MUSCOVITE, the most common variety of mica (q.v.); synonyms—Muscovy glass, biaxial mica, oblique mica, potash mica„common mica, verve de Muscorie. Trimetric crystallization. usually in hernihedral forms, with a monoclinic aspect; hexagonal prisms; cleavage parallel to the base, and easily separated, forming very thin, elastic plates, which are used in stoves under the name of " isinglass," and in Russia in windows. whence called Muscovy glass. The leaves are sometimes aggregated together in stellate, plumose (plumose mica), or globular forms, or in scales, which are sometimes in masses. Hardness. 2 to 2.5; sp. gr., 2.75 to 2.31 (Dana). Luster, pearly; color, white, gray, pale green, violet-yellow. brown and dark olive-green, and the colors vary in axial and dia metral directions. In transmission of light it from transparent to translucent. In general terms it is a silicate of 'potash and alumina, containing iron, and frequently small quantities of manganese, and hydrofluoric acid (see FLUORINE, ante). A specimen from Uto, analyzed by Rose. gave: silica, 47.50; alumina, 37.30; peroxide of iron, 3.20; peroxide of manganese, 0.90; potash. 9.60; hydrofluoric acid, 0.56; it contained also 2.63 of water. A specimen from Abborfoss contained: silica, 39.45; alumina, 9.27; peroxide
of iron, 35.73; magnesia, 3,29; potash. 5.06; fluorine, 0.29; calcium, 0.32. iron, 1.45, manganese, 2.57 90.59 (Svattherg), Mica fuses with some difilcinty before the mouth of the blow-pipe to a grayish,. blebby masa; easily dissolvea m borax and phosphorus salt.
Fine crystals of Muscovite occur in granite at Acworth, Grafton, and Alstcad, N. H., the plates being sometimes 3 ft. across and perfectly transparent. It occurs in Massa chusetts at Chesterfield with albite, and in brown, hexagonal crystals at the Conn., feldspar quarry. At Warwick, N. Y., crystals and plates a foot and more hi diameter occur in a vein of feldspar. In St. Lawrence co., 8 m. from Potsdam, on the road to Pierrepont, it occurs in plates 7 in. across; and near Saratoga in reddish brown crystals with chrysoheryl; on the Croton aqueduct, near Yonkers, in rhombic prisms, with transverse cleavage; in line, hexagonal crystals of dark brown in Chester co., Penn.; in Philadelphia co., smoky brown, with hexagonal internal bands; and at Chestnut hill, near the Wissahickon, is a green variety. It is found in Maryland, at Jones's falls, 2 in. from Baltimore, and various other localities, for which see Dana's Mineralogy.