Muscovite

Muscovite (also known as common mica, isinglass, or potash mica[5]) 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.[6]

Muscovite with beryl (var. morganite) from Paprok, Afghanistan
Muscovite (var. alurgite), from Prabornaz Mine, Aosta Valley, Italy
Tabular muscovite crystals in a gneiss in thin section viewed under cross-polarized light at 2x magnification.
For other uses, see Muscovite (disambiguation).
Muscovite
Muscovite with albite from Doce valley, Minas Gerais, Brazil (dimensions: 6×5.3×3.9 cm)
General
CategoryPhyllosilicate
Formula
(repeating unit)		KAl2(AlSi3O10)(F,OH)2
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
Identification
ColorWhite, grey, silvery
Crystal habitMassive to platy
TwinningCommon on the [310], less common on the {001}
CleavagePerfect on the {001}
FractureMicaceous
TenacityElastic
Mohs scale hardness2–2.5 parallel to {001}
4 right angle to {001}
LusterVitreous, silky, pearly
StreakWhite
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
References[1][2][3][4]

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.[7] 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 muscovite comes from Muscovy-glass, a name given to the mineral in Elizabethan England due to its use in medieval Russia (Muscovy) as a cheaper alternative to glass in windows. This usage became widely known in England during the sixteenth century with its first mention appearing in letters by George Turberville, the secretary of England's ambassador to the Muscovite tsar Ivan the Terrible, in 1568.

Stereo image
Right frame 
Small specimen of Muscovite (fuchsite) from Brazil.

References
  1. Mineralienatlas
  2. Muscovite mineral information and data Mindat
  3. Muscovite Mineral Data Webmineral
  4. Handbook of Mineralogy
  5. Encyclopædia Britannica
  6. P. C. Rickwood (1981). "The largest crystals" (PDF). American Mineralogist. 66: 885–907.
  7. Blatt, Harvey and Robert J. Tracy, Petrology, Freeman, 2nd ed., 1995, p. 516 ISBN 0-7167-2438-3
  • Media related to Muscovite at Wikimedia Commons
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