Muscovite

{{Infobox mineral | name = Muscovite | category = Mineral Species | boxwidth = | boxbgcolor = | image = Muscovite2.jpg | imagesize = 200px | caption = Muscovite from a pegmatite outcrop along the Patapsco River in Baltimore County, Maryland. | formula = KAl₂(AlSi₃O₁₀)(F,OH)₂ | molweight = 398.71 gm | color = White, grey, silvery | habit = massive to platy | system = Monoclinic-prismatic | twinning = common on the [310] less common on the {001} | cleavage = Perfect on the {001} | fracture = Micaeous | tenacity = Elastic | mohs = 2–2.5, 4 | luster = Vitreous | polish = | refractive = n_α = 1.552–1.576 n_β = 1.582–1.615 n_γ = 1.587–1.618 | opticalprop = Biaxial | birefringence = δ = 0.035 - 0.042 | dispersion = r > v weak | pleochroism = weak when colored | fluorescence= None | absorption = | streak = White | gravity = 2.76–3 | density = 2.83 g/cm₃ | melt = about 1320 °C | fusibility = | diagnostic = | solubility = | diaphaneity = transparent or translucent | other = | references = }}

Muscovite (also known as Common mica, Isinglass, or Potash mica) is a phyllosilicate mineral of aluminium and potassium with formula KAl₂(AlSi₃O₁₀)(F,OH)₂, or (KF)₂(Al₂O₃)₃(SiO₂)₆(H₂O). It has a highly-perfect basal cleavage yielding remarkably-thin laminæ (sheets) which are often highly elastic. Sheets of muscovite 5 metres by 3 metres have been found in Nellore, India.

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 high birefringence. Its crystal system is monoclinic.

See also

• List of minerals

References

• Mineral Handbook
• Mindat
• Webmineral