Minerals in rock sections; the practical methods of identifying minerals in rock sections with the microscope, especially arranged for students in technical and scientific schools . parallel to base ; seen insections showing cleavage by variations in extinction, in basalsections by distorted interference figures. Micas of different kinds often associated together in parallel-position, also intergrown with hornblende, pyroxene, chlorite andquartz. MICA GROUP. ^7 Color. — Depends on chemical composition. Biotites, brown,green or red to almost opaque. Phlogopites, colorless or yel-lowish. Muscovi
Minerals in rock sections; the practical methods of identifying minerals in rock sections with the microscope, especially arranged for students in technical and scientific schools . parallel to base ; seen insections showing cleavage by variations in extinction, in basalsections by distorted interference figures. Micas of different kinds often associated together in parallel-position, also intergrown with hornblende, pyroxene, chlorite andquartz. MICA GROUP. ^7 Color. — Depends on chemical composition. Biotites, brown,green or red to almost opaque. Phlogopites, colorless or yel-lowish. Muscovites colorless. Index of Refraction.— //= to , hence somewhatmarked relief diwd surface varies in appearance from slightly roughto fairly rough. In polarized light the surface appears roughestwhen the cleavage cracks are parallel to the plane of the polarizer. Biotite has more marked relief Cleavage.—Very perfect, parallel to base (OP, 001), Fig. Fig. 60. —Biotite, showing basal cleavage, in biotite-granite. (From Cohen.) Basal sections show no cleavage, but all other sections show manysharp, parallel cleavage cracks. For precussion and pressure figures, see reference given below.* Inclusions. — !May be arranged parallel to lines of pressure needles, tourmaline, apatite, etc., common in magnesiummica. Zircon inclusions often surrounded by pleochroic halos. Polarized Light: Pleochroisni. — Varies with the color, being \ery marked in thecolored varieties (from pale yellow to chestnut-brown or black).The strong absorption, about parallel to the cleavage lines, is verycharacteristic of the colored micas. Fig. 59 b. Strong absorptionis also noticed in hornblende, tourmaline and allanite. Absorptionmay even be noticed around inclusions (pleochroic halos) in color-less, non-pleochroic micas. Cleavage plates of biotite are notpleochroic unless the axial angle is large. * Idding s Rosenbusch, p. 274. 88 CflARACTERS
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