CHAPTER IV.
THE MICROSCOPIC AND OPTICAL CHARACTERS OF MINERALS.
OPAL.
ISOTROPIC. AMORPHOUS.
COMPOSITION: SiO_{2}.nH_{2}O, generally soluble in caustic alkalies.
▄Usual Appearance in Sections▄: Colorless patches, incrustations or veins, also at times with sphærulitic structure (hyalite), showing interference cross, of negative character, between crossed nicols. Often shows anomalous double refraction due to strains. The refractive index is very low (1.46) so that the surface of the opal appears rough.
REMARKS: Found as a secondary mineral in many acid volcanic rocks, rhyolite, trachyte, andesite, etc., and also in basic basalts. H., 5.5 to 6.5. Sp. gr., 2.2.
LIMONITE.
AMORPHOUS.
COMPOSITION: Fe_{2}(OH)_{6}, Fe_{2}O_{3}, frequently quite impure.
▄Usual Appearance in Sections▄: Brownish and opaque, in very thin sections may be translucent.
REMARKS: Limonite is essentially a decomposition product, often forming pseudomorphs after ferruginous silicates or halos about the iron ores.
PYRITE, Pyrites.
ISOMETRIC. COMPOSITION: FeS_{2}.
▄Usual Appearance in Sections▄: Cubes, pyritohedrons, combinations of these forms; or in irregular grains. Outline of cross-sections generally square.
_Opaque_, and by reflected light, bright yellow, with strong metallic lustre.
Alters very easily to the oxides of iron (rust).
REMARKS: May be present in all kinds of rocks, igneous, metamorphic and sedimentary. Not noticeably acted on by hydrochloric acid. H., 6 to 6.5. Sp. gr., 4.9 to 5.2.
PYRRHOTITE, Magnetic Pyrites.
COMPOSITION: FeS. Distinguished from pyrite by being practically always in irregular masses and not in crystals, and by bronze yellow color with reflected light. Found in basic eruptive rocks, more rarely in schists.
MAGNETITE, Magnetic Iron Ore.
ISOMETRIC.
COMPOSITION: Fe_{3}O_{4}, often contains Ti.
▄Usual Appearance in Sections▄: Grains and crystals (generally octahedra), Fig. 33 B. Skeleton crystals frequent in highly ferruginous eruptive rocks.
_Twinning._—Common, according to _Spinel_ law.
[Illustration:
FIG. 33.—_A_, Zircon crystals (isolated from granite) in balsam, showing high relief. _B_, Magnetite crystals. _C_, Ilmenite, showing partial decomposition to _leucoxene_ along crystallographic directions. ]
_Opaque_, and by reflected light, bluish-black, with strong metallic lustre.
▄Distinguished from▄: HEMATITE, CHROMITE, ILMENITE and GRAPHITE, by being easily separated from powdered rock by weak magnet.
REMARKS: Very widely distributed in eruptive rocks and crystalline schists. In the eruptive rocks magnetite belongs to the oldest secretions from the magma, immediately followed by chrysolite, biotite, hornblende, augite, etc.; hence often appears as inclusions in these and other minerals. Magnetite grains may form with other substances pseudomorphs after hornblende, biotite, hypersthene, etc. Such pseudomorphs appear to be caused by “resorption.” Magnetite is strongly magnetic and soluble in hydrochloric acid. H., 5.5 to 6.5. Sp. gr., 4.9 to 5.2.
CHROMITE.
ISOMETRIC.
COMPOSITION: FeCr_{2}O_{4}.
▄Usual Appearance in Sections▄: Octahedral crystals, grains and in the olivine rocks sometimes in dense aggregates. May be surrounded by green, pleochroic halo of chrome ochre.
_Opaque_, and by reflected light, brownish-black to black, with general absence of metallic lustre. Usually translucent and brownish on the edges (by transmitted light), with a very rough surface due to high index of refraction (_n_ = 2.1).
▄Distinguished from▄:
(_a_) MAGNETITE by brownish-black to black color and general absence of metallic lustre (by reflected light) and by grains being usually translucent and brownish on the edges (by transmitted light).
(_b_) SPINEL (Picotite), see under Spinel.
REMARKS: Common in crystalline rocks, rich in magnesia, and in serpentine. Chromite is not acted on by acids, is non-magnetic and gives chromium bead test. H., 5.5. Sp. gr., 4.3 to 5.6.
SPINEL.
ISOTROPIC. ISOMETRIC.
COMPOSITION: Mg(AlO_{2})_{2}. Pleonaste (Fe, Mg spinel), Picotite (Cr spinel).
▄Usual Appearance in Sections▄: Octahedral crystals and twins (after spinel law), less often in grains. Fracture cracks. Always optically normal and never decomposed in rocks. Usually colorless or dark green (pleonaste) to brown (picotite). The refractive index is high (_n_ = 1.72, spinel proper, to 2.00, chrome spinel), hence the _relief_ is marked and the surface rough.
▄Distinguished from▄:
(_a_) GARNET when colorless by octahedral shape of crystals (garnet forms being 110 and 211), when brown (picotite) from melanite garnet by common zonal coloration of the latter, but may require chemical tests. Furthermore spinel may have green color and is never decomposed.
(_b_) PEROVSKITE by the lower index of refraction and the absence of reaction for Ti.
(_c_) CHROMITE chemically or by density or hardness.
REMARKS: Found in gneiss, granulite, lherzolite and in regions of contact metamorphism and secondary bedding formations (picotite), olivine-basalt and serpentine. Spinels are insoluble in hydrochloric acid. H., 8. Sp. gr., 3.5 to 4.1.
GARNET.
ISOTROPIC. ISOMETRIC.
COMPOSITION: R″_{3}R‴_{2}(SiO_{4})_{3}. R″ is Ca, Mg, Fe or Mn; R‴ is Al, Fe‴, or Cr, rarely Ti.
▄Usual Appearance in Sections▄: Irregular grains, Fig. 13, or simple crystals, showing forms (110) and (211), alone or in combination, Fig. 14 _b_. Zonal structure not infrequent, especially in the titanium varieties, Fig. 34.
_Color._—Colorless, or nearly so, to yellowish, reddish or brownish.
_Index of Refraction._—_n_ = 1.750–1.856, hence _relief_ high and surface very rough.
_Fracture._—Irregular cracks occur, but no cleavage noticed.
▄Crossed Nicols▄: As garnets are isotropic, sections remain dark during complete rotation. Optical anomalies may however occur, but are generally confined to, titanium free, lime garnets and manganese garnets. The effect being to divide the crystal symmetrically into different areas, “dodecahedral structure.”
[Illustration:
FIG. 34.—Garnet, with zonal structure, in gneiss. (From Cohen.) ]
▄Alteration▄: Garnets are usually fresh, but may be found altered to chlorite or hornblende.
▄Distinguished from▄: SPINEL and PEROVSKITE.—See under the latter.
REMARKS: Found principally in granulites, metamorphic rocks, contact rocks, crystalline schists, etc. Certain varieties may be found in eruptive rocks or olivine rocks. May form pegmatitic borders with pyroxene, spinel, etc. Garnets are practically insoluble in hydrochloric acid. H., 6.5 to 7.5. Sp. gr., 3.4 to 4.3. The insolubility in acids and the high sp. gr. help in separating garnet from a powdered rock.
LEUCITE.
ISOTROPIC. ISOMETRIC.[80]
COMPOSITION: KAl(SiO_{3})_{2}.
▄Usual Appearance in Sections▄: Crystals or grains, which vary greatly in size. Cross-sections often nearly round. When very small and free from inclusions may be easily overlooked. Sometimes the grains are surrounded by tangentially arranged needles of different minerals.
[Illustration:
FIG. 35.—Leucite, with radial and tangential inclusions, Vesuvius Lava. (From Cohen.) ]
_Color._—Colorless.
_Index of Refraction._—_n_ = 1.509, hence no _relief_ and generally smooth surface.
_Fracture._—May be noticed, but no cleavage observed.
_Inclusions._—Common, radially or zonally arranged, consisting of minerals or glass, Fig. 35.
▄Crossed Nicols▄: The smaller crystals appear isotropic; the larger crystals show characteristic intersecting systems of twin lamellæ, Fig. 36.
[Illustration:
FIG. 36.—Leucite, showing complicated, interpenetration twinning between crossed nicols. ]
_Double Refraction._—Very weak (γ − α = 0.001). In thin sections it may be necessary to use a sensitive color plate to prove double refraction.
_Interference Colors._—Very low 1st order, dark gray, etc.
▄Alteration▄: Quite frequent to fibrous or granular zeolites.
▄Distinguished from▄: ANALCITE—see under analcite.
REMARKS: Almost entirely confined to younger eruptive rocks, phonolite, tephrite and other leucite rocks and their tuffs. Often found with plagioclase, nephelite, augite, etc. It is more or less attacked by hot hydrochloric acid. H., 5.5 to 6. Sp. gr., 2.4 to 2.5. The isolation of leucite from rock powder can be better accomplished by specific gravity than by chemical methods.
ANALCITE.
ISOTROPIC. ISOMETRIC.
COMPOSITION: NaAlSi_{2}O_{6} + H_{2}O.
▄Usual Appearance in Sections▄: Secondary colorless grains, with no very characteristic microstructure or properties. Cleavage parallel to cube (100) usually seen. Index of refraction low (n = 1.488), hence rather rough surface. Between _crossed nicols_ may show optical anomalies, but not so marked as in garnet.
_Distinguished from_: LEUCITE, SODALITE and NEPHELITE. These minerals are most easily confused with analcite and recourse must be had to chemical tests, detection of optical anomalies, gelatinization test or turbidity by heating.
REMARKS: Occurs as a secondary product (commonly from nephelite or leucite) in alkali-rich eruptive rocks. Considered also as a primary mineral in igneous rocks.[81] Gelatinizes with hydrochloric acid, and becomes turbid by heating. H., 5.5. Sp. gr., 2.25.
SODALITE GROUP. Sodalite, Haüynite (Haüyne) and Noselite (Nosean).
ISOTROPIC. ISOMETRIC.
COMPOSITION:
Sodalite, 3NaAlSiO_{4} + NaCl. Haüynite, 2(Na_{2}Ca)Al_{2}(SiO_{4})_{2} + (Na_{2}Ca)SO_{4}. Noselite, 2Na_{2}Al_{2}Si_{2}O_{8} + Na_{2}SO_{4}.
▄Usual Appearance in Sections▄: Dodecahedral rounded crystals or (S) irregular grains. Colorless, yellowish, brownish, greenish to deep blue. Refractive index low [_n_ = 1.483(S) to 1.503(H)], hence the surface appears rather rough in sodalite and slightly rough in haüynite. Inclusions, abundant and characteristic, often rod-like and arranged regularly, making section translucent and especially dark at border, Fig. 37. Dodecahedral cleavage sometimes seen. Optical anomalies may occur.
▄Alteration▄: Takes place easily to aggregates of natrolite, other zeolites, mica, etc.
▄Distinguished from▄:
(_a_) ONE ANOTHER only by chemical tests. Gelatinization test with hydrochloric acid will show in addition to jelly, salt crystals for (S), abundant gypsum crystals (CaSO_{2} + 2H_{2}O) for (H) and few if any gypsum crystals (absence of Ca) for (N). (H) and (N) turn blue when heated, but test will not work if minerals are decomposed. When treated with hydrochloric acid and nitrate of silver the black sulphide of silver will show on (H) and (N), but the white chloride on (S).
(_b_) NEPHELITE (Elæolite) by being isotropic.
(_c_) ANALCITE by no turbidity when heated.
REMARKS: These minerals are found in the basic, soda-rich rocks. (S) in elæolite-syenite also in trachyte and phonolite, (H) and (N) common in phonolite and leucite-porphyry. H., 5.5 to 6. Sp. gr., 2.3.
[Illustration:
FIG. 37.—Haüynite, showing dark centre and border, in nepelinite. (From Cohen.) ]
PEROVSKITE, Perofskite.
ISOTROPIC. ISOMETRIC. COMPOSITION: CaTiO_{3}.
▄Usual Appearance in Sections▄: Microscopic, octahedral crystals or larger grains, pale brownish in color and not very transparent, darker colored in the larger grains. In reflected light grains appear yellowish with adamantine lustre. Refractive index very high (_n_ = 2.38), hence _relief_ very strong.
Between _crossed nicols_, the little crystals generally appear optically normal and remain dark; but the larger crystals may show a complicated penetration twinning.
▄Distinguished from▄: GARNET (melanite) and SPINEL (picotite) by much higher refractive index and reaction for titanium and by zonal coloration of melanite. When opaque it might be mistaken for the iron ores, but has no metallic lustre.
REMARKS: Found in the younger basic eruptive rocks, especially melilite-basalt. Commonly associated with the iron ores nephelite, augite and chrysolite. Insoluble in hydrochloric acid. H., 5.5. Sp. gr., 4.1.
RUTILE.
ANISOTROPIC. UNIAXIAL. TETRAGONAL. COMPOSITION: TiO_{2}. _ć_ = c ELONGATION ∥ _ć_.
▄Usual Appearance in Sections▄: Sharp, elongated, prismatic crystals when microscopic, but granular when the individuals are large. Grains may be almost opaque, with adamantine lustre by reflected light. Knee- or heart-shaped twins, Figs. 38 and 39, common in the smaller crystals, the larger individuals may also show geniculated twinning. Small crystals sometimes form net-shaped groups (sagenite), by crossing one another at angles of 60°. Pleochroic halos may surround crystals. Color, yellowish to reddish-brown. Index of refraction very high (_n′_ = 2.712, α = 2.616, γ = 2.903), hence _relief_ marked and surface very rough. Prismatic cleavage present in larger individuals, not observed in microscopic crystals. Pleochroism and strong absorption may be noticed, especially in the larger grains, but may fail entirely.
▄Crossed Nicols▄: Double refraction _very_ strong (γ − α = 0.287). Interference colors[82] very high order, only seen in the microlitic crystals which do not appear dark due to total reflection; in other cases may not show at all. _Extinction_ parallel to prisms. In _convergent light_ optical character (+).
▄Alteration▄: May take place to a white or yellowish, fibrous or granular substance, strongly refracting, and similar to the alteration product of ilmenite. May be surrounded by grains of titanite.
▄Distinguished from▄: The OPAQUE ORES by adamantine lustre with reflected light; ZIRCON and CASSITERITE in concentrates by chemical tests. May not be possible to distinguish from cassiterite in sections.
REMARKS: Found in the metamorphic schists, amphibolites, slates, contact and fragmentary rocks, etc.; also as inclusions in quartz and mica. Especially common as a secondary product of titaniferous hornblende and biotite. The “sagenite” webs of the decomposed micas are probably secondary. Rutile is insoluble in hydrochloric acid. H., 6 to 6.5. Sp. gr., 4.2. It is easily separated from rock powder by its insolubility in acid and its high sp. gr.
[Illustration:
Rutile twins.
FIG. 38.—Twin plane (101). FIG. 39.—Twin plane (301). ]
ZIRCON.
ANISOTROPIC. UNIAXIAL. TETRAGONAL. COMPOSITION: ZrSiO_{4}. _c_ = ć. ELONGATION ∥ c′.
▄Usual Appearance in Sections▄: Small, short prismatic crystals, Fig. 33A, and grains. Shell-like (zonal) structure may be noticed. When enclosed in black-mica, hornblende, cordierite, etc., often surrounded by characteristic pleochroic halos.[83]
_Color._—Colorless, rarely pale brownish.
_Index of Refraction._—_n′_ = 1.95, (α = 1.931, γ = 1.993) hence _relief_ very high and surface rough.
▄Polarized Light▄:
_Pleochroism._—Not usually noticeable.
▄_Crossed Nicols_▄:
_Double Refraction._—Very strong (γ − α = 0.062).
_Interference Colors._—Very high (4th) order, minute crystals show brilliant colors.
_Extinction._—As zircon is uniaxial, basal sections remain dark during rotation of stage. In all other sections extinction is parallel to _ć_.
▄_Convergent Light_▄: Basal sections, which are large enough to give interference figures, show several rings in addition to dark cross. Optical character (+).
▄Alteration▄: Very rarely takes place.
▄Distinguished from▄:
(_a_) APATITE.—By much higher relief and stronger double refraction.
(_b_) TITANITE.—By uniaxial character.
(_c_) RUTILE.—See under the latter.
Easily confused with xenotime, which, however, has higher interference colors and more distinct pleochroism; but chemical tests may be necessary.
REMARKS: Found widely distributed but not in quantity in eruptive and metamorphic rocks. Occurs in granite, syenite, diorite, gabbro, gneiss, etc. It is one of the oldest constituents of the rocks in which it occurs, and may often be found as inclusions in the ferro-magnesium minerals. Zircon is insoluble in hydrochloric acid. H., 7.5. Sp. gr., 4.5 to 4.7. It can easily be separated from rock powder on account of its high sp. gr., insolubility in acid and non-magnetic properties. The crystals can then be examined separately, or chemical tests made to prove the presence of Zr.
SCAPOLITE GROUP, Wernerite, etc.
ANISOTROPIC. UNIAXIAL. TETRAGONAL. COMPOSITION: Silicates of Ca, Al _ć_ = a. ELONGATION ∥ a′. and Na.
▄Usual Appearance in Sections▄: Colorless grains, lath-like individuals or prisms (dipyre, in contact metamorphic limestone). Index of refraction (_n′_ = 1.551 to 1.584;, α = 1.542 to 1.558, γ = 1.555 to 1.597) about the same as quartz, hence usually no _relief_ and surface smooth. Cleavage distinct parallel to square prism. Inclusions (carbonaceous) may be abundant in contact rocks.
▄Crossed Nicols▄: Double refraction usually strong, but varies (γ − α = 0.013 to 0.039), increases with the Ca percentage. Interference colors upper 1st or 2d order, more brilliant than those of most of the colorless minerals. Basal sections (showing cleavages intersecting at 90°) isotropic. _Extinction_ parallel in longitudinal sections. In _convergent light_ basal sections show distinct uniaxial interference figure; optical character (−).
▄Alteration▄: Takes place easily to a fibrous substance or to kaolin, muscovite, etc.
▄Distinguished from▄:
(_a_) FELDSPARS (not showing twinning) and IOLITE (Cordierite) by uniaxial character, cleavage and higher order interference colors.
(_b_) QUARTZ by cleavage, higher order interference colors and optical character; quartz is (+)
(_c_) APATITE (in grains) by lower index of refraction, cleavage and higher order interference colors.
REMARKS: Found especially in metamorphosed diabases and gabbros (Norwegian); also in gneisses, crystalline schists, metamorphosed limestones, etc. Dipyre occurs in contact zones of limestones and schists, where it might be confused with andalusite, but cross-sections show uniaxial character. The minerals of this group are more or less soluble in hydrochloric acid. When the scapolite contains Cl, the following test can be made on fresh material. Treat with a solution of silver nitrate in hydrofluoric acid and the jelly will be impregnated with chloride of silver which will turn brown. H., 5.5. Sp. gr., 2.68.
VESUVIANITE, Idocrase.
ANISOTROPIC. UNIAXIAL. TETRAGONAL. COMPOSITION: _ć_ = a. ELONGATION ∥ a′. Ca_{6}Al_{3}(OH.F)(SiO_{4})_{6}.
▄Usual Appearance in Sections▄: Grains or prismatic crystals. Almost colorless to reddish (when containing Mn). Index of refraction high (_n′_ = 1.715, α = 1.701 to 1.726, γ = 1.705 to 1.732), hence _relief_ marked and surface rough. Cleavage imperfect, parallel to prism. Pleochroism generally very faint.
▄Crossed Nicols▄: Double refraction very weak (γ − α = 0.001 to 0.006), may vary in different portions of the same crystal (optical anomalies due to the mineral being at times a mixture of isomorphous individuals). Interference colors very low 1st order, dark gray, etc., may often appear zonal. Basal sections isotropic when normal, but may show division into biaxial portions. _Extinction_ parallel in sections elongated ∥ _ć_ axis. In convergent light basal sections show a faint cross when normal; optical character generally (−).
▄Alteration▄: Not known in rock-making vesuvianite.
▄Distinguished from▄: EPIDOTE (Pistacite) by the very low order interference colors. CORUNDUM by weaker double refraction. GARNET (Grossularite), ZOISITE, and APATITE may be easily confused with this mineral and hard to distinguish from it.
REMARKS: Found in limestones, that have undergone alteration by contact with igneous rocks, and in metamorphic schists. Also may occur in dense (nephrite-like) aggregates in serpentine. It is insoluble in hydrochloric acid. H., 6.5. Sp. gr., 3.3 to 3.8.
MELILITE.
ANISOTROPIC. UNIAXIAL. TETRAGONAL. COMPOSITION: Na_{2}(Ca,Mg)_{11}(Al,Fe)_{4}Si_{9}O_{36}?. _ć_ = a. ELONGATION ∥ c′.
▄Usual Appearance in Sections▄: Almost colorless, tabular (∥ base) crystals or irregular grains or shreds. Sections very commonly lath-shaped, and often characterized by the peculiar “peg-structure,”[84] the lines or markings being ∥ _ć_ (⟂ elongation of the section). Index of refraction (_n′_ = 1.630, α = 1.629, γ = 1.631) higher than that of the other associated colorless materials, hence _relief_ rather marked. Cleavage, parallel to base, very imperfect.
▄Crossed Nicols▄: Double refraction very weak (γ − α = 0.003), and diminishes with a decrease of Al. Interference colors the lower 1st order, grays, etc.; anomalous interference colors may show. _Extinction_ parallel to cleavage or the peculiar markings or lines. Optical character usually (−), but when poor in Al (+).
▄Alteration▄: Takes place frequently to a fibrous aggregate.
▄Distinguished from▄: NEPHELITE and FELDSPAR by higher relief, shape, “peg-structure” and usual dull appearance with reflected light.
REMARKS: Abundant in the leucite and nephelite rocks (associated with these minerals and with augite, perovskite and chrysolite), and takes the place of a feldspar in the melilite-basalt. It gelatinizes easily with hydrochloric acid. H., 5. Sp. gr., 2.9.
GRAPHITE.
HEXAGONAL.
COMPOSITION: C.
▄Usual Appearance in Sections▄: Minute particles, or flakes and grains of irregular shape, seldom crystallized.
_Opaque_, and by reflected light, black with metallic lustre.
▄Distinguished from▄: The similarly appearing ores by its insolubility in acids and the possibility of making it disappear by heating.
REMARKS: Graphite is widely distributed in the oldest rock formations, especially in the schists. It is often associated with rutile and the iron oxides. Graphite is not acted on by acids. H., 1 to 2. Sp. gr., 2.09 to 2.25. It is burnt with great difficulty in thin sections on platinum foil; but this test may vary, in many cases the graphite (when in bladed flakes) not being consumed even after long heating. When heated it may expand into worm-like forms.
_Carbonaceous Matter._—Occurs in opaque, grayish-black particles having no lustre; and is found finely disseminated, sometimes in larger aggregations, in clay slates, limestones, etc.
HEMATITE.
HEXAGONAL.
COMPOSITION: Fe_{2}O_{3}.
▄Usual Appearance in Sections▄: Irregular scales, minute grains or earthy. Distinct crystalline forms not often observed in rocks.
_Opaque_, and by reflected light, black with metallic lustre, or red without lustre. May also be transparent in red tints. No marked pleochroism observed.
REMARKS: Found widely distributed in acid eruptive rocks, crystalline schists, etc. Also as inclusions in minerals, and as a red pigment in many rocks. It is insoluble in hydrochloric acid, and non-magnetic, unless attached to grains of magnetite. H., 5.5 to 6.5. Sp. gr., 4.9 to 5.3.
ILMENITE, Menaccanite.
HEXAGONAL.
COMPOSITION: (FeTi)_{2}O_{3}.
▄Usual Appearance in Sections▄: Irregular masses, without crystallographic outline, rhombohedral crystals, or skeleton-like growths. Also in brownish, translucent mica-like forms.
_Opaque_, and by reflected light, iron-black with metallic lustre.
When _translucent_: pleochroism brown to yellow; double refraction not very strong; optically (−).
▄Alteration▄: Often takes place to a whitish, strongly refracting, substance only slightly transparent, called _leucoxene_. This alteration product frequently develops along definite rhombohedral directions, Fig. 33 C. Also a change to titanite or rutile may occur, or the ilmenite may be surrounded by these minerals.
▄Distinguished from▄: MAGNETITE and HEMATITE.—By whitish, strongly refracting decomposition product. At times the distinction may be very difficult.
REMARKS: Ilmenite occurs principally in the soda-rich and basic eruptive rocks. The mica-like form is limited to the porphyritic eruptives. The brown pigment in the plagioclase of certain gabbros may be ilmenite. It is attacked slowly by hot hydrochloric acid, and the solution when heated with tin becomes violet. Pure ilmenite is indifferent towards the magnet, hence strong magnetic properties would indicate a mixture with magnetite. H., 5 to 6. Sp. gr., 4.5 to 5.
CORUNDUM.
ANISOTROPIC. UNIAXIAL. HEXAGONAL. COMPOSITION: Al_{2}O_{3}. _c_ = a.
▄Usual Appearance in Sections▄: Pyramidal or prismatic crystals, grains or basal plates. Zonal structure or twinning may be noticed. Colorless or with patches of blue. Index of refraction high (_n′_ = 1.766, α = 1.760, γ = 1.769), hence _relief_ well marked and surface very rough. Rhombohedral cleavage may show in larger individuals. Pleochroism only marked when color is deep.
▄Crossed Nicols▄: Double refraction weak (γ − α = 0.009), like quartz. Interference colors middle 1st order, white to yellow. _Extinction_ parallel in elongated sections. Optical anomalies very rarely noticed in microscopic individuals. In _convergent light_ basal sections show a rather indistinct cross; optical character (−).
▄Distinguished from▄:
(_a_) APATITE and VESUVIANITE by brighter interference colors.
(_b_) TOURMALINE (light colored) by not having such strong absorption.
(_c_) CYANITE by uniaxial character.
Corundum may need to be isolated from the rock in order to be determined with certainty.
REMARKS: found in contact metamorphic rocks, eruptive rocks, granular limestones, etc. It is insoluble in hydrochloric acid. When rock sections are ground with emery, care must be taken not to confuse grains of emery with corundum in the rock. H., 9. Sp. gr., 3.9 to 4.
QUARTZ.
ANISOTROPIC. UNIAXIAL. HEXAGONAL. COMPOSITION: SiO_{2}. _ć_ = c.
▄Usual Appearance in Sections:▄ Allotriomorphic in the granitoid rocks, when apparently the last mineral to form, Fig. 5. More or less chemically corroded pyramidal crystals (with cross-sections six-sided or rhombic with an angle of about 100°) in the porphyritic rocks. Rounded or angular grains in the “clastic” rocks; granular mosaic in crystalline schists and contact rocks, very rarely in distinct crystals in any rocks. May at times be mutually interpenetrated with an acid feldspar (the areas of quartz and feldspar extinguishing as entire crystals), producing “micro-pegmatitic” structure, Fig. 67. Finally may appear as pseudomorphs after other minerals, but may then consist of some of the other forms of silica.
_Color._—Colorless, although by reflected light it may appear colored or cloudy if it contain many inclusions.
_Index of Refraction._—_n′_ = 1.547 (α = 1.544, γ = 1.553) hence no _relief_ and surface smooth.
_Cleavage._.—Rarely noticed, an important fact in determining quartz. Quartz breaks irregularly.
_Inclusions._—Minute fluid, gas and mineral inclusions, often in irregular trains, are very characteristic of quartz in granite rocks and crystalline schists. The inclusions are not so abundant in porphyritic rocks, but a few glass inclusions may occur, filling up “negative” crystals in the quartz. Rutile, amphibole, etc., may occur as needle-like inclusions in quartz.
_Polarized Light:_
_Pleochroism._—None.
_▄Crossed Nicols:▄_
_Double Refraction_.—Weak (γ − α = 0.009).
_Interference Colors._—The middle 1st order, white, yellow, etc.
_Extinction._—As quartz is uniaxial, basal sections remain dark during a complete rotation of stage. In the other sections extinction is not characteristic, due to the absence of cleavage and crystallographic outlines. Thin sections do not show circular polarization.
_▄Convergent Light:▄_ Basal sections show a dark cross, without any rings. Optical character (+).
▄Alteration:▄ Does not take place, so quartz always appears fresh and unweathered in sections.
▄Distinguished from:▄
(_a_) SANIDINE (in fresh grains).—By use of convergent light. Feldspar is biaxial, or sections which appear uniaxial are (−).
(_b_) NEPHELITE.—By almost entire absence of hexagonal outline, stronger double refraction, fresh, unweathered appearance and (+) optical character.
(_c_) IOLITE (Cordierite), SCAPOLITE and TOPAZ.—See under the latter minerals.
Quartz may be distinguished from all silicates by being dissolved without residue in hydrofluoric acid.
REMARKS: Quartz occurs widely distributed, as in the great sandstone formations. It is also a characteristic mineral of all acidic rocks, being common in granite, aplite, rhyolite, quartz-porphyry, quartz-diorite, dacite, etc. Quartz is very brittle and hence is a good indicator of the dynamic forces which have affected the rocks. It may show traces of mechanical deformation by peripheral shattering of the larger grains or by “wavy extinction”;[85] and also evidences of chemical corrosion by curved and looped contours. In some diabases the quartz may be surrounded by a rim of hornblende or augite needles (“quartz augen”). “Cataclastic” quartz may be biaxial. The “secondary enlargement” of quartz in clastic rocks may be noticed by the deposition of silica in crystallographic orientation around the clastic grains,[86] the new portion extinguishing at the same time as the core. Quartz is not attacked by ordinary acids. H., 7. Sp. gr., 2.6 to 2.7.
_Chalcedony._—This variety of SiO_{2} has a radially fibrous structure and shelly parting. It may form sphærulites, central sections through which show a dark cross between crossed nicols, or line cavities in rocks.
The index of refraction is a little lower than for ordinary quartz. The optical character is (−), which must be determined by a mica or gypsum plate, _ć_ = a. Elongation ∥ a′.
Chalcedony occurs in the ground mass of very silicious porphyritic rocks, which have microfelsitic development; and is found as a secondary mineral in all kinds of silicate rocks.
TRIDYMITE.
▄Usual Appearance in Sections▄: This form of SiO_{2}, which is soluble in boiling caustic soda, appears in “tile-like” aggregates of minute colorless plates (pseudo-hexagonal) and is always secondary. The refractive index is extremely low (_n′_ = 1.477), hence the surface appears rough.
Between _crossed nicols_ the interference colors are very low in order (γ-α = 0.002), and the tablets may show a division into different areas (optical anomalies). In _convergent light_ an indistinct biaxial figure is generally seen.
REMARKS: Chiefly a volcanic mineral, found in rhyolite, trachyte and andesite. Commonly associated with opal and chalcedony.
CALCITE.
ANISOTROPIC. UNIAXIAL. HEXAGONAL.
COMPOSITION: CaCO_{3}. Ca may be replaced by small quantities of Mg, Fe, Mn, etc. _ć_ = a.
▄Usual Appearance in Sections:▄ Grains and aggregates. May be fibrous or oölitic. Only in crystals in certain rocks.[87]
[Illustration:
FIG. 40.—Calcite, crossed twin lamellæ, in granular limestone. (From Cohen.) ]
_Twinning._—Polysynthetic, parallel to one or more faces of −½ R. (10̄12). Very common in crystalline limestones, and may have been produced by pressure or by the grinding of the section. Shows itself between crossed nicols as a series of light and dark bands, parallel or intersecting, Fig. 40, about parallel to longer diagonal of cleavage rhombs. When the composition face of the twins is oblique to the face of the section, interference colors can be seen without the analyzer.
_Color._—Colorless when pure, but may appear colored by transmitted light, due to organic pigments.
_Index of Refraction._—_n′_ = 1.601 (α = 1.487, γ = 1.659) hence with ordinary light _relief_ not marked. Due to the great variation in refractive indices of the two rays, with polarized light, the surface will appear either quite smooth or rather rough, depending upon which vibration direction lies over the plane of the polarizer. This marked variation in appearance (sometimes called “twinkling”) serves as a good test for calcite.
[Illustration:
FIG. 41.—Calcite, section parallel to face of rhombohedron, showing rhombohedral cleavage. (From Cohen.) ]
_Cleavage._—Parallel to unit rhombohedron (10̄11), appearing in thin sections as many sharp cracks, whose angles of intersection depend on the position of the section, Fig. 41. Newton’s colors may be seen along cleavage cracks.
▄Polarized Light:▄
_Pleochroism._—None.
_▄Crossed Nicols:▄_
_Double Refraction._—Very strong (γ − α = 0.172).
_Interference Colors._—Pale, iridescent colors of very high order.
_Extinction._—As calcite is uniaxial, basal sections remain dark during rotation. Extinction angles with respect to the cleavage cracks vary with the position of the section.
_▄Convergent Light:▄_ Basal sections, even when very thin, show distinct interference figure, with cross and rings. Optical character (−).
▄Distinguished from▄:
(_a_) Other CARBONATES.—By ease with which it is attacked by cold dilute acids, test can be made on slide after removing cover.
(_b_) MAGNESIUM-BEARING CALCITE.—By micro-chemical tests.
(_c_) TITANITE (Sphene).—See under the latter.
REMARKS: Calcite is very widely distributed, in addition to the extensive sedimentary limestone deposits. Common limestone consists of dense aggregates of crystalline grains. Calcite is often a secondary product of the lime-bearing silicates in the more basic eruptive rocks. Pseudomorphs of calcite after olivine are noteworthy. Coarse aggregates of calcite occur in the crystalline schists and contact rocks. Calcite is exceedingly plastic to pressure and mechanical deformation may be recognized by curving of the cleavage cracks, crumpling of the twin lamellæ and “wavy” extinction. Calcite is easily attacked and completely dissolved with effervescence by cold dilute acids. H., 3. Sp. gr., 2.72.
DOLOMITE.
ANISOTROPIC. UNIAXIAL. HEXAGONAL.
COMPOSITION: CaMg(CO_{3})_{2}, when pure CaO = 30.4, MgO = 21.7, CO_{2} = 47.8. Proportions of Mg and Ca vary, and Fe and Mn also occur. _ć_ = α.
▄Usual Appearance in Sections:▄ In rocks chiefly as crystals, even dense homogeneous aggregates showing tendency towards crystalline boundaries (saccharoidal structure). Crystals almost always unit rhombohedron (10̄11) with tendency to curved surfaces.
_Index of Refraction._—_n′_ = 1.622 (α = 1.503, γ = 1.682, γ − α = 0.179), a little higher than that of calcite. For variation in appearance of surface with polarized light, see under calcite.
The microscopic characters are similar to those of calcite, from which it may be _distinguished_ by not being so easily attacked by cold dilute acid (test can be made on slide with cover off), by tendency towards crystalline boundaries, by absence of twin lamellæ (or when present parallel to _−2R._ (20̄21), hence about parallel to shorter diagonals of cleavage rhombs), and by micro-chemical tests. The distinction at times may be very difficult.
REMARKS: Occurs in sedimentary formations and as crystals in limestone and other rocks. In certain rocks the dolomite crystals may not have a very good “bond,” and a “drusy” structure may also be characteristic of the cavities between dolomite crystals in rocks. Only slightly attacked by cold dilute acids, but if acid is heated it dissolves easily with effervescence.
APATITE.
ANISOTROPIC. UNIAXIAL. HEXAGONAL. COMPOSITION: _ć_ = a. ELONGATION ∥ a′. Ca_{5}(Cl.F)(PO_{4})_{3}.
▄Usual Appearance in Sections▄: Minute, slender hexagonal prisms, cross-sections having regular hexagonal boundaries, needles, and grains. Figs. 14 _a_ and 42.
[Illustration:
FIG. 42.—Apatite, showing cross fracture, in nepheline-basalt. (From Cohen.) ]
_Color._—Generally colorless, seldom bluish or brownish (only in eruptive rocks).
_Index of Refraction._—_n′_ = 1.635 (α = 1.634, γ = 1.637), hence _relief_ more marked than that of the colorless associated minerals.
_Cleavage._—Seldom observed microscopically.
_Parting._—Long columnar crystals generally show a transverse jointing, so that the pieces may be more or less separated.
_Inclusions._—Gas and fluid may be present.
▄Polarized Light▄:
_Pleochroism._—None shown by the colorless crystals, the colored crystals show stronger absorption parallel to _ć_.
_▄Crossed Nicols▄_:
_Double Refraction._—Weak (γ − α = 0.003).
_Interference Colors._—The lower first order, generally grayish-blue or white.
_Extinction._—As apatite is uniaxial, basal sections remain dark during rotation of stage. In all other sections extinction is parallel to _ć_ axis.
▄_Convergent light_▄: Basal sections show a cross, without rings. Optical character (−).
▄Alteration▄: Does not usually take place, apatite being found perfectly fresh in decomposed rocks, which is quite remarkable considering its easy solubility in acids.
▄Distinguished from▄:
(_a_) SILLIMANITE and TREMOLITE.—By weak double refraction and elongation ∥ a′.
(_b_) NEPHELITE.—By being relatively much smaller and longer than the nephelite crystals, which are often decomposed. Also by higher relief and negative results with gelatinization test.
(_c_) ZIRCON.—By lower relief and much weaker double refraction.
(_d_) FELDSPARS (when granular and undecomposed).—By higher relief and uniaxial interference figure.
(_e_) VESUVIANITE and ZOISITE.—May be only possible by chemical tests.
(_f_) CORUNDUM.—See under the latter.
REMARKS: Found in most igneous rocks and crystalline schists. In the eruptive rocks it appears as one of the oldest secretions from the magma, and hence is often found as inclusions in other minerals, especially biotite, hornblende, etc. Apatite is easily soluble in hydrochloric and nitric acids. H., 4.5 to 5. Sp. gr., 3.19. On account of its high sp. gr., apatite, in rock powder, comes down in heavy solutions with the metallic minerals, and can be separated from them by the use of a magnet. This residue can also be tested for phosphorus in the wet way with ammonium molybdate.
NEPHELITE,[88] Nepheline, Elæolite.
ANISOTROPIC. UNIAXIAL. HEXAGONAL. COMPOSITION: 7NaAlSiO_{4} + _ć_ = α. NaAl(SiO_{3})_{2}, with partial replacement of Na by K.
▄Usual Appearance in Sections▄: Nephelite in short hexagonal prisms and grains in the younger volcanic rocks, hence sections rectangular or hexagonal, Fig. 43; elæolite allotriomorphic in the older plutonic rocks.
_Color._—Colorless.
_Index of Refraction._—_n′_ = 1.539 (α = 1.538, γ = 1.542), hence no relief and surface smooth.
_Cleavage._—Imperfect, parallel to prism (10̄10) and base (0001). More marked in elæolite than in nephelite, especially when decomposition has commenced.
[Illustration:
FIG. 43.—Nephelite sections, showing zonal inclusions. (From Reinisch.) ]
_Inclusions._—Microscopic needles of augite, etc., also fluid and gas. Mostly in zones. Elæolite may be much clouded by inclusions and alteration products.
▄Polarized Light▄:
_Pleochroism._—None.
_▄Crossed Nicols▄_:
_Double Refraction._—Very weak (γ − α = 0.004), may only be detected by using a test-plate.
_Interference Colors._—The lower first order, grayish-white, etc., a little lower than the feldspar colors.
_Extinction._—As the mineral is uniaxial, basal sections remain dark during rotation of stage. In all other sections extinction takes place and is parallel to cleavage lines when these appear.
_▄Convergent Light▄_: Basal sections show a broad cross, without rings. Optical character (−).
▄Alteration▄: Takes place easily to fibrous zeolites (natrolite), or in certain rocks to mica.
▄Distinguished from▄: OTHER MINERALS by gelatinization test and staining with fuchsine. When present in small interstitial individuals (as is often the case in basalts) it is very difficult to distinguish without this test; but it must be remembered that other minerals, zeolites, etc., will also gelatinize. QUARTZ has stronger double refraction, rarely shows hexagonal outline, is always fresh and optically (+). FELDSPAR is biaxial and often shows twinning. ANALCITE.—See under the latter.
REMARKS: Nephelite bears the same relation to elæolite as sanidine does to orthoclase. It occurs only in the younger volcanic rocks; with sanidine in phonolite, with plagioclase in tephrite, without feldspar in nepheline-basalt, and with leucite in leucite-basalt. It is not found with primary quartz. Elæolite occurs with orthoclase in elæolite-syenite, etc. nephelite and elæolite frequently occur with the sodalite group. Nephelite gelatinizes with acids. H., 5.5 to 6. Sp. gr., 2.5 to 2.6.
TOURMALINE, Schorl.
ANISOTROPIC. UNIAXIAL. HEXAGONAL. COMPOSITION: Uncertain, _ć_ = a. ELONGATION ∥ a′. R_{18}B_{2}(SiO_{5})_{4}. R chiefly Al, K, Mn, Ca, Mg, Li.
▄Usual Appearance in Sections▄: Staff-like individuals, bunched or in radiating aggregates, Fig. 44 B, or prismatic crystals, Fig. 44 A. Basal sections may be nine-sided.
_Color._—Varies greatly, grayish-blue, brown and green most common. Li-tourmaline (rare in rocks) is colorless. Zonal structure may be indicated by differences in color.
[Illustration:
FIG. 44.—_A_, Tourmaline, showing strong absorption at right angles to direction of elongation (_P_ = plane of vibration of polarizer). _Quartzite_, Black Hills, D. _B_, Tourmaline in radiate aggregate. _Granite_, Cornwall. ]
_Index of Refraction._—_n′_ = 1.633 (precious) to 1.674 (α = 1.620 to 1.651, γ = 1.640 to 1.685), hence _relief_ is marked and surface rough.
_Cleavage._—Not seen in thin sections, but irregular, transverse and longitudinal cracks may appear.
▄Polarized Light▄:
_Pleochroism._—Distinct, even in light colored varieties, increasing with the depth of color. The greatest absorption takes place at right angles to the direction of elongation of the crystal, Fig. 44 A. The other minerals having this very strong absorption are hornblende, dark colored mica (distinguished by cleavage and lamellar form) and allanite. Pleochroic halos may be noticed surrounding inclusions.
▄_Crossed Nicols_▄:
_Double Refraction._—Quite strong (γ − α = 0.017 (precious) to 0.034).
_Interference Colors._—Bright upper first or second order, but may not be noticeable, due to absorption of parts of the light.
_Extinction._—As tourmaline is uniaxial, basal sections remain dark during rotation of stage. In all other sections extinction is parallel to _ć_ axis.
▄_Convergent Light_▄: Cross-sections show a sharp cross. Optical character (−).
▄Alteration▄: Does not take place.
▄Distinguished from▄:
(_a_) HORNBLENDE.—By absence of cleavage, and by the fact that the greatest absorption takes place at right angles to the longitudinal axis, while in hornblende it takes place approximately parallel to the longitudinal axis, or to the cleavage lines which are parallel to this axis.
(_b_) APATITE (when colored).—By strong absorption at right angles to longitudinal axis.
(_c_) CORUNDUM.—See under the latter.
In some cases where recognition is difficult, chemical tests, to prove presence of boracic acid, must be made.
REMARKS: The black schorl is the only primary tourmaline and is found in granitoid rocks. Tourmaline in other rocks results from “fumarole” action; hence occurs in pegmatite, tin and copper veins, clay deposits, also (light colored) in contact rocks and crystalline schists. The hemimorphic terminations may sometimes be noticed. Tourmaline is not acted on by acids. H., 7 to 7.5. Sp. gr., 3 to 3.2. It can be separated from powdered rock by sp. gr. solutions combined with magnetic methods.
ANDALUSITE
ANISOTROPIC. BIAXIAL. ORTHORHOMBIC. COMPOSITION: Al_{2}SiO_{5}. _ć_ = a. ELONGATION ∥ a′.
▄Usual Appearance in Sections▄: In short, rounded, prismatic crystals, with almost square cross-section. Colorless or at times pale reddish and spotted. Index of refraction medium (_n′_ = 1.637, α = 1.632, γ = 1.643), hence _relief_ well marked and surface rough. Cleavage, parallel to almost square prism, may show. Pleochroism only marked in colored varieties, being reddish ∥ _ć_ (the direction of elongation or cleavage). Carbonaceous inclusions are characteristic, arranged as in macroscopic specimens (_Chiastolite_), Fig. 45. Pleochroic halos may surround inclusions.
▄Crossed Nicols▄: Double refraction weak (γ − α = 0.001). Interference colors middle 1st order, white to yellow. _Extinction_ in general parallel to _ć_ axis in longitudinal sections, symmetrical in cross-sections. In _convergent light_ Ax. pl. ∥ (010), Bx_{_a0_}. ∥ _ć_; axial angle very large (2_E_ > 180°); optical character (−).
▄Alteration▄: Often takes place to dense aggregate of mica, when the pseudomorph may be hard to recognize.
▄Distinguished from▄:
(_a_) SILLIMANITE by much weaker double refraction, less elongated crystals and by elongation ∥ a′ (Sillimanite elong. ∥ c′).
(_b_) DIOPSIDE by weaker double refraction, rhombic cross-section and parallel extinction in longitudinal sections.
REMARKS: Very characteristic of metamorphic schists and of contact zones of clay slates with granite, etc., but not found in rocks which have been formed at great pressure. The andalusite grains may often be arranged in divergent or finger-like manner. May also form parallel growths with sillimanite. It is insoluble in hydrochloric acid. H., 7 to 7.5. Sp. gr., 3.18.
[Illustration:
FIG. 45.—Chiastolite, showing characteristic carbonaceous inclusions. (From Cohen.) ]
[Illustration:
FIG. 46.—Sillimanite aggregate, showing cross fracture, in mica schist. (From Cohen.) ]
SILLIMANITE, Fibrolite.
ANISOTROPIC. BIAXIAL. ORTHORHOMBIC. COMPOSITION: Al_{2}SiO_{5}. _c_ = c′. ELONGATION ∥ c′.
▄Usual Appearance in Sections▄: Long, slender, colorless prisms or needles; often in felt-like aggregates. Crystals often bent. Index of refraction rather high (_n′_ = 1.664, α = 1.656, γ = 1.677), hence _relief_ marked. Transverse fractures common, Fig. 46.
▄Crossed Nicols▄: Double refraction rather strong (γ − α = 0.021). Interference colors upper first or lower second order, red, purple, blue, etc. _Extinction_ parallel to prisms. Ax. pl. ∥ (100), Bx_{_a_}. ∥ _ć_, 2_E_ = 35° to 55°. Optical character (+).
▄Distinguished from▄:
(_a_) APATITE by higher order interference colors and by elongation ∥ c′ (apatite has elongation ∥ a′).
(_b_) TREMOLITE by always parallel extinction and small size of axial angle.
(_c_) ANDALUSITE, see under the latter mineral.
REMARKS: Found especially in clay-rich contact rocks, gneisses and schists, often occurring with iolite (cordierite). Crystals may appear in bands. It is insoluble in hydrochloric acid. H., 6 to 7. Sp. gr., 3.24.
TOPAZ.
ANISOTROPIC. BIAXIAL. ORTHORHOMBIC. COMPOSITION: _ć_ = c. Al(Al(O.F_{2}))SiO_{4}.
▄Usual Appearance in Sections▄: Colorless crystals of short prismatic habit, grains or rod-like radiating aggregates. Index of refraction about the same as that of calcite (_n′_= 1.608 to 1.632, α = 1.607 to 1.629, γ = 1.618 to 1.637), hence _relief_ medium. Cleavage perfect, parallel to base, but does not show as many cracks. Fluid inclusions abundant.
▄Crossed Nicols▄: Double refraction weak (γ − α = 0.008 to 0.011), about the same as that of quartz. Interference colors middle first order, white, yellow, etc. _Extinction_ parallel to cleavage. In _convergent light_, Ax. pl. ∥ (010), Bx_{_a_}. ∥ _ć_, axial angle large (2_E_ = 86° to 126°); interference figure obtained from basal sections (_i. e._, from sections showing no cleavage); optical character (+).
▄Alteration▄: May take place to kaolin or muscovite, by loss of F and taking up of H_{2}O and alkalies.
▄Distinguished from▄:
(_a_) QUARTZ by higher relief, cleavage and biaxial character.
(_b_) SILLIMANITE (when topaz is in radiating aggregates) by lower refraction and double refraction.
REMARKS: Common in greisen and all granite rocks containing tin ore. When formed by “fumarole” action (tin veins) the mineral shows rod-like radiating forms. It is insoluble in hydrochloric acid. H., 8. Sp. gr., 3.5.
STAUROLITE.
ANISOTROPIC. BIAXIAL. ORTHORHOMBIC. COMPOSITION: ELONGATION ∥ ć. Fe(AlO)_{4}(AlOH)(SiO_{4})_{2}, but varying, may contain Mg or Mn. _ć_ = c.
[Illustration:
FIG. 47.—Staurolite, showing twinning at 90° _b_ and 60° _c_, also granular quartz inclusions. (From Reinisch.) ]
▄Usual Appearance in Sections▄: Short, flat prisms, which may be twinned at 90° or 60°, Fig. 47, or grains. Color yellowish to reddish-brown. Index of refraction rather high (_n′_ = 1.741, α = 1.736, γ = 1.746), hence _relief_ marked and surface rough. Cleavage, both prismatic and pinacoidal, variable. Inclusions of minute quartz grains and carbonaceous matter found in larger crystals, but not in microscopic crystals. Pleochroism distinct but not strong, showing red ∥ _c_ (direction of elongation). Pleochroic halos may surround inclusions.
▄Crossed Nicols▄: Double refraction weak (γ − α = 0.010). Interference colors middle first order, white to yellow, etc. (about like quartz). _Extinction_ in general parallel or symmetrical (in cross-sections) to cleavages or crystal outline. In _convergent light_, Ax. pl. ∥ (100), Fig. 48. Bx_{_a_}. ∥_ć_, axial angle large (2_E_ > 180°); optical character (+).
[Illustration:
FIG. 48.—Staurolite, cross-section. ]
▄Alteration▄: Rarely takes place.
▄Distinguished from▄: TITANITE, see under the latter mineral.
REMARKS: Found in metamorphic schists, associated with cyanite (disthene), iolite (cordierite), andalusite, etc. It is one of the minerals produced by thermal metamorphism, hence found in rocks of granite contact zones. It does not occur in the eruptive rocks or in schists rich in amphibole. Staurolite is not acted on by hydrochloric acid. H., 7 to 7.5. Sp. gr., 3.4 to 3.8.
THE ORTHORHOMBIC PYROXENES. Enstatite and Hypersthene.
ANISOTROPIC. BIAXIAL. ORTHORHOMBIC. COMPOSITION: (Mg.Fe)SiO_{3}. _ć_ = c. ELONGATION ∥ c′.
Enstatite contains little, if any, Fe. Hypersthene contains more Fe, its optical characters beginning to show with about 10 per cent.
[Illustration:
FIG. 49.—Enstatite, showing columnar or fibrous structure ∥ _ć_ axis. _Norite_, Harzburg. ]
▄Usual Appearance in Sections▄: Irregularly bounded individuals (E) or rounded prismatic-pyramidal crystals (H). Columnar or fibrous structure ∥ _ć_ often shows in (E), Fig. 49. Prism angle about 92°. Outline of crystal sections very similar to that of monoclinic pyroxenes.
_Twinning._—Not so common as in monoclinic pyroxenes. Parallel growths with monoclinic pyroxene (diallage) occur.[89]
_Color._—Varies with Fe per cent., (E) colorless, (H) brownish.
_Index of refraction._—_n′_ = 1.665 (E) to 1.723 (H) (α = 1.660 to 1.716, γ = 1.670 to 1.729) (about the same as in monoclinic pyroxene), hence _relief_ marked and surface rough.
_Cleavage._—Variable, parallel to prism (angle 92°) common to all pyroxenes. Also cleavage or parting parallel to brachy pinacoid (010) (prominent) and macro pinacoid (100).[89]
_Inclusions._—Parallel oriented, brownish plates and rods, producing “schiller” structure on the principal cleavage faces, Fig. 15. Glass inclusions abundant in (H).
▄Polarized Light▄:
_Pleochroism._—Almost absent in (E), but distinct in (H), increasing with Fe per cent. The change in color may be very marked, from brownish-red to greenish ∥ _ć_.
_▄Crossed Nicols▄_:
_Double Refraction._—Weak, much weaker than in the monoclinic pyroxenes, increasing with Fe per cent. (γ − α = 0.010 (E) to 0.013 (H).)
_Interference Colors._—Higher first order, about the same or a little higher than quartz.
_Extinction._—Parallel to cleavages in longitudinal sections, which are parallel to _a_ or _b_, and bisecting angles of intersecting prismatic cleavages in basal sections.
_▄Convergent Light▄_: Axial plane parallel to brachy pinacoid (010),[89] _i. e._, parallel to best pinacoidal cleavage. Bx_{_a_}. ∥ _ć_ (E), ∥ _a_(H). Axial angles large (2_E_ = 95° to > 180°). Optical character for (_E_)(+), for (_H_)(−). On account of weak double refraction the interference figures are not very marked.
▄Alteration▄: Takes place to bastite, serpentine, etc.
▄Distinguished from▄: The MONOCLINIC PYROXENES and AMPHIBOLES.—See under these species.
REMARKS: Found in the granular rocks of the gabbro-peridotite series, also in the olivine basalts (E); and in crystals in porphyritic andesite (H). These minerals are in general not attacked by acids. H., 5 to 6. Sp. gr., 3.1 to 3.5.
_Bronzite_ is the name give to the variety containing about 5% Fe and having the characteristic bronzy lustre due to inclusions.
_Bastite_ (an alteration product of the orthorhombic pyroxenes poor in Fe).—Composed of fibers, often traversed by irregular cracks. Color light yellowish or greenish and index of refraction about the same as Canada balsam. Pleochroism faint (only seen in thick sections), the greatest absorption taking place parallel to the fibers. Double refraction weak and extinction parallel to the fibers. Axial angle large and axial plane at right angles to principal cleavage face (010). The position of the axial plane is the surest distinction between bastite and the orthorhombic pyroxenes.
CHRYSOLITE, Olivine.
ANISOTROPIC. BIAXIAL. ORTHORHOMBIC. COMPOSITION: (Mg.Fe)_{2}SiO_{4}. ELONGATION ∥ a′ or c′.
▄Usual Appearance in Sections▄: Prismatic crystals or in large angular fragments or grains. Longitudinal sections more or less lath-shaped, with pointed ends, Figs. 50 and 51, cross-sections six or eight-sided. Outlines of crystals often rounded or corroded. Skeleton forms may occur, and sometimes twinning may be observed.
[Illustration:
Chrysolite.
FIG. 50. FIG. 51. Basal section. Macro pinacoid section. ]
_Color._—Nearly colorless, may be reddish (with high Fe per cent.).
_Index of Refraction._—_n′_ = 1.675 (α = 1.661, γ = 1.697), hence _relief_ marked and surface rough.
_Cleavage._—Parallel to brachy pinacoid (010), less distinct parallel to macro pinacoid (100), Fig. 50. Often only made visible by decomposition. An irregular fracturing occurs, which increases with alteration into serpentine.
_Inclusions._—Chromite, opaque earths, apatite and the brown plates so common in hypersthene; also glass and slag (in basaltic rocks) and fluid (in peridotites and olivinfels).
▄Polarized Light▄:
_Pleochroism._—In general none, but noticed in the reddish varieties, when the absorption is a little stronger parallel to _ć_.
▄_Crossed Nicols_▄:
_Double Refraction._—Very strong (γ − α = 0.036).
_Interference Colors._—Rather high in order (second or third), higher than the colors of augite.
_Extinction._—In general parallel to cleavage lines.
_▄Convergent Light▄_: Axial plane parallel to base (001) and always at right angles to cleavage cracks, Fig. 51. Bx_{_a_}. ∥ _a_. Axial angle very large (2_E_ > 180°). Optical character (+).
▄Alteration▄: Into serpentine very common,[90] producing “mesh-” or “lattice-” structure (see under serpentine, p. 114); also into amphibole, etc. In certain basaltic rocks the rims of grains may be changed into gœthite?, and in certain gabbros the crystals may be surrounded by a radial rim of amphibole.
▄Distinguished from▄:
Light colored MONOCLINIC PYROXENES.— By the absence of extinction angles, cleavage (the intersecting prismatic cleavages of augite being of equal distinctness), stronger double refraction and by axial plane being parallel to base, hence always at right angles to best cleavage (in augite axial plane lies in clino pinacoid, bisecting angles of intersecting prismatic cleavages). Also by gelatinization with acids.
REMARKS: Found only in basic rocks, as peridotite, diabase, gabbro, norite, basalts, etc. Chrysolite (olivine) is a very brittle mineral and shows under mountain making pressure “cataclastic” structure. Chromite is a characteristic associated mineral. When not too poor in Fe, chrysolite becomes permanently red and pleochroic when strongly heated. Chrysolite is decomposed by hydrochloric and sulphuric acids, with separation of gelatinous silica. H., 6.5 to 7. Sp. gr., 3.3 to 3.4.
_Hyalosiderite_ (a more ferruginous chrysolite) and _Fayalite_ (Fe_{2}SiO_{4}) may be reddish in sections, and common in the basic porphyritic eruptive rocks.
IOLITE, Cordierite, Dichroite.
ANISOTROPIC. BIAXIAL. ORTHORHOMBIC. COMPOSITION: _ć_ = a. ELONGATION ∥ a′. Mg_{3}(Al.Fe)_{6}Si_{8}O_{28}.
▄Usual Appearance in Sections▄: Grains, more rarely crystals of short prismatic habit, which often form pseudo-hexagonal interpenetration twins. Crystals may have edges rounded or corroded. Colorless, but may be bluish. Index of refraction a little lower than quartz (_n′_ = 1.539, α = 1.535, γ = 1.544), hence _relief_ low and surface smooth. Cleavage very variable, parallel to brachy pinacoid (010), especially noticeable when decomposition has taken place. Inclusions of sillimanite, zircon, rutile, etc., may be seen. Pleochroism usually not observed, but noticed in blue sections (yellowish white ∥ _ć_ to blue). Pleochroic halos (yellow) surrounding inclusions common, see p. 59.
▄Crossed Nicols▄: Double refraction weak (γ − α =0.009), like quartz. Interference colors middle first order, white to yellow. _Extinction_ in general parallel to cleavage cracks. In _convergent light_, Ax. pl. ∥ (100), Bx_{_a_}. ∥ _ć_; axial angle large (hyperbolas only seen without ellipses) (2_E_ = 64° to 150°); optical character (−).
▄Alteration▄: Takes place readily, forming greenish mica-like decomposition products, the decomposition commencing along the crevices or about the inclusions.
▄Distinguished from▄: QUARTZ by observation in convergent light (quartz is uniaxial), decomposition and pleochroism or pleochroic halos. The section can also be treated with hydrofluosilicic acid, when the evaporated solution yields characteristic prismatic crystals of magnesium fluosilicate.
REMARKS: Found in gneiss, hornstone, granite, granulite, etc., and in some volcanic rocks. It is often associated with garnet, biotite, sillimanite, etc. In a thick section heating to redness makes the pleochroism more distinct. Iolite is only slightly acted on by acids. H., 7 to 7.5. Sp. gr., 2.6. It is hard to make a mechanical separation from quartz, on account of similarity in sp. gr.
NATROLITE.
ANISOTROPIC. BIAXIAL. ORTHORHOMBIC. COMPOSITION: _ć_ = c. ELONGATION ∥ c′. Na_{2}Al_{2}Si_{2}O_{10} + 2H_{2}O.
▄Usual Appearance in Sections▄: Aggregates of colorless, fibrous crystals, which may have sphærulitic structure, showing a dark cross between crossed nicols. Index of refraction lower than balsam (_n′_ = 1.483, α = 1.478, γ = 1.490), hence (in large crystals) the surface would appear rather rough.
▄Crossed Nicols▄: Double refraction weak (γ − α = 0.012). Interference colors the middle first order (yellow, etc.), a little higher than those of quartz. _Extinction_ parallel to fibres. Optical character (+).
REMARKS: Never a primary mineral in rocks, but found in igneous rocks filling amygdaloidal cavities, and also as a very common alteration product of sodalite, noselite, nephelite and acid plagioclases. It gelatinizes easily with hydrochloric acid. H., 5 to 5.5. Sp. gr., 2.2.
OTHER ZEOLITES.
COMPOSITION: Hydrous silicates; Al, Ca and Na being the chief bases.
▄Usual Appearance in Sections▄: The form depends on the individual mineral species, but the majority appear in elongated crystals or fibers. They are all colorless and most of them have a small index of refraction, hence no _relief_ (prehnite has distinct relief).
▄Crossed Nicols▄: The double refraction is generally very weak (between that of nephelite and quartz), giving very low order interference colors (prehnite and thomsonite have strong double refraction).
REMARKS: The zeolites are always secondary minerals in rocks. They gelatinize with hydrochloric acid.
GYPSUM.
ANISOTROPIC. BIAXIAL. MONOCLINIC. COMPOSITION: CaSO_{4} + 2H_{2}O.
▄Usual Appearance in Sections▄: Colorless grains or fibers. May be colored, however, by inclusions of carbonaceous matter, iron oxides, etc. Index of refraction about the same as orthoclase (_n′_ = 1.525, α = 1.521, γ = 1.531), hence no _relief_ and surface smooth. Twinning lamellæ abundant. Cleavage parallel to (010) gives abundant cracks, other cleavages may also be noticed.
▄Crossed Nicols▄: Double refraction weak (γ − α = 0.010), the same as quartz. Interference colors middle first order, white to yellow. _Extinction_ parallel to most perfect cleavage cracks in sections parallel to _b_ axis; large extinction angles noticed with reference to less perfect cleavages. In _convergent light_, Ax. pl. ∥ (010), _i.e._, ∥ to most distinct cleavages; Bx_{_a_}. (c) Λ = 54° front; 2_E_ = 104°; optical character (+). As the characters of gypsum are not always very marked it may be necessary to employ micro-chemical tests.
REMARKS: Forms a rock by itself, often associated with rock salt. It also occurs as an alteration product of anhydrite. Gypsum is soluble in hydrochloric acid. H., 1.5 to 2. Sp. gr., 2.2 to 2.4.
MONOCLINIC PYROXENES, Augite, etc.
Including the monoclinic minerals of the Pyroxene Group, which show distinctly the characteristic cleavage parallel to an almost right-angled prism.
ANISOTROPIC. BIAXIAL. MONOCLINIC. ELONGATION ∥ c′.[91]
COMPOSITION: RSiO_{3}, R = Ca, Mg, Mn, Fe, Al chiefly, with the Ca predominating over the Mg.
▄Usual Appearance in Sections▄: Both in crystals and more or less irregular grains, Figs. 4 and 12, the habit varying with the chemical composition as follows:
_Diopside_ (Ca, Mg varieties), long columnar crystals and grains.
_Augite_ (ditto, but containing also Al and Fe), short prismatic crystals and grains.
_Diallage_, granular or lamellar (|| (100)), may show fibrous structure ∥ _ć_.
Prism angle = 87° 06′ (important in cross-sections). Sections of crystals nearly at right angles to the vertical axis _ć_ are octagonal or square with truncated corners, Figs. 4 and 53, while those parallel to the _ć_ axis are lath-shaped. Pyroxene also occurs in skeleton crystals and acicular microlites in eruptive rocks.
[Illustration:
FIG. 52.—Augite, section parallel to _ć_ axis showing prismatic cleavage, in leucite-basal. (From Cohen.) ]
Zonal structure (especially in augites) may be marked by differences in color or extinction, and in some basalts the crystals have the “hour-glass” structure.
_Twinning._—Common, usually the twinning plane being the ortho pinacoid (100). Twin lamellæ may be noticed. Intergrowths occur with orthorhombic pyroxene and amphibole.
_Color._—From almost colorless through green (diopsides, Na pyroxenes, etc.) to brown (augites); the red to brownish-red color of certain augites has been considered due to manganese. Yellow color very rare.
_Index of Refraction._—_n′_ = 1.68 to 1.72 (α = 1.671 to 1.706, γ = 1.700 to 1.728), hence _relief_ high and surface rough.
_Cleavage._—More or less perfect parallel to prism of 87° 06′. Cleavage cracks distinct and numerous, but not generally running uninterruptedly through crystal, Figs. 12 and 52. Cleavage not so perfect as that of amphibole.
_Parting._—Diallage and diopside have distinct parting parallel to ortho pinacoid (100), Fig. 53. Some crystals may show parting parallel to base (001).
_Inclusions._—Tabular microscopic interpositions, similar to those in bronzite, may occur in diallage. The iron ores, apatite, etc., may occur in augite.
▄Polarized Light▄:
[Illustration:
FIG. 53.—Diallage, cross-section. ]
_Pleochroism._—Usually not noticed, and in general only appearing as different shades of the same color. In some cases (diallage, fassaite and Na rich augite) well marked, a and c green to yellowish green and b brownish to reddish-brown; hence pleochroism not intense in sections showing extinction angles. When Ti is present, violet parallel to _b_.
▄_Crossed Nicols_▄:
_Double Refraction._—Strong (γ − α = 0.022 to 0.029), being stronger in the pale or colorless pyroxenes.
_Interference Colors._—Second order, hence always bright tints.
_Extinction._—Symmetrical in sections (through _b_ axis) showing intersecting cleavage lines, in such cases bisecting the angles of the cleavage. In sections showing parallel cleavage lines, only parallel in ortho pinacoid (100) sections, in all other sections an extinction angle being observed. The maximum extinction angle is large, lies in the obtuse angle, varies with the chemical composition from 36° 30′ to 54°, and is only obtained when the section of the crystal is parallel to the clino pinacoid (010), Fig. 54, varying from this angle to 0°, when the section is parallel to the ortho pinacoid (100). In Ti and Na pyroxenes the inclined dispersion is so great that extinctions are not sharp, but instead a change takes place in the interference color from bluish to brownish.
▄_Convergent Light_▄: Axial plane parallel to clino pinacoid (010). Fig. 54. A cleavage flake parallel to ortho pinacoid (100) shows the emergence of an optic axis (orthorhombic pyroxene parallel to best pinacoidal cleavage would not show figure). Bx_{_a_}.(c) Λ _ć_ = 36° to 54° front. Axial angles large (2_E_ = 70° to 112°). Optical character (+). The interference figures are distinct on account of the strong double refraction.
▄Alteration▄: May take place to chlorite, serpentine or amphibole (uralitization[92]), depending on the chemical composition and the conditions producing the change.
[Illustration:
FIG. 54.—Diopside, clino pinacoid section. ]
▄Distinguished from▄:
(_a_) ORTHORHOMBIC PYROXENES.—By extinction angle, the orthorhombic pyroxenes having always parallel or symmetrical extinction in sections parallel to _a_, _b_, or _c_, and by higher order interference colors. Also from hypersthene by absence of, or much fainter, pleochroism. Diallage and bronzite might be confused on account of pronounced pinacoidal parting, fibrous structure and inclusions; but may be distinguished by the presence or absence of extinction angles and also by the position of the optic axes relative to the best cleavage plates.
(_b_) AMPHIBOLE.—See under amphibole.
(_c_) EPIDOTE and CHRYSOLITE (Olivine). When light colored and granular, by examination in convergent light. The plane of the optic axes is parallel to the clino pinacoid (010), hence to the longitudinal axis and prismatic cleavage cracks, while in epidote it is at right angles to these directions and in chrysolite parallel to the base. Also yellow color is common in epidote but rare in pyroxene.
REMARKS: Next to the feldspars pryoxene is the most common constituent of the igneous rocks. Diopside and fassaite (green) are found in contact rocks; also, what appear to be the same pyroxenes, in many eruptive rocks, as andesites, monzonites, etc. Malacolite (light green) is found in amphibolites and eclogites, where it may be associated with a greenish amphibole (smaragdite). Diallage (bladed and twinned) occurs in gabbros and pyroxenites. Common augite (brown) is found in the remaining basic eruptive rocks. In the schists the pyroxene is colorless.
Finally augite occurs as a secondary product resulting from the “magmatic resorption” of hornblende and biotite.
Chemical corrosion and mechanical deformation may occur. The green and brown augites when heated to redness on platinum foil may become red in color. In general the pyroxenes are not attacked by acids. H., 5 to 6. Sp. gr., 3.3 to 3.5. The sp. gr. of the pyroxenes is considerably higher than that of the amphiboles of similar composition, hence mechanical separations are possible.
_Acmite_ (_Ægirine_) (Na pyroxenes).—Occur in green or brown, elongated prismatic crystals, often not very transparent and with marked pleochroism (like amphibole). Zonal coloring is common. When zonally intergrown with pyroxene the outer zone is ægirine. The elongation is ∥ a′ (distinction from amphibole whose elongation is ∥ c′). The index of refraction is higher than in the other pyroxenes (_n′_ = 1.792, α = 1.763, γ = 1.813) and the double refraction stronger (γ − α = 0.050). The _extinction_ angle is small (5°) and the optical character (—).
The term _Ægirine-augite_ may be used to describe a soda, pleochroic augite with a large extinction angle.
These pyroxenes are only found in the eruptive rocks rich in alkalies, as elæolite-syenite, phonolite, certain trachytes, etc.; hence are associated with elæolite, sodalite, leucite, etc. The small, second generation, crystals, in the ground mass of a rock, are always the richest in Na of the pyroxenes in that rock.
AMPHIBOLE, Hornblende, etc.
ANISOTROPIC. BIAXIAL. MONOCLINIC. ELONGATION ∥ c′.
COMPOSITION: RSiO_{3}. R = Mg, Ca, Fe chiefly also may contain Al, Na, Mn. The Mg predominates over the Ca.
[Illustration:
FIG. 55.—Hornblende, showing twinning between crossed nicols, in amphibole-biotite-granite. (From Cohen.) ]
▄Usual Appearance in Sections▄: Both in crystals and more or less irregular grains, often fibrous, Figs. 55 and 56, the habit varying with the chemical composition as follows:
_Tremolite_ (Mg_{3}Ca) and _Actinolite_ ((MgFe)_{3}Ca varieties), in long columnar to needle-like individuals, with no terminal planes or with frayed out ends. May be in dense aggregates.
_Pargasite_ in well developed crystals.
_Common green Hornblende_ (aluminous varieties) in crystals, compact grains or shreds.
_Basaltic Hornblende_ (iron rich, aluminous varieties) in prismatic crystals of varying length, which may often show “magmatic resorption” (to augite and magnetite) around outer zone or throughout whole crystal.
[Illustration:
FIG. 56.—Hornblende, section parallel to _ć_ axis, showing prismatic cleavage, in hornblende-diorite. (From Cohen.) ]
Crystals are simple in form of prismatic habit, with prism angle 124° 30′. Cross-sections are acutely rhombic, generally with acute angles truncated, hence six-sided (pyroxene being eight-sided). Longitudinal sections are lath-shaped and fibrous structure may be noticed. Skeleton crystals may also occur, being very fine in certain pitchstones.
Zonal structure and parallel growth may be noticed in the amphiboles.
_Twinning._—Frequent, parallel to ortho pinacoid (100). Twins dual, less often multiple, Fig. 55. Intergrowths with pyroxene and biotite occur.
_Color._—From colorless (tremolite), through green (actinolite, pargasite and hornblende) to brown (basaltic hornblende). Yellow in some varieties and bluish in the soda varieties.
_Index of Refraction._—_n′_ = 1.621 to 1.641 (α = 1.607 to 1.629, γ = 1.634 to 1.653) (1.719, in the basaltic hornblende), hence _relief_ distinct and surface rough.
_Cleavage._—Perfect, parallel to prism of 124° 30′. Generally appears in thin sections as sharp cracks crowded close together, Figs. 56 and 57. More perfect than in pyroxene.
Some of the long prisms (actinolite and tremolite) may show transverse parting.
_Inclusions._—The iron ores, apatite, etc., may be found in hornblende.
[Illustration:
FIG. 57.—Hornblende, cross-section. ]
▄Polarized Light▄:
_Pleochroism._—All colored amphiboles show pleochroism, which in general is stronger the darker the color of the variety (actinolite and pargasite show but little). The absorption is very marked in the hornblendes, being greatest in the general direction of the cleavage lines in longitudinal sections. Marked differences in absorption are also characteristic of the mineral species biotite, tourmaline and allanite. Pleochroic halos (brownish) surrounding inclusions may be noticed.
_▄Crossed Nicols▄_:
_Double Refraction._—Quite strong, but a little weaker than in pyroxene (γ-α = 0.019 to 0.027). Ferruginous basaltic hornblende has strong double refraction (γ-α = 0.072).
[Illustration:
FIG. 58.—Actinolite, clino pinacoid section. ]
_Interference Colors._—Second order, hence bright tints, but in darker colored varieties not so noticeable as in pyroxenes, due to the stronger absorption of parts of the light. The colors of basaltic hornblende are so high that they show no bright tints.
_Extinction._—Always symmetrical in sections (through _b_ axis) showing intersecting cleavage lines, in such cases bisecting the angles of the cleavage. In sections showing parallel cleavage lines, only parallel in ortho pinacoid (100) sections, in all other sections an extinction angle being observed. The maximum extinction angle lies in the acute angle and is much smaller than in pyroxene, varying with the chemical composition from 0°–20°. In hornblende, actinolite and tremolite 12°–20°, Fig. 58; in the basaltic hornblende 0°–10°. The maximum extinction angle is only obtained when the section of the crystal is parallel to the clino pinacoid (010), varying from this angle to 0°, when the section is parallel to the ortho pinacoid (100).
_▄Convergent Light▄_: Axial plane parallel to clino pinacoid (110), Fig. 58. Bx_{_o·_}(c) Λ _ć_ = 0°–20° behind. Axial angles large (2_E_ = 77° to >180°). Optical character (−). Pargasite is (+).
▄Alteration▄: May take place to chlorite, talc, serpentine, asbestus, etc., depending on the chemical composition. Amphibole frays out and becomes fibrous during alteration, and may also lose color.
▄Distinguished from▄:
(_a_) PYROXENE.—By usually much stronger pleochroism in the colored varieties, and by cleavage and extinction angle. In pyroxene the cleavage (parallel to prism of 87° 06′) is less perfect; and the extinction angle is much larger, varying from 36° to 54°.
(_b_) BIOTITE.—By the extinction in the mica being always about parallel to the cleavage. Both have strong absorption, but biotite shows very slight pleochroism in sections parallel to the cleavage, and has only the one cleavage parallel to the base. Also the biotite has lower index of refraction and generally shows uniaxial interference figure.
Colorless tremolite may be distinguished from muscovite and talc by extinction angles, relief and lower order interference colors.
(_c_) TOURMALINE.—By presence of cleavage, and by the fact that absorption is most marked about parallel to the elongation (also parallel to cleavage lines), while in tourmaline the absorption is strongest at right angles to the elongation.
(_d_) The ORTHORHOMBIC PYROXENES.—By extinction angles, the latter having parallel extinction in all sections parallel to _a_, _b_ and _c_, and by prismatic cleavage of 124° 30′. Pleochroism is strong in the colored varieties of both species, but in amphibole it appears more generally as a variation of the same color; while in hypersthene a change in color is often noticed, from brownish-red to greenish parallel to _ć_ axis.
(_e_) SILLIMANITE and CYANITE.—See under the latter.
REMARKS: Amphibole comes next to pyroxene in importance and distribution of the dark colored ferruginous rock-forming minerals. As a rule it occurs in rocks with a large percentage of SiO_{2}, associated with quartz and orthoclase; while augite generally occurs in rocks of a basic nature, associated with plagioclase and little or no free SiO_{2}. Furthermore amphibole contains hydroxyl and is therefore naturally found in the deep eruptive rocks; its place being taken by augite in the effusives. By application of heat hornblende changes to augite, while hydrochemical processes bring about the opposite result “uralitization.”
Tremolite and actinolite are found in contact rocks and crystalline schists, also as a result of the alteration of olivine into serpentine. Pargasite occurs in contact rocks. Common green hornblende is found in the plutonic rocks (Na poor and SiO_{2} rich), also in contact rocks and crystalline schists (amphibolites). Brown hornblende replaces the green variety in the basic plutonic rocks. Basaltic hornblende is found in many effusive rocks.
The hornblende crystals in eruptive rocks, being among the first formed constituents, have often suffered subsequent corrosion by the magma, giving rise to the dark border already mentioned. The brown primary hornblende in some rocks may be changed by a process analogous to “uralitization” into a green, reed-like hornblende. Mechanical deformations are found in massive and schistose rocks. Light green amphiboles, with weak pleochroism, may often be colored intensely reddish-brown and made strongly pleochroic by heating to redness on platinum foil. In general the amphiboles are not affected by acids. H., 5 to 6. Sp. gr., 2.9 to 3.3.
_Glaucophane_, _Arfvedsonite_, etc. (Na rich amphiboles).—Occur blue to bluish-green in color, with pleochroism and weaker double refraction than the other amphiboles. Extinction angles vary from 4°–6° (glaucophane) to 14° (arfvedsonite). They are found in contact rocks, crystalline schists, eclogite, etc.
For the rarer and less known members of the amphibole group, resource should be had to more elaborate works.
_Uralite._—Pyroxene altered to amphibole, having the outward crystal form of pyroxene and the physical characters and cleavage of amphibole. The change usually commences on the surface and the uralite does not form a single compact crystal, but consists of numerous slender columns parallel to one another. These little columns or fibers have their _c_ and [_=b_] axes parallel to the positions of these axes in the parent mineral. The color is green and the pleochroism weak to strong.
This change is called “_uralitization_” and results from hydrochemical processes. When the alteration is not complete, portions of the original pyroxene may be left, having all the characteristic optical properties of this latter mineral.
_Anthophyllite_, the orthorhombic amphibole, with always parallel extinction, is sometimes found in colorless to brownish, blade- to rod-like aggregates in crystalline schists and serpentine.
MICA GROUP.
ANISOTROPIC. BIAXIAL. MONOCLINIC. May appear hexagonal or orthorhombic. COMPOSITION: ELONGATION (∥ cleavage) ∥ c′.
_Biotite_ (black or ferro-magnesium mica) = (H.K)_{2}(Mg.Fe)_{2}Al_{2}(SiO_{4})_{3}, approx.
_Phlogopite_ = a magnesium mica, near biotite, but containing little Fe.
[Illustration:
FIG. 59.—Mica. _A_, biotite, showing hexagonal cross-section and zonal markings. _Minette_, Freiburg. _B_, Biotite, showing strong absorption parallel to cleavage and also zonal marking (_P_ = plane of vibration of polarizer). _Minette_, Cumberland. _C_, Muscovite in bent shreds in gneiss. ]
_Muscovite_ (white or potash mica) = H_{2}(K.Na)Al_{3}(SiO_{4})_{3}, with some replacement by Mg or Fe.
▄Usual Appearance in Sections▄: Scales, which may be notched or jagged, with lateral sections lath-shaped; or shreds, Fig. 59 C. When distinctly crystallized (magnesium micas) the thin hexagonal plates have plane angles of 120°, Fig. 59 A. Phlogopite crystals may be extended in direction of _ć_ axis.
Zonal structure not uncommon in the magnesium micas, Fig. 59 A, which may also have dark iron ore border like hornblende.
_Twinning._—Common, generally parallel to base; seen in sections showing cleavage by variations in extinction, in basal sections by distorted interference figures.
Micas of different kinds often associated together in parallel position, also intergrown with hornblende, pyroxene, chlorite and quartz.
_Color._—Depends on chemical composition. Biotites, brown, green or red to almost opaque. Phlogopites, colorless or yellowish. Muscovites colorless.
_Index of Refraction._—_n′_ = 1.564 to 1.619 (α = 1.541 to 1.580, γ = 1.575 to 1.638), hence somewhat marked _relief_ and surface varies in appearance from slightly rough to fairly rough. In polarized light the surface appears roughest when the cleavage cracks are parallel to the plane of the polarizer.
Biotite has more marked relief.
_Cleavage._—Very perfect, parallel to base (001), Fig. 60. Basal sections show no cleavage, but all other sections show many sharp, parallel cleavage cracks.
[Illustration:
FIG. 60.—Biotite, showing basal cleavage, in biotite-granite. (From Cohen). ]
For percussion and pressure figures, see reference given below.[93]
_Inclusions._—May be arranged parallel to lines of pressure figure. Rutile needles, tourmaline, apatite, etc., common in magnesium mica. Zircon inclusions often surrounded by pleochroic halos.
▄Polarized Light▄:
_Pleochroism._—Varies with the color, being very marked in the colored varieties (from pale yellow to chestnut-brown or black). The strong absorption, about parallel to the cleavage lines, is very characteristic of the colored mica, Fig. 59 B. Strong absorption is also noticed in hornblende, tourmaline and allanite. Absorption may even be noticed around inclusions (pleochroic halos) in colorless, non-pleochroic micas. Cleavage plates of biotite are not pleochroic unless the axial angle is large.
_▄Crossed Nicols▄_:
_Double Refraction._—Very strong (γ − α = 0.034 to 0.058).
_Interference Colors._—High order (third). May be very bright in thin sections of the colorless micas, and at times be so high in order as not to show any marked color tints. May not be noticeable in sections of the colored varieties, due to absorption of parts of the light.
_Extinction._—About parallel to cleavage lines. Very small extinction angles may be noticed in biotites. Basal sections of biotite (the approximately hexagonal mica) usually appear isotropic.
Mottled appearance (like “Birds-eye” maple) characteristic, caused by distortion of the flexible laminæ during grinding. Most noticeable in sections inclined to cleavage and near position of extinction.
_▄Convergent Light▄_: Axial plane[94] and Bx_{_a_}, practically at right angles to basal cleavage; therefore cleavage plates always show well defined interference figures, generally biaxial in character. The axial angles vary greatly, being usually small for biotite and phlogopite (may appear uniaxial) and large for muscovite (2_E_ = 55° to 90°). Optical character for all micas (−).
▄Alteration▄: Biotites decompose quite easily, lose color and may become completely bleached, which appears to be due to a leaching out of the iron. May also alter to green chlorite, with a fraying out of the mica and a change to chloritic structure.
Phlogopites may alter to fibrous, scaly masses, apparently chiefly talc. “Sagenite” webs of rutile may accompany the alteration.
Muscovites are characterized by their freshness, and do not seem to suffer from weathering.
▄Distinguished from▄:
(_a_) HORNBLENDE.—Magnesium mica has extinction about parallel to the cleavage, while hornblende may have extinction angles of from 0° to 20°. Both have strong absorption, but biotite shows very slight pleochroism in basal sections, which also give approximately uniaxial interference figures in convergent light.
(_b_) TOURMALINE.—Magnesium mica shreds show absorption parallel to elongation, while in tourmaline the absorption is at right angles to elongation. There is also an absence of cleavage in tourmaline.
(_c_) CHLORITE.—By strong double refraction, the very high order colors, however, being often not noticed. Chlorite also shows aggregate structure and is almost always greenish in color.
(_d_) TALC.—White mica by large axial angle of scales in convergent light and by micro-chemical tests. The distinction may be very difficult.
REMARKS: Muscovite is a rare primary mineral in eruptive rocks, except in two-mica granite, etc. As a secondary mineral it occurs in dense scaly aggregates or as pseudomorphs after feldspar, nephelite, etc. It is frequent in crystalline schists and is probably also the mica in amphibolite and eclogite. Phlogopite is found chiefly in contact metamorphic limestone; and may be distinguished from muscovite by nearly uniaxial character and less sharp cleavage. Biotite is much more widely distributed, occurring especially in eruptive rocks, crystalline schists and contact rocks.
Chemical corrosion occurs in original biotite of porphyritic rocks, producing a “resorption border” of augite and magnetite. Mechanical deformations, producing bending and slipping along “gliding” planes (oblique to cleavage), are common to all varieties of mica and may produce change to chlorite. The muscovites, together with the feldspars, are the most characteristic minerals of dynamo metamorphic origin. Biotites and phlogopites are attacked by sulphuric acid at high temperatures. Muscovite is but slightly attacked by acids. H., 2 to 3. Sp. gr., 2.7 to 3.2. The specific gravity separation between the micas is difficult on account of the scaly nature of the minerals.
Other micas occur, some being alteration products of those already described. Among these may be mentioned:
_Lithia Mica._—Both light and dark colored, occurring in granitic rocks and often only distinguished chemically from muscovite and biotite.
_Damourite_ (_Sericite_) (hydrous K mica).—A secondary product usually in colorless, fine scaly aggregates in phillites, sericite-schists, etc.
CHLORITE GROUP.
Embracing the members of the Chlorite Group, commonly occurring in rocks.
ANISOTROPIC. BIAXIAL. MONOCLINIC.
The minerals of this group usually appear uniaxial, and crystallize in part with hexagonal symmetry.
COMPOSITION: May be considered as isomorphous mixtures of H_{4}(MgFe)_{3}Si_{2}O_{9} and H_{4}(MgFe)_{2}(AlFe)_{2}SiO_{9} (Rosenbusch).
▄Usual Appearance in Sections▄: Minute, scaly aggregates, which may incline to radial grouping; or in minute grains as a pigment (veridite) in other minerals.
_Twinning._—May be seen as in mica.
_Color._—Generally green, varying from greenish white to dark green, rarely colorless or red.
_Index of Refraction._—_n′_ = 1.567 to 1.589 (α = 1.560 to 1.585, γ = 1.571 to 1.596), hence no marked relief and only slightly rough surface.
_Cleavage._—Like mica, very perfect; parallel to flat face, which is considered to be the basal plane. This cleavage may not be noticed, especially in secondary chlorite in rocks.
▄Polarized Light▄:
_Pleochroism._—In green and yellow tints (green ∥ cleavage), being more marked in dark colored varieties. Basal sections are non-pleochroic, the mineral being practically uniaxial. Pleochroic halos may be seen.
_▄Crossed Nicols▄_:
_Double Refraction._—Generally very weak (γ − α = 0.001 to 0.011).
_Interference Colors._—Very low first order, gray or white, at times scarcely noticed. Anomalous colors, however, often seen (deep blue or brown.)
_Extinction._—Plates parallel to cleavage generally appear isotropic or only show faint color. In other sections extinction is apparently parallel to the cleavage in the uniaxial type, but extinction angles may be noticed when type is biaxial. Complete extinction may not be noticed, due to aggregate structure.
_▄Convergent Light▄_: Plates parallel to cleavage show, at times, an indistinct interference cross, which may open into two hyperbolas, indicating biaxial nature of crystallization. Ax. pl. ∥ (010); Bx_{_a_}. Λ _ć_ = 0° to 15°; 2_E_ variable. Optical character (±).
▄Distinguished from▄: SERPENTINE.—By general green color (serpentine, with exception of Fe rich variety, is colorless), pleochroism and frequent anomalous interference colors; but the distinction between these two minerals may be very difficult. Chlorite may resemble decomposed or green mica (mica has, however, strong double refraction). The different species in the chlorite group cannot usually be distinguished in rocks.
REMARKS: The chlorites are essentially secondary minerals, derived from the aluminous silicates, biotite, augite, garnet, feldspar, etc. They are found abundantly in chlorite-schists, contact rocks, etc., and as pigment (veridite) in altered eruptive rocks. May occur as a primary constituent of eruptive rocks, often in parallel growth with biotite. Chlorides are acted on by hot hydrochloric acid, and decomposed easily by sulphuric acid. H., 2 to 3. Sp. gr., 2.6 to 2.96. A thin section heated to redness on platinum foil loses water and becomes opaque. Ferruginous varieties are turned reddish-brown to black by heating (serpentine, as it contains less iron, may give negative results with this test).
_Delessite._—Found in sphærulites, filling cavities in amygdaloidal basic rocks, and in pseudomorphs. It appears to be much altered to other minerals.
TALC.
ANISOTROPIC. MONOCLINIC (Pseudo-Hexagonal). COMPOSITION: ELONGATION ∥ c′. H_{2}Mg_{3}(SiO_{3})_{4}.
▄Usual Appearance in Sections▄: In fine scaly, colorless aggregates. Sections, cutting across the scales, would show rod-like forms. Index of refraction only a little higher than balsam (_n′_ = 1.572, α = 1.539, γ = 1.589), hence no marked _relief_ and only slightly rough surface. Cleavage perfect parallel to base, like mica.
▄Crossed Nicols▄: Double refraction very strong (γ − α = 0.050). Interference colors third order, like muscovite. _Extinction_ parallel to basal cleavage lines. In _convergent light_, Ax. pl. ∥ (100), Bx_{_a_}. ∥ _ć_; axial angle small (2_E_ = small); optical character (−).
▄Distinguished from▄: MUSCOVITE (with which it is easily confused) by micro-chemical tests, proving absence of alkalies and Al; and often by the more aggregate structure of the talc. Also by smaller axial angle of scales in convergent light.
REMARKS: Found mainly in metamorphic schists, etc., always as a secondary product. It is insoluble in hydrochloric acid. H., 1 to 1.5. Sp. gr., 2.6 to 2.8.
EPIDOTE.
ANISOTROPIC. BIAXIAL. MONOCLINIC. ELONGATION ∥ a′ or c′.
COMPOSITION: Ca_{2}Al_{2}(AlOH)(SiO_{4})_{3}, with some Fe replacing Al.
▄Usual Appearance in Sections▄: Columnar to thick tabular crystals, more or less elongated parallel to ortho axis [_=b_], Fig. 61, or in granular aggregates.
[Illustration:
Epidote.
FIG. 61. FIG. 62. Ortho pinacoid section. Clino pinacoid section. ]
_Twinning._—May occur but rarely noticed. Irregular interpenetrations common with other members of the group; also parallel growths.
_Color._—Colorless to yellowish (Fe poor) or yellow to greenish to yellow-brown (Fe rich).
_Index of Refraction._—_n′_ = 1.751 but may be lower (α = 1.731, γ = 1.768), hence _relief_ high and surface rough.
_Cleavage._—Parallel to base (001), Figs. 61 and 62, imperfect parallel to ortho pinacoid (100). Basal cleavage cracks not very numerous and appear parallel to general direction of elongation.
▄Polarized light▄:
_Pleochroism._—Varies with the color, being faint in the light colored varieties, but strong when the color is marked (Fe rich).
_▄Crossed Nicols▄_:
_Double Refraction._—Variable, often very strong (γ − α = 0.037).
_Interference Colors._—Variable, often high (third) orders. Intergrowths with other members of the group are clearly shown by the “flecked” interference colors.
_Extinction._—Parallel to cleavage in sections parallel to [_=b_] axis. In other sections extinction angles vary, see Fig. 62.
_▄Convergent Light▄_: Axial plane ∥ (010), _i. e._, at right angles to the elongation of crystal and cleavage cracks, Fig. 61. Bx_{_a_.}-(a) Λ _ć_= 3° behind. Basal cleavage flakes show the almost ⟂ emergence of an optic axis. Axial angles very large (2_E_ > 180°). Optical character (−).
▄Alteration▄: Does not take place readily.
▄Distinguished from▄: Light colored MONOCLINIC PYROXENE.—By having plane of optic axes at right angles to cleavage cracks and direction of elongation; while in pyroxene plane of optic axes is parallel to parallel prismatic cleavage cracks or bisects the angle between intersecting cracks. Furthermore the yellow color is rare in pyroxene.
REMARKS: Epidote is essentially a secondary mineral, resulting from the alteration of the feldspars and the ferro-magnesium silicates. It is found in crystalline schists (especially those containing hornblende), gneiss, gabbro, diorite, diabase, lime-silicate hornstones, contact rocks, etc. Epidote is partially decomposed by hydrochloric acid. The Fe rich epidote can be changed to an intense color by “glowing” in the air. H., 6 to 7. Sp. gr., 3.32 to 3.45.
_Piedmontite_ (containing Mn).—Red in color. Very pleochroic, red to yellow. Found in crystalline schists, the porphyrite of Scotland, the famous “Porfido rosso antico” of Egypt and in certain Japanese mica schists.
ZOISITE.
Essentially orthorhombic? members of Epidote group.[95]
COMPOSITION: Like epidote but without any Fe.
▄Usual Appearance in Sections▄: Similar to epidote or in columnar aggregates. Often intergrown with epidote.
Distinguished from epidote by general absence of color (colorless to yellowish) and pleochroism; by slightly lower refractive index (_n′_ = 1.699 to 1.720) and by much weaker double refraction (γ − α = 0.005 and less). The interference colors are very low order, gray to white, but anomalous colors are often seen (yellow or prussian blue). Extinction is in general parallel to pinacoidal cleavage cracks (except in clinozoisite).
The plane of the optic axes may be parallel or at right angles to cleavage cracks, and the optical character is (+).
REMARKS: Generally a secondary mineral. Found in crystalline schists, amphibolites, contact rocks, eclogite, etc., and in “saussurite.” May be hard to distinguish from vesuvianite and apatite.
ALLANITE, Orthite.
Monoclinic member of Epidote group.
COMPOSITION: Like epidote but containing cerium.
▄Usual Appearance in Sections▄: Similar to epidote in form; but distinguished by brown color (may be also almost colorless), strong pleochroism and absorption, and medium to weak double refraction (γ − α = 0.002 to 0.030). Lamellar twinning clearly seen on account of oblique extinction. When included in hornblende and mica it is surrounded by pleochroic halos. _n′_ = 1.78 about. Optical character (±). Usually perfectly fresh.
REMARKS: Found as an accessory mineral in SiO_{2} rich eruptive rocks and connected crystalline schists, and (light colored) in amphibolite and eclogite.
TITANITE, Sphene.
ANISOTROPIC. BIAXIAL. MONOCLINIC. COMPOSITION: CaTiSiO_{5}. ELONGATION ∥ a′.[96]
▄Usual Appearance in Sections▄: Wedge-shaped crystals (in Na rich rocks, more prismatically developed); grains, which may be elongated; and aggregates of small rounded particles, which appear nearly opaque. Sections of crystals commonly acute rhombs, Fig. 63.
_Twinning._—Occurs, the twinning boundary bisecting the acute angles of the rhomb (only noticed between crossed nicols), Fig. 64.
_Color._—Reddish-brown to yellowish to colorless.
[Illustration:
FIG. 63.—Titanite, showing acute rhombic cross-section. ]
_Index of Refraction._—_n′_ = 1.920 to 1.963 (α = 1.888 to 1.913, γ = 1.978 to 2.054), hence _relief_ very marked and surface very rough.
_Cleavage._—Imperfect and not parallel to predominant form, hence only appears as a few rough cracks, which are not parallel to any crystallographic boundary, Fig. 63. Cleavage rarely observed in secondary grains.
▄Polarized Light▄:
_Pleochroism._—Varies with the color, being more distinct in colored crystals, yellowish (the lighter color) ∥ a′ and reddish-brown ∥ c′. Scarcely noticed when the color is light.
_▄Crossed Nicols▄_:
_Double Refraction._—Very strong (γ − α = 0.090 to 0.141).
[Illustration:
FIG. 64.—Titanite, showing twinning in nepheline-syenite. (From Cohen.) ]
_Interference Colors._—Very high order, like those of calcite. Due to the fact that the refractive indices of two of the rays are very nearly alike, some sections may show very low order colors.
_Extinction._—Extinction angles not characteristic. There may be no complete extinction in white light, owing to dispersion.
_▄Convergent Light▄_: On account of the very strong characteristic dispersion of the optic axes (ρ > ν), the axial angle varies a good deal with the color of the light used, (St. Gotthard) 2_E_{IA}_ = 57°, 2_E_{Tl}_ = 47°. By using colored glasses this variation in the axial angle can be seen. The axial plane lies in the clino pinacoid (010), hence bisects the obtuse angle in the rhombic cross-section, Fig. 63. Bx_{_a_}.(c) Λ _ć_ = 51° front. The optical character is (+).
▄Alteration▄: May take place.
▄Distinguished from▄:
(_a_) STAUROLITE.—In convergent light the axial plane is shown to be in the shorter diagonal of the rhombic cross-section, while in staurolite it is in the longer diagonal.
(_b_) RUTILE.—By biaxial character.
(_c_) CALCITE.—The light colored titanite (sphene), in absence of twinning, by higher index of refraction.
Titanite may easily be confused with some of the rarer minerals.
REMARKS: Titanite is always an accessory mineral and is found distributed in all rocks, except SiO_{2} rich eruptive and magnesia silicate rocks. As a secondary mineral it forms rims around other titanium minerals or pseudomorphs after them and also the principal part of _leucoxene_. It is partly soluble in hot hydrochloric acid and completely decomposed by sulphuric acid. H., 5 to 5.5. Sp. gr., 3.3 to 3.7. In a specific gravity separation it falls with the ferruginous minerals (on account of its density) and from these can generally be separated by electromagnetic methods.
FELDSPAR GROUP. Orthoclase, Microcline and the Plagioclases.
ORTHOCLASE.
ANISOTROPIC. BIAXIAL. MONOCLINIC. ELONGATION (∥ cleavage) ∥ a′.
COMPOSITION: KAlSi_{3}O_{8}, with some replacement by Na.
▄Usual Appearance in Sections▄: In crystals and grains. In porphyritic rocks habit of crystals more or less tabular parallel to clino pinacoid (010), or rectangular much extended parallel to clino axis _à_, with cross-sections six-sided, or rectangular to long lath shape, Figs. 65 and 66. Crystals may be changed into rounded or looped grains by chemical corrosion, Fig. 6. Adularia crystals more prismatically developed, giving rhombic sections. Dimensions of crystals vary extremely; microlites occur, at times forming sphærulitic structure. The very fine grained ground-mass “microfelsite” (not resolved by the microscope) consists largely of feldspar.
[Illustration:
Orthoclase cleavage plates.
FIG. 65.—Basal. FIG. 66.—Clino pinacoid. ]
Intergrowths with microcline and plagioclase common, forming “microperthite” when lamellæ are microscopic. May be in zonal formation with plagioclase (the orthoclase on the periphery). Also intergrown with quartz[97] forming “pegmatite” and “micro-pegmatite,” Fig. 67.
[Illustration:
FIG. 67.—Micro-pegmatitic structure, in granophyric quartz-porphyry. (From Cohen.) ]
Zonal structure often seen, Fig. 20, especially when decomposition has commenced; and in fresh crystals may be indicated by zonal arrangement of inclusions.
_Twinning._—Very common, generally after _Carlsbad_ law, Figs. 18 and 69; the twinning boundary, dividing the section longitudinally, either being parallel to edges of crystal or bent or jagged. Twinning after _Baveno_ (twinning boundary diagonal, with the two parts extinguishing at the same time, but having a and c directions crossed in the two portions) and _Manebach_ laws less common.[98]
[Illustration:
FIG. 68.—Orthoclase, ortho pinacoid section showing cleavages intersecting at 90°, in augite-syenite. ]
_Color._—Colorless or tinged by oxide of iron. Cloudy if decomposed.
_Index of Refraction._—_n′_ = 1.523 (α = 1.519, γ = 1.526), hence no _relief_ and surface smooth.
_Cleavage._—Varies and sometimes only seen in very thin sections, but is an important character and should always be searched for. It occurs perfect, parallel to base (001), and almost as perfect parallel to clino pinacoid (010). The two cleavages intersect at 90° in section parallel to the [_=b_] axis, Fig. 68.
_Inclusions._—May be present and arranged in regular or zonal order, but not important. Do not occur in individuals of a second generation.
▄Polarized Light▄:
_Pleochroism._—None.
_▄Crossed Nicols▄_:
_Double Refraction._—Very weak (γ − α = 0.007).
_Interference Colors._—Lower first order, gray, white, etc., not quite so bright as colors of quartz and plagioclase.
_Extinction._—Being monoclinic the extinction angle on base (001), with reference to clino pinacoid (010) cleavage cracks, is 0°. On clino pinacoid with reference to basal cleavage cracks, it is 5°. Some sections (notably in glassy sanidine grains) may appear dark during complete rotation. This is due to the fact that the axial angle is very small and the sections act approximately like those of a uniaxial mineral at right angles to the optic axis.
_▄Convergent Light▄_:[99] Plane of optic axes in general at right angles to clino pinacoid (010) (plane of symmetry), Fig. 65, hence parallel to trace of basal cleavage; but in some sanidines parallel to plane of symmetry. Bx_{_a_} (a) Λ _a_ = 5° above. Axial angles vary, 2_E_ = 125° (orthoclase), 0°–50° (sanidine).[100] May appear uniaxial when axial angle is very small. Optical character (−).
▄Alteration▄: Very common to clay,[101] muscovite, hydrargillite, etc. Generally commences along the cleavage cracks, and when it has progressed very far the whole feldspar appears opaque or cloudy, and no perceptible change may take place between crossed nicols. As decomposition is very prevalent in many rocks, the orthoclase is rarely clear or pellucid. Epidote is often formed when accessory solutions are present.
▄Distinguished from▄:
(_a_) The other FELDSPARS and MELILITE.—See under the latter minerals.
(_b_) QUARTZ.—Feldspar is biaxial but when occurring in clear glassy grains (notably sanidine), which appear uniaxial in convergent light, may resemble quartz. When tested the optical character is (−), while that of quartz is (+).
REMARKS: The members of the feldspar group are the widest distributed of the rock-forming minerals and their recognition is of the utmost importance on account of their bearing on the systematic classification of rocks. Orthoclase is found as an essential constituent in the more acid plutonic and older volcanic rocks, as granite, syenite, trachyte, porphyry, and also in gneiss, crystalline schists, more seldom in contact rocks and subordinate in clastic rocks.
Chemical corrosion (producing rounded or looped grains), Fig. 6, and mechanical deformation (producing angular, sharp-edged, broken grains, bending and undulatory extinction[102]), Fig. 7, occur in orthoclase. When a rock containing feldspar crystals is shattered, the orthoclase breaks parallel to basal cleavage and plagioclase parallel to twinning plane. Orthoclase is practically insoluble in acids. H., 6 to 6.5. Sp. gr., 2.56.
[Illustration:
FIG. 69. —Sanidine, showing Carlsbad twin and cross-parting, in nepheline-phonolite. (From Cohen.) ]
_Sanidine._—This clear, glassy variety of orthoclase occurs in the later eruptive rocks, rhyolite, trachyte, obsidian, etc. Sanidine often has a parting parallel to ortho pinacoid (100), which may be noticed in sections so thick that the cleavage is not seen, Fig. 69. In general it shows no sign of decomposition, and has a smaller axial angle than orthoclase. Inclusions of glass are more abundant than in orthoclase.
MICROCLINE.
ANISOTROPIC. BIAXIAL. TRICLINIC. COMPOSITION: KAlSi_{3}O_{8}.
▄Usual Appearance in Sections▄: As a rock constituent in irregular grains.
In general characters like orthoclase and _distinguished_ from it and the plagioclases by characteristic “_gridiron_” structure between crossed nicols, resulting from the polysynthetic twinning after both _Albite_ and _Pericline_ laws, Fig. 19. This crossed twinning will show in all sections except those parallel to the brachy pinacoid (010). The lamellæ are generally thinner than in the plagioclases and more “spindle-shaped.”[103]
Furthermore the rather obscure triclinic crystallization is shown by an extinction angle of + 15° on basal cleavage plates with reference to brachy pinacoid (010) cleavage lines (distinction from orthoclase, which has O° extinction angle).
REMARKS: Found with orthoclase, often almost replacing it, in granite, syenite, gneiss, etc., and is one of the last minerals to form. It is notably resistant to decomposition. A structure like the microcline twin structure may be produced in orthoclase by dynamic action.[104]
THE PLAGIOCLASES. Albite, Oligoclase, Labradorite, Anorthite.
ANISOTROPIC. BIAXIAL. TRICLINIC.
ELONGATION (∥ Albite twin lamellæ) ∥ a′ (except in anorthite when it may be ∥ a′ or c′).
COMPOSITION:[105]
Albite, NaAlSi_{3}O_{8}.
Oligoclase, _n_(NaAlSi_{3}O_{8}) + CaAl_{2}Si_{2}O_{8}, or _n_ Ab + An, _n_ = 2 to 6.
Labradorite, NaAlSi_{3}O_{8} + _n_(CaAl_{2}Si_{2}O_{8}), or Ab + _n_ An, _n_ = 1, 2 or 3.
Anorthite, CaAl_{2}Si_{2}O_{8}.
▄Usual Appearance in Sections▄: Much the same as orthoclase. Lath-shaped[106] forms and microlites very common, especially in the acid series.
[Illustration:
FIG. 70.—Plagioclase, showing narrow lamellæ, in diabase. (From Cohen.) ]
[Illustration:
FIG. 71.—Plagioclase, showing broad lamellæ, in gabbro. (From Cohen.) ]
_Twinning._—Polysynthetic, after _Albite_ law, almost universal; the twinning appearing between crossed nicols as a series of dark and light bands, bounded by parallel edges, Figs. 70 and 71. The twin lamellæ are parallel to brachy pinacoid (010), hence not observed in sections parallel to this pinacoid. The lamellæ may appear irregular and interrupted, and seem to be broader in the basic than in the acid series. When this twinning fails, however, as in the basic plagioclases in certain metamorphic rocks, the determination becomes very difficult. In some cases polysynthetic twinning, after both _Albite_ and _Pericline_ laws, may take place at the same time, giving rise to a structure somewhat similar to that of microcline, Fig. 72. In addition the polysynthetic crystals may be twinned like orthoclase after _Carlsbad_ and _Baveno_ laws.
The general characters are the same as in orthoclase with the following differences:
_Indices of Refraction_: _n′_ = 1.535 (α = 1.532, γ = 1.540) Albite. _n′_ = 1.541 (α = 1.537, γ = 1.545) Oligoclase, Ab_{4}An_{1}. _n′_ = 1.559 (α = 1.555. γ = 1.563) Labradorite, Ab_{1}An_{1}. _n′_ = 1.582 (α = 1.575, γ = 1.588) Anorthite.
The surface of anorthite appears slightly rougher than that of orthoclase.
_Cleavages_, parallel to base (001) and brachy pinacoid (010), never intersect at right angles, as is the case in sections of orthoclase parallel to [_=b_] axis. This is due to the triclinic system of crystallization, but the divergence from a right angle is small (93° 36′ to 94° 10′).
[Illustration:
FIG. 72.—Plagioclase, showing crossed lamellæ, in olivine-gabbro. (From Cohen.) ]
_Inclusions_ at times may be quite important, as the vitreous inclusions of oligoclase in andesites, etc., and the iron ore inclusions and other microlites in labradorite. The arrangement of these inclusions may be zonal or in parallel orientation.
_Double refraction_ is a little stronger than for orthoclase (γ − α = 0.008 to 0.013 (anorthite)), hence producing slightly brighter interference colors in sections of the same thickness.
_Extinction_ takes place in all sections unsymmetrically with respect to crystallographic, twinning or cleavage lines (as these minerals are triclinic); hence extinction angles are always observed.
_▄Convergent Light▄_: All plagioclases show the emergence of a bisectrix,[107] more or less oblique, on brachy pinacoid (010) cleavage faces. These cleavage faces show no twin lamellæ, unless twinning after _Pericline_ law occurs, in which case the determination is much more complicated. The axial angle is large, 2_E_ = 155° (Albite). Optical character, depending on variety, (+) or (−).
▄Alteration▄: Partly the same as in orthoclase, forming clay, muscovite, etc. Calcite and epidote are more common as side-products, and zeolitization also occurs in some rocks. The plagioclases decompose more easily than orthoclase.
▄Distinguished from▄:
(_a_) ORTHOCLASE.—By repeated twinning after _Albite_ law, giving between crossed nicols a series of alternate dark and light bands.
When _Albite_ twinning is absent the distinction is very difficult.
(_b_) MICROCLINE.—By common absence of the microcline “_gridiron_” structure between crossed nicols.
Methods for Optical Determination of the Plagioclases.[108]
The correct determination of the particular plagioclase is of the greatest importance in the classification of rocks, and it is no longer sufficient to simply determine the feldspar as either orthoclase or plagioclase.
A quantitative analysis of isolated material would lead most surely to the desired result, but has many objections.
Modern optical methods now permit of a very accurate and convenient determination under ordinary circumstances. But of course these methods involve a knowledge of the approximate orientation of the section tested. When this section is not a definite cleavage fragment, its orientation can best be determined by convergent light tests.
Only an outline of these methods can be here given, and reference should be made to more complete works for an elaborate discussion of the subject.
It is very convenient to have at hand a set of glass models of the plagioclases, showing location of plane of optic axes, vibration directions and crystal axes.[109]
(1) _Schuster’s_ method of recognizing the different feldspars by extinction angles measured on the cleavage plates[110] is very precise, but not always applicable for crystals in rock sections.
EXTINCTION ANGLES:
EXTINCTION ANGLES: ON BASE, MEASURED FROM TRACE OF ON BRACHY PINACOID, MEASURED FROM PINACOIDAL CLEAVAGE. TRACE OF BASAL CLEAVAGE. Albite + 4° Albite +19½° Oligoclase, Ab_{4}An_{1} + 2° Oligoclase, Ab_{4}An_{1} + 8° Labradorite, Ab_{1}An_{1} − 5½° Labradorite, Ab_{1}An_{1} −20° Anorthite −36½° Anorthite −41½°
Confusion may here arise between albite and labradorite if disregard be had to signs, but the more acid oligoclase is readily distinguished from the basic anorthite.
By convention the angles on base and pinacoids are (+) when the direction of extinction has apparently moved as the hands of a watch, with reference to the upper right hand edge (between base and pinacoid) of the crystal. When the reverse is true the angles are (−), see Fig. 73.
(2) The statistical method of _Michel Lévy_ and others is often applicable, especially in the following case:
Sections at right angles to the brachy pinacoid (010) and hence showing _Albite_ twinning.—These sections, as nearly perpendicular to the lamellæ as possible, are known by the sharp dividing lines, by the extinction angles on each side of the trace of the twinning plane being approximately equal and by the fact that the two adjacent lamellæ are of the same color when the trace of the twinning plane is parallel to the plane of vibration of either nicol. Also in the 45° position the lamellæ are exactly the same color and the dividing lines disappear. Measure the extinction angles in as many sections thus selected as possible and take the maximum value.[111] This should be very close to the maximum extinction angle, which is a constant for each kind of feldspar.
[Illustration:
FIG. 73.—Showing conventional signs of extinction angles. ]
MAXIMUM EXTINCTION ANGLES IN SECTIONS PERPENDICULAR TO ALBITE TWINS. Albite 16° Oligoclase, Ab_{4}An_{1} 5° Labradorite, Ab_{1}An_{1} 27° Anorthite 53°
In the determination of rod-like microlites,[112] oligoclase extinguishes almost parallel to its length, while anorthite may show extinction angles of over 27°. When these microlites show _Albite_ twinning use the method just described.
(3) _Fouqué’s_ method[113] can be used when the optical orientation of the section is known (as the result of a test with convergent light). The extinction angles of these known sections are of great diagnostic importance.
The best sections are those at right angles to the two bisectrices, and these may be obtained by rapidly testing those sections, in the rock, which show an interference color about half as high as the maximum color in the rock section, in this way avoiding the sections parallel to the optic axes.
Having found such a section, test it with a gypsum or ¼ undulation mica plate to prove whether the bisectrix is ⟂ a or c. If ⟂ a (these sections show sharp twinning striations) measure extinction angle between trace of axial plane and _albite_ twinning; if ⟂ c measure extinction angle between trace of axial plane and basal cleavage cracks.
EXTINCTION ANGLES IN SECTIONS:[114] ⟂ a, MEASURED FROM ALBITE STRIATIONS. ⟂ c, MEASURED FROM CLEAVAGE CRACKS. Albite 74° 19½° Oligoclase, Ab_{4}An_{1} 88° 5° Labradorite, Ab_{1}An_{1} 60° 22° Anorthite 55½° 48°
When both extinction angles can be obtained, the determination of the plagioclase is very certain, but the result cannot be regarded as definite when only one is found; and the method becomes more difficult as the crystals become smaller.
The position of the axial plane should be determined by convergent light test and not simply by the direction of extinction in parallel polarized light.
(4) _Michel Lévy’s_ method[115] can be employed when twinning is present after both _Carlsbad_ and _Albite_ laws.
[Illustration:
FIG. 74.—Extinction Angles in the Zone normal to (010) in Carlsbad Twins of the Plagioclases. ]
The section to be tested should be in the zone perpendicular to (010). Such sections show sharp boundary lines between the twin lamellæ and, between crossed nicols, the _Albite_ lamellæ show the same interference color when the trace of (010) is parallel to the cross-wires of the ocular, and also in the 45° position the lamellæ are the same color and no dividing lines show. The two parts of the _Carlsbad_ twin exhibit different interference colors in the 45° position, this difference being more marked as the composition approaches that of Anorthite. The extinction angles are measured from the trace of the _Albite_ twinning plane (010), paying regard to the + and − signs, and the _concurrent series_ of angles are to be obtained from the two parts of the _Carlsbad_ twin. The range of these angles (for four type compositions, Ab, Ab_{4}An_{1}, Ab_{1}An_{1} and An) is given in the accompanying diagram for all positions of the section in the zone normal to (010).
In the curves of this diagram (Fig. 74) the vertical distances are the extinction angles for every ten degrees measured from the trace of the _Albite_ twinning plane, and the horizontal distances represent varying positions of the section in the zone normal to (010) for every ten degrees of rotation from the position ∥ to the edge (100)(010) (that is ∥ _ć_ axis), through a revolution of 180° to again ∥ to the same position. The concurrent angles in one part of a _Carlsbad_ twin are represented by a heavy line and in the other part by a broken line. It will be observed that the difference between these concurrent angles is very small in Albite (3°) and increases markedly towards Anorthite (60°).
TABLE FOR BECKE METHOD.[116] Orthoclase │ α │< ω Quartz Microcline │ β │ „ Albite │ γ │ „ ───────────────────────────────┼───────┼─────────────────────────────── Oligoclase, Ab_{4}An_{1} │ α │< ω; γ > ω Quartz. ───────────────────────────────┼───────┼─────────────────────────────── Labradorite, Ab_{1}An_{1} │ α │>ε Quartz Anorthite │ β │ „ „ │ γ │ „
(5) _Becke’s_ method may be employed to identify the feldspar, by determining the relative values of the indices of refraction of the feldspar grain when it lies in contact with a quartz grain (best results) or with the balsam (not such good results.) The grains should have vibration directions in parallel position.
Other methods that may be employed are here simply referred to: Determination (in convergent light) of the emergence of an optic axis with reference to a known plane, the basic plagioclases show an optic axis about parallel to _ć_ of the crystal; determination of total reflection by Wallerant’s total reflectometer; determination of the value of the mean index of refraction of crushed isolated grains by Schrœder van der Kolk’s method;[117] determinations by specific gravity separations with use of heavy solutions, and by chemical and micro-chemical tests (for the relative amounts of K, Na and Ca).[118]
REMARKS: The plagioclases may have the same two general habits as orthoclase, being glassy and colorless in the younger eruptive rocks, and dull and cloudy in the granular and porphyritic, older, massive and schistose rocks. They occur in rocks of intermediate and basic composition.
Albite is found in granite (commonly intergrown with orthoclase), gneiss, etc., and frequently as a secondary constituent (secondary feldspar[119]) in the feldspar-quartz mosaic of mechanically metamorphosed rocks. It may also be present in acid eruptive rocks.
Oligoclase is very frequent in granite, syenite, gneiss, diorite, trachyte, andesite, diabase, etc.; and particularly accompanies orthoclase.
Labradorite is confined more to the gabbros,[120] basic eruptive rocks and crystalline schists, rich in amphibole and pyroxene.
Anorthite occurs in gabbros, the most basic porphyrites, basalts, etc.
Chemical corrosion and mechanical deformation[121] may take place as in orthoclase.
Anorthite and labradorite are more or less decomposed by hydrochloric acid, while albite and oligoclase are not acted on by the acid.
Especially interesting is the alteration of the plagioclase that takes place in gabbros, accompanied by “uralitization” of the pyroxene, forming “_saussurite_.” This consists of a white to greenish confused aggregate, chiefly of zoisite, grossularite, vesuvianite, chlorite, secondary feldspar (albite), etc.
_Anorthoclase_ (a Na K, triclinic, feldspar).—Shows between crossed nicols intersecting areas of exceedingly fine composite twin structure and others of homogeneous structure, producing a watery or “moiré” appearance. The twin structure may be only seen in very thin sections. All possible kinds of perthitic intergrowth occur. Further distinguished from orthoclase by small extinction angle (4°) on base and by smaller axial angle (2_E_ = 72° to 88°).
Replaces orthoclase in the Na rich eruptives. Found in augite-syenite and “Rhombenporphyr” of Norway (with rhombic cross-section), acid augite-andesite of Pantelleria and in the porphyries of the Hartz.
CYANITE, Disthene.
ANISOTROPIC. BIAXIAL. TRICLINIC. COMPOSITION: Al_{2}SiO_{5}. ELONGATION ∥ c′.
[Illustration:
FIG. 75.—Cyanite, macro pinacoid cleavage section. ]
▄Usual Appearance in Sections▄: Blade-like crystals without terminal planes, but with cross-section (six-sided) showing two long parallel edges and four shorter edges; also in columnar aggregates. Twinning common, with generally twinning plane parallel to macro pinacoid (100). Colorless or bluish and spotted. The index of refraction is high (_n′_ = 1.720, α = 1.712, γ = 1.728), hence _relief_ marked and surface rough. Cleavage perfect, parallel to macro pinacoid (100), appearing as sharp cracks, parallel to longest edges in cross-sections; less distinct, parallel to brachy pinacoid (010). Fibrous parting parallel to base (001), Fig. 75. Pleochroism (colorless to blue ∥ c′) not noticed except in colored crystals.
▄Crossed Nicols▄: Double refraction quite strong (γ − α = 0.016). Interference colors upper first order, yellow, red, violet, etc. _Extinction_ angles observed in all sections (being triclinic), reaching a maximum of 30° on macro pinacoid (100), Fig. 75. Extinction on base, about parallel to most perfect cleavage. In _convergent light_ axial angle large; axial plane and Bx_{_a_}. about perpendicular to best cleavage (100); optical character (−).
▄Alteration▄: Seldom observed, but may take place to mica.
▄Distinguished from▄:
(_a_) AMPHIBOLE by cleavage (intersecting cleavages at 124° in amphibole and 90° in cyanite) and by (100) cleavage plates of cyanite showing emergence of acute bisectrix.
(_b_) CORUNDUM by being biaxial.
Distinction from similar appearing minerals may be difficult.
REMARKS: Found in gneiss, granulite, metamorphic schists, ecolgite, etc., commonly associated with garnet. It is not attacked by acids. H., 5 to 7. Sp. gr., 3.6.
SERPENTINE.
AGGREGATE. ELONGATION (of fibers) ∥ c′.
COMPOSITION: H_{4}Mg_{3}Si_{2}O_{9}, with replacement by Fe.
▄Usual Appearance in Sections▄: Dense, fibrous (chrysotile) or scaly (antigorite) aggregates.
_Color._—Colorless to light greenish, except the Fe rich variety which is green.
_Index of Refraction._—_n′_ = 1.55 to 1.56 (α = 1.56, γ = 1.571 for antigorite), hence no _relief_ and surface smooth.
▄Polarized Light▄:
_Pleochroism._—Not seen or very feeble, except in the Fe rich variety.
_▄Crossed Nicols▄_:
_Double Refraction._—Rather weak (γ − α = 0.009 to 0.011).
_Interference Colors._—Middle first order, gray, white, yellow, etc. Anomalous colors do not appear. The aggregate structure is distinctly seen between crossed nicols. Due to compensation aggregates may appear isotropic.
▄Distinguished from▄: CHLORITE.—By more usual absence of color, pleochroism and anomalous interference colors; but this distinction may be very difficult.
REMARKS: Serpentine (both antigorite and chrysotile) is essentially a secondary mineral, resulting in most cases from the alteration of chrysolite (olivine), Fig. 22, more rarely of pyroxene or amphibole.[122] The alteration of olivine to antigorite leads to the characteristic “lattice structure,” the alteration to chrysotile to “mesh structure.” In the case of the “mesh” formation the alteration starts from the surface and cracks, producing fibres of chrysotile, which stand at right angles to these edges and cracks. As serpentinization proceeds new cracks form, due to increase in volume, and the process may continue until complete pseudomorphism takes place. When this subsequent serpentinization of the meshes takes place the resulting serpentine may appear almost isotropic[123] and is certainly different from the chrysotile of the first formed veins (Weinschenk). Pieces of the parent mineral are often present.
Serpentine is found in ophiolites, the altered basic igneous rocks, pyroxenites, peridotites, etc., and as a primary mineral in the Central Alps peridotite, intergrown with fresh olivine (Weinschenk). It may also form a rock by itself. Serpentine is attacked quite strongly by hydrochloric acid, still more so by sulphuric acid. Common serpentine is not altered by heating (distinction from chlorite), but the Fe rich variety becomes brown and opaque. H., 2.5 to 4. Sp. gr., 2.5 to 2.7.
CLAY, Kaolin.
COMPOSITION: AGGREGATE. H_{4}Al_{2}Si_{2}O_{9} (kaolinite).
▄Usual Appearance in Sections▄: Fine, scaly, colorless aggregates, which appear opaque (due to porous structure). The scales show basal cleavage. Index of refraction is about the same as balsam (_n′_ = 1.55), hence no _relief_. The double refraction is weak (γ − α = 0.008).
▄Distinguished from▄: Colorless MICA and HYDRARGILLITE [(Al(OH)_{3}), which as an alteration product of the feldspars is often confused with clay] by weak double refraction.
REMARKS: Clay results from the alteration of the feldspars (especially the plagioclases), elæolite, scapolite and other silicates. Kaolinite is insoluble in hydrochloric but decomposed by sulphuric acid. H., 2.5. Sp. gr., 2.6.