Minerals in rock sections

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).

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) 2E_IA = 57°, 2E_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 SiO₂ 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.

Composition: KAlSi₃O₈, 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.

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.

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]

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, 2E = 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.

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.

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.

Elongation (∥ Albite twin lamellæ) ∥ a′ (except in anorthite when it may be ∥ a′ or c′).

Composition:^[105]

Albite, NaAlSi₃O₈.

Oligoclase, n(NaAlSi₃O₈) + CaAl₂Si₂O₈, or n Ab + An, n = 2 to 6.

Labradorite, NaAlSi₃O₈ + n(CaAl₂Si₂O₈), or Ab + n An, n = 1, 2 or 3.

Anorthite, CaAl₂Si₂O₈.

Usual Appearance in Sections: Much the same as orthoclase. Lath-shaped^[106] forms and microlites very common, especially in the acid series.

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:

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′).

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, 2E = 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:

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.

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.

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.

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, Ab₄An₁, Ab₁An₁ 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°).

(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 (2E = 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.

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.

Composition: H₄Mg₃Si₂O₉, 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.

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.