▄Usual Appearance in Sections▄: In short, rounded, prismatic crystals, with almost square cross-section. Colorless or at times pale reddish and spotted. Index of refraction medium (_n′_ = 1.637, α = 1.632, γ = 1.643), hence _relief_ well marked and surface rough. Cleavage, parallel to almost square prism, may show. Pleochroism only marked in colored varieties, being reddish ∥ _ć_ (the direction of elongation or cleavage). Carbonaceous inclusions are characteristic, arranged as in macroscopic specimens (_Chiastolite_), Fig. 45. Pleochroic halos may surround inclusions. ▄Crossed Nicols▄: Double refraction weak (γ − α = 0.001). Interference colors middle 1st order, white to yellow. _Extinction_ in general parallel to _ć_ axis in longitudinal sections, symmetrical in cross-sections. In _convergent light_ Ax. pl. ∥ (010), Bx_{_a0_}. ∥ _ć_; axial angle very large (2_E_ > 180°); optical character (−). ▄Alteration▄: Often takes place to dense aggregate of mica, when the pseudomorph may be hard to recognize. ▄Distinguished from▄: (_a_) SILLIMANITE by much weaker double refraction, less elongated crystals and by elongation ∥ a′ (Sillimanite elong. ∥ c′). (_b_) DIOPSIDE by weaker double refraction, rhombic cross-section and parallel extinction in longitudinal sections. REMARKS: Very characteristic of metamorphic schists and of contact zones of clay slates with granite, etc., but not found in rocks which have been formed at great pressure. The andalusite grains may often be arranged in divergent or finger-like manner. May also form parallel growths with sillimanite. It is insoluble in hydrochloric acid. H., 7 to 7.5. Sp. gr., 3.18. [Illustration: FIG. 45.—Chiastolite, showing characteristic carbonaceous inclusions. (From Cohen.) ] [Illustration: FIG. 46.—Sillimanite aggregate, showing cross fracture, in mica schist. (From Cohen.) ] SILLIMANITE, Fibrolite. ANISOTROPIC. BIAXIAL. ORTHORHOMBIC. COMPOSITION: Al_{2}SiO_{5}. _c_ = c′. ELONGATION ∥ c′. ▄Usual Appearance in Sections▄: Long, slender, colorless prisms or needles; often in felt-like aggregates. Crystals often bent. Index of refraction rather high (_n′_ = 1.664, α = 1.656, γ = 1.677), hence _relief_ marked. Transverse fractures common, Fig. 46. ▄Crossed Nicols▄: Double refraction rather strong (γ − α = 0.021). Interference colors upper first or lower second order, red, purple, blue, etc. _Extinction_ parallel to prisms. Ax. pl. ∥ (100), Bx_{_a_}. ∥ _ć_, 2_E_ = 35° to 55°. Optical character (+). ▄Distinguished from▄: (_a_) APATITE by higher order interference colors and by elongation ∥ c′ (apatite has elongation ∥ a′). (_b_) TREMOLITE by always parallel extinction and small size of axial angle. (_c_) ANDALUSITE, see under the latter mineral.