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, general term for a large group of minerals, hydrous silicates of aluminum and potassium, often containing magnesium, ferrous iron, ferric iron, sodium, and lithium and more rarely containing barium, chromium, and fluorine.
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(from Muscovy, the old name for Russia, from where large sheets of the mineral were exported to the West under the name of Muscovy glass), a mineral from the mica group with the empirical formula KA12[[AlSi3O10] • (OH)2. Crystals are tabular and in the monoclinic system. Basal cleavage is extremely perfect. Muscovite is easily split into very thin sheets. This is caused by its crystalline structure, which is composed of three-layer packets of two sheets of aluminum-oxygen and silicon-oxygen tetrahedrons connected through a layer of octahedrons, in the center of which are located aluminum ions, each surrounded by four oxygen ions and two OH groups. One-third of the octahedrons are not filled with aluminum ions. The packets are interconnected through potassium ions.
Muscovite has a hardness on Mohs’ scale of 2.5–3; its density is 2,760–3,100 kg/m3. The mineral is usually colorless; sometimes, light brown, pale green, or other colors. Its luster is vitreous; on the cleavage planes the luster is pearly or silvery. Muscovite is sometimes found as cryptocrystalline masses with a silky luster. In this form it is called sericite.
Muscovite is a widespread mineral and is found as a component of magmagene rocks and also metamorphic rocks such as granites, granite pegmatites, syenites, greisens, crystalline schists, and gneisses. Of industrial significance is the muscovite recovered from pegmatite veins, where the mineral occurs as large crystals or as accumulations measuring 1–2 m thick.
Muscovite is most frequently used in electrical insulation. In industry, it is used in the form of sheet mica (for insulators, condensers, and telephones), mica powder (for roofing paper, mica board, and fireproof paints), and mica products (for electrical insulating gaskets in electric instruments). In the USSR, muscovite deposits are found on the Kola Peninsula and in Eastern Siberia near Mama and Kanska. Muscovite is also found in India, the Malagasy Republic, Canada, the United States, and Brazil.