Muscovite

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Muscovite
Muscovite-Albite-122887.jpg
Muscovite with albite from Doce valley, Minas Gerais, Brazil (dimensions: 6×5.3×3.9 cm)
General
CategorySilicate mineral Phyllosilicate
Formula
(repeating unit)
KAl2(AlSi3O10)(F,OH)2
Strunz classification09.EC.15
Dana classification71.02.02a.01
Crystal symmetry2/m – prismatic
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
Crystal systemMonoclinic (2/m), space group C 2/m
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]
 
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Muscovite
Muscovite-Albite-122887.jpg
Muscovite with albite from Doce valley, Minas Gerais, Brazil (dimensions: 6×5.3×3.9 cm)
General
CategorySilicate mineral Phyllosilicate
Formula
(repeating unit)
KAl2(AlSi3O10)(F,OH)2
Strunz classification09.EC.15
Dana classification71.02.02a.01
Crystal symmetry2/m – prismatic
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
Crystal systemMonoclinic (2/m), space group C 2/m
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]

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 5×3 m have been found in Nellore, India.[5]

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

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 tzar Ivan the Terrible, in 1568.

References[edit]

Right frame 
Fuschite3d.jpg
Small specimen of Muscovite (fuchsite) from Brazil.