▄Crossed Nicols▄: Double refraction weak (γ − α = 0.010). Interference colors middle first order, white to yellow, etc. (about like quartz). _Extinction_ in general parallel or symmetrical (in cross-sections) to cleavages or crystal outline. In _convergent light_, Ax. pl. ∥ (100), Fig. 48. Bx_{_a_}. ∥_ć_, axial angle large (2_E_ > 180°); optical character (+). [Illustration: FIG. 48.—Staurolite, cross-section. ] ▄Alteration▄: Rarely takes place. ▄Distinguished from▄: TITANITE, see under the latter mineral. REMARKS: Found in metamorphic schists, associated with cyanite (disthene), iolite (cordierite), andalusite, etc. It is one of the minerals produced by thermal metamorphism, hence found in rocks of granite contact zones. It does not occur in the eruptive rocks or in schists rich in amphibole. Staurolite is not acted on by hydrochloric acid. H., 7 to 7.5. Sp. gr., 3.4 to 3.8. THE ORTHORHOMBIC PYROXENES. Enstatite and Hypersthene. ANISOTROPIC. BIAXIAL. ORTHORHOMBIC. COMPOSITION: (Mg.Fe)SiO_{3}. _ć_ = c. ELONGATION ∥ c′. Enstatite contains little, if any, Fe. Hypersthene contains more Fe, its optical characters beginning to show with about 10 per cent. [Illustration: FIG. 49.—Enstatite, showing columnar or fibrous structure ∥ _ć_ axis. _Norite_, Harzburg. ] ▄Usual Appearance in Sections▄: Irregularly bounded individuals (E) or rounded prismatic-pyramidal crystals (H). Columnar or fibrous structure ∥ _ć_ often shows in (E), Fig. 49. Prism angle about 92°. Outline of crystal sections very similar to that of monoclinic pyroxenes. _Twinning._—Not so common as in monoclinic pyroxenes. Parallel growths with monoclinic pyroxene (diallage) occur.[89] _Color._—Varies with Fe per cent., (E) colorless, (H) brownish. _Index of refraction._—_n′_ = 1.665 (E) to 1.723 (H) (α = 1.660 to 1.716, γ = 1.670 to 1.729) (about the same as in monoclinic pyroxene), hence _relief_ marked and surface rough. _Cleavage._—Variable, parallel to prism (angle 92°) common to all pyroxenes. Also cleavage or parting parallel to brachy pinacoid (010) (prominent) and macro pinacoid (100).[89] _Inclusions._—Parallel oriented, brownish plates and rods, producing “schiller” structure on the principal cleavage faces, Fig. 15. Glass inclusions abundant in (H). ▄Polarized Light▄: _Pleochroism._—Almost absent in (E), but distinct in (H), increasing with Fe per cent. The change in color may be very marked, from brownish-red to greenish ∥ _ć_. _▄Crossed Nicols▄_: _Double Refraction._—Weak, much weaker than in the monoclinic pyroxenes, increasing with Fe per cent. (γ − α = 0.010 (E) to 0.013 (H).) _Interference Colors._—Higher first order, about the same or a little higher than quartz.