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 . or-tions of the original pyroxene may be left, having all the char-acteristic optical properties of this latter mineral. A)itJiopJiyllitt\ the orthorhombic amphibole, with always parallel-extinction, is sometimes found in colorless to brownish, blade- torod-like aggregates in crystalline schists and serpentine. 86 CHARACTERS OF MINERALS. MICA GROUP. Anisotropic. Biaxial. Moxoclinic. May appear hexagonal or orthorho
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 . or-tions of the original pyroxene may be left, having all the char-acteristic optical properties of this latter mineral. A)itJiopJiyllitt\ the orthorhombic amphibole, with always parallel-extinction, is sometimes found in colorless to brownish, blade- torod-like aggregates in crystalline schists and serpentine. 86 CHARACTERS OF MINERALS. MICA GROUP. Anisotropic. Biaxial. Moxoclinic. May appear hexagonal or orthorhombic. Composition : Elongation ( || cleavage) || c. Biotitc (black or ferro-magnesium mica) = ().,()2Al2(SiOJ.,, approx. Phlogopitc = a magnesium mica, near biotite, but containinglittle Fe. Miiscovitc (white or potash mica) = H2()Al3(SiOJ3, withsome replacement by Mg or Fe. Usual Appearance in Sections : Scales, which may be notchedor jagged, with lateral sections lath-shaped ; or shreds, Fig. 59 distincdy crystallized (magnesium micas) the thin hexagonalplates have plane angles of 120, Fig. 59 a. Phlogopite crystalsmay be extended in direction of c Fig. 59. — Mica. A, Biotite, showing liexagonal cross-section and zonal , Freiberg. B, Biotite, showing strong absorption parallel to cleavage and alsozonal marking (/=: plane of vibration of polarizer). Minette, Cumberland. C, Mus-covite in bent shreds in gneiss. Zonal structure not uncommon in the magnesium micas, A, which may also have dark iron ore border like hornblende. Tz^iiiiiiiig. — Common, generall} 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. Ph
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