▄Usual Appearance in Sections▄: Similar to epidote in form; but distinguished by brown color (may be also almost colorless), strong pleochroism and absorption, and medium to weak double refraction (γ − α = 0.002 to 0.030). Lamellar twinning clearly seen on account of oblique extinction. When included in hornblende and mica it is surrounded by pleochroic halos. _n′_ = 1.78 about. Optical character (±). Usually perfectly fresh. REMARKS: Found as an accessory mineral in SiO_{2} rich eruptive rocks and connected crystalline schists, and (light colored) in amphibolite and eclogite. TITANITE, Sphene. ANISOTROPIC. BIAXIAL. MONOCLINIC. COMPOSITION: CaTiSiO_{5}. ELONGATION ∥ a′.[96] ▄Usual Appearance in Sections▄: Wedge-shaped crystals (in Na rich rocks, more prismatically developed); grains, which may be elongated; and aggregates of small rounded particles, which appear nearly opaque. Sections of crystals commonly acute rhombs, Fig. 63. _Twinning._—Occurs, the twinning boundary bisecting the acute angles of the rhomb (only noticed between crossed nicols), Fig. 64. _Color._—Reddish-brown to yellowish to colorless. [Illustration: FIG. 63.—Titanite, showing acute rhombic cross-section. ] _Index of Refraction._—_n′_ = 1.920 to 1.963 (α = 1.888 to 1.913, γ = 1.978 to 2.054), hence _relief_ very marked and surface very rough. _Cleavage._—Imperfect and not parallel to predominant form, hence only appears as a few rough cracks, which are not parallel to any crystallographic boundary, Fig. 63. Cleavage rarely observed in secondary grains. ▄Polarized Light▄: _Pleochroism._—Varies with the color, being more distinct in colored crystals, yellowish (the lighter color) ∥ a′ and reddish-brown ∥ c′. Scarcely noticed when the color is light. _▄Crossed Nicols▄_: _Double Refraction._—Very strong (γ − α = 0.090 to 0.141). [Illustration: FIG. 64.—Titanite, showing twinning in nepheline-syenite. (From Cohen.) ] _Interference Colors._—Very high order, like those of calcite. Due to the fact that the refractive indices of two of the rays are very nearly alike, some sections may show very low order colors. _Extinction._—Extinction angles not characteristic. There may be no complete extinction in white light, owing to dispersion. _▄Convergent Light▄_: On account of the very strong characteristic dispersion of the optic axes (ρ > ν), the axial angle varies a good deal with the color of the light used, (St. Gotthard) 2_E_{IA}_ = 57°, 2_E_{Tl}_ = 47°. By using colored glasses this variation in the axial angle can be seen. The axial plane lies in the clino pinacoid (010), hence bisects the obtuse angle in the rhombic cross-section, Fig. 63. Bx_{_a_}.(c) Λ _ć_ = 51° front. The optical character is (+). ▄Alteration▄: May take place. ▄Distinguished from▄: