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. [Illustration: FIG. 69. —Sanidine, showing Carlsbad twin and cross-parting, in nepheline-phonolite. (From Cohen.) ] _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. ANISOTROPIC. BIAXIAL. TRICLINIC. COMPOSITION: KAlSi_{3}O_{8}. ▄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. ANISOTROPIC. BIAXIAL. TRICLINIC. ELONGATION (∥ Albite twin lamellæ) ∥ a′ (except in anorthite when it may be ∥ a′ or c′). COMPOSITION:[105] Albite, NaAlSi_{3}O_{8}.