Muscovite
Common mica with perfect cleavage, used historically as window glass.
Muscovite, also known as common mica, isinglass, or potash mica, is a hydrated phyllosilicate mineral of aluminium and potassium with the formula KAl2(AlSi3O10)(OH)2. It is the most common mica, found in granites, pegmatites, gneisses, and schists, and is valued for its highly perfect basal cleavage, which yields remarkably thin, elastic laminae. Its name derives from Muscovy-glass, a term used in Elizabethan England due to its use in medieval Russia as a cheaper alternative to glass in windows.
- formula
- KAl2(AlSi3O10)(OH)2
- specific_gravity
- 2.76–3
- crystal_system
- Monoclinic
- color
- Colorless, gray, violet, red; transparent or translucent
- known_for
- Perfect basal cleavage, elastic sheets, use as glass substitute and insulator
Lore & Background
Muscovite has been known since at least the sixteenth century, when its use in Russia as window panes became widely known in England. In cold countries, mica was inserted into windows as large slabs because it does not crack from the cold, serving as a cheaper alternative to glass. Sheets of muscovite as large as 5 by 3 meters have been found in Nellore, India.
Reader's Guide
Muscovite is the most common mica and is characteristic of peraluminous rock, where aluminum content is high. It is found in immense sheets in pegmatites, which are commercially valuable. Its perfect basal cleavage allows it to be split into thin, elastic sheets, distinguishing it from softer minerals like pyrophyllite and talc, and from darker biotite. Muscovite is in demand for fireproofing and insulating materials, as a lubricant, and for high-temperature applications such as industrial furnace windows. It is also used in electronics, as a filler in paints, plastic, and wallboard, in tire manufacturing as a mold release agent, in drilling mud, and in cosmetics for its luster. The green, chromium-rich variety is called fuchsite; mariposite is also a chromium-rich type. Muscovite from Brazil is red due to manganese(3+).
Did You Know?
- Muscovite sheets as large as 5 by 3 meters have been found in Nellore, India.
- The name muscovite comes from Muscovy-glass, used in Elizabethan England due to its use in medieval Russia as a cheaper alternative to glass.
- The green, chromium-rich variety of muscovite is called fuchsite.
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