Minerals in rock sections

OPAL.

Composition: SiO₂.nH₂O, 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.

Composition: Fe₂(OH)₆, Fe₂O₃, 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.

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.

Composition: Fe₃O₄, 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.

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.

Composition: FeCr₂O₄.

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.

Composition: Mg(AlO₂)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.

Composition: R″₃R‴₂(SiO₄)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.”

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.

Composition: KAl(SiO₃)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.

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.

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.

Composition: NaAlSi₂O₆ + H₂O.

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).

Composition:

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 (CaSO₂ + 2H₂O) 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.

PEROVSKITE, Perofskite.

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.

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.

ZIRCON.

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.

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.

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.

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.

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.

Composition: Fe₂O₃.

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.

Composition: (FeTi)₂O₃.

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.

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.

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 SiO₂ 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.