[Illustration: FIG. 44.—_A_, Tourmaline, showing strong absorption at right angles to direction of elongation (_P_ = plane of vibration of polarizer). _Quartzite_, Black Hills, D. _B_, Tourmaline in radiate aggregate. _Granite_, Cornwall. ] _Index of Refraction._—_n′_ = 1.633 (precious) to 1.674 (α = 1.620 to 1.651, γ = 1.640 to 1.685), hence _relief_ is marked and surface rough. _Cleavage._—Not seen in thin sections, but irregular, transverse and longitudinal cracks may appear. ▄Polarized Light▄: _Pleochroism._—Distinct, even in light colored varieties, increasing with the depth of color. The greatest absorption takes place at right angles to the direction of elongation of the crystal, Fig. 44 A. The other minerals having this very strong absorption are hornblende, dark colored mica (distinguished by cleavage and lamellar form) and allanite. Pleochroic halos may be noticed surrounding inclusions. ▄_Crossed Nicols_▄: _Double Refraction._—Quite strong (γ − α = 0.017 (precious) to 0.034). _Interference Colors._—Bright upper first or second order, but may not be noticeable, due to absorption of parts of the light. _Extinction._—As tourmaline is uniaxial, basal sections remain dark during rotation of stage. In all other sections extinction is parallel to _ć_ axis. ▄_Convergent Light_▄: Cross-sections show a sharp cross. Optical character (−). ▄Alteration▄: Does not take place. ▄Distinguished from▄: (_a_) HORNBLENDE.—By absence of cleavage, and by the fact that the greatest absorption takes place at right angles to the longitudinal axis, while in hornblende it takes place approximately parallel to the longitudinal axis, or to the cleavage lines which are parallel to this axis. (_b_) APATITE (when colored).—By strong absorption at right angles to longitudinal axis. (_c_) CORUNDUM.—See under the latter. In some cases where recognition is difficult, chemical tests, to prove presence of boracic acid, must be made. REMARKS: The black schorl is the only primary tourmaline and is found in granitoid rocks. Tourmaline in other rocks results from “fumarole” action; hence occurs in pegmatite, tin and copper veins, clay deposits, also (light colored) in contact rocks and crystalline schists. The hemimorphic terminations may sometimes be noticed. Tourmaline is not acted on by acids. H., 7 to 7.5. Sp. gr., 3 to 3.2. It can be separated from powdered rock by sp. gr. solutions combined with magnetic methods. ANDALUSITE ANISOTROPIC. BIAXIAL. ORTHORHOMBIC. COMPOSITION: Al_{2}SiO_{5}. _ć_ = a. ELONGATION ∥ a′.