_Extinction._—Parallel to cleavages in longitudinal sections, which are parallel to _a_ or _b_, and bisecting angles of intersecting prismatic cleavages in basal sections. _▄Convergent Light▄_: Axial plane parallel to brachy pinacoid (010),[89] _i. e._, parallel to best pinacoidal cleavage. Bx_{_a_}. ∥ _ć_ (E), ∥ _a_(H). Axial angles large (2_E_ = 95° to > 180°). Optical character for (_E_)(+), for (_H_)(−). On account of weak double refraction the interference figures are not very marked. ▄Alteration▄: Takes place to bastite, serpentine, etc. ▄Distinguished from▄: The MONOCLINIC PYROXENES and AMPHIBOLES.—See under these species. REMARKS: Found in the granular rocks of the gabbro-peridotite series, also in the olivine basalts (E); and in crystals in porphyritic andesite (H). These minerals are in general not attacked by acids. H., 5 to 6. Sp. gr., 3.1 to 3.5. _Bronzite_ is the name give to the variety containing about 5% Fe and having the characteristic bronzy lustre due to inclusions. _Bastite_ (an alteration product of the orthorhombic pyroxenes poor in Fe).—Composed of fibers, often traversed by irregular cracks. Color light yellowish or greenish and index of refraction about the same as Canada balsam. Pleochroism faint (only seen in thick sections), the greatest absorption taking place parallel to the fibers. Double refraction weak and extinction parallel to the fibers. Axial angle large and axial plane at right angles to principal cleavage face (010). The position of the axial plane is the surest distinction between bastite and the orthorhombic pyroxenes. CHRYSOLITE, Olivine. ANISOTROPIC. BIAXIAL. ORTHORHOMBIC. COMPOSITION: (Mg.Fe)_{2}SiO_{4}. ELONGATION ∥ a′ or c′. ▄Usual Appearance in Sections▄: Prismatic crystals or in large angular fragments or grains. Longitudinal sections more or less lath-shaped, with pointed ends, Figs. 50 and 51, cross-sections six or eight-sided. Outlines of crystals often rounded or corroded. Skeleton forms may occur, and sometimes twinning may be observed. [Illustration: Chrysolite. FIG. 50. FIG. 51. Basal section. Macro pinacoid section. ] _Color._—Nearly colorless, may be reddish (with high Fe per cent.). _Index of Refraction._—_n′_ = 1.675 (α = 1.661, γ = 1.697), hence _relief_ marked and surface rough. _Cleavage._—Parallel to brachy pinacoid (010), less distinct parallel to macro pinacoid (100), Fig. 50. Often only made visible by decomposition. An irregular fracturing occurs, which increases with alteration into serpentine. _Inclusions._—Chromite, opaque earths, apatite and the brown plates so common in hypersthene; also glass and slag (in basaltic rocks) and fluid (in peridotites and olivinfels). ▄Polarized Light▄: