MUSCOVITE, or COMMON MICA, a native silicate of aluminum, potassium and hy drogen, occurring in crystals that belong to the monoclinic system, though usually hexagonal or rhombic in general form, and distinguished by the facility with which it may be split into thin laminae. Muscovite also occurs in massive forms and in scaly aggregates. It occurs in various colors, and has a lustre that is vitreous or pearly. It is transparent or translucent, with a hardness of from 2 to 3, and a specific gravity of from 2.8 to 3. It is a very com mon mineral and is an essential constituent of granite, gneiss and numerous other rocks, The transparent varieties, when obtainable in considerable size, are used for the manu facture of gas-burners chimneys, for cov ering the windows of stoves and the °peep holes" of furnaces, and for other purposes where transparency must be combined with in fusibility and with a considerable power of re sisting the prolonged action of heat. Consid
erable quantities of it are mined in Bengal and in Switzerland, and fine deposits occur in many parts of the United States. Perfectly trans parent plates a yard in diameter are sometimes found at Grafton, N. H., and similar specimens also occur in the western part of North Caro lina. Pulverized muscovite is employed in the preparation of certain kinds of paint and in the manufacture of insulating material for use about electric machinery. Muscovite is not af fected by acids, but decomposes upon being fused with the alkaline carbonates. The name "muscovite" is derived from the older popular name "muscovy-glass," which is supposed to refer to the fact that the Russians used it for window-panes. See MICA.