pleochroism, = deep blue or orange-yellow, E = light blur or light yellow, but it may lw very weak so that tin- mineral appears eolorlr in thin sections. Color- less or yellow portions are usually normal, while blue portions show anomalous opening of the interference cross and do not fully extinguish. Perofskite differs in form, and the anoma- lou- interference colors are lower than those in anatase. Calotte, dolomite, and magnesite cannot be separated under the microscope, but may be by chemical means (Manual, p. 565). Aragon- it e is biaxial with 2V =18, and it differs in certain chemical reactions (Manual, p. 568). B r u c i t e differs chemically (Manual, p. 567), and has much lower double refraction. Calcite is a common alteration mineral in all kinds of igneous rocks, and is said to be primary in some granites. Both calcite and dolonu'te occur as vein minerals, and in widespread and thick st rata. Magnesite occurs as a secondary mineral derived from the alteration of magnesia-bearing minerals. It also occurs in talc-schists, serpen- tines, etc., often as veins. 'ite has higher indices of refraction than the three preceding carbonates, and is usually somewhat yellowish or brownish. It is a com- mon mineral of ore veins and of limestones. It is also found in gneisses, slates, shales, gray-wackes, etc. ESSENTIALS FOR THE MICROSCOPICAL DETERMINATION The Mineral it The Mineral ii NEGATIVE (- \NISOTROPIC, COLC -). Tl .ase in Index of Refract RLESS. BIAXIAL, ic Mineral ii POSITIVE (+). ooooin o in o in E o in m o o> oo h. i* to (o ing in ^ lit crcasg BIREFH. I in o Em o m o inoooo -a n'ri. - - -J VerjH. High. Medium. Not Marked, o LOW. LOW. Not Marked. Medium. High. VertH. i Inorthoc Caolinj- igoclase, nite. * las _ ^r' - M .TnrH anidine. rthoclas - Stilbi icrocline Epistilbi >aumonti .005 Quartz. .010 .015 .020 O Bytow e, '. Hel landite. ' Andesinc. Labradorite. / Bro izite. Clino oisite. -^ elite. - r n< m. ^ dony." ntine. / Enstatite r Dist Hyp lene. - ersthene. jllastonit Vnorthite ' nl igorite I tc. Chalce Serpe ^ Sp odumene. _ Sillimanite. . Anthophyllite.i Tremo] Actinoi lie. ite. 030 Diopsi ie. _ Diallage. Epido te. 035 __ For sterite. Mus Par: :ovite. i goni'te. 1 ^^ ] 'ectolite. Dat Jlite. _ ___ I >h -CF ogopit idolite e. _ Anhj drite. M .085 .090 .095 .100 .120 Titanite.- Aragonit .160 .200 ,250 Broo lite. ~ OP ROCK-FORMINO MINERALS ANT> ROCKS The Mineral is Negative Kaolin usually appears as a flour-like, white, opaque alteration protlm-t of feldspar. It may l>e stained yellow or red l>y iron oxides. When crystallized it occurs in the form of leaves and scales with an extinction angle of 13, and may be mistaken for sericite, although its bire- fringence is lower. Much so-called kaolin is colloidal aluminium silicate, and not kaolin. Muscovite (sericite) and h y d r a r g i 1 1 i te have higher birefringence^. .1 nurthodase (soda-microcline), orthoclase, san- . oligoclase, and microdine are feldspars. Social methods for their separation are given on pages 30-34. Anorthocla.se, orthoclase, sani- dine, and mieroclino have indices less than ( anada balsam. Anorthoclase has 2V = 43-53, orthoclase 2V = 6943', 2E= 1216', sanidine 2V = small to 0, microcline 2V = 83. It may be impossible to separate anorthoclase and ortho- clase under the microscope. Microcline is sepa- rated by the "grating" or plaid structure, due to the combination of polysynthetic albite and pericline twinning. Oligoclase usually has poly- synthetic twinning with characteristic extinction angles, and refractive indices very near Canada balsam, in some sections greater in one direction and less in the other. All of these feldspars are separated from albite by the positive char- acter of the latter, or by its extinction angle. < >rt hoclase and microcline may contain inter- grown lamellae of albite (less commonly oligo- clase), and are then called microperthite and microcline-microperthite. In anti-perthite, orthoclase forms lamellae in oligoclase or andesine. Stilbile, one of the zeolites, occurs in rods, leaves, and sheaflike or radiating groups, in cavities in basalts, and less commonly in granites. The extinction angle c:o is about 8", and 2V is about 33. It is decomposed by HC1 without gelatinization. Cordieriie, orthorhombic, 2V from 40 to 84, 2E from 63 to 150, occurs in gneisses and various schiste, rarely as a primary mineral in granites, andeaites, etc. As a contact mineral it is found at the contact of acid igneous rocks with shales and slates. When treated with HF it gives characteristic prismatic crystals of magnesium fluosilicate. Pleochroic halos are occasionally seen in sections parallel to the c axis. Trillings and polysynthetic twins occur. Q u a r t z is uni- axial and positive, albite is positive and has lower indices of refraction, nephelite is uni- axial. The Mineral is Positive Heulanditf, a zeolite, occurs in leaves, plates, or rosettes, in basaltic rocks, rarely in gneiss. Its biaxial character (2E = 0-55), low indices and birefringence, and its habit separate it from other minerals. Andesine and labradorite are plagioclase feld- spars. They usually show polysynthetic twin- ning with characteristic angles. See pages 31-34. Clinozoisite is an iron-poor or iron-free epidote with the composition of zoisite. It is colorless or reddish with weak or no pleochroism, extinc- tion angle of 3, and a large optic angle (2V = 80-90). It occurs in prisms or rods elongated on l>. and in grains. Abnormal Berlin blue interference colors are common, as in zoisite, but this has parallel extinction and smaller optic angle (2V = 0-60). It may be impossible to separate the usual grains found in igneous rocks from zoisite. P i s t a c i t e has higher double refraction. Topaz occurs as a pneumatolytic mineral in granite dikes, granites, rhyolites, and cassiterite- pegmatites, either in cavities or scattered through the rock. It is also found in the adjacent schists and gneisses. Cleavage is basal, but may not show in thin sections. Quartz has lower relief and is uniaxial. Andalusite is nega- tive and usually has different mode of occurrence. Apatite is uniaxial and negative, while topaz has a large optic angle (2E = 71-129). Vesuvianite is usually slightly pleochroic and is uniaxial. Disthene is negative, 2V = 82, and elongation is positive. Corundum is uniaxial and negative. Enslaiite has the usual pyroxene cleavage, parallel extinction (see note under hypersthene), 2E = 135, and is non-pleochroic. Mono- clinic pyroxenes have higher birefring- ence and inclined extinction in sections at right angles to the principal optic sections. In basal sections (which have sharp cleavage lines at angles of approximately 90 with each other) monoclinic pyroxenes show the emergence of an axis while orthorhombic pyroxenes show the emergence of a bisectrix. Bronzite is slightly pleochroic, hypersthene is pleo- chroic and negative. [ironzite is essentially like enstatite but it is slightly pleochroic in green and pink tones. 2E= =t 106. Hypersthene has similar but stronger pleochroism and is negative. For sepa- ration from monoclinic pyroxenes, see under enstatite above. 10 ESSENTIALS FOR THE MICROSCOPICAL DETERMINATION The Mineral is Negative Epistilbite, a zeolite with inclined extinction, c:c= 9, occurs in basaltic rocks. It is im- perfectly soluble in concentrated HC1 without gelatinization. May not be possible to separate from s t i 1 b i t e , although the birefringence of the latter is lower. Andalusite has characteristic though fre- quently faint pleochroism, a = rose, b = c = color- less to light green, resembling that seen in hyper- sthene. Hypersthene, however, has posi- tive elongation, while andalusite has negative. Cleavage also differs, but the good (110) and less distinct (100) cleavage of andalusite may not be seen in thin sections, where the mineral often appears in irregular grains. In the schists the mineral usually occurs in characteristic irregular oval grains associated with grains of magnetite. In chiastolite the inclusions are found in regular arrangement, in the forms of rhombs, crosses, etc., in cross-sections and parallel to the long axes of prisms, and the material is often altered to a mica-like mineral. The higher relief separates andalusite from cordierite. Andalusite is found in a few granites, but is essentially a mineral of slates, schists, and gneisses. Chiastolite is a contact mineral in argillites near granitic intrusions. Antigorite, the massive, lamellar serpentine, differs from common serpentine, which is fibrous, in being negative, and in its habit. Pennine, when optically negative, is sepa- rated by its optical character, when positive, by a chemical test for A1 2 3 . Pennine also has lower birefringence, usually abnormal interference colors, and pleochroism. Serpentines are always secondary and occur as an alteration product of olivine, less commonly of pyroxene or amphibole, and possibly also of other ferromagnesian min- erals. Disthene (cyanite) does not occur in igneous rocks, but chiefly in muscovite or paragonite schists, gneisses, eclogites, etc., often associated with garnets or corundum. The color is faint blue in thin sections, in some cases almost color- less. Cleavages, (100) perfect, (010) distinct, making an angle of 74, are very characteristic, although they do not show in all sections. Orien- tation, a is nearly at right angles to (100), c is inclined 30 on (100) to the edge (100): (010). S i 1 1 i m a n i t e and andalusite are orthorhombic and have different cleavages, topaz has basal cleavage only, z o i s i t e usually has abnormal interference colors and occurs in grains. The Mineral is Positive Zoisite, orthorhombic, is a mineral of the crystalline schist formations, produced by the dynamo-metamorphism of igneous rocks contain- ing basic plagioclase. It also occurs in pegma- tite dikes. Abnormal Berlin blue interference colors are common. Cleavage (010) good, (100) distinct. Clinozoisite has an extinction angle of 3 and an optic axial angle of 2V = 80-90, while zoisite has an angle of 0-60. M e 1 i - lite gelatinizes with acids, occurs only in quartz-free rocks, and has a characteristic habit. Vesuvianite has poorer cleavage and higher relief. Gypsum is a mineral of the stratified rocks and occurs in connection with limestones and other rocks, but is rarely found in the crystalline schists. It resembles muscovite but the birefringence is lower, its extinction angle (c:c = -52 to -53) is higher (muscovite, c:c = 0-2), and it does not show the "bird's-eye maple" effect. Anhydrite is not so shredded and has higher double refraction as well as parallel extinction. Albite, a plagioclase, usually shows poly- synthetic twinning with characteristic extinc- tion angles. When untwinned it resembles orthoclase, but is optically positive. Other plagioclases have higher re- fractive indices. See pages 31-32. Ottrelite is a mineral almost exclusively confined to argillites altered by dynamo- metamorphism. It occurs in leaves and plates and usually shows hour-glass structure. Pleo- chroism may be rather weak or wanting, c= yel- lowish green, colorless, b = blue, a = olive-green. Cleavage (001) good. The low double refrac- tion and high relief as well as its mode of occur- rence separate it from all other minerals. Zoi- site has parallel extinction and has different color and smaller optic angle. Clinozoi- site has different cleavage (001:100 = 6437'), and usually abnormal interference colors. Chalcedony fills or lines cavities in rocks, or occurs in threadlike aggregates, concretionary masses, or spherulites. It is insoluble in HC1. 2V = 10^0. Z e o 1 i t e s do not have thread- like habit and are soluble or gelatinize in acid. Pseudochalcedony is negative and has a small value for 2V. HIII K-Fi'ltMIM. MlNKRALS AKD ROCKS 11 The Mineral is Negative 'lontitc li:is an extinction angle of + 20. :iinl occurs M- -mall |>risns in cavities in basalts and other basic extrusive*, in pegma- tites. >ycliites. etc. From e p i s t i 1 li i t e it arated by its gelatinization in HC1 and its extinction angle. ( ' a n c r i n i t e has higher birefringence and is uniaxial. t h o m s o n i t e is positive and has higher birefringence. Hi/l><-r*lhene has characteristic pleochroism. c = greenish, n = re y r o x e n e s show the emergence of an axis while orthorhombic pyroxenes show the emer- gence of a bisectrix. The cleavage, (110): (110) = '.M Hi', is characteristic of all pyroxenes. Hyper- sthene occurs in all rocks from gabbroic rocks to granites. Bronzite is positive and has weaker pleochroism; a n d a 1 u s i t e, with similar pleochroism, has negative elongation and different cleavage. Anorthite, the calcium plagioclase, shows polysynthetic twinning in most cases. See pages 30-34. WoUastonile has (100) good and (001) dis- tinct cleavages with an angle between them of 84.5. Extinction angle c: a = +3212'. It usu- ally occurs in tablets or rods along the 6 axis, or reticulated or parallel in masses in granular limestones, at igneous contacts in lime rocks, but very rarely in igneous rocks themselves, then usually in calcareous inclusions. It gelat- ini/es with HC1. Pectolite and tremo- 1 i t e differ in not having the plane of the optic axes at right angles to the elongation, which is very cha ract erist ic of sections in the orthodiagonal zone of wollastonite. P i s t a c i t e has higher refraction and higher birefringence. The Mineral is Positive Serpentine is always secondary and occurs as an alteration product of olivine, less commonly of pyroxene or amphibole, and possibly of other ferromagnesian minerals. Antigorite is the mas>ive, lamellar variety; here is included the fibrous variety. Antigorite is nega- tive and massive. P e n n i n e has lower double refraction, usually abnormal interference colors, and is pleochroic. Spodumene has typical pyroxene cleavage, has extinction c:c= 23 to 26, and is generally non-pleochroic unless the sections are thick when a = amethyst, b = amethyst, c = colorless. In many cases it is altered to a mixture of albite and muscovite. It occurs in pegmatite veins, often of great size, and in granites and gneisses. Pleo- chroism, moderate birefringence, and occurrence separate it from other pyroxenes. Sillimanite occurs as a contact mineral, and in long, slender, fine needles without terminal faces in the quartz of granites and gneisses. May also occur in prisms or aggregates of needles. Its (010) cleavage is perfect, and there are transverse fractures. Apatite, with similar cross-parting, has much lower double refraction and negative elongation. Andalusite is negative, has negative elongation, lower bire- fringence, and the relation of the axial plane to cleavage is different. Scapolites are nega- tive, have negative elongation, and are uniaxial. Z o i s i t e has weaker double refraction and different orientation. Anthophyllite, an orthorhombic amphibole, usually fibrous, occurs in mica- and other schists as a contact mineral, and as an alteration product of olivine in serpentines, gabbros, peridotites, etc. It is usually non-pleochroic in thin sec- tions, but may show c = yellowish, b = clove- brown, reddish, a = yellowish, greenish, colorless. Typical amphibole cleavage and parallel extinc- tion separate it from other minerals. In basal sections showing sharp cleavage lines, a bisectrix appears in the center of the field ; in m o n o - clinic amphiboles this lies from a few degrees to twenty-two to the side. Augile is usually green, brown, reddish, violet, or yellowish, but is rarely colorless. Pyroxene cleavage and high extinction angle (c:c= 45 to 55) characterizes it. In sections showing parallel extinction, the plane of the optic axes is parallel to (010), in olivine it is parallel to (001). Augite is a common pyroxene in igneous rocks. It also occurs in metamorphosed sediments and metamorphosed igneous rocks. 12 ESSENTIALS FOR THE MICROSCOPICAL DETERMINATION The Mineral is Negative Tremolite has typical amphibole cleavage, extinction c : c = 16, and occurs as crystals, long or short, often bladed or fibrous, or com- pact, in metamorphosed magnesian limestones with little iron. Where iron is abundant, actino- lite occurs, or where iron is the only carbonate, griinerite. Tremolite is hardly affected by HC1. Actinolite is pale green and slightly pleo- chroic, wollastonite gelatinizes with HC1 and has the trace of the plane of the optic axes at right angles to the elongation. Actinolite, with amphibole cleavage, extinc- tion c:c= 15, and similar in habit to tremolite, is rather a common mineral in certain schists and metamorphosed magnesian limestones containing much ferrous iron. It is green in color and has a faint pleochroism, sometimes hardly noticeable in thin sections, green to yellowish green. Epidote (pistacite, green epidote) is a com- mon contact or dynamo-metamorphic mineral in impure calcareous rocks, and a secondary mineral in the feldspars of many igneous rocks. It is often associated with clinozoisite. It has been described as primary in certain granites. It is sometimes very abundant with quartz in the rock called epidosite. Pistacite is the iron-rich epidote, clinozoisite the iron-poor or iron-free variety. The characteristic pistachio green color, brilliant interference colors, and high relief sepa- rate it from all other minerals. Pleochroism a = colorless to yellowish or greenish, b = yellow- ish to yellowish gray, c = green to light yellowish brown, sometimes rather weak. The plane of the optic axes lies at right angles to the elonga- tion of the crystal. Muscovite has a characteristic "bird's-eye maple" appearance, which separates it from all other minerals except the micas and talc. The optic axial angle (2E = 60-70) separates it from bleached biotite (2E = small to 0). Paragonite can be separated only by chemical tests. Lepidolite usually has a smaller optic angle (2E = 32-84), but in some cases may not be distinguishable except by chemical tests. Talc has 2E = 6-20, but in shreds it cannot be distinguished from muscovite except by chemical or physical tests, or by asso- ciated minerals. Primary muscovite never occurs with pyroxenes, talc usually does. The fine, shredded muscovite, secondary in potash feld- spars, is called s e r i c i t e. Do not call the secondary mica in plagioclase sericite unless you are certain that potash is present; the mica prob- ably is secondary paragonite. The Mineral is Positive Diallage and diopside are monoclinic pyrox- enes. The former has very perfect and abun- dant (100) cleavage in addition to the (110) cleavage of the latter. Both are pale green to colorless, and have extinction angles c:c= 39. Diopside occurs in pyroxene-granites, diorites, lamprophyres, crystalline schists, and magnesia- rich marbles; diallage is common in gabbros and related rocks, and in peridotites and the ser- pentines derived from them. Pyroxene cleavage separates these minerals from all but other pyrox- enes, from which the extinction angle separates them. In sections showing parallel extinction, the plane of the optic axes is parallel to (010), in o 1 i v i n e it is parallel to (001). Forsterite, the magnesia olivine, rarely occurs in igneous rocks, but is a mineral of dynamic and contact metamorphosed marbles, basic schists, and gneisses. It is colorless, and while it has a distinct cleavage (010), (001), this is usually seen only as heavy, irregular cracks. The mode of occurrence separates forsterite from olivine. The orientation of the interference figure (see under olivine) separates it from the pyroxenes. Olivine, the intermediate magnesia-iron va- riety, is a common primary mineral in basic rocks, and an accessory in basic schists, gneisses, and marbles. It alters to actinolite, anthophyl- lite, iddingsite, magnetite, chromite, opal, quartz, serpentine, tremolite, and other minerals. Alter- ation to serpentine and magnetite are most common. It gelatinizes slowly in HC1. The interference figure lies parallel to (001), while in pyroxenes, in sections showing parallel extinction, it is parallel to (010). Fayalite has 2V = 50, higher birefringence, and is nega- tive. Forsterite has a different mode of occurrence. Pectolite usually occurs in tablets, rods along the b axis which are rarely terminated, or fibrous aggregates of acicular crystals, sometimes radiat- ing. 2V = 60 and c : c = 5. It occurs as a secondary mineral, like the zeolites, in cavities in basic igneous rocks, sometimes in metamor- phosed rocks, and in nephelite-syenites. Wol- lastonite is negative, has ( ) elongation, lower birefringence, and higher refractive indices, and c:a=+32. Pyroxenes have differ- ent orientation of the interference figure. OF ROCK-FOKMIM; Ml\KHU.s \M) RoCKS 13 The Mineral is Negative iiti-, the white soda mica analogous to muscovite, is common in certain schists (paragonite- sehi>N> aiul probably as a secondary mineral from plagioclatii>it< is paler brown than biotite, red- dish brown, or yellowish brown, sometimes greenish or colorless. 2V, as in biotite, is small to 0. Biotite has stronger pleochroism, but when bleached may not be distinguishable from it. I'hlogopite is essentially a mineral of marbles ad' crystalline dolomites, but does occur in the leucite rocks of Wyoming and in the mica- peridot itcs of southern Illinois. For separation from other minerals, see under muscovite. Lepidolite, colorless to reddish, pink, or violet . in many cases resembles muscovite, from which its optic axial angle (2E = 32-84), when low, may separate it. The optic axial angle also separates it from bleached biotite (2V = small to 0). In most cases it can \IG separated only from these micas by the reaction for Li. Lepidolite occurs in granite- pegmatite \eins, greisen, and gneiss, often with tourmaline, cassiterite, etc. Fayalite, the iron olivine, may be colorless or yellowish, greenish, reddish, with weak or no pleochroism in yellow and red tones. Oliv- ine has 2V = 88 (fayalite, 2V = 50), is opti- cally positive, and has lower birefringence. Forsterite is positive, has 2V = 86, lower indices, and different mode of occurrence (con- tact mineral in metamorphic limestones). Talc, orthorhombic, closely resembles mus- covite in thin sections, and it may be neces- sary to use chemical means to separate them. The optic angle (2E = 6-20) is smaller than usual in mu-covite, and the mode of occurrence is different, primary muscovite never occurring with pyroxene, while talc commonly does. It has the same "bird's-eye maple" appearance so common in mica. Araganite, under the microscope, resembles calcite, in refractive indices and double refraction, but it is biaxial (2V = 18) and shows no cleavage. It occurs in gypsum deposits, occasionally in ves- icules in basalt, and as the material of certain fossil shells and corals. For chemical separation from calcite see Manual, p. 568. The Mineral is Positive Anhydrite occurs in grains, sharp blades, seldom in threads in .sedimentary beds associated with gypsum, in limestone, or with rock salt, and rarely in cavities in lava (Santorin). Gyp- sum has lower double refraction and 53 extinction, d i s t h e n e has lower birefringence and is negative. Monazite occurs in granites, in gneissoid rocks, and in sediments, but most commonly in sands and gravels. The crystals are small, tabular parallel to (100) or elongated on the 6 axis, rounded grains, occasionally in larger masses. The yellow, non-pleochroic color, high birefringence, and high relief, separate it from most minerals. T i t a n i t e has higher bire- fringence, and the extinction angle is 39 (mona- zite, c:c = 2-6). Brookite has () elonga- tion (monazite negative), and 2E is somewhat larger. R u t i 1 e has positive elongation, is usually of a deeper red or orange color, is uni- axial, has higher indices, and may show genicu- lated or heart-shaped twins. Titanite is a very common accessory mineral in the acid plutonites, such as granites and syenites, abundant in nephelite-syenites, and less common in diorites. It is also abundant in gneisses and schists, and in some limestones. As a secondary mineral (leucoxene) it is derived from titaniferous magnetite, ilmenite, rutile, and other titanium-bearing minerals. The pleochroism is weak, c>b>a, in brown and yellow tones, c:c +39, and 2E = 45-68. The strong dispersion produces colored isogyres. It occurs in the form of prisms, rhombs, and grains. Monazite has lower birefringence, smaller extinction angle, and weak dispersion. Brookite has parallel extinction, 2V =0 to 23. Rutile is uniaxial. Brookite occurs in veins with various other minerals albite, quartz, nephelite, garnets, rutile, chalcopyrite, galena, etc. and in gold washings, always in the form of crystals of various habits, often tabular. The acute bisectrix is normal to (100) but the axial plane is parallel to (001) for red and yellow and parallel to (010) for green and blue. For red (670 w) 2E = 580', yellow (589 w) 2E = 3810', yellowish green (555^) 2E = 0, green (535-525 MM) 2E = 2140'-330' (Manual, pp. 444-45, and Figs. 619-23). Interference figure for white light is a combination of these, giving a peculiar form (Manual, Fig. (524). Cassiterite and rutile have different habit, and brookite has very different strength of double refraction in (100) and (010) sections. 14 ESSENTIALS FOR THE MICROSCOPICAL DETERMINATION The Mineral i. AN I SOT R( The Miner*! it NEGATIVE )PIC, COLORED, NON-I (-) 'Th cre*e in Index of Refracti LEOCHROIC. UNIAX1AL. c Mineral U POSITIVE (+). o 01 oo r* r- to Nol Marked. Medium. High. Van High.. Eucol Apatite ite. ^ - Eudii T lite. v< suvianite . ^ -T /\ 015 2j rcon. , 090 095 .100 120 Cas siter ite. . Magne Dolom 160 .200 250 Ruti e. _ HOCK-FORMING MINERALS AND ROCKS 15 The Mineral is Negative 'He occurs in v;iriK) pleochroism ur none. Cleavage (0001) is distinct. Anomalous 'JI', to 50. Eudialyte is optically positive, and lias negative elongation. Topaz is biaxial. Apatite lias poor cleavage, long crystals -how parting, and elongation is negative. M e 1 i 1 i t e has characteristic abnormal Berlin blue interference color and basal cleavage. Apatite has characteristic basal parting in long prisms. It is a very common accessory, in the form of small prisms, in most igneous rocks. In large crystals it occurs in pegmatites, some lamprophyres, etc. It is also found in the crys- talline schists, limestones, argillites, etc. Silli- m unite has higher double refraction and positive elongation. Mil'lite is a feldspathoid and does not occur in quart /.-iH-aring rocks. It usually shows ab- normal Berlin blue interference colors. The (001) and (110) cleavages are poor; only the basal cleavage is generally seen in thin sections, and this occurs as a single cleft along the middle of the lath-shaped section. Peg structure, due to inclusions growing inward from basal sections, is characteristic. It gelatinizes with HC1 i Manned, p. 564). Vesuvianite and z o i - site, both of which may give the abnormal interference color, are insoluble in acids. Vesuvi- anite has higher relief, and usually occurs as a contact mineral in limestone. Zoisite is biaxial and occurs as a secondary mineral. Vesuvianite has poor (110), (100) cleavages. It usually occurs as a contact mineral derived from limestone, but has also been found in ancient ejected blocks among the dolomite masses of Vesuvius. In some cases it shows abnormal blue interference colors, or biaxial character. Zoisite has better cleavage and a different mode of occurrence. Calcite, dolomite, and magnesite cannot be separated under the microscope, but may be by chemical means (Manual, p. 565). A r a g o n - it e is biaxial with 2V =18, and it differs in certain chemical reactions (Manual, p. 568). B r u c i t e differs chemically (Manual, p. 567), and has much lower double refraction. Calcite is .a common alteration mineral in all kinds of igneous rocks, and is said to be primary in some granites. Both calcite and dolomite occur as vein minerals, and in widespread and thick strata. Magnesite occurs as a secondary mineral derived from the alteration of magnesia-bearing minerals. It also occurs in talc-schists, ser- pentines, etc., often as veins. The Mineral is Positive l-'.nilinlyte is optically positive and has nega- tive elongation, while eucolite is negative and has po-itivr elongation. Anomalous 2E may be as high as 50 (see eucolite). It is commonly associated with nephelite. ///con has weak, seldom noticeable pleo- chroism. It occurs in small characteristic crys- tals which are shorter and stouter than those of apatite and which have brilliant interference colors. In larger grains the interference colors are very high and pale, and the mineral is brown- ish. Zircon is especially common in acidic and sodic igneous rocks, but is also found in schists and gneisses, and as a residual mineral in decomposed igneous rocks. Cajusiterite may be pleochroic in weak brown- ish tones. Cleavage (110) is poor, (100) distinct. Gcniculated twins are common. It occurs as a pncumatolytic mineral in acid dikes and quartz veins, and as a rare primary mineral in some igneous rocks. R u t i 1 e has better cleavage and is not so brown. Anatase is negative. Brookite is biaxial. Perofskite is isotropic. Rutile occurs as an accessory mineral in granites, syenites, gneisses, and mica-schists, and as secondary microlites in argillites. It is also found in granular limestones, and has been found forming a dike with apatite. It occurs in grains, sometimes in geniculated twins, though usually in acicular crystals in quartz. It is also found regularly intorgrown in phlogopite, biotite, and hematite, in so-called s a g e n i t e webs. Pleochroism seldom noticeable in thin sections, O = yellowish to brownish, E = brownish yellow to greenish yellow. Cassiteritc has lower birefringence, poorer cleavage, a n a - t a s e is negative and has much lower bire- fringence, b r o o k i te is biaxial and has different crystal form, and perofskiteis isotropic. 16 ESSENTIALS FOR THE MICROSCOPICAL DETERMINATION The Mineral it ANISOTR The Mineral it NEGATIVE OPIC, COLORED, NON-F (->. The rease in Index of Refractio LEOCHROIC, BIAXIAL Mineral is POSITIVE (+). 0000 W O U)| in o ot oo t* r- to to in N"N ~ - " - " - ~3 In oieas'j BIREFR. I EIA o in o in c gin (o to t- r^ a 3 - - ;o o o o o -" N N Verj High. High. Medium. Not Marked, u " Not Marked. Medium. High, Very High. Ka olin. - ordieritc e. ^ i Clii Zoisite ozoisite. Disthe 1C. Antigc >rite. Clinochlc re. Spo iumene. Hedenbt rgit Gedrite ^ Sillimanite. _ Anthophyllitc. Actinol _ te. _ rtuj Diallag( Diopsid ite. e. Mu Pai scovite. agonite. Phlo gopite. . onaz ite ?ay alitc Tal< .090 .095 .100 120 nite ] .160 .200 .250 Tita iroo dte OF ROCK-FORMING MINERALS AND ROCKS 17 The Mineral is Negative usually appears as a flour-like, while, upaque alteration pro.lurt of feldspar. It may he stained yellow or red l>y iron oxides. When crvstalli/ed it occurs in the form of leaves or M-ales with an extinction angle of 13, and may l.e mistaken for sericite, although its l>ire- fringence is lower. Much so-called kaolin is colloidal aluminium silicate, and not kaolin. M u s e o v i t e (scricite) and h y d r a r g i 1 1 i t e have higher birefringences. Cordierite, orlhorhombic, 2V from 40-84, _'K from 63-150, occurs in gneisses and various schists, rarely as a primary mineral in granites, andcsites, etc. As a metamorphic mineral it is found at the contact of acid igneous rocks with -hales and slates. When treated with HF it gives characteriMic prismatic crystals of magnesium lluosilicatr Pleochroic halos are occasionally seen in sections parallel to the c axis. Trillings and polysynthetic twins occur. Quart/, is uniaxial and positive, a 1 b i t e is jxwitive and has a lower index of refraction, nephelite is uniaxial and negative. Anlitjorite, the massive, lamellar serpentine, differs from common serpentine, which is fil.rous, in being negative, and in its habit. Pennine, when optically negative, is sepa- rated by its optical character, when positive, by a chemical test for AljOj. Pennine also has lower birefringence, usually abnormal inter- ference colors, and pleochroism. Serpentines are always secondary and occur as an alteration product of olivine, less commonly of pyroxene or amphibole, and possibly also of other ferro- magnesian minerals. Disthene (cyanite) does not occur in igneous rocks, but chiefly in muscovite or paragonite schists, gneisses, eclogites, etc., often associated with garnets or corundum. The color is faint blue hi thin sections, in some cases almost color- less. Cleavages, (100) perfect, (010) distinct, making an angle of 74, are very characteristic, although they do not show in all sections. Orien- tation, a is nearly at right angles to (100), c is inclined 30 on (100) to the edge (100): (010). S i 1 1 i m a n i t e and andalusite are ortho- rhombic and have different cleavages, topaz has basal cleavage only, z o i s i t e usually has abnormal interference colors and occurs in grains. The Mineral is Positive Cliitozoinile is an iron-poor or iron-free epi- dote, with the composition of zoisite. It is colorless to reddish with weak or no pleochroism, has extinction angle of 3, and a large optic angle (2V=80-90). It occurs in prisms or rods elongated on b, and in grains. Abnormal inter- ference colors are common, as in zoisite, but zoisite has parallel extinction and smaller optic angle (2V = 0H30). It may be impos- sible to separate the usual grains found in igneous rocks from /.oisit. Pistacite lias higher double refraction. Bronzite has the usual pyroxene cleavage, parallel extinction (see under hypersthene), and is slightly pleochroic in green and pink tones. 2E= 106. Hypersthene has similar but stronger pleochroism and is negative. Mono- clinic pyroxenes have higher bire- fringence and inclined extinction in sections at right angles to the principal optic sections. In basal sections which show sharp cleavage lines at approximately 90, monoclinic pyroxenes show the emergence of an axis while orthorhomhic pyroxenes show the emergence of a bisectrix. E n s t a t i t e is non-pleochroic. Zoisite, orthorhombic, is a mineral of the crystalline schist formation, produced by the dynarao-metamorphism of igneous rocks con- taining basic plagioclase. It also occurs in peg- matite dikes. Abnormal blue interference colors are common. Cleavage (010) good, (100) distinct. Clinozoisite has an extinction angle of 3 and an optic angle of 2V = 80-90, while zoisite has an angle of 0-60. M e 1 i 1 i t e gelatinizes with acids, occurs only in quartz-free rocks, anil has a characteristic habit. Vesuvianite has poorer cleavage, and high relief. Clinochlore, one of the chlorites, occurs in leaves, scales, plates, or leafy aggregates as a mineral of schists and serpentines, and in igneous rocks from the alteration of ferromagnesian silicates. 2E = 32-90, c:C=-2 to-9, maxi- mum birefringence =0.011. Pennine is (^), 2E = 61, has parallel extinction, and maxi- mum birefringence of 0.002. Spodumene has typical pyroxene cleavage, extinction c:c= 23 to 26, is generally non- pleochroic unless the sections are thick, when a = amethyst, b = amethyst, c = colorless. In many cases it is altered to a mixture of albite and muscovite. It occurs in pegmatite veins, often of great size, and in granites and gneisses. Pleo- chroism, moderate birefringence, and mode of occurrence separate it from other pyroxenes. ESSENTIALS FOR THE MICROSCOPICAL DETERMINATION The Mineral is Negative Gedrite, an aluminium-bearing orthorhombic amphibole, occurs in metamorphic schists and gneisses, and as a contact mineral. It is usually pleochroic, c = yellowish, brownish, b = clove- brown, reddish, a = yellowish, greenish, colorless. Anthophyllite, the other orthorhombic amphibole, has 2V = 84, while gedrite has 2V = 57-79. Actinolite, with amphibole cleavage, extinc- tion c:c= 15, and similar in habit to tremolite, is rather a common mineral in certain schists and metamorphosed magnesian limestones con- taining much ferrous iron. It is green in color and has a faint green to yellowish green pleo- chroism, sometimes hardly noticeable in thin sections. Muscovite has a characteristic "bird's-eye maple" appearance, which separates it from all other minerals except the micas and talc. The optic angle (2E = 60-70) separates it from bleached biotite (2E = small to 0). Paragonite can be separated only by chemical tests. Lepidolite usually has a smaller optic angle (2E = 32-84), but in some cases may not be distinguishable except by chemical means. T a 1 c has 2E = 6-20, but in shreds it cannot be distinguished from muscovite except by chemical or physical tests, or by associ- ated minerals. Primary muscovite never occurs with pyroxene, talc usually does. The fine shredded muscovite, secondary in potash feld- spars is called s e r i c i t e . Do not call the secondary mica in plagioclase sericite unless you are certain that potash is present; the mica probably is secondary paragonite. Paragonite is the white soda mica analogous to muscovite. It is common in certain schists (paragonite-schists) and probably as a secondary mineral derived from plagioclase (see under mus- covite). It cannot be distinguished from musco- vite optically. Phlogopite is paler brown than biotite, reddish brown, or yellowish brown, sometimes greenish or colorless. 2V, as in biotite, is small to 0. Biotite has stronger pleochroism, but when bleached may not be distinguishable from it. Phlogopite is essentially a mineral of marbles and crystalline dolomites, but it does occur in the leucite rocks of Wyoming and in the mica-peridotites of southern Illinois. For sepa- ration from other minerals, see under muscovite. The Mineral is Positive Hedenbergite shows typical pyroxene cleav- age. It occurs in some nephelite- and other basic syenites. 2V = 5952', c:c=-44. Separated from other pyroxenes by its lower double refrac- tion and by its extinction angle. O 1 i v i n e has different orientation of the interference figure. Sillimanite occurs as a contact mineral, and in long, slender, fine needles without terminal faces in the quartz of granites and gneisses. It may also occur in prisms or aggregates of needles. Its (010) cleavage is perfect, and there are transverse fractures. Apatite, with similar cross-parting, has much lower double refraction and negative elongation. A n d a - 1 u s i t e is negative, has negative elongation, lower birefringence, and the relation of the axial plane to the cleavage is different. S c a p o - 1 i t e s are negative, have negative elongation, and are uniaxial. Z o i s i t e has weaker double refraction and different orientation. Anthophyllite, an orthorhombic pyroxene, usu- ally fibrous, occurs as a contact mineral in mica- and other schists, and as an alteration product of olivine in serpentines, gabbros, peridotites, etc. It is usually non-pleochroic in thin sections, but may show c = yellowish, b = clove-brown, reddish, a = yellowish, greenish, colorless. Typical amphi- bole cleavage and parallel extinction separate it from other minerals. Augite is usually green, brown, reddish, violet, or yellowish, but rarely colorless. Pyroxene cleavage and high extinction angle (c:c= 45 to 55) characterizes it. In sections showing parallel extinction, the plane of the optic axes is parallel to (010), in olivine it is parallel to (001). Augite is common in igneous and meta- morphic rocks. It is of a brownish-purplish color when titaniferous, and, of that color, it is a com- mon constituent of diabases and basalts. Diallage and diopside are monoclinic pyrox- enes. The former has very perfect and abun- dant (100) cleavage in addition to the (110) cleavage of the latter. Both are pale green to colorless, and have extinction angles of c:c= 39. Diopside occurs in pyroxene-granites, diorites, lamprophyres, crystalline schists, and magnesia- rich marbles; diallage is common in gabbros and related rocks, and peridotites and the serpentines derived from them. Pyroxene cleav- age separates these minerals from all but other pyroxenes, from which the extinction angle separates them. In sections showing parallel extinction, the plane of the optic axes is parallel to (010), in olivine it is parallel to (001). OF ROCK-FORMINO MINERALS AND ROOKS 19 The Mineral is Negative Fayalitr. the iron olivine, may !> colorless Of yellowish, greenish, reddish, with weak or no plcocliroism in yellow and red tones. O 1 i v i n e has 2V=SS (fayalite, 2V=*50), is optically IMtsitive, anil has lower birefringence. Fors- t e r i t e is positive, has 2V = 80, lower indices, an. I different mode of occurrence (contact in metamurphic limestones). Talc, orthorhombic, closely resembles m u s - covite in thin sections, and it may l>e neces- -ary to use chemical means to separate them. The optic angle (2E = 6-20) is smaller than usual in museovite, and the mode of occurrence is different, primary muscovite never occurring with pyroxene, while talc commonly does. It has the same "bird's-eye maple" appearance so common in mica. The Mineral is Positive Olivine, the intermediate magnesia-iron va- riety, occurs as a common primary mineral in basic rocks, and as an accessory in basic schists, gneisses, and marbles. It alters to actinolite. anthophyllite, iddingsite, magnetite, cliromite. opal, quartz, serpentine, tremolite, and other minerals. Alteration to serpentine and magnet- ite are most cojnmon. It gelatinizes slowly in HC1. Th intnrf. -miBiMnn parallel to (001) while in pyroxenes, in sections show- ing parallel extinction, it is parallel to (010). Fayalite has 2V = 50, higher birefringence, and is negative. Forsterite has a different mode of occurrence. Monazite occurs in granites, in gneissoid rocks, in sediments, and most commonly in sands and gravels. The yellow, non-pleochrioc color, high birefringence and high relief, separate it from most minerals. T i t a n i t e has higher birefrin- gence, the extinction angle is 39 (monazite, c:c = 2 -6), b rook ite has () elongation (monazite negative), 2E is somewhat larger. R u t i 1 e has positive elongation, is usually of a deeper red or orange color, is uniaxial, has higher indices, and may show geniculated or heart-shaped twins. Titanite, in the form of prisms, rhombs, and grains, is a very common mineral in acid pluton- ites. such as granites and syenites, abundant in nephelite-syenites, and less common in diorites. It is also abundant in gneisses and schists, and in some limestones. As a secondary mineral (leucoxene) it is derived from titaniferous magne- tite, ilmenite, rutile, and other titanium-bearing minerals. Pleochroism is weak c>b>o, in brown and yellow tones, c:c=+39, and 2E = 45-68. Monazite has lower birefringence, smaller extinction angle, and weak dispersion. Brook- i t e has parallel extinction, 2V =0 to 23, ami rutile is uniaxial. Brookite occurs in veins with various other minerals such as albite, quartz, nephelite, garnets, rutile, chalcopyrite, galena, etc., and in gold washings, always in the form of crystals. The acute bisectrix is normal to (100) but the axial plane is parallel to (001) for red and yellow and parallel to (010) for green and- blue. For red (670 MM) 2E = 580', yellow (589 MM) 2E=3810', yellowish green (555 MM) 2E = 0, green (535 to 525 MM) 2E = 2140'-330' (Manual, pp. 444-^5, Figs. 619-23). Interference figure for white light is a peculiar combination of all these (Manual, Fig. 624). Cassiterite and rutile have different habit, and brookite has very different strength of double refraction in (100) and (010) sections. 20 ESSENTIALS FOR THE MICROSCOPICAL DETERMINATION The Mineral U ANISO1 The Mineral is NEGATIVE ROPIC, COLORED, PLEO (-) The create in Index of Refractio CHROIC, UNIAX1AL. Mineral .. POSITIVE (+). a eooo in o m o w m o o> co t- r- to (0 10 e (N OJ ^ M i- "J In cieas'g BIREFR. 4. E w o in o in oooo u; v i r~ cooiom r- - ^ *- ^ _ r^ rj Verj High. High. Medium. Not Marked, u ONot Marked. Medium. High. Very High. Apatite ^ T 3run Vesuvi um. ^ anite. ^ e. T 010 .015 020 .025 T ourmalin e. .035 .040 .045 Biotite. - / nat; se. 090 095 100 .120 _ Ht mat Magn Dolom .140 160 200 250 Ru tile. - te. 1 OF ROCK-FORMI\<; MtNKRALS AND ROCKS 21 The Mineral is Negative Apatite has characteristic basal part inn in long prisms. It i< easily soluble in II ; S() 4 and the solution gives a \ello\v precipitate with ammonium molybdatc (Mnininl, p. 565). Apa- tite, in the form of small prisms, is a very ( unon accessory in most igneous rocks. In large cr\>tal- it nccur> ill pegmatites, some lampro- phyres. etc. It is also found in crystalline schists, limestones, argillites, etc. S i 1 1 i m a n - ite has higher double refraction and positive elongation. \t>!>titr is a feldspathoid and does not occur in quartz-bearing rocks. It usually shows ab- normal blue interference colors. The (001) and (110) cleavages are poor; only the Iwsal cleavage is generally seen in thin sections, and this occurs as a single cleft along the middle of the lath-shai>ed section. Peg structure, due to inclusions growing inward from basal sections, is characteristic. It gelatinizes easily with HC1 ntal, p. 564). Ves u vi an i t e and zois- i t e . both of which may give the abnormal blue interference color, are insoluble in acids. Yesuvianite has higher relief, and usually occurs as a contact mineral in limestone. Zoisite is biaxial and occurs as a secondary mineral. Vesuvianite has poor (110), (100) cleavages. It usually occurs as a contact mineral derived from limestone, but has also been found in ancient ejected blocks among the dolomite masses of Vesuvius and Monte Somma. In some cases it shows abnormal Berlin blue interference colors, or biaxial character. It is insoluble in acids unless first fused. Z o i s i t e has better cleav- age and different mode of occurrence. Corundum occurs as a primary mineral in alumina-rich igneous rocks, both acid and basic, such as pegmatites, syenites, anorthosites, and dunites. It is rare as a contact mineral, but occurs in granular limestones and dolomites, gneisses, mica-schists, etc. The pleochroism, O = blue, red, E= sea-green, yellow, or greenish yellow, is seen only in deeply colored specimens. It has a poor parting (1011), (0001). The high relief separates it from similar minerals except vesuvianite from which it is separated by its hardness, higher double refraction, and by chemical means. The Mineral is Positive Kutile occurs as an accessory mineral in granites, syenites, gneisses, and mica-schists, and as secondary microlites in argillites. It is also found in granular limestones, and has been found forming a dike with apatite. It occurs in grains, sometimes in gcniculatcd twins, though usually in acicular crystal- in quart/. It is also found regularly intergrown in phlogopite, bio- t it e, and hematite, in so-called sagenite- webs. Pleochroism seldom noticeable in thin sections, O yellowish to brownish, E brownish yellow to greenish yellow. Cassiterite has lower birefringence, poorer cleavage; ana- t a s e is negative and has much lower birefrin- gence; brookite is biaxial and has different crystal form, and pcrofskite is isotropic. 22 ESSENTIALS FOR THE MICROSCOPICAL DETERMINATION The Mineral is Negative Tourmaline, a pneumatolytic mineral, occurs in granites and pegmatites, and rocks in contact with these, in schists, gneisses, talc-schists, and limestones and marbles. That found in marble is usually brown; in greisen and with tin ores usually blue-black; and in association with lepidolite red, yet these colors are not confined to the rocks mentioned, and two colors may occur together. Red and green transparent, and black opaque varieties also occur. It forms prisms, grains, and needles, the latter in many cases in radiating groups, so-called tourmaline suns, but the needles are not necessarily con- fined to one plane, but may radiate in all direc- tions, giving in thin sections a central portion showing basal sections, characteristically three-, six-, nine-sided, etc., often zonal, surrounded by radiating crystals. The uniaxial character and the strong pleochroism, greatest in the direction at right angles to the vibration direction of the lower nicol, separate it from all other minerals, most of which are dark when the elongation is in the direction of vibration of the lower nicol. Biotite is a common mineral of the acid and intermediate rocks, both plutonic and extrusive, and of some of the lamprophyres, and occurs as a metamorphic mineral in gneisses, schists, and various other rocks. In many cases it is inter- grown with muscovite, either in parallel inter- growth or with muscovite forming the outer zone. Pleochroism, strong ca; c and b = deep brown to red-brown, deep green, a = light yellow to reddish, light greenish. It has a golden brown color in some nephelite-syenites. Basal sections are non-pleochroic and give nearly to quite uniaxial figures. "Bird's-eye maple" appearance is characteristic of all micas. Pleo- chroic halos about minute inclusions of zircon, etc., are common (Manual, p. 323). Tour- maline is darkest when its long direction is at right angles to the polarizer, lepidolite is non-pleochroic, zinnwaldite has less pleochroism and occurs in greisen and with tin deposits, but may require test for Li to distinguish, phlogopite is less pleochroic and generally occurs in crystalline limestones. Horn- blende is biaxial, has inclined extinction, and does not show the "bird's-eye maple" effect. or l;>" k-l-'.'UMiSi. MIM.UM.S AMI ROCKS 23 The Mineral is Negative .ln/1/n.vr occurs in pyramids .-mil taMets, and is found in some granite-pegmatites. I' usually lias pleochroism, O = deep liluc or orange-yellow. K = light blue or light yellow, lull it may l>i- very weak, M> thai tin- mineral appears colorless in (liin sections. Colorless or yellow portions arc usually normal, while Niie portions show anoma- lous opening of the interference cross and do not fully extinguish. I* e r o f s k i t c differs in form and the anomalous interference colors are lower. Ctilciti. ilnliiniiti-, and magnesite cannot be separated under the microscope, hut may lx- liy chemical means (Manual, p. 565). A r a % o n - ite is liiaxial with 2V= 18, and differs in certain chemical reactions (Manual, p. 568). Hrucite differs chemically (Manual, p. 567), and has much lower doultle refraction. Calcite is a common alteration mineral in all kinds of rocks, ami is said to l>e primary in some granites. Roth ealcite and dolomite occur as vein minerals, and in widespread and thick strata. Magnesite occurs as a secondary mineral from magnesia- liearing varieties. It also occurs in talc-schists, serpentines, etc., often as veins. Siilerite has higher indices of refraction than the preceding three carbonates, and is usually somewhat yellowish or brownish. It is a common mineral of ore veins and of limestones. It is also found in gneisses, slates, shales, gray- wackes, etc. Hi' mat ite is found in rocks of all kinds, either as small hexagonal crystals, rare in igneous rocks, as pseudomorphs after magnetite, a* rims around magnetite, as an alteration product from various ferromaitnesian minerals, and as stains in cleav- age cracks. Also in immense deposits among sedimentarics. Pleochroism, () = brownish red, E = light yellowish red, not seen, of course, in basal sections nor in earthy varieties. Mag- netite is black by incident light, hematite red; 1 i m o n i t e is usually yellow, though it may be red, in which case it may be confused with hematite. In such cases it is customary to speak of the material as red-, brown-, or yellow iron oxide. Basal sections of some biotite may appear blood-red and closely resemble hematite, but the interference figure of hematite shows more rings in sections of the same thickness. 24 ESSENTIALS FOR THE MICROSCOPICAL DETERMINATION The Mineral it ANISOTR The Mineral ii NEGATIVE OPIC, COLORED, PLEO (-) Th reae in Index of Refractk CHROIC, BIAXIAL Miner*! it POSITIVE (+). < ' So o o in o in o m c o ff> oo r* f* tf to r* r* cooiOin -; ^ p^ *.'^MN Vrj High High. Medium. Not Marked.u Not Marked. Medium. High. Very High. - - Pennine iiebeckit - Thulit ( orditrite nligorite usite. Quartz. c. mortieri n,- sthene. relite." Slauroli Distht Hyper L i "^Anda Clinochl >re. Comm on hornblende Glaucopham Spo dumenc. Barkev ikite. 1" Bas Ge altic hor drite. nblende. ^ Anlhc phyllit*. Actinoli te. Dullaf lea Aegiri e-augite Pistacite 035 040 Titanoli 'Orthite vine. Pai Zin agonitt nwaldite L f>hl< Biotite >gopite. . .045 .050 .055 .060 Fay alile 1 \egirite. Semite trophylJ P edrr onti (e. .075 .080 .085 .090 .095 .100 .120 .140 .160 .200 .250 nite Titi Bro okit 7 (IK HIM K-KoKMINC MINERALS AND ROCKS 25 The Mineral is Negative Cordiiritr, orthorhombie, 2V fnmi 40 to 84, 21". from tiH to l.'iii . occurs in gneisses and various schists, rarely as a primary mineral in granites, andesiies. i-tc. As a metamurphic mineral it is found at the ( tact of acid igneous rucks witli shales and slates. I'leochroie halos are occasion- ally seen in sections parallel to the c axis. Tril- lings and polysynthetic twins occur. Quart / is uniaxial and positive, alhite is positive and ha* lower indices of refraction, and n e p h e 1 i t e is uniaxial and negative. l)ist> mite) does not occur in igneous nicks. l>ut chiefly in muscovite- and paragon it <> schist-, gneix-cs, cclogites. etc., often associated with garnets or corundum. The color is faint blue in thin sections, in some cases almost color- less. Cleavages, (100) perfect, (010) distinct, making an angle of 74, are very characteristic, although they do not show in all sections. Orien- tation, a is nearly at. right angles to (100), c is inclined 30 on (100) to the edge (100): (010). S i 1 1 i in a n i t e and andalusite are ortho- rhombic and have different cleavages, topaz has basal cleavage only, zoisite usually has abnormal interference colors and occurs in grains. Diininrtierite occurs in a few gneisses and similar rocks. It is characterized by its pleo- chroism, a = blue, b = yellowish, reddish violet, greenish, c = colorless; parallel extinction; good i 1 1 i) cleavage; and small optic angle, 2V = 30. Blue amphibole is monoclinic, anda- lusite and hypersthene have pink to green pleochroism, staurolite has higher relief, is positive, and is pleochroic in brown tones. Spodumene, when pleochroic, has amethystine colors and is positive. A/iilnhi.-iili has characteristic though fre- quently faint pleochroism, a = rose, b = c = color- less to light green, resembling that seen in hypersthene. Hypersthene, however, has positive elongation and more marked cleavage. Andalusite frequently appears in irregular grains, or in more or less irregular oval forms associated with grains of magnetite in schists. In the variety chiastolite the inclusions are found in regular arrangement in the forms of rhombs, crosses, etc. in cross-sections and parallel to the long axes of prisms, and the material is altered to a mica-like mineral. The higher relief separates andalusite from cordierite. Andalusite is found in a few granites, but is essentially a mineral of slates, schists, and gneisses. As chiastolite it is a contact mineral in argillites near granitic contacts. The Mineral is Positive Pennine, one of the chlorites, occurs as an alteration product of biotite ami other ferro- magnesian minerals. It is usually nearly uni- axial, usually negative, sometimes positive, green in color with distinct pleochroism, b and a -green. c = yellowish, and with parallel extinction. Ab- normal Herlin blue interference colora^re com- mon. Habit: leaves, scales, leafy aggregates, etc. Micas have higher birefringence and different pleochroism, serpentine has higher birefringence and lower indices and is seldom pleochroic. The separation from a n t i g o r i t e may be difficult and in some cases may l>e ]x>ssible only chemically. Riebeckite, an iron- and alkali-rich amphibole, occurs in igneous rocks rich in soda and iron, such as alkali-granites, and in metamorphosed igneous rocks and sediments. Extinction, r:a = 5. Pleochroism, a = deep blue, b = lighter blue, c = yellowish green. Riebeckite is separated from other minerals by its amphibole cleavage, from other amphiboles except glaucophane, gastaldite, and arfvedsonite, by its blue color. Pleochroism in glaucophane is in- violet tones, in arfvedsonite greenish blue and lavender. The latter also has a higher extinction angle, and both have higher birefringence than riebeckite, though this may be concealed by the deep color yet indicated by the numl>er of rings in the interference figure (see top of page 39). Thulile, the manganese zoisite, has a faint pleochroism, a = nearly colorless, b = rose, c= yel- lowish. It occurs in pegmatites and in crystal- line schists. Zoisite is non-pleochroic, andalusite is optically negative, and green epidote has much higher bire- fringence. Bronzite has the usual pyroxene cleavage, parallel extinction (see bottom, page 37), 2E = =*=106, and it is slightly pleochroic in pink and green tones. Hypersthene has similar but stronger pleochroism and is negative. Mono- clinic pyroxenes have higher birefrin- gences and inclined extinction in sections at right angles to the principal optic sections. In basal sections (which have sharp cleavage lines at angles of approximately 90 with each other) monoclinic pyroxenes show the emergence of an axis while orthorhombic pyroxenes show the emergence of a bisectrix. Enstatite is non- pleochroic. 26 ESSENTIALS FOR THE MICROSCOPICAL DETERMINATION The Mineral is Negative Antigorite, the massive, lamellar serpentine, differs from common serpentine, which is fibrous, in being negative, and in its habit. Pennine, when optically negative, is sepa- rated by its optical character, when positive, by a chemical test for AUOs. Pennine also has lower biijgfringence, usually abnormal interference colors, and pleochroism. Serpentine is always secondary and occurs as an alteration product of olivine, less commonly of pyroxene or amphi- bole, and possibly also of other ferromagnesian minerals. Hypersthene has characteristic pleochroism, c = greenish, a = reddish yellow, b = pink, fairly strong in iron-rich specimens but fainter in bronzite. The extinction is parallel, but sections in which only one set of cleavage lines is brought out by grinding show inclined extinction, as do also, of course, all sections cutting the three axes. In basal sections (these show sharp cleavage lines at approximately right angles to each other) monoclinic pyroxenes show the emer- gence of an axis while orthorhombic pyroxenes show the emergence of a bisectrix. The cleav- age, (110):(1TO)=9140', is characteristic of all pyroxenes. Hypersthene occurs in all rocks from the gabbro family to granites. Bronz- ite is positive and has weaker pleochroism; andalusite, with similar pleochroism, has negative elongation and different cleavage. Common hornblende, a widespread monoclinic mineral in acid and intermediate igneous rocks, is strongly pleochroic in green, rarely brown, tones. Pargasite is the name applied to the green varieties, common hornblende to the brown and black varieties, although the name hornblende is applied to all. Hornblende occurs also in limestones and is widespread among the schists and other metamorphic rocks. Arfvedsonite has negative elongation ; other amphiboles have different extinction angles. Amphibole cleavage separates hornblende from all other minerals. Glaucophane, with (110):(1TO)=5516', re- sembles amphibole in habit. It occurs in grains, prisms, and fibers. The pleochroism is character- istic, a = nearly colorless to yellowish green, b = reddish to bluish violet, c = blue, c : c = 4 to -6, 2V = 45, 2E = 85.5, but the mineral some- times appears nearly uniaxial. Glaucophane is a metamorphic mineral of mica-schists, amphibo- lites, and gneisses, especially those derived from basic rocks which formerly contained much soda. Garnets, mica, omphacite, epidote, zoisite, etc., are frequent associates. The Mineral is Positive Ottrelite is a mineral almost exclusively confined to argillites altered by dynamo- metamorphism. It occurs in leaves and plates, and usually shows hour-glass structure. Pleo- chroism may be rather weak or wanting, c = yellowish green, colorless, 6 = blue, a = olive- green. Cleavage (001) good. The low double refraction and high relief, as well as the mode of occurrence, separate it from all other minerals. Zoisite has parallel extinction, different color, and smaller optic angle. Clinozoisite has different cleavage (001:100 = 6437'), and usually abnormal interference colors. Staurolite occurs in crystalline schists as a contact or dynamo-metamorphic mineral. In- clusions symmetrically arranged or in subparallel bands are common. The yellow, red, or brown color, the pleochroism, c = red-brown, a and b = yellow, the parallel extinction, the large optic angle, 2V = 89, giving a straight bar in sections at right angles to an optic axis, separate it from other minerals. Andalusite is negative and has different pleochroism, g e d r i t c has smaller optic axial angle (2V = 57-79) and amphibole cleavage, anthophyllite has amphibole cleavage and is rarely pleochroic in thin sections, vesuvianite has a pale yellow color, is negative, and has lower birefringence. Clinochlore, one of the chlorites, occurs in leaves, scales, plates, or leafy aggregates as a min- eral of schists and serpentines, and as a secondary mineral in igneous rocks from the alteration of ferromagnesian silicates. 2E = 32-90, c : c = 2 to 9, maximum birefringence = 0.011. Pen- nine is (=*=), 2E = 0-6T, has parallel extinction, and the maximum birefringence is 0.002. Spodumene has typical pyroxene cleavage, extinction c:c= 23 to 26, is generally non- pleochroic unless the sections are thick, when a = amethyst, b = amethyst, c = colorless. In many cases it is altered to a mixture of albite and muscovite. It occurs in pegmatite veins, often in very large crystals, and in granites and gneisses. Pleochroism, moderate birefringence, and mode of occurrence separate it from other pyroxenes. , *~^>^j-- Anthophyllite, an orthorhombic pyroxene, usually fibrous, occurs in mica- and other schists as a contact mineral, and as an alteration product of olivine in serpentines, gabbros, peridotites, etc. It is usually non-pleochroic in thin sections, but may show c = yellowish, b = clove-brown, reddish, a = yellowish, greenish, colorless. Typical amphi- bole cleavage and parallel extinction separate it from other minerals. I;... K-| ..I:MI\(. Mi\mu.s \\n ROCKS 27 The Mineral is Negative Arfirdmtniti; rather a ran' mineral, occurs iti soda-bearing igneous rocks. MX nephelite-syemtes, phonolites. tingiiaites. pantellerites, and alkali- pegmatites. It lias ainpliiliolc cleavage and is characterize! by .strong plt'ocliroisni, a- pair greenish yellow. b = lavender, o ^ deep greenish blue. Absorption C>b>0. 2V is large, and the mineral is probably optically |x>sitive. c:c on nii)] = i4. Glauoophane has positive elongation ami c:f = I' to 0. II ic beck i t c has c:c= 85 (c:a = 5), aegiritc has differ- ent color, b a r k e v i k i t < has 2V = about 54, c:c= 11. and the color is brown. Hlc, has c:c=14, and is strongly pleochroic in brown tones. A sharp separation between barkcvikite and basaltic hornblende is not possible. Other amphit>oles differ as mentioned under arfved- sonite. ti\ an aluminium-bearing orthorhombic amphibolc. occurs in metamorphic schists and gneisses, and as a contact mineral. It is usually pleochroic, c = yellowish, brownish, b = clove- brown. reddish, a = yellowish, greenish, colorless. A n t h o p h y 1 1 i t e , the other orthorhombic amphibolc. has 2V = 84, while gedrite has 2V = 57-79. Basaltic hornblende is common in basic extrusives. It frequently shows absorption rims. It is pleochroic in strong brown and yellow tones, with c>b>a, also in green and brown tones with a = green, b and c = brown. Common horn- blende has c:c = 12 to 20, sometimes positive character, lower double refraction, and sometimes lower 2V. B i o t i t e generally has the ''bird's-eye maple" effect. See under arfved- sonite. Actinolite, with amphibole cleavage, extinc- tion c:c= 15, and a habit similar to tremolitc, is rather a common mineral in certain schists and metamorphic magnesian limestones containing much ferrous iron. It is green in color and has a faint green to yellowish green pleochroism. sometimes hardly noticeable in thin sections. The Mineral is Positive Augite is usually green, brown, reddish, violet, or yellowish, but rarely colorless. Pyroxene cleavage and high extinction angle (c:c- 45 to .W) characterize it. In sect inns showing paral- lel extinction, the plane of the optic axes j< parallel to (010), in olivine it is parallel to (001). Augite is a common pyroxene in igneous rocks, and it also occurs in metamorphosed sediments and igneous rocks. It is of a brownish-purplish color when titaniferous, and. with that color, it is a common constituent of diabases and basalts. Diallage and diopxide are monoclinic pyrox- enes. The former has very perfect and abundant (100) cleavage in addition to the (110) cleavage of the latter. Both are pale green to colorless, and have extinction angles of c:c= 39. Diop- side occurs in pyroxene-granites, diorites, lampro- phyres, crystalline schists, ami magnesia-rich marbles; diallage is common in gabbros and related rocks, and in jx-ridotites and the serpen- tines derived from them. Pyroxene cleavage separates these two minerals from all but other pyroxenes, from which the extinction angle separates them. In sections showing parallel extinction, the plane of the optic axes is parallel to (010), in olivine it is parallel to (001). Aegirite-augite, a pyroxene of the igneous rocks rich in sodium, especially of nephelite- syenitcs, phonolites, leucitophyres, etc., also of some alkali-granites and -syenites, shows the extinction angle of augite but the peculiar green color of aegiritc. Pleochroism is the same as in aegirite, a = grass-green, b = light green, c = yel- lowish to brownish. Optically it is probably positive. Aegirite is negative and has an extinction angle, c:o = about 5, augite has different color and slight pleochroism. Orthite (allanite), the cerium epidote, occurs in various granitic rocks, in the form of grains, prisms, and rods along 6 or c, with high (0.032) birefringence in fresh material but sinking to zero when the mineral is altered to a megascopi- cally gumlike substance. Cleavage (001) is dis- tinct. Zones not uncommon. Pleochroism, strong, in fresh material, o = greenish brown, b = reddish brown, c = brownish yellow. Brown hornblende has smaller extinction angle, and distinct cleavage, r u t i 1 e and cassiter- i t e are uniaxial and have higher birefringences anil indices of refraction. , THanolimne resembles olivine in having no cleavage, but it is pleochroic with o = rcd, b = c= light yellow. 2V=G2-{>3 . The transition be- tween olivine and titanolivine is usually gradual. 28 ESSENTIALS FOR THE MICROSCOPICAL DETERMINATION The Mineral is Negative Epidote (pistacite, green epidote) is a com- mon contact or dynamo-metamorphic mineral in impure calcareous rocks, and a secondary mineral in the feldspars of many igneous rocks. It is often associated with clinozoisite. It has been described as primary in certain granites. Pistacite is the iron-rich epidote, clinozoisite the iron-poor or iron-free variety. The characteristic pistachio green color, brilliant interference colors, and high relief separate it from all other minerals. Pleochroism, a = colorless to yellowish or greenish, b = yellowish to yellowish gray, c = green to light yellowish brown, sometimes rather weak. The plane of the optic axes lies at right angles to the elongation of the crystal. Paragonite is common in certain schists and probably also occurs as a secondary mineral from plagioclase (see under muscovite). It cannot be distinguished optically from muscovite. Zinnwaldite is the lithia-iron mica between lepidolite and biotite. It is found in greisens and rocks associated with tin ores. It has weaker pleochroism than biotite, c>b>a, with c and a = dark brown, brownish gray, b = yellowish brown or reddish, nearly colorless. Orientation as in biotite: 6 = b, c:a=0 to +7, 2E=10-60. Lepidolite has a different position of the plane of the optic axes (6 = c, c:a = to +2, rarely. b = b, c:c = to +2). It may be impossible to separate this mineral from biotite except by the reaction for lithium. Biotite is a common mineral of the acid and intermediate igneous rocks. It also occurs as a metamorphic mineral in gneisses, schists, and various other rocks. In many cases it is inter- grown with muscovite, either in parallel position or with muscovite forming the outer zone. Pleo- chroism, strong c < b>a; c and b = deep brown to red-brown, deep green, a = light yellow to reddish, light greenish. Has a golden brown color in some nephelite-syenites. Basal sections are non- pleochroic and give nearly or quite uniaxial figures. "Bird's-eye maple" appearance is char- acteristic of all micas. Pleochroic halos about minute inclusions of zircon, etc., are common (Manual, p. 323). Tourmaline is darkest when its long direction is at right angles to the polarizer; lepidolite is non-pleochroic; zinnwaldite has less pleochroism, and occurs in greisens and with tin deposits, but may require test for Li to distinguish. P h 1 o g o - p i t e is less pleochroic and generally occurs in crystalline limestones. Hornblende is bi- axial, has inclined extinction, and does not show the "bird's-eye maple" appearance. The Mineral is Positive Olivine, the intermediate magnesia-iron variety, is a common primary mineral in basic rocks. It also occurs as an accessory in basic schists, gneisses, and marbles. It alters to actino- lite, anthophyllite, iddingsite, magnetite, chromite, opal, quartz, serpentine, tremolite, and other minerals. Alteration to serpentine and magne- tite are most common. It gelatinizes slowly with HC1. The plane of the optic axes is parallel to (001) while in pyroxenes, in sections show- ing parallel extinction, it is parallel to (010). F a y a 1 i t e has 2V = 50, higher birefringence, and is negative. Forsterite has a different mode of occurrence. Astrophyllite, a rare mineral of the nephelite- syenites, has pleochroism, a = yellow to red, b = orange, c = citron-yellow. 2V = 75, 2E = ca. 160. It occurs in plates, laths along b, leaves, and rosettes. Cleavage (010) perfect. Micas have smaller axial angles and different pleo- chroism, and are negative. Lavenite is negative. Staurolite has lower birefrin- gence and a different mode of occurrence. Titanite, in the form of prisms, rhombs, and grains, is a very common mineral in acid plu- tonites, such as granites and syenites, abundant in nephelite-syenites, and less common in diorites. It is also abundant in some schists, gneisses, and limestones. As a secondary mineral (leucoxene) it is derived from titaniferous minerals. Pleo- chroism weak, c>b>d, in brown and yellow tones, c:c=+39, and 2E = 45-68. The strong dispersion produces colored isogyres. M o n a - z i t e has lower birefringence, smaller extinction angle, and weak dispersion. Brookite has parallel extinction, 2V = to 23, and r u t i 1 e is uniaxial. Brookite occurs in veins with various other minerals such as albite, quartz, nephelite, rutile, garnets, etc., and in gold washings, always in the form of crystals. The acute bisectrix is normal to (100) but the axial plane is parallel to (001) for red and yellow, and parallel to (010) for green and blue. For red (670 MM) 2E = 580', yellow (589 MM) 2E = 3810', yellowish green (555 MM) 2E = 0, green (535-525 MM) 2E = 2140'-330' (Manual, p. 444, Figs. 619-23). The interfer- ence figure for white light is a combination of all of these (Manual, Fig. 624). Cassiterite and rutile have different habits, and brookite has very different strengths of double refraction in (100) and (010) sections. OF ROCK-FORMIN'. Ml\KHM.-> SSI) ROCKS 29 The Mineral is Negative I'hlogopiti' is paler brown than biotitr, red- dish brown, or yellowish brown, sometimes green- ish or colorless. L'V, as in biolite, is small to 0. Biotitc has stronger pleochmNm, but when bleached may not In- distinguishable from it. Phlogopite is essentially a mineral of marble- ainl crystalline dolomites, but 'does occur in the leucite rocks of Wyoming and in the mica- peridotites of southern Illinois. For separation from other minerals, see under muscovite. brown iron amphibole of fibrous, leafy, lamellar, or granular form, is a constituent of metamorphosed carbonate rocks whose chief or only carl>onate is of iron. Pleochroism c = light brown, b = a = colorless. Amphibole cleav- parates it from other minerals, plcochroism and extinction angle (11-15) from other amphi- boln. I'lviuliti. the iron olivine, may be colorless or yellowish, greenish, reddish, with weak or no pleochroism in yellow and red tones. Oliv- ine has 2V = 88 (fayalite, 2V==t50), is optically positive, and has lower birefringence. Forsterite is positive, has 2V = 86, lower indices, and different occurrence (contact mineral in nietaniorphic limestones). . Aegirile, a constituent of sodium-rich igneous rocks, especially nephelite-syenites, phonolites, and leucitophyres, but also of some granites and syenites, occurs in the form of thin needles or crystals bluntly terminated. The pleochroism, a = deep green, b = lighter green to yellowish green, c = yellowish to brownish, and the extinction angle, c:a = 3-6, separate it from other pyroxenes; the pyroxene cleavage from other minerals. A c - mite occurs in crystals with acute terminations and is brownish. Aegirite-augite has a large extinction angle. Acmite occurs in long prismatic crystals with characteristic acute terminations. Occurrence same as aegirite. Color, brownish to reddish brown, often zonal around green centers of aegirite. Pleochroism, o = brown, b = light brown, c = greenish yellow. Extinction, c:o = 3-6. A c g i r i t e has different pleochroism, and occurs in bluntly terminated crystals. Other pyrox- enes have little or no pleochroism and larger extinction angles. Piedmontite, a manganese epidote, occurs in glaueophane- and other schists, rarely in certain porphyries, for example, porfido rosso antico. Pleochroism, strong and characteristic, o = orange, b = violet, amethyst, c = red. The epidote-like character of piedmontite and its characteristic pleochroism, separate it from all other minerals. 30 ESSENTIALS FOR THE MICROSCOPICAL, DETERMINATION THE DETERMINATION OF THE FELDSPARS The general characteristics of all members of the feldspar group are the same. They are usually colorless, belong to the monoclinic or triclinic systems (with close resemblance in angles, twinning, etc.), have a cleavage of from 86 to 90, a hardness of from 6.0 to 6.5, and a specific gravity of from 3.84 in celsian, through 2.55 in orthoclase, to 2.76 in anorthite. They may be classified as follows: Monoclinic Composition Triclinic Celsian Orthoclase Soda orthoclase BaO-Al 2 O 3 -2SiO 2 K 2 O-Al 2 O 3 -6SiOj> (K, Na) 2 OAl 2 O 3 -6SiO 2 Na 2 O-Al 2 O 3 -6SiO 2 Microcline Anorthoclase Albite CaO-Al 2 O 3 -2SiO 2 The twinning is one of the most important characteristics of the feldspars, and by the different extinction angles, the various members may be distinguished. The three most important kinds of twinning are Carlsbad, in which the composition plane is one parallel to the c axis, usually near (010), and the twinning axis the c axis; a 1 b i t e twinning in which the composition plane is (010) and the twinning axis normal to this face, and pericline twinning in which the 6 axis is the twin- ning axis and the composition plane is an inclined plane approximately parallel to the basal plane though tilted backward in albite and down in front in anorthite. All of these kinds of twinning may be combined in a single crystal, and they may be repeated many times to' form the so-called poly- synthetic twinning. When albite and Carlsbad twins are combined, the albite twinning may be recognized upon the (001) face by the fact that the elongation of the twinning lamellae lies in the direction of the faster ray, while in pericline twinning this length is the direction of the slower ray. Orthoclase (Fig. 1) is negative, 6 = c, a:a= 5, extinction on (001) from (010) cleavage = 0, on (010) from (001) cleavage = 5, 2V = 70 to 80, 2E = 120 ca., dispersion p>v, indices of refraction less than Canada balsam. The most common form of twinning in orthoclase is on the Carlsbad law. The twins are turned 180 with respect to each other. In the (001) : (100) zone the 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 6357' Q3) with the (001) cleavage and of 21 with the extinction of each individual (c:b = 19-23). In this zone, as the sections depart from (010) and approach (100), the angle of the cleavage with the twinning line naturally increases from 6357' to 90. The extinction angle also changes from 21 to 90, the increase being slight at first, but, as the section approaches 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. The (001) : (010) zone of one individual of a Carlsbad twin almost coincides with the (T01) : (010) of the other. In all sections in this zone the cleavage cracks of one individual are parallel to the twinning line, and the extinction angle from this line varies from on (001) to 3 to 7 (12 in soda-orthoclase) 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 and Mannebach twins are less common. In the former the twinning axis is the line normal to (021) which is also the composition plane. In sections at right angles to this plane, the Fio. 1. Section through a crystal of orthoclase, par- allel to (010). "I l;.n K-l-'tiltMIM. MlM. KM.-* \M> ll()CK8 31 twinning line is diagonal to the cleavage. The two parts extinguish :it tin- -ame time and arc parallel to the cleavage, Imt the direction of n in one individual is at right angles to a in the other, and the interference figures lie at right angles t,, ,. :l ,.|, other. Manneliach twinning is comparatively rare. The (Hll i plane i^ the composition plane and the twinning axis is a line at right angles to it. S a n i d i n e is like orthoclase in all its properties except that L'Y is much smaller, varying from very small to 0. The orientation may U> different. It is either as in orthoclase or in some cases 6 = b, a:a=-f-5, in which case the dispersion is pv as in orthoclase. Microcline, chemically like orthocla.-e, is likewise negative. The extinction angle on (010) = -f-o, on (001) = +10. Indices of refraction, birefringence, and dis|>ersion like orthoclase. _'\ -71 to 84. Combined polysynthctic twinning on albitc and periclinc laws is almost universally ut, giving rise to a plaid effect or so-called "grating" texture. Anort hoclase, negative, slightly higher in refractive indices but with the same birefrin- gence as orthoclase. ha- the same dispersion, 2V = 43-53, extinction on (010) = +4 to +10, on (Mil) = +1 to +4. In some cases it is polysynt helically twinned on the albite and pericline laws like microcline, from which it is then separated by the extinction angles on (001) and (010), and by the smaller optic axial angle. It is separated from all plagioclases but albite by its low refractive indices; from albite by its optically negative character. Plagioclase feldspars. The plagioolase feldspars form an isomorphous series. In this book the divisions are made as shown in the feldspar diagrams inside the back cover. The cleavage (001) to (010) is practically the same in all, varying from 8624' in albite to 8550' in anor- thite. The angle d- between crystallographic a and c, is 11629' in albite and 11555.5' in anorthitc. All of the plagioclases are found in all kinds of igneous rocks, both plutonites and extrusives. It is to be noted, however, that only one kind of plagioclase of the same generation occurs in any igneous rock. The feldspar of a plutonite may l>c zonal, the more basic plagioclase in the center, and the zones progressively more acid toward the (K-riphery, but two independent crystals of different plagioclases do not occur. It is true that a section cut through and parallel to the outer rone of a banded plagioclase may show nothing but acid plagioclase, for example, yet zonal growth in other crystals will show that the section was not cut from a crystal entirely of acid plagioclase. In the extrusive rocks the plagioclases of the pheno- crysts and those of the groundmass may be, though they need not be, different, but they represent two generations; all of the material of each is of its own kind. The phcnocrysts, having crystallized first, are usually the more basic. Besides occurring in igneous rocks, albite occurs as a secondary mineral in gneisses and schists, and is also found disseminated through certain limestones. Anorthitc has been found in meteorites and in slags. Plagioclase is almost invariably twinned on the albite law and shows varying antl characteristic extinction angles for each meinlx>r of the group. Many methods for their separation have been pro- posed. The most useful of these are given below and are shown graphically, where possible, in the back of this book. 1. By specific granites. Curve A. The value of the specific gravity is constant so long as the material used is pure and unaltered. Glassy inclusions or alteration to kaolin reduce the value, while the inclusion of most other minerals or the alteration to carbonates, sericite, paragonite, or saussurite increases it. 2. By the optical character of the mineral. Curve B. The optical character alone does not deter- mine the kind of plagioclase, but it is of value when taken in connection with other properties. When the value of 2V is between 85 and 90 the curvature of the isogyre is too slight to be seen, conse- quently the position of the acute bisectrix (which is always on the convex side of the isogyre when it is placed at 45 to the cross-hairs) cannot be determined. 32 ESSENTIALS FOR THE MICROSCOPICAL DETERMINATION 3. By the relative indices of refraction of the feldspar and some known mineral with which it is in contact (Becke method). Curve C. When the feldspar lies in contact with a known mineral, their relative indices may be determined by the movement of the Becke line. By making use of several sections, the indices in different directions may be determined (Manual, pp. 277-83). The most common substance used for comparison is Canada balsam although its index varies slightly in differ- ent sections, depending upon age and the original solvent or amount of heat used in mounting. The index in good sections should lie between 1.534 and 1.540 (Manual, pp. 283-85). In this book the Canada balsam line is shown at the mean, 1.537. It is shown by the broken line in the figure. The lines e and w, shown in the same place, are the indices of quartz. 4. By determining the refractive indices by immersion in various liquids. This method and a list of various immersion fluids are given elsewhere (Manual, pp. 249-65). The method has been very extensively used, and recently Larsen published a complete list of all minerals arranged according to their refractive indices (Bull. 679, U.S. Geol. Survey, 1921). Tsuboi, in a paper published in the Japanese language (Jour. Geol. Soc. Tokyo, Vol. XXVII, 1920), used the method in connection with cleavage flakes of feldspar, and gives the curves shown in D. These are of much greater practical value than those giving a, /3, and 7. 5. By extinction angles on cleavage flakes parallel to (010). Curve E. These values were deter- mined by Schuster (Tscherm. Min. Petr. Mitt., Ill [1880], 117). He considered extinction angles measured clockwise from cleavage on (010) and (001) as positive, and counter-clockwise as negative. The (010) face may be recognized by the fact that albite twinning lamellae are wanting, although those according to the pericline law are occasionally seen. The crystal form is often shown in outline or by 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 plagioclases the elongation, as defined by cleavage, is nearly parallel to a. The extinction is measured from the (001) cleavage. 6. By extinction angles on cleavage flakes parallel to (001). Curve F. These values were also determined by Schuster. Plates on (001) cannot be recognized in random fragments in rock sections, but must be obtained by crushing, not grinding, a fragment of the feldspar. Breaking along the cleavage, many of the flakes will be found to be parallel to (010) or (001). Only flakes of less than 0.5 mm. in thickness and with parallel faces (which may be recognized by their uniform interference colors) are of use. The (001) flakes show albite twinning, while those parallel to (010) do not. In the (001) sections the extinction is measured from the twinning lamellae. 7. By the position of the bisectrix in (010) plates. (Becke, Tscherm. Min. Petr. Mitt., XIV [1894], 375, 415; XVI [1897], 180.) In convergent light the different plagioclases show different positions of emergence of the bisectrix in (010) plates. In air they are as follows when the flake is oriented with the c axis vertical and the (001) face sloping from southwest to northeast: In albite the inclination is small and the positive bisectrix (c) emerges below the center and slightly to the right. Inoligoclase the bisectrix is nearly normal to the face but slightly above the center and to the left. In andesine the bisectrix lies 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. The bisectrix lies off the field above and to the right. Inbytownite the figure is similar to that in labradorite except that no lemnis- cate curves appear, only the circular rings about the melatope, which is off the stage but near the edge of the field in the southwest, are seen. The bisectrix lies to the northeast. In anorthite the melatope appears at the edge of the field at the southwest. 8. By the extinction angles on sections cut at right angles to both (001) and (010). This method was used by Becker (Eighteenth Ann. Rept. U.S. Geol. Survey, III [1898], 34) and by Becke (Tscherm. Min. Petr. Mitt., XVIII [1900], 556). Curve G. These sections are easily recognizable in microlites and in phenocrysts of extrusive rocks by their nearly quadratic sections, and in plutonites by zonal growths OK KiK-k-FoRMiNc MINERALS AND ROCKS 33 with quadratic outlines. Sections :it right angles to both ltd! i and (010) have tin- division lines between the albite twinning lamellae and tin- i(M)l) cleavage lines extending at right angles to the section: consequently when the tul>e of the microscope is slightly raised or lowered, there will be no lateral displacement of these lines. The small cross-sections, shown at the side of the curve, indicate the directions of i -f i and i - ) extinction to a. 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 slightly is not great. This method is good where applicable, because the increase in the extinction angles from albite to anorthitc is rapid and uniform. !i. />'// tin ijrtincliiin angles on sections from the (001) (010) zone. (Extinction angles of micro- htes, after Wiilfing, Mikroxkop. Physiog., I 1 , 361-62.) Curve H. Spherulite rays and the micro- lites of the extrusive rocks are Ixmnded by these cleavages. The curve alx>ve 12 is good if albite and andesine are separated by their refractive indices. 10. By the <-xtincti<>n nmjles on sections at right angles to the optic normal (b). Curve I. This is the method <>f Fedorow. The interference colors between crossed nicols of sections at right angles to the optic normal are the highest of any in that mineral, though rarely exceeding pale yellow in normal sections of feldspar. In the acid plagioclases, the extinction angles vary only from +2 to 2, but from andesine to anorthite they change rapidly and may be used. 11. liy the extinction angles on sections at right angles to either bisectrix, as used by Fouqu6 (Bull. Soc. \\ II [1894], 306). Sections cut at right angles to either bisectrix may l>e recogni/ed l>y their intermediate birefringence. In convergent light the interference figure will close as a cross in the center of the field when the principal sections of the slide and nicols are parallel. Rotate the section to the diagonal position and test by the gypsum plate whether it is at right angles to the a or the c axis (negative or positive, disregarding whether the acute or the obtuse bisectrix appears in the field i. In sections at right angles to a the extinction angle is measured from the twinning lines (solid line, Curve K). 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 (Curve M, dotted line). In the acid feldspars the section at right angles to c is very near the (010) face, and therefore shows neither twin- ning lamellae nor (010) cleavage; the extinction is measured from the (001) cleavage (Curve L, broken line). 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 directions of extinction. The values in sections at right angles to c are good in the basic feldspars. 12. By the extinction angles on sections from the zone at right angles to (010) or the symmetrical zone (Statistical method of Michel-Levy, Ann. d. Mines, 1877, pp. 392-471.) Curve N. Sections in this zone may be recognized by the fact that (1) albite-t winning lamellae" are separated by very sharp lines which are not laterally displaced when the microscope is focussed, (2) the extinction angle from the twinning line is the same on each side, (3) if the section is turned to the 45 position the two systems of twins become of uniform interference color. This method is one of the most valuable for the determination of the feldspars although some confusion may arise from the positive and negative directions of extinction in albite on the one hand and andesine on the other. The acid end, however, to about Ab^Aiiu, in sections in this zone, has refractive indices less tharr Canada balsam, so the area of confusion falls entirely within oligoclasc. It is not necessary that the sections used be absolutely in the symmetrical zone, for there is but slight error if they vary no more than 10 or 15 from the true i>osition. In such sections the extinc- tion angles on either side of the twinning lamellae will not be the same, but half of the sum of the two angles very nearly coincides with the true values found in the zone. For determination, there- fore, read the extinction angle of one twin at one side of the vertical cross-hair, turn the section past 34 ESSENTIALS FOR THE MICROSCOPICAL DETERMINATION the vertical cross-hair to the position of extinction of the other twin on the other side, and divide the angle thus obtained by two. From albite to bytownite (extinction 45), the extinction angles are read from the twinning lamellae to the intermediate vibration direction, but since a is nearly normal to the section, the third direction is near c, consequently the measured angle is toward the faster ray. For a short distance beyond the point where the extinction angle is 45, the angle is read toward the other and slower axis of vibration. In anorthite, however, the nearest vibration direction is again the faster way. 13. By the extinction angles on sections from the zone at right angles to (010), when the albite twinning is combined with Carlsbad twinning. (Michel-Levy, Etude sur la determ. d. felds., Troisieme fasc., 1904.) Occasionally the albite twinning is combined with Carlsbad twinning in the same section. In such cases the combined extinction angles are characteristic, without considering the direction of rotation, and it is not necessary to search for the maximum angle. A section is chosen which has approximately symmetrical extinction in the albite twins in each half of the Carlsbad twin. The angles are measured in each half as in the preceding method and divided by two. The angle of the smaller pair is found in the column to the left in Figure 23, the larger on the curves. The vertical line at the intersection gives the feldspar. In the figure the broken lines indicate the angles which the sections make with the (100) face. The size of this angle is indicated in the diagram by the figures which are not followed by the degree () mark. CO' 20' 20' Fio. 2. Maximum extinction angles in the Pyroxene and Amphibole groups. Solid lines indicate extinction angles from c toT] broken lines from c toj( OF KnrK-I "KMIV. \llM .KM..- \\l> 88 PYROXENES AND AMPHIBOLES Pyroxenes differ from amphilioles in having a prismatic angle of 87 (amphiboles 124), and !<>- prrfn-t cleavage. '1'ln- crystals arc usually stouter, the extinetioii angles are greater, ami ]ilrorhn>ism is generally weaker, often wanting except in aegirite-augite, aegirile. and acmite. Augite, also, is sometimes quite strongly pleoehroie in purple tones, and hypersthene in pink to green tones. The monoelinie pyroxenes are separated from the orthorhombic by having inclined extinc- tion. Acmitr, aegirite, and pectolite have extinction angles usually less than 5, and may be con- fused with orthorhombic pyroxenes, rxcrpt for their pleochrnism. Upsides differing in pleochroism, aegirite may l>e separated from orthorhombic pyroxenes by its much higher double refraction, and its negative elongation (all orthorhombic pyroxenes have positive elongation). Basal sections of orthorhomliir sections in convergent light show the emergence of a positive bisectrix in the center of the field, monoelinie pyroxenes which have low extinction angles show a negative bisectrix, while the other monoelinie pyroxmrs show the emergence of an axis. The chief mode of separation of the pyroxenes from each other is by means of extinction angles as shown by Figure 2. All pyroxenes are separated from other minerals by their characteristic cleavage. PYROXENES Name tjM m Mtal I 'll.ir acter Orientation Optic Angle BlreMn- gvnce Pleochrolon Enstatite ... Hnmzitc. . Hypersthene ... 1 iin|)-ii|i' . . . Ortho. Ortho. Ortho. Mono. Mono. Mono. Mono. Mono. Mono. Mono. Mono. Mono. Mono. Mono. + + + + + + + - + + + e C c-C CC c:C--39 c:c--39 c:e--44 c:c--45to -55 c:t--55 to -87 c:tt--3 to -6 c:a--3 to -6 c:c--23 to -26* c:C--33.5 c:a-+32 c:o--5 2E-135* 2E-106 2E-85 2V -59 2V -59 and less 2V -60 2V -60 0.009 0.009 0.013 0.029 0.029 0.015 0.025 0.029 0.050 0.050 0.016 0.029 0.015 0.038 None Faint Pink, green None None Weak, green May be purple Green Green Brown None None Nonr None DialloKt- nnl<-iiU-rj!il- Aunitr AegiriteHuigite AegJrite' Acmite SpodumeiK- Jadeitc. . . \Vollastoitilt- 2V -62 2V -54 to 60 2V -72 2V-40to69 e 2V = 60 PectoliU- AMPHI HOLES Name Synutn Optical CTiar- actcr Orientation Optic Angle Birefrin- gence I'kxjchrolwn Anthophyllite ... Ortho. rii e e 2V- -90 0.024 Yellow, brown, green GtHlriU- Ortho. C-t 2V-5r-79 0.021 Yellow, brown, green Tremolite ... Mono. c:c--16 2V -87.5 0.026 Non-pleochroic Aotinolite (iriinorite .... Mono. Mono. c:c--15 c:e--ll e to -15 2V -80 2V -82 0.027 0.045 Faint green Colorless, brownish Common hornblende . . Piirgnsite Katophorite Uasaltir homblemlr Karkcvikite. ... Glaucouhane . . Mono. Mono. Mono. Mono. Mono. Mono. * + + c:c--12to -20 c:e--18to -21 c:t--23to -60 c:C- to -12 c:c--14 c:c--4 to -6 ,.,_ R 2V -54 to 84 2V = 52 to 60 2V -small 2V -80 2V -54 2V-50 2E=-70 0.016 0.019 low 0.021 0.021 . 0.018 Strong green, yellowish Green, yellow Red, yellow Strong, green, brown Brown Blue, violet, greonLsh Blue, violet, greenish Kirlx-rkite Mono. A c:c--85 2V -large 0.005 Mine, yellowish Kn-en Arf vedsonite . . . Mono. * ^ M L_ + int +20j 2V -large 0.021 Blue to greenish 36 ESSENTIALS FOR THE MICROSCOPICAL DETERMINATION MODES OF OCCURRENCE OF VARIOUS MINERALS Minerals which occur in needle-like crystals. Actinolite, aegirite, apatite, aragonite, cancrinite, datolite, dumortierite, hydromagnesite, hydronephelite, natrolite, pectolite, sillimanite, stilbite, tremolite, topaz, tourmaline, wollastonite. Minerals which occur in fibrous aggregates. -Chalcedony, datolite, gypsum, hydrargillite, kaolin, natrolite, prehnite, sericite, serpentine, sillimanite, talc. Minerals which occur in radiating groups of fibers, or as spherulites. Brucite, chlorite, chalcedony, delessite, natrolite, pectolite, quartz-orthoclase, stilbite, thomsonite, other zeolites. Minerals which occur as cavity and interspace fillings. Analcite, carbonates, chalcedony, quartz, sodalite(?), zeolites.. Minerals soluble in HCl without gelatinization. Brucite, epistilbite, hematite (hot cone.), limonite (cone.), magnetite (cone.), monazite (cone.), pyrrhotite (cone.), stilbite. Minerals which gelatinize with HCl.- Analcite, anorthite, cancrinite, chlorite, datolite, fayalite, glass, haiiynite, hydronephelite, kaolin, laumontite, lazurite, leucite (part), melilite, nephelite, nose- lite, olivine, scapolite (Ca end), sodalite, wollastonite, zeolites. Effervesce with HCl. Calcite, cancrinite (slightly), dolomite (hot), hydromagnesite, magnesite (hot), siderite (hot). Secondary minerals.- Albite (in metamorphic rocks), analcite, antigorite, brucite, calcite, can- crinite, chlorite, clinochlore, epidote, hematite, kaolin, leucoxene (titanite, etc.), limonite, magnetite, opal, paragonite, pennine, pyrite, quartz, rutile, saussurite, sericite, serpentine, talc, titanite, uralite (amphibole), zeolites. Pneumatolytic minerals (frequently associated). Fluorite, lepidolite, muscovite, topaz, tourma- line, zinnwaldite. Metamorphic minerals by assimilation of other material in argillaceous rocks. Almandite, andalu- site, biotite, cordierite, corundum, disthene, ottrelite, pleonast, sillimanite, spinel, staurolite. Metamorphic minerals by assimilation in calcareous argillites or argillaceous limestones. Epidote, grossular, scapolites, vesuvianite, zoisite. Metamorphic minerals by assimilation in calcareous rocks. Calcite, datolite(?) wollastonite. Metamorphic minerals by assimilation in magnesian rocks. Actinolite, anthophyllite, brucite, diopside, forsterite, periclase, serpentine(?), talc(?), tremolite. Other metamorphic minerals.- Graphite, magnetite, pyrite. Minerals never secondary. Apatite, cassiterite, eucolite, eudialyte, fluorite, haiiynite, leucite, melilite, nephelite, noselite, pyrope, sodalite, tridymite, zircon. Minerals never primary. Chlorite, serpentine. Never occur in igneous rock^- Anhydrite, disthene, gypsum, ottrelite. Rare yellow minerals of allmi-syenites and nephelite-syenites. Astrophyllite, hjortdahlite, laaven- ite, lamprophyllite, mosandrite, rinkite, rosenbuschite. They are always accompanied by fluorite. Minerals which usually give abnormal Berlin blue interference colors. Chlorite, clinozoisite, melilite, vesuvianite, zoisite. Alteration products which occur in minute shreds. Kaolin, talc, white mica (paragonite, sericite). Alteration products which occur in grains. Albite, calcite, epidote, leucoxene, quartz, saussurite, titanite, zoisite. Minerals which never occur together. Quartz with nephelite, leucite, sodalite, haiiynite, noselite, melilite, etc. Primary muscovite with pyroxene. Two different plagioclases of the same generation (see p. 31). Quartz is rare with olivine except in a few basalts. Significant mineral association. If aegirite, acmite, or golden brown biotite is present, you will frequently find nephelite, leucite, sodalite, analcite, etc. If one pneumatolytic mineral is present there are usually others also. OK ROCK-FORMINC MlNKUAI.S AND HoCKS 37 A SUMMARY OF PETROGRAPHIC METHODS In the fr\v pap- following, reference can only l>e made to those points in the manipulation of the microscope which the occasional worker constantly needs and is likely to have forgotten. Kjrniniiiiilion l>i/ nnlinnnj liijht. By ordinary light is meant light which is not polarized. As 11 matter of fact one uses plane-polari/ed light for practically all examinations which could also be made liy ordinary light. Pleochroism only is affected (229-32 1 )- Without the analy/.er, therefore, the microscope is tested and adjusted, and the vibration directions ate determined in the accessories (229-32). Tn dtli-rmini the ribration direction of the glow ray in the oceeMoriet. Obtain an interference figure in the mica plate. The line ronnrrtiiig tin' nielatoprs is tin- itirrrtinn nf c. I'lace the gy|>uiii plate nearly parallel to it, consequently in grinding, the mineral will lx> separated more or less along the former but not along the latter, with the result that a single set of lines, inclined to the extinction, appear. Usually at the same angle on the other side of the extinction, traces of the corMppnding cleavage may be seen in the form of rough cracks or broken lines. Furthermore, in minerals of these systems with prismatic cleavage, the traces of this cleavage are parallel or sym- metrical with respect to crystallographic axes only in the zones at right angles to the three principal optic sections, that is, at right angles to the three planes containing the crystaUographic axes. Interference figures on the sections in two of these (ones will show a bar passing directly through the center of the field when the bar is horizontal or vertical, consequently they may l)e so identified. In other .sections the bar when vertical or horizontal will lie off to one side. In all sections which cut the three crystallographic axes, the extinction will lie inclined with respect to the cleavage, the term parallel extinction referring, of course, to parallelism with the crystallographic axes and not with the traces of cleavage planes. Another determination made between crossed nicols is birefringence (348-59, 369-88). The interference color produced by a mineral depends upon two factors, thickness of section and difference between the refractive indices in two directions. 1 The numbers in this section refer to pages in the writer's Manual of Petrographic Method*, 2d ed., New York, 1918. 38 ESSENTIALS FOR THE MICROSCOPICAL DETERMINATION This is brought out by the Michel-Le'vy color chart shown in Figure 3. The numbers beneath the figure represent M(nini) in millionths of millimeters. The ordinates represent thickness of section. The value of unit birefringence, iii n t (that is 7 a or u t) remains constant for any mineral, but as the section increases in thickness so does the retardation increase. The diagonal lines in the diagram represent, therefore, the retardation produced by sections of different thicknesses (371-72). Of course the interference color in a mineral depends also upon the orientation of the section, being zero or nearly zero along the optic axis and increasing to a maximum in the plane of the optic axes. To measure the maximum birefringence of a mineral, therefore, choose the section which gives the highest interference colors. . FIG. 3. Outline of Michel-Levy's chart of birefringences; the positions of the colors modified according to the Kraft scale for a clear sky. The optical elongation is often useful in the separation of minerals (361). Place the unknown mineral on the stage so that its vibration directions make angles of 45 with the vibration direc- tions in the nicol prisms. The light is then at its maximum. Place above it, in the slot provided for the purpose, the gypsum plate (Red of the first order) or the quartz wedge, the former for low colors, the latter for high. If the inter- ference color increases, the vibration directions are parallel, if it decreases, they are at right angles. It is usually well to determine the colors with the mineral successively in two positions at right angles to each other. From the known vibration directions in the accessory, it is thus determined whether the long direction (elongation) of the mineral is parallel to the fast or slow ray. If the elongation of the mineral is parallel to the direc- tion of c, the mineral is said to have positive (+) elongation; if parallel to the fast ray, n e g a t i ve () Examination between crossed nicols by convergent light. By converting the microscope into a conoscope (413), it is possible to determine whether a mineral is isotropic, uniaxial, or biaxial. If it is isotropic (415) no interference figure is produced. The student should familiarize himself with the interference figure produced by the microscope when set up properly as a conoscope but with only a blank object glass on the stage. All objectives give more or less well-defined uniaxial interference figures due to polarization by the glass of the objective or condenser (415). This should not be confused with the figure produced by uniaxial minerals. In crystals of the tetragonal and hexagonal systems, a cross with a greater or smaller number of colored rings is produced (416-19, 425-26). The section which will give the best interference figure with the cross in the center of the field is one which is com- pletely isotropic between crossed nicols. If no such section can be found, choose the one giving the lowest inter- ference colors. If the center of the cross lies only a little beyond the field of view, the fact that the mineral is uniaxial, or nearly so, is shown by the appearance of the bars which remain parallel to the cross-hairs during their passage or HIM k- MISKKALS AND ROCKS across the stage The greater th.- di-t inn- fri>m th. . , nt. r of the -tae to the center of the cross, the greater the flora of the l>:ir ll.s, Im ."..".I 'I'liere is thus :in iipp:irent curvature. Simie idea uf the strength <>f the birefringence may IK' iil)t:iiinil I iy tin- number of rings in the interference figure 'j:ti. This is .sometimes of value. i-s|-ii;(lly in colored minerals whose interference colors an- liiililen by the color of the mineral ljuart/.. in a section of normal thicknes- (MU.'inim . with eu 0.009, iihoWH a black crntui anil the first yellow rini at the |>eriphery of the Held of \ i< Hiotite with y O-0.040, give* two colored rings, and calcite, with ui t = 0.17-, give- i, MI many to count. In crystals of the ortliorhoinbic, monoclinic, and triclinic systems, the interference figures are ( I'.'O L'l. I _''. I'nless tin- optic axial angle is very small, only a portion of the figure can l>e r'li;. I Quartz (-(-) under a .m plate. .">. A positive uniaxial in- terference figure. The arrow.s inili- cate the movement taking place upon inserting a quartz wedge al lovi- the gcetion. . (jinirlz ( + ), with the center of the cross outside the tieM of the micro.sco|x > , an seen under a plate. .-< -i -n. The average microscope will show the two melatopes of topaz just at the limits of the field (2E=130ca.). l'"r the determination of the uniaxial or biaxial character, a figure with both bars off the stage may suffice, but to ileterinine the optical character, it is necessary (unless the optic axis be measured) that a melatope remain in the field of view during a complete rotation of the stage, for I he melittopc is at its greatest distance from the center in the 4. r > position, which is the position in which the curvature of the bar is to be determined. In the 45 position the convex sideof the bar is toward the acute h i > e c t r i x . For the determination of the optical character, therefore, it is not the most symmetrical figure (at right angles to a bisectrix) which is best, but the one most nearly at right angles to an optic axis. Since light is dispersed in all biaxial crystals, such a section can be actually at right angles to an optic axis only for a given color, consequently is never actually isotropic. As in uniaxial crystals, so here also, the section giving the lowest color gives the best figure for the determination of the optical character. I ni. 7. The appearance of a biaxial positive (+) crystal, showing one melatope in the field, under a gypsum plate. FIGS. 8-9. Movement of the colors upon inserting a quartz wedge above the interference figure of a positive ( + ) biaxial mineral. The lower nr- rows indicate the direction of c in the wedges. Five the section. The mineral is augite (+). The optical character of both uniaxial (457-62) and biaxial (462-65) crystals is obtained, as just mentioned, in sections which show the emergence of an optic axis. The determinations may be made by means of various accessories, the ones most commonly employed being the gypsum plate and the quartz wedge. 40 ESSENTIALS FOR THE MICROSCOPICAL DETERMINATION If one considers a uniaxial crystal as only a special case of a biaxial in which the optic axial angle is 0, or con- siders the biaxial as a special case of the uniaxial in which the axis has been split apart, one need only remember the phenomena in two cases. Figure 4 shows a uniaxial interference figure with the emergence of the optic axis in the center of the field. In a positive mineral, with the gypsum plate orientated as shown in the figure, the blue spots appear in the northeast and southwest quadrants. (If the orientation of the slow ray in the gypsum plate is at right angles to that shown in the figure, the phenomenon is, of course, reversed, and yellow appears in the northeast, etc., quadrants.) Figure 5 shows the phenomenon appearing when a quartz wedge whose slow direction vibrates across the accessory is pushed across a positive uniaxial figure. One needs but remember that the colors in the south- east quadrant move toward the hand. (With negative minerals or with the orientation of the slow ray in a different direction in the wedge, the phenomenon is reversed.) All other cases can be reduced to these two. Thus Figures 6 and 7 are the same as Figure 4, and Figures 8, 9, and 10 the same as Figure 5. Measurement of the optic axial angle. The methods for measuring the optic axial angle should be looked up elsewhere (466-502). A QUANTITATIVE MINERALOGICAL CLASSIFICATION OF IGNEOUS ROCKS The main division lines in the classification of igneous rocks which is generally accepted are the result of a gradual development through the hundred and twenty years during which were produced the systems of Werner (1787), von Leonhard (1823), Zirkel (1866 and 1894), and Rosenbusch (1877, 1887, 1897, and 1907-8). All of these systems were qualitative and more or less mineralogical, but they lacked the quantitative element now deemed essential. As a consequence, rock terms have been used loosely or with different meanings. Thus dolerite, originally applied to a coarse basalt, has been used for any dark rock, and in England is used for rocks which we call diabase. The term diabase in the United States means a dike-rock with an ophitic texture, yet it was originally used for Paleozoic basalts and is still so used in various countries. Basalt has been applied to plagioclase rocks with augite and olivine and irrespective of the kind of feldspar, to labradorite-pyribole rocks with or without olivine, to the darker labradorite-pyribole rocks, to post-Tertiary extrusives of gabbroic magma, etc. The system to be presented here 1 is strictly mineralogical, quantitative, and modal, and is directly applicable to all plutonites and to practically all extrusives. No attempt has been made to change the general basis of classification of the old system, although the additional factor of the ratio of the dark to the h'sht constituents is used. As an objection to a quantitative mineralogical system, it will be said that it is not always possible to determine the exact com- Foide position of rocks with glassy bases, or extrusives of the alkali series. p ia 11 Subdivisions of the double But the percentage of indeterminable rocks is comparatively tetrahedron into classes, representing small, and for these there still remain, if necessary, chemical light to dark rocks. methods for determining the composition of the base. Most glassy rocks are leucocratic, and a recalculation into the minerals which would have crystallized had the conditions been right is easy. The basis of the classification here proposed is a double tetrahedon (Fig. 11), each trihedral angle of which represents certain mineral constituents. Since there is no geometrical figure having 1 This classification of igneous rocks has been gradually developed by the writer since 1909, and is described in considerable detail in the following papers, to which the reader is referred: "Suggestions for a Quantitative Mineralogical Classification of Igneous Rocks," Jour. Geol., XXV (1917), 63-97. Figs. 27. "A Quantitative Mineralogical Classification of Igneous Rocks, Revised," ibid., XXVIII (1920), 38-60, 159-77, 210-32. Figs. 7. OF ROCK-FORHINO MINERALS AND ROCKS 41 as many comers as there are minerals in the rocks, it was fount! necessary to divide the minerals into certain groups. Since quartz and the feldspathoids never occur together, it is possible to make the classification in five dimensions by usinn two tetrahedrons with a common base. The groups of minerals represented by the corners of the double tetrahedron are (1) quartz (symbol t^u 1 ); (2) jxitash feldspars, including also anorthoclase, microperthite, etc. .(symbol Kf), (3) all planioelases, (4) all fcldspathoids, (5) the mafites, 1 including the ferromagnesian constituents, the "ores," etc., as given below. As shown in Figure 11, the double tetrahedron is unsymmetrically divided by the traces of planes, some parallel to the qiiarfeloid* face, others converging to one of the angles. The divisions were so made to conform to the rock names of the older classifications. It is true that new names might have l>oen devised for symmetrical subdivisions, but it was not thought desirable to discard entirely the old and well-tried- lines of separation which have very much to recommend them besides the fact that they have been so long in use. The old classifications are unsymmetrical, for we speak of a rock as a quart /->\enite, quartz-monzonite, quartz-diorite, etc., when it contains any amount of quartz. With resjH'ct to this mineral, therefore, the classification is based upon its ratio to the sum of all the other constituents, and the lines of division must be parallel to the side of the tetrahedron. The same is true also of the feldspathoids. In the divisions according to the feldspars, however, we find for example that a rock is a quartz-monzonite whether the total feldspar percentage is 10 or 90. Here the divisions are based upon the ratio of the feldspars to each other, irrespective of what their amount may be in the rock. The division lines, therefore, Order must converge toward the quartz and feldspathoid corners, as shown in Figures 11, 15, etc. Classes. The igneous rocks may be divided into various classes according to the percentage of dark constituents present. Four divisions are here made: (1) rocks with less than 5 per cent of dark constituents, (2) dark constituents between 5 and 50 per cent, (3) dark constituents between 50 and 95 per cent, and (4) dark constituents more than 95 per cent. Since these Folds division lines represent planes parallel to the two quarfeloid Fio. 12. Subdivisions of the second- planes (quartz-feldspars and feldspars-feldspathoids), Figure 11, wy double tetrahedron into orders, , . . , , .1 representing differences in the kind of they form similar double tnangles whose sizes represent the piLrioclase amounts of light constituents, consequently decrease with in- crease in dark constituents and with approach to the mafite comer. For convenience, however, since they are similar, they may be represented by triangles of the same size. Orders. Thus far the classification is one of five dimensions. But this is not enough. The kind of plagioclase in the rock must be taken into consideration. Imagine that the lozenge-shaped quarfe- loid plane consists of two sheets of paper fastened together only along the Qu-Kf-Foids edge. If now the loose corners at the right of the two sheets be separated a distance equal to a side of the original 1 In the figures following, the quartz corner is indicated by the symbol Qu. The letter/ is used for feldspar, there- fore Kf indicates the potash-feldspars orthoclase, microcline, anorthoclase, microperthite, etc. Naf indicates albite, CaA'of represents the acid plagioclases, XaCaf the basic plagioclaaes, and Caf anorthite. In CaJVof and NaCof, the element in excess is indicated by italics and the symbols are to be read, calcium-bearing soda-feldspar, and soda-bearing calcium feldspar. Folds is the symbol used for the feldspathoids. ' The term mafite is here used for the dark minerals of a rock. This term includes not only the mafic (ferromag- nesian) minerals of C.I.P.W., but certain iron minerals listed below, as well. ' Quarfeloids (Ql'ARU, FELdspar, feldspathUlDS) is used as a noun for minerals in the front face* of the double tetrahedron, "felsite" being unavailable from its use as a rock name. " Leucocrates " cannot be used, since all light- colored minerals are not included. 42 ESSENTIALS FOR THE MICROSCOPICAL DETERMINATION triangle, a new double tetrahedron will be developed, the horizontal line along which it was opened representing all plagioclases, the ends being formed by the Ab and the An molecules (Fig. 12). The same thing is done in each of the first three classes, the fourth being differently divided as shown below. The classification will now be made up of three double tetrahedrons (and a single tetrahedron for the fourth class), one for each class, the corners being formed by quartz, potash feldspar (including microperthite, anorthoclase, etc.), albite, anorthite, and the feldspathoids. But these tetrahedrons may be subdivided into orders, depending upon the proportions of the albite to the anorthite mole- cule; consequently the divisions must be made by planes all of which cut the quartz-potash-feldspar- feldspathoid edge but which separate across the central plane of the double tetrahedron as shown by the dotted lines in Figure 12 and by Figure 13. The edge Qu-Kf-Foids remains common to all of the divisions, the plagioclase corner simply having been changed. Now while the triangles formed by the intersections of these planes with the tetrahedron are not all equilateral, the relative position of any rock plotted on any intersecting plane is the same as it would be in the equilateral triangle, since the divisions are 100 each way in every possible triangle. Equilateral triangles, consequently, may Pyroxend FIG. 13. A section through the central plane of Figure 12. FIG. 14. Subdivisions of the single tetrahedron of Class 4 into orders. be substituted for any triangle. In this manner the different orders may be represented by a series of double equilateral triangles whose right-hand corners vary with the kind of feldspar. Figures 16-19 show the plutonic rocks in Class 2. Each of the first three classes is hereby divided into orders according to the Ab-An ratio in the plagioclase. The division points are Abi 00 An , Ab 9 6An 6 , Ab 6 oAn 6 o, AbsAngs, AboAnioo. There are thus formed, for each class, four double triangles in each of which three angles represent (1) quartz (Qu), (2) all feldspars except plagioclase (Kf), and (3) the feldspathoids (Foids). The remaining angle (Flag) represents albite (Naf) in Order 1, oligoclase to andesine (CaNat) in Order 2, labradorite to bytownite (NaCaf) in Order 3, and anorthite (Caf) in Order 4, making the divisions conform to the present lines of separation between the alkali rocks, the acid plagioclase (dioritic) rocks, the basic plagio- clase (gabbroic) rocks, and the anorthite rocks. Figures 16-19 represent the triangles of Class 2. There are similar sets for Classes 1 and 3. Zonal feldspars may be determined by considering the approximate amounts of each kind and obtaining the average Ab-An value. This will be necessary in but few cases, for ordinarily a simple inspection will show whether the average runs across the AbsoAriso line. Of course if the nucleus as well as the rim falls entirely between the and 5, 5 and 50, 50 and 95, or 95 and 100 lines, there is no need for computation. Class 4 : Owing to the practical absence of light constituents in Class 4, the subdivisions into orders must be made on a different basis from those of the first three. They are here made by dividing the tetrahedron by planes parallel to the left-hand face, forming four orders, depending upon the amount of ores present (Fig. 14). The division points for these planes, as in the other classes, are 0-5-50-95-100. There are now six dimensions in the classification, and since each pigeonhole will represent not only a plutonic rock but also a hypabyssal and an extrusive, we may say we have a classification in OF RnrK-FoRMiN-o MIVKRALS AVD 43 seven dimensions, yet every rock is shown by a single point on a drawing in a single plane. The more detailed description which follows may make this clearer. Families. The quarfeloid face of the double tetrahedron will appear a- >hown in Figures Hi I'.i. The families in the first three classes are to be mimlx-rod as shown in Figure 15. The object of begin- ning with is to make the positions easier to remember, since they run in groups of four. The division points are at 0-5-50-95-100 on lx>th the feldspar base line and in the vertical direction, except for tin- few intermediate mon/onitic families to lx mentioned in a moment. Families 0, 1, 5, 9, 13, 17, 21, and 25 occur but once in each class, since the amount of plagioclase in each, whether albite, acid plagioclasc, basic plagioclaso, or anorthite, is too small to make an essential difference in the rock. They form the hinge alwut which the order tetrahedron (Fig. 12) was opened, and are the same in all orders. For convenience these "hinge families" are classed with Order 1. This is shown in Figures 16-10 where these families arc omitted and are represented by dot tod lines. In the original article describing this classification there were thirty-two families, an intermediate family having been inserted along the Ab so Anjo line, namely, the family of the monzonites. Since in general descriptions these monzonitic rocks are unneces- sary, they are omitted from all orders. For special rock de- scriptions the additional monzonitic families, adamellite and mon/onite, are used as shown in Figure 15. Certainly the intermediate families are not necessary between Families 2 and 3, 22 and 23, and doubtfully between 14 and 15, and 18 and 19. Whether they are used or not need cause no confusion, the rock names 1 as indicated show what divisions have been made. That these are smaller families is shown even in the numbers by the marks (') and (") after them. They may thus be used or not as desired. Class 4 : The single tetrahedrons of the four orders of Class 4 are subdivided on the basis of the dark minerals present since the light-colored constituents are practically wanting. In Orders 1, 2, and 3, of Class 4, the corners represent respectively olivine, biotitc and (or) amphibole, and pyroxene (Figs. 14 and 20). In Order 4, if thought desirable, they may be taken to represent the various ores: the writer, however, groups these in one family, for, considered as rocks, they are unimportant. The various hematite, magnetite, ilmenite, etc., ores may be made subfamilies. THE MINERAL GROUPS The constituents of the rock are divided into three primary groups: Fio. 15. Family numbers in Classes 1-3. QUARFELOID8 (Qu) Quartz (Kf) Orthoclase, microcline, microperthitc, anorthoclase, etc. (Flag) The whole isomorpHous Ab-An series of plagioclases (Foids) The feldspathoids (nephelite, leucite, sodalite, haflynite, noselite, melilite, primary analcite, etc.) 1 Granodiorite is given the original significance intended by Lindgren, being applied to a rock intermediate between granite and diorite. Syenodiorite, as originally used by the writer, is the quartz-free equivalent. In the limited MOM rocks between adamellites and quarti diorites are called monzotonalites, and lietween monzonites and diorite* mon- zodiorites. On the granite side the limited granite, between orthogranite and adamcllitc is monzogranite, and between monzonite and orthoeyenite, monzosyenite. The name indicates at once its intermediate position. 44 ESSENTIALS FOB THE MICROSCOPICAL DETERMINATION Ortbotyenii Kf( ' ( Atbite-dmellite ) | Ennyenitc i ( Ubilc-monwinil* ) W'Albitt mnninrtiori aittynile _ | \ X PubUctte \ Nat Fig. 16. Folds / N>;.hf1itf-bcrlnl Fig. 17 v , Mnmoptabbro ) X- G.bbro, Nmiw '1""" bl " : 4-A NaCaf Fig. 19 x - Foids Foidi OK Ho. K-I'uiiMiMi MINERALS AND ROCKS 45 UAFITB8 Dark mica.- (biotitr, phlogopitc, zimiwaMite. Amphibolr- Pyroxenes (incluilini: uralitizeil pyroxene) Olivine Iron "ores" (magnetite, ilmenite, elirumite, pyrite, lii-matite, etc.) Cassitcritc {darnel Primary epiilote Allanite. zircon, rutile, primary titanitc, spinel, and other dark minor constituents AUXILIARY CONSTITUENTS Tlif auxiliary constituents are seldom of importance. Topaz Corundum Primary scapolite Lcpidolite Tourmaline Fluorite Primary calcite Apatite, Conlierite Andalusite Muscovite etc. Most of the auxiliary constituents arc light in color; they are, consequently, computed among the IriK-ocrates. SECONDARY CONSTITUENTS inlary constituents arc to be calculated as the originals from which they came. Thus ore replacement- of the mufites are computed as mafitcs, kaolin as feldspar, etc., chlorite as a biopyribole, analcitc as fcldspathoid, pscudoleucite as leucite, etc. GLASS Glass must be computed from an analysis. One can usually surmise its composition from the character of the phenocrysts and the appearance of the rock as a whole. When undetermined, the rock must be given a tentative name, such as hyaline-rhyolite, etc. Glassy rocks are rare. RULES FOR COMPUTING ROCKS FROM THEIR MODES 1. The sum of the minerals in the mode should be 1000.5. If greater or less, it should be recal- culated to 100. The sum of the leucocratcs (quarfeloids plus auxiliary minerals) determines the r!a-- . Class 1. Leucocratcs form more than 95 per cent of the total components Class 2. Leucocrates between 95 (inclusive) and 50 per cent Class 3. Leucocrates between 50 (inclusive) and 5 per cent Class 4. Leucocratcs between 5 (inclusive) and per cent 2. Determine the orders in Classes 1, 2, and 3 directly from the Ab-An ratio in the plagioclase. Order 1. Ab,wAn to AbAn, Order 2. Ab.iAn t (inclusive) to AhioAn M Order 3. AbwAn M (inclusive) to AbtAnu Order 4. AbtAn* (inclusive) to AboAnm In Class 4 the orders are determined by the percentage of "ores." Reduce the sum of biotite, olivine, pyribole, and "ores" (including cassiteritc, chromite, etc.) to 100, dropping the minor mafites, apatite, garnet, perofskite, any small amount of quarfeloids, etc. The percentage of "ores" determines the order. Order 1. to 5 per cent "ores" Order 2. 5 (inclusive) to 50 per cent "ores" Order 3. 50 (inclusive) to 95 per cent "ores" Order 4. 95 (inclusive) to 100 per cent "ores" 46 ESSENTIALS FOR THE MICROSCOPICAL DETERMINATION 3. Determine the family. In Classes 1, 2, and 3, first recalculate the quarfeloids to 100. The amount of quartz (or feldspathoid) thus determined immediately locates a row of horizontal pigeon- holes, in one of which the rock belongs. Recalculate Kf plus plagioclase to 100 and determine the proper point on the Kf-Plag base line. This determines the vertical series of pigeonholes, and its intersection with the horizontal series gives the proper position for the family. Still simpler is the location of the family graphically. This is discussed below. In Class 4, Orders 1, 2, and 3, recalculate the olivine, pyrox- ene, and biotite plus amphibole to 100 (Fig. 20) and find the Biotttal 10 I 11 ^5& position by taking the relation of olivine to the sum of the bio- AopniDOl pyroxene FIG. 20.-Family numbers in Class 4 tite and Pyriboles for the horizontal line, and of biotite plus amphibole to pyroxene for the intersecting line. Graphically the method is the same as for the other classes. 4. Subfamilies. In all classes, subfamilies are based on 0-5-50-95-100 division points after the manner shown in Figure 21. Thus we may have biotite-granite, hornblende-bearing biotite-granite, biotite-bearing hornblende-granite, and hornblende-granite. Biotite- Hornblende bearing biotite- Biotite bearing hornblende- Hornblende granite granlta , granite , granite | SO FIG. 21. Method of naming subfamilies 06 50 9B 100 A FEW POINTS TO BE OBSERVED A rock whose percentage value falls exactly on a family line should be given the name of the family in the pigeonhole toward the opposite apex of the triangle, except as indicated below. Thus a syenite with 5 per cent quartz is called a granite. On the 50-50 line of quartz they are moved upward, and on the 50-50 line of foids downward, toward the apices; that is, they are placed in Families 1-4 or 21-24. On the 50-50 line between Kf and Flag they are included in the Flag side. Rocks falling on the line separating the two triangles, namely, on the feldspar base line, usually should be classed on the quartz side, that is, on the normal side, but if the rock has affinities with alkalic rocks, it should be placed on the Foid side. For classificatory purposes, it is seldom necessary to make exact determinations of the mineral percentages. Unless the proportions are such that the rock falls near a division line, a simple inspec- tion will answer. EXAMPLE A granodiorite having the composition : Quartz.. . 14.0 = 18.7 Orthoclase 15.0 = 20.0= 24.6 Oligoclase (Ab 90 An 10 ) 46.0 = 61 .3= 75.4 Total quarfeloids 75.0 100.0 100.0 Biotite 12.0 Hornblende 12.0 Magnetite 0.6 Titanite.., 0.4 Total mantes 25.0 100.0 Percentage quarfeloids = 75. The rock belongs to Class 2. Ab 90 Anio falls between 95 and 50. The order, therefore, is 2. OF ROCK-FORMING MINERALS AND ROCKS 47 The family, with 18.7 quartz (light constituents reduced to 100), falls in the row of families 5 to 8 The nilin orthodase t<> oligoclase is 24.6 to 75.4, consequently the family is 7, granodiorite. If the mon/onitic families are included, it is 7', mmizotonalite. The rock number is 227 (or 227'), to be read two, two, seven (or two, two, seven prime). ( Jraphieally 1 the rock may l>e shown by a point and a line as indicated in Figure 22. Draw three lines, parallel to the sides of the triangle, through the points representing the amounts of the corner minerals in the rock. In this case a horizontal line through 14 for quartz, a line sloping northeast and M nit h west through 40 for oligoclase, and a line sloping northwest and southeast through 15 for ortho- clase. Lay the side of a straight-edge on the apices of the two triangles and draw a short line through the small triangle from its apex to its base. Connect either of the lower corners of the large triangle with the >imilar corner of the small one, and indicate its intersection with the line first drawn by a dot. /'. The line M and the spot /' represent the rock. The point /' is the same as that which was obtained by computation above. It gives the relative proportions of the light constituents, quartz i nil' i. orthoclasc (kF), and plagioclase (IF), in the rock by its distances from the sides of the triangle opposite these names at the corners. The actual percent- Quartz ages of the minerals in the rock are also represented: be or pk for orthoclase, ab or to for plagioclase, '<rtliiit;ir:uitiilito (-111) (-111) (-111) _ T:tr:m1iilitc i liMiiitf-Kreisen 3 Grnnodiorite-greisen 4 Tonal itc-greiiieD .-, llrtli..:d:i.-kite (-115) (-115) (-115) 6 Ala-kit,' Leurogrenite 7 I.iMin>-:dl>iti'-itr!iin>diiirite Lcucogranodiorite Leucogranogabbro S 1., 11, ,,-:ill,itf-ton;llite Leucotonalitc- Quarti-anorthositc 9 Orthosite (-119) (-119) (-119) in I..'iiri>-iill)ili-i.yi-iiitc IxMioosycnite 1 1 I.euco-albite-sycnodiorit<' Leucoeyenodiorite Iveucoeyenogabbro Lcuco-anorthitc-ycnogabbro 12 Albitite Ix!ucxliorite Anorthositc Anorthitite 13 (-1113) (-1113) (-1113) 14 15 16 Dungannonite 17 (-1117) (-1117) (-1117) 18 19.LeucolitchfieIdite JO Ix'iicoinariupolite 21 (-1121) (-1121) (-1121) 22 23 24 Craigmontite 25 (-1125) (-1125) (-1125) : With the additional families: 6' Lcucomonzogranite 6"-7" Lcuco-albitc-adamellite Ixmcoadamellite 7' I/eucomonzotonalite 10' I^uconionzosyeniti- 10"-11" Leuco-albite-monzonite Leucomonzonite 11' Leucomonzodiorite 50 ESSENTIALS FOR THE MICROSCOPICAL DETERMINATION CLASS 2. TABLE II Quarfeloids Mafites between -=- and _-. 5 50 Order 1 AbiooAno to AbjsAm Order 2 AbisAm to AbsoAnio Order 3 AbwAnso to AbsAnss Order 4 AbsAni to AboAniw Mesosilexite ( = 210) ( = 210) ( = 210) 1 Moyite (=211) ( = 211) (=211) 2 Quartz-granite 3 Quartz-granodiorite 4 Rockallite Quartz-tonalite 5 Orthogranite ( = 215) ( = 215) (=215) 6 Albite-granite Granite Calcigranite Anorthite-granite 7 Albite-granodiorite Granodiorite Granogabbro Anorthite-granogabbro 8 Albite-tonalite Tonalite Quartz-gabbro Quartz-anorthite-gabbro 9 Orthosyenite ( = 219) ( = 219) ( = 219) 10 Albite-syenite Syenite Calcisyenite Anorthite-syenite 11 Albite-syenodiorite Syenodiorite Syenogabbro Anorthite-syenogabbro 12 Albite-diorite Diorite Gabbro, Norite Anorthite-gabbro 13 Pulaskite (=2113) ( = 2113) ( = 2113) 14 Nephelite-bearing syenite 15 Nephelite-bearing syenodiorite 16 Nephelite-bearin g diorite 17 Ortho-nephelite-syenite (=2117) ( = 2117) ( = 2117) 18 Albite-nephelite-syenite Nephelite-syenite 19 Litchfieldite Nephelite-syenodiorite Nephelite-syenogabbro 20 Mariupolite Nephelite-diorite Nephelite-gabbro 21 Naujaite (=2121) ( = 2121) (. = 2121) 22 Beloeilite Heronite 23 24 Toryhillite Lugarite 25 Urtite, Fergusite, Uncompahgrite ( = 2125) ( = 2125) ( = 2125) With the additional families: 6' Monzogranite 6"-7" Albite-adamellite Adamellite Calciadamellite Anorthite-adamelh'te 7' Monzotonalite 10' Monzosyenite 10"-11" Albite-monzonite Monzonite Calcimonzonite Anorthite-monzonite 11' Albite-monzodiorite Monzodiorite Monzogabbro Anorthite-monzogabbro OF RncK-FoRMi\<: MIVV.RALH AND ROCKS 51 TABLE III Quarfcloi.ls CLAM3 ' Mafite. 50 OrdiT 1 AbioAni to AlHi.Mu Order 2 AhitAm to AbwAnH Orders At)- An- to AbiAnH Order 4 AbAn to AthAm. (-310) (-310) (-310) 1 (-311) (-311) (-311) 2 3 4 .". Mi'laorthogranit*- (-315) (-315) (-315) i; Mi'la-albite-graaito Mrlagranite Melacalcigranite 7 Mi-la-albiU>-Kranodiorite M I'laftranodiorite M elagranogabbro 8 Mela-albite-tonalite Melatonalite M eU-quarti-gabbro 9 Melaorthosyenite (-319) (-319) (-319) 10 Mrla-albite-yenite Melasyenite 1 1 M rln-albite-ycnodiorite MrLaayenodiorite MelaHypnogabhro llirol<-ttait<- IJ Mi-lii-nll>it(Mliorite Moladiorite Melagabbro Yamaakitc 13 OrthoKhiuikinitf (-3113) (-3113) (-3113) 14 Shonkinite OligoclaBe-(andeeine-) shonkinite Labradorite- (by town- ite-) shonkinite 15 16 17 Nephelite-ehonkinite (-3117) (-3117) (-3117) 18 19 Melalitchfieldite Mela-nephelite- syenogabbro 20 Melamariupolite Theralite 21 (-3121) (-3121) (-3121) 22 23 24 25 Bekinkinite, Missouritf.Farrisite (-3125) (-3125) (-3125) With the additional families: 8' Melamonzogranite 6"-7" MeU-albite-adamellite Mela-ad amellite Melacalciadamellite 7' Melamonzotonalite 10* Melamonxoeyenite 10"-11" Mekpalbite-monsonite Melamonzonite Melacalcimonxonite 11' Melamonrodiorite 52 ESSENTIALS FOR THE MICROSCOPICAL DETERMINATION TABLE IV Quarfeloids , 5 .0 CLASS 4. -- between == and ,,.7: Mafites 95 100 Order I "Ores" less than 5 per cent Order 2 "Ores" between 5 and 50 per cent Order 3 "Ores" between 50 and 95 per cent Order 4 "Ores" more than 95 per cent Dunite Chromite-dunite, Magnetite-dunite Olivinc-chromitite, Olivine-magnetitite Chromitite, Magnetitite 1 Hornblende-dunite 2 Pyroxene-bearing-hornblende- dunite 3 Hornblende-bearing-pyroxene- dunite 4 Pyroxene-dunite 5 Mica-peridotite, Amphibole- peridotite, Scyelite, Cortlandite 6 Olivinite ( ?) Valbellite ( ?) 7 Wehrlite Koswite 8 Lherzolite, Diallage-peridotite, Saxonite Harzbergite 9 Amphibolites, Hornblendites, Biotite-pyroxenites 10 Olivine-bahiaite 11 Bahiaite, Cromaltite 12 Diallagite, Bronzitite, Hypersthenite, Websterite Ilmenite-enstatitite, Magnctite-pyroxenite MINERAL INDEX in Ixil.l-fare ty|n- refer to the group deacriptiomi) Arinit.- _>'.. 36 Actinolitc I'.', is. .'7. 35 te -".'. 35 Argirile-iiuijile '27. 36 Albilc ID. 30 .Miami.- See Drtliilc) 27 Aliinitc ."i Aiiiphil>lc-. iiinnnclinie 36 AniphilHiie*. iirtlinrh ..... Me 36 Analcitc :i Anala-e 7 Andaliisite in. .'."> AndeMiic '.i Anhydrite i:i Anorthita II. 30 Aniirthorla-e '.i. 30, 31 AnthophyOite 11, is, 2t>, 36 Antiisoriic Id. 17. jr. Anti|M-rthite '.I Apatite .'). l.V Jl A rf \vil.-omtc 27. 36 Actrophyllil mte II. is. 27. 36 Sec Gthnite) Harkevikitc 27. 36 tic ImniMciide 27, 36 Heckelite :< Biotil Bronzite 9, 17, 2T>, 36 Hr.M.kitc 13, 19, 28 BruciteO Bytownite 31 Calcite 7, 15, 23 Cancrinite 6 Cassiterite 6, 15 CcUi:in 30 ( 'li.'ilccilmiy 10 Chiiuttnlitc (Sec Andalusite) Chlorite (See Pennine and Clinochlore) Chronutc 1 Chrysolite (See Olivinc) ChryHotile (See Serpentine) Clin'ochlorelT, 26 Clinozoisitc 9, 17 Common hornblende 26, 36 Cordicrite 9, 17, 25 Corundum 6, 21 Cyanite (See Disthene )10, 17, 25 Datolite 13 Desminc (See StUbite) DiallaRp 12, 18, 27, 36 Dii'hroite (See Cordieritc) Diopmdc 12, 18, 27, 36 Dipyr 6 Disthene 10, 17,25 Dolomite 7, 15, 23 Dumortierite 25 Enstatite 9, 36 Kpiilote, Green (See Pbtacitc) 12, 28 Epixtilbite 10 Kneolitc 5, 15 Eudialyte 5, 15 ite L3. I' 1 Kihni Iliiuanitc) Muorile J .rite 12 Garnet* (See the different varieties) 3 Qutaldite 36 Gedrite 18, 27, 36 Glass 3 Glaucophane '2(>, 35 < Iraphitc 1 ' Grime-rite 29, 36 Gypsum 10 HaOyno (Sec HaOynito) Haii^iito 2 IfedenlMTKito IS, 36 lleiiiiititt- 1. _'.'! llereynite :i Heulandite 9 Hnrnlilcnile, baHaltic 27, 35 llnnihlendo, common 26, 36 Hydroncphelite 5 Hypcrsthene 11, 26, 35 Idocrasc (Sec Vesuvianitc) Ilnienite 1 lolitc (Sec Cordierite) Jadcite 36 Kaolin 9, 17 Kaolinite (Sec Kaolin) Katophoritc 36 Labradorite 9 Laumontite 11 Lcpidolite 13 I .en, -iii- 2, 5 Leucoxenc 13, 19, 28 Magnesite 7, 15, 23 Magnetite 1 Mcinnitc Melanite 3 Melilite 5, 15, 21 Menaccanit^ (Spr Ilmcnitr) Mica (Sop (lifTerent varieties) Mieroeline '.I, 30, 31 Micnilinc microperthitc 9 Microperthite 9 Mizzonite (i Mi.na/itc 13, 19 Muscovite 12, 18 Ncphclinc (Sec Ncphclite) Xephelitc 5 Noeean (See Noeelite) Noselitc 2 Octahedrito (See Anatase) ( Hinorlase 9 ( )liKi>clase-albitc v Olivinc 12, 19, 28 Opal 3 Orthitc .'7 OrthoclaM' I'erii'lasc 3 IVrofskite 3 Phlogopitc 7, 13, 18, 2!l Picotitc 1, H Pfodmoatra -".' PisUriti 1'J. L's I Mam. H-la-c 31 34 ricdnimte 3 I'yrit45 1 Pyroxene.s, MoniM-linic 36 Pyrojcenes, OrtboritomUc 35 PynUtg :{ Pyrrhotite 1 Quartz 5 Ricbcckite 25, 36 Rutile 1,5, 21 Sagcnitc, 15, 21 Sanidinc 9, 31 Scaiwlitc 6 Sencitc, 12, 18 Serpentine (Fibrous variety, Chryso- lite) 11 Serpentine (Leafy variety, see Antigo- rite) Siilc-ritc 7, 23 Silliniatiitc 11, IS Soda orthoclusc 30 Sxxlalite 2 SpcsHartitc 3 Sphenc (Sec Titniuto Spinel, Precious 3 SiMNliiuieiie 11, 17, 26, 36 Stiturolitc 26 Still.ite 9 Talc 13, 19 Thulite 25 Titanitc 13, 19, 28 Titanolivine 27 Topai 9 Tourmaline -- Tremolitc 12, 36 Tridymite 5 Uwarowite 3 Vesuvianite 6, 15, 21 \\ erncrite 6 WoUaatonitc 11.35 Zinnwiildite 28 Zircon 6, 15 Zoisite 10, 17 PUHTED IN TBX UJ.A. 53 De irst, de geiht, Dit is de tweit'; Will wiinschen dat de't ok noch deiht. Un wenn hei't dauhn deiht, kann hei gahn, Ick heww an em dat Minig dahn. Wenn Einer dauhn deiht, wat hei deiht, Denn kann hei nich mihr dauhn, as hei deiht. FRITZ REUTER 14 DAY USE RETURN TO DESK FROM WHICH BORROWED EARTH SCIENCES LIBRARY This book is due on the last date stamped below, or on the date to which renewed. Renewed books are subject to immediate recall. m<&&x~ JUN 31969 ^tnr nMo ' General Library LD 21-40m-5, 65 University of California (F4308slO)476 Berkeley J