_Uralite._—Pyroxene altered to amphibole, having the outward crystal form of pyroxene and the physical characters and cleavage of amphibole. The change usually commences on the surface and the uralite does not form a single compact crystal, but consists of numerous slender columns parallel to one another. These little columns or fibers have their _c_ and [_=b_] axes parallel to the positions of these axes in the parent mineral. The color is green and the pleochroism weak to strong. This change is called “_uralitization_” and results from hydrochemical processes. When the alteration is not complete, portions of the original pyroxene may be left, having all the characteristic optical properties of this latter mineral. _Anthophyllite_, the orthorhombic amphibole, with always parallel extinction, is sometimes found in colorless to brownish, blade- to rod-like aggregates in crystalline schists and serpentine. MICA GROUP. ANISOTROPIC. BIAXIAL. MONOCLINIC. May appear hexagonal or orthorhombic. COMPOSITION: ELONGATION (∥ cleavage) ∥ c′. _Biotite_ (black or ferro-magnesium mica) = (H.K)_{2}(Mg.Fe)_{2}Al_{2}(SiO_{4})_{3}, approx. _Phlogopite_ = a magnesium mica, near biotite, but containing little Fe. [Illustration: FIG. 59.—Mica. _A_, biotite, showing hexagonal cross-section and zonal markings. _Minette_, Freiburg. _B_, Biotite, showing strong absorption parallel to cleavage and also zonal marking (_P_ = plane of vibration of polarizer). _Minette_, Cumberland. _C_, Muscovite in bent shreds in gneiss. ] _Muscovite_ (white or potash mica) = H_{2}(K.Na)Al_{3}(SiO_{4})_{3}, with some replacement by Mg or Fe. ▄Usual Appearance in Sections▄: Scales, which may be notched or jagged, with lateral sections lath-shaped; or shreds, Fig. 59 C. When distinctly crystallized (magnesium micas) the thin hexagonal plates have plane angles of 120°, Fig. 59 A. Phlogopite crystals may be extended in direction of _ć_ axis. Zonal structure not uncommon in the magnesium micas, Fig. 59 A, which may also have dark iron ore border like hornblende. _Twinning._—Common, generally parallel to base; seen in sections showing cleavage by variations in extinction, in basal sections by distorted interference figures. Micas of different kinds often associated together in parallel position, also intergrown with hornblende, pyroxene, chlorite and quartz. _Color._—Depends on chemical composition. Biotites, brown, green or red to almost opaque. Phlogopites, colorless or yellowish. Muscovites colorless. _Index of Refraction._—_n′_ = 1.564 to 1.619 (α = 1.541 to 1.580, γ = 1.575 to 1.638), hence somewhat marked _relief_ and surface varies in appearance from slightly rough to fairly rough. In polarized light the surface appears roughest when the cleavage cracks are parallel to the plane of the polarizer. Biotite has more marked relief.