GYPSUM. ANISOTROPIC. BIAXIAL. MONOCLINIC. COMPOSITION: CaSO_{4} + 2H_{2}O. ▄Usual Appearance in Sections▄: Colorless grains or fibers. May be colored, however, by inclusions of carbonaceous matter, iron oxides, etc. Index of refraction about the same as orthoclase (_n′_ = 1.525, α = 1.521, γ = 1.531), hence no _relief_ and surface smooth. Twinning lamellæ abundant. Cleavage parallel to (010) gives abundant cracks, other cleavages may also be noticed. ▄Crossed Nicols▄: Double refraction weak (γ − α = 0.010), the same as quartz. Interference colors middle first order, white to yellow. _Extinction_ parallel to most perfect cleavage cracks in sections parallel to _b_ axis; large extinction angles noticed with reference to less perfect cleavages. In _convergent light_, Ax. pl. ∥ (010), _i.e._, ∥ to most distinct cleavages; Bx_{_a_}. (c) Λ = 54° front; 2_E_ = 104°; optical character (+). As the characters of gypsum are not always very marked it may be necessary to employ micro-chemical tests. REMARKS: Forms a rock by itself, often associated with rock salt. It also occurs as an alteration product of anhydrite. Gypsum is soluble in hydrochloric acid. H., 1.5 to 2. Sp. gr., 2.2 to 2.4. MONOCLINIC PYROXENES, Augite, etc. Including the monoclinic minerals of the Pyroxene Group, which show distinctly the characteristic cleavage parallel to an almost right-angled prism. ANISOTROPIC. BIAXIAL. MONOCLINIC. ELONGATION ∥ c′.[91] COMPOSITION: RSiO_{3}, R = Ca, Mg, Mn, Fe, Al chiefly, with the Ca predominating over the Mg. ▄Usual Appearance in Sections▄: Both in crystals and more or less irregular grains, Figs. 4 and 12, the habit varying with the chemical composition as follows: _Diopside_ (Ca, Mg varieties), long columnar crystals and grains. _Augite_ (ditto, but containing also Al and Fe), short prismatic crystals and grains. _Diallage_, granular or lamellar (|| (100)), may show fibrous structure ∥ _ć_. Prism angle = 87° 06′ (important in cross-sections). Sections of crystals nearly at right angles to the vertical axis _ć_ are octagonal or square with truncated corners, Figs. 4 and 53, while those parallel to the _ć_ axis are lath-shaped. Pyroxene also occurs in skeleton crystals and acicular microlites in eruptive rocks. [Illustration: FIG. 52.—Augite, section parallel to _ć_ axis showing prismatic cleavage, in leucite-basal. (From Cohen.) ] Zonal structure (especially in augites) may be marked by differences in color or extinction, and in some basalts the crystals have the “hour-glass” structure. _Twinning._—Common, usually the twinning plane being the ortho pinacoid (100). Twin lamellæ may be noticed. Intergrowths occur with orthorhombic pyroxene and amphibole.