| Muscovite | |
|---|---|
Muscovite with albite from Doce valley, Minas Gerais, Brazil (dimensions: 6×5.3×3.9 cm) |
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| General | |
| Category | Silicate mineral Phyllosilicate |
| Chemical formula | KAl2(AlSi3O10)(F,OH)2 |
| Strunz classification | 09.EC.15 |
| Dana classification | 71.02.02a.01 |
| Crystal symmetry | 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 |
| Crystal system | Monoclinic (2/m), space group C 2/m |
| 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 metres by 3 metres 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 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.
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| Small specimen of Muscovite (fuchsite) from Brazil. |
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