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Muscovite from a pegmatite outcrop along the Patapsco River in Baltimore County, Maryland.
CategoryMineral Species
Chemical formulaKAl2(AlSi3O10)(F,OH)2
Molecular Weight398.71 gm
ColorWhite, grey, silvery
Crystal habitmassive to platy Crystaline
Crystal systemMonoclinic - Prismatic
Twinningcommon on the [310] less common on the {001}
CleavagePerfect on the {001}
Mohs Scale hardness2-2.5, 4
Refractive indexnα = 1.552 - 1.576 nβ = 1.582 - 1.615 nγ = 1.587 - 1.618
Optical PropertiesBiaxial
Birefringenceδ = 0.035 - 0.042
Dispersionr > v weak
Pleochroismweak when colored
Ultraviolet fluorescenceNone
Specific gravity2.76 - 3
Density2.83 g/cm3
Melting pointapproximately 1320°C
Diaphaneitytransparent or translucent

Muscovite (also known as Common mica, Isinglass, or Potash mica[3]) is a phyllosilicate mineral of aluminium and potassium with formula: KAl2(AlSi3O10)(F,OH)2. It has a highly perfect basal cleavage yielding remarkably thin laminae (sheets), which are often highly elastic. Sheets of muscovite 5 metres by 3 metres have been found in Nellore, India.[4]

Muscovite melts at approximately 1320°C, has a Mohs hardness of 2 - 2.25 parallel to the [001] face, 4 perpendicular to the [001] and a specific gravity of 2.76 - 3. It can be colorless or tinted through grays, browns, greens, yellows, or (rarely) violet or red, and can be transparent or translucent. The green chromium rich variety is called fuchsite.     Muscovite is the most common mica, found in granites, pegmatites, gneisses and schists, and as a contact metamorphic rock or as a secondary mineral resulting from the alteration of topaz, feldspar, kyanite, etc. In pegmatites, it is often found in immense sheets that are commercially valuable. Muscovite is in demand for the manufacture of fireproofing and insulating materials and to some extent as a lubricant.

The name of muscovite comes from Muscovy-glass, a name formerly used for the mineral because of its use in Russia for windows. It is anisotropic, and has quite a high birefringence. Its crystal system is monoclinic.

See also


  • Mineral Handbook
  • Mindat
  • Webmineral
  1. ^
  2. ^
  3. ^ Encyclopædia Britannica
  4. ^ The Complete Encyclopedia of Minerals by P. Korbel and M. Novak
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Muscovite". A list of authors is available in Wikipedia.
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