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 . on base and by smaller axial angle {^2E= 72° to 88°). * These tests are only possible on pure and fresh material. The specific gravity in-creases with the Ca % (albite , anorthite ). f In clear unstriated granules, which may be distinguished from quartz by biaxialinterference figure in convergent light. J The tendency of labradorite in gabbros to twinning, after both Albile and Periclinelaws, is to be noted
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 . on base and by smaller axial angle {^2E= 72° to 88°). * These tests are only possible on pure and fresh material. The specific gravity in-creases with the Ca % (albite , anorthite ). f In clear unstriated granules, which may be distinguished from quartz by biaxialinterference figure in convergent light. J The tendency of labradorite in gabbros to twinning, after both Albile and Periclinelaws, is to be noted. ^ Wervekes (N. J. B., 1883, 11, 97) theory is that a twin lamination maybecaused by the forces producing mechanical deformations, as movement in the magmaand mountain making pressure. Such lamellae are characterized by the fact that theirextent and course seem to depend on fracture lines in the crystal. io8 CHARACTERS OF MINERALS. Replaces orthoclase in the Na rich eruptives. Found in augite-syenite and Rhom-henporphyr of Norway (with rhombic cross-section), acid augite-andesite of Pantel-leria and in the porphyries of the llartz. Anisotropic. CYANITE, Disthene. Triclinic. Composition : (AlO), SiO.,. Elongation 1| c. Usual Appearance in Sections : Blade-like crystals without terminal planes, butwith cross-sections (six sided) showing two long parallel edges and four shorter edges;also in columnar aggregates. Twinning common, with generally twinning plane paral-lel to macro pinacoid (oo Pec , lOo). Colorless or bluish and spotted. The index of refraction is high («= ), hence ;Y//£y marked\ f and surface rough. Cleavage perfect, parallel to ^^-<- macro pinacoid (00 Poo, icx)), appearing as sharp cracks, parallel to longest edges in cross-sections ; lessdistinct, parallel to brachy pinacoid (00 Poo, oio).Fibrous parting parallel to base (OP, 001), Fig. 74-Pleochroism (colorless to blue || c) not noticed ex-cept in colored crystals. Crossed Nicols : Double r
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