Minerals in rock sections

Usual Appearance in Sections: This form of SiO₂, 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.

Composition: CaCO₃. 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]

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.

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.

Composition: CaMg(CO₃)₂, when pure CaO = 30.4, MgO = 21.7, CO₂ = 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.

Usual Appearance in Sections: Minute, slender hexagonal prisms, cross-sections having regular hexagonal boundaries, needles, and grains. Figs. 14 a and 42.

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.

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.

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.

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.

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

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 (2E > 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.

SILLIMANITE, Fibrolite.

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, 2E = 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.

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 (2E = 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 H₂O 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.

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 (2E > 180°); optical character (+).

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.

Enstatite contains little, if any, Fe. Hypersthene contains more Fe, its optical characters beginning to show with about 10 per cent.

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 (2E = 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.

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.

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 (2E > 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 (Fe₂SiO₄) may be reddish in sections, and common in the basic porphyritic eruptive rocks.

IOLITE, Cordierite, Dichroite.

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) (2E = 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.

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.

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; 2E = 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.