_Pleochroism._—In general none, but noticed in the reddish varieties, when the absorption is a little stronger parallel to _ć_. ▄_Crossed Nicols_▄: _Double Refraction._—Very strong (γ − α = 0.036). _Interference Colors._—Rather high in order (second or third), higher than the colors of augite. _Extinction._—In general parallel to cleavage lines. _▄Convergent Light▄_: Axial plane parallel to base (001) and always at right angles to cleavage cracks, Fig. 51. Bx_{_a_}. ∥ _a_. Axial angle very large (2_E_ > 180°). Optical character (+). ▄Alteration▄: Into serpentine very common,[90] producing “mesh-” or “lattice-” structure (see under serpentine, p. 114); also into amphibole, etc. In certain basaltic rocks the rims of grains may be changed into gœthite?, and in certain gabbros the crystals may be surrounded by a radial rim of amphibole. ▄Distinguished from▄: Light colored MONOCLINIC PYROXENES.— By the absence of extinction angles, cleavage (the intersecting prismatic cleavages of augite being of equal distinctness), stronger double refraction and by axial plane being parallel to base, hence always at right angles to best cleavage (in augite axial plane lies in clino pinacoid, bisecting angles of intersecting prismatic cleavages). Also by gelatinization with acids. REMARKS: Found only in basic rocks, as peridotite, diabase, gabbro, norite, basalts, etc. Chrysolite (olivine) is a very brittle mineral and shows under mountain making pressure “cataclastic” structure. Chromite is a characteristic associated mineral. When not too poor in Fe, chrysolite becomes permanently red and pleochroic when strongly heated. Chrysolite is decomposed by hydrochloric and sulphuric acids, with separation of gelatinous silica. H., 6.5 to 7. Sp. gr., 3.3 to 3.4. _Hyalosiderite_ (a more ferruginous chrysolite) and _Fayalite_ (Fe_{2}SiO_{4}) may be reddish in sections, and common in the basic porphyritic eruptive rocks. IOLITE, Cordierite, Dichroite. ANISOTROPIC. BIAXIAL. ORTHORHOMBIC. COMPOSITION: _ć_ = a. ELONGATION ∥ a′. Mg_{3}(Al.Fe)_{6}Si_{8}O_{28}. ▄Usual Appearance in Sections▄: Grains, more rarely crystals of short prismatic habit, which often form pseudo-hexagonal interpenetration twins. Crystals may have edges rounded or corroded. Colorless, but may be bluish. Index of refraction a little lower than quartz (_n′_ = 1.539, α = 1.535, γ = 1.544), hence _relief_ low and surface smooth. Cleavage very variable, parallel to brachy pinacoid (010), especially noticeable when decomposition has taken place. Inclusions of sillimanite, zircon, rutile, etc., may be seen. Pleochroism usually not observed, but noticed in blue sections (yellowish white ∥ _ć_ to blue). Pleochroic halos (yellow) surrounding inclusions common, see p. 59.