Muscovite

For other uses, see Muscovite (disambiguation).
Muscovite

Muscovite with albite from Doce valley, Minas Gerais, Brazil (dimensions: 6×5.3×3.9 cm)
General
Category Silicate mineral Phyllosilicate
Formula
(repeating unit)		 KAl2(AlSi3O10)(F,OH)2
Strunz classification 09.EC.15
Dana classification 71.02.02a.01
Crystal system Monoclinic (2/m), space group C 2/m
Space group 2/m – prismatic
Unit cell a = 5.199 Å, b = 9.027 Å, c = 20.106 Å, β = 95.78°; Z = 4
Identification
Color White, grey, silvery
Crystal habit Massive to platy
Twinning Common on the [310], less common on the {001}
Cleavage Perfect on the {001}
Fracture Micaceous
Tenacity Elastic
Mohs scale hardness 2–2.5 parallel to {001}
4 right angle to {001}
Luster Vitreous, silky, pearly
Streak White
Diaphaneity Transparent to translucent
Specific gravity 2.76–3
Optical properties Biaxial (-)
Refractive index nα = 1.552–1.576
nβ = 1.582–1.615
nγ = 1.587–1.618
Birefringence δ = 0.035 – 0.042
Pleochroism Weak when colored
Dispersion r > v weak
Ultraviolet fluorescence None
References [1][2][3]

Muscovite (also known as common mica, isinglass, or potash mica[4]) 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 m × 3 m have been found in Nellore, India.[5]

Muscovite with beryl (var. morganite) from Paprok, Afghanistan (dimensions: 5.9×4.8×3.4 cm)

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. 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 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 Russian tsar Ivan the Terrible, in 1568.

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

References

  1. ↑ Muscovite mineral information and data Mindat
  2. ↑ Muscovite Mineral Data Webmineral
  3. ↑ Handbook of Mineralogy
  4. ↑ Encyclopædia Britannica
  5. ↑ P. C. Rickwood (1981). "The largest crystals" (PDF). American Mineralogist. 66: 885–907.
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