TALC. ANISOTROPIC. MONOCLINIC (Pseudo-Hexagonal). COMPOSITION: ELONGATION ∥ c′. H_{2}Mg_{3}(SiO_{3})_{4}. ▄Usual Appearance in Sections▄: In fine scaly, colorless aggregates. Sections, cutting across the scales, would show rod-like forms. Index of refraction only a little higher than balsam (_n′_ = 1.572, α = 1.539, γ = 1.589), hence no marked _relief_ and only slightly rough surface. Cleavage perfect parallel to base, like mica. ▄Crossed Nicols▄: Double refraction very strong (γ − α = 0.050). Interference colors third order, like muscovite. _Extinction_ parallel to basal cleavage lines. In _convergent light_, Ax. pl. ∥ (100), Bx_{_a_}. ∥ _ć_; axial angle small (2_E_ = small); optical character (−). ▄Distinguished from▄: MUSCOVITE (with which it is easily confused) by micro-chemical tests, proving absence of alkalies and Al; and often by the more aggregate structure of the talc. Also by smaller axial angle of scales in convergent light. REMARKS: Found mainly in metamorphic schists, etc., always as a secondary product. It is insoluble in hydrochloric acid. H., 1 to 1.5. Sp. gr., 2.6 to 2.8. EPIDOTE. ANISOTROPIC. BIAXIAL. MONOCLINIC. ELONGATION ∥ a′ or c′. COMPOSITION: Ca_{2}Al_{2}(AlOH)(SiO_{4})_{3}, with some Fe replacing Al. ▄Usual Appearance in Sections▄: Columnar to thick tabular crystals, more or less elongated parallel to ortho axis [_=b_], Fig. 61, or in granular aggregates. [Illustration: Epidote. FIG. 61. FIG. 62. Ortho pinacoid section. Clino pinacoid section. ] _Twinning._—May occur but rarely noticed. Irregular interpenetrations common with other members of the group; also parallel growths. _Color._—Colorless to yellowish (Fe poor) or yellow to greenish to yellow-brown (Fe rich). _Index of Refraction._—_n′_ = 1.751 but may be lower (α = 1.731, γ = 1.768), hence _relief_ high and surface rough. _Cleavage._—Parallel to base (001), Figs. 61 and 62, imperfect parallel to ortho pinacoid (100). Basal cleavage cracks not very numerous and appear parallel to general direction of elongation. ▄Polarized light▄: _Pleochroism._—Varies with the color, being faint in the light colored varieties, but strong when the color is marked (Fe rich). _▄Crossed Nicols▄_: _Double Refraction._—Variable, often very strong (γ − α = 0.037). _Interference Colors._—Variable, often high (third) orders. Intergrowths with other members of the group are clearly shown by the “flecked” interference colors. _Extinction._—Parallel to cleavage in sections parallel to [_=b_] axis. In other sections extinction angles vary, see Fig. 62.