REMARKS: Found especially in clay-rich contact rocks, gneisses and schists, often occurring with iolite (cordierite). Crystals may appear in bands. It is insoluble in hydrochloric acid. H., 6 to 7. Sp. gr., 3.24. TOPAZ. ANISOTROPIC. BIAXIAL. ORTHORHOMBIC. COMPOSITION: _ć_ = c. Al(Al(O.F_{2}))SiO_{4}. ▄Usual Appearance in Sections▄: Colorless crystals of short prismatic habit, grains or rod-like radiating aggregates. Index of refraction about the same as that of calcite (_n′_= 1.608 to 1.632, α = 1.607 to 1.629, γ = 1.618 to 1.637), hence _relief_ medium. Cleavage perfect, parallel to base, but does not show as many cracks. Fluid inclusions abundant. ▄Crossed Nicols▄: Double refraction weak (γ − α = 0.008 to 0.011), about the same as that of quartz. Interference colors middle first order, white, yellow, etc. _Extinction_ parallel to cleavage. In _convergent light_, Ax. pl. ∥ (010), Bx_{_a_}. ∥ _ć_, axial angle large (2_E_ = 86° to 126°); interference figure obtained from basal sections (_i. e._, from sections showing no cleavage); optical character (+). ▄Alteration▄: May take place to kaolin or muscovite, by loss of F and taking up of H_{2}O and alkalies. ▄Distinguished from▄: (_a_) QUARTZ by higher relief, cleavage and biaxial character. (_b_) SILLIMANITE (when topaz is in radiating aggregates) by lower refraction and double refraction. REMARKS: Common in greisen and all granite rocks containing tin ore. When formed by “fumarole” action (tin veins) the mineral shows rod-like radiating forms. It is insoluble in hydrochloric acid. H., 8. Sp. gr., 3.5. STAUROLITE. ANISOTROPIC. BIAXIAL. ORTHORHOMBIC. COMPOSITION: ELONGATION ∥ ć. Fe(AlO)_{4}(AlOH)(SiO_{4})_{2}, but varying, may contain Mg or Mn. _ć_ = c. [Illustration: FIG. 47.—Staurolite, showing twinning at 90° _b_ and 60° _c_, also granular quartz inclusions. (From Reinisch.) ] ▄Usual Appearance in Sections▄: Short, flat prisms, which may be twinned at 90° or 60°, Fig. 47, or grains. Color yellowish to reddish-brown. Index of refraction rather high (_n′_ = 1.741, α = 1.736, γ = 1.746), hence _relief_ marked and surface rough. Cleavage, both prismatic and pinacoidal, variable. Inclusions of minute quartz grains and carbonaceous matter found in larger crystals, but not in microscopic crystals. Pleochroism distinct but not strong, showing red ∥ _c_ (direction of elongation). Pleochroic halos may surround inclusions.