_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 (2_E_ = 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. ANISOTROPIC. BIAXIAL. TRICLINIC. COMPOSITION: Al_{2}SiO_{5}. ELONGATION ∥ c′. [Illustration: FIG. 75.—Cyanite, macro pinacoid cleavage section. ] ▄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. AGGREGATE. ELONGATION (of fibers) ∥ c′. COMPOSITION: H_{4}Mg_{3}Si_{2}O_{9}, with replacement by Fe. ▄Usual Appearance in Sections▄: Dense, fibrous (chrysotile) or scaly (antigorite) aggregates.