Muscovite (also known as common mica, isinglass, or potash mica[6]) is a hydrated 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 laminae (sheets) which are often highly elastic. Sheets of muscovite 5 meters × 3 meters (16.5 feet × 10 feet) have been found in Nellore, India.[7]

Muscovite with albite from Doce valley, Minas Gerais, Brazil (dimensions: 6×5.3×3.9 cm)
(repeating unit)
IMA symbolMs[1]
Strunz classification9.EC.15
Dana classification71.02.02a.01
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupC2/c
Unit cella = 5.199 Å, b = 9.027 Å,
c = 20.106 Å, β = 95.78°; Z = 4
ColorWhite, grey, silvery
Crystal habitMassive to platy
TwinningCommon on the [310], less common on the {001}
CleavagePerfect on the {001}
Mohs scale hardness2–2.5 parallel to {001}
4 right angle to {001}
LusterVitreous, silky, pearly
DiaphaneityTransparent to translucent
Specific gravity2.76–3
Optical propertiesBiaxial (-)
Refractive indexnα = 1.552–1.576
nβ = 1.582–1.615
nγ = 1.587–1.618
Birefringenceδ = 0.035 – 0.042
PleochroismWeak when colored
Dispersionr > v weak
Ultraviolet fluorescenceNone

Muscovite 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. It is anisotropic and has high birefringence. Its crystal system is monoclinic. The green, chromium-rich variety is called fuchsite; mariposite is also a chromium-rich type of muscovite.

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. It is characteristic of peraluminous rock, in which the content of aluminum is relatively high.[8] 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.

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