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 abundant mica, occurring in granites, pegmatites, gneisses, and schists, as well as in contact metamorphic rocks and as a secondary mineral formed from the alteration of minerals such as topaz, feldspar, and kyanite. Its crystal system is monoclinic, and it exhibits a highly perfect basal cleavage that allows it to be split into remarkably thin, elastic sheets. These sheets can be colorless, gray, violet, or red, and are either transparent or translucent. The mineral has a Mohs hardness of 2–2.25 parallel to its [001] face and 4 perpendicular to it, with a specific gravity between 2.76 and 3. It is anisotropic with high birefringence. Notable varieties include fuchsite, a green chromium-rich type, and mariposite, also chromium-rich. The name "muscovite" originates from "Muscovy-glass," a term used in Elizabethan England because the mineral was employed in medieval Russia (Muscovy) as an inexpensive substitute for window glass. This usage was first recorded in 1568 in letters by George Turberville, secretary to England’s ambassador to Tsar Ivan the Terrible. In cold climates, mica was preferred over glass for windows as it does not crack from freezing temperatures. Muscovite is distinguished from other micas by its pseudohexagonal crystal shape and perfect cleavage into elastic sheets; it is lighter in color than biotite and lacks the greasy feel of pyrophyllite or talc. Paragonite is similar but far less common. Commercially, immense sheets found in pegmatites are valuable for manufacturing fireproofing and insulating materials, and it is also used as a lubricant, in electronics, paints, plastics, wallboard, cosmetics, and as a mold release agent in tire production.
- 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, also known as common mica, is a hydrated phyllosilicate mineral of aluminium and potassium. Its defining characteristic is a highly perfect basal cleavage that allows it to be split into remarkably thin, elastic laminae. The mineral is anisotropic with high birefringence and belongs to the monoclinic crystal system. It has a Mohs hardness of 2–2.25 parallel to its perfect cleavage face and 4 perpendicular to it, with a specific gravity between 2.76 and 3. Muscovite can be colorless, translucent, or transparent, and may be tinted through grays, violet, or red; a chromium-rich green variety is called fuchsite. It is the most common mica, found in granites, pegmatites, gneisses, and schists, and also as a contact metamorphic rock or secondary mineral from the alteration of topaz, feldspar, or kyanite. It is characteristic of peraluminous rock with high aluminium content. In pegmatites, immense commercially valuable sheets occur. The mineral’s name derives from “Muscovy-glass,” a term used in Elizabethan England for its use in medieval Russia as a cheaper, cold-resistant alternative to glass in windows. This usage became known in England through letters by George Turberville in 1568. Muscovite is in demand for fireproofing and insulating materials, and as a lubricant.
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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