Muscovite

Fluorapatite with Muscovite Chumar Bakhoor, Hunza Valley, Gilgit District, Gilgit-Baltistan, Pakistan Size: 8.5 x 5.0 x 5.0 cm © Dan Weinrich

Chemical Formula: KAl2(AlSi3O10)(OH)2
Locality: Common world wide.
Name Origin: From Muscovy glass, alluding to the Russian province of Muscovy.

Muscovite  is a phyllosilicate mineral of aluminium and potassium . 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.

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 tzar Ivan the Terrible, in 1568.

History

Discovery date : 1850

Optical properties

Optical and misc. Properties: Transparent – Translucent
Refractive Index : from 1,55 to 1,61
Axial angle 2V : 30-47°

Physical Properties

Cleavage: {001} Perfect
Color:     White, Gray, Silver white, Brownish white, Greenish white.
Density: 2.77 – 2.88, Average = 2.82
Diaphaneity: Transparent to translucent
Fracture: Brittle – Sectile – Brittle fracture with slightly sectile shavings possible.
Hardness: 2-2.5 – Gypsum-Finger Nail
Luminescence: Non-fluorescent.
Luster: Vitreous (Glassy)
Streak: white

Photos :

Muscovite, Albite (Var: Cleavelandite) Locality: Divino das Laranjeiras, Doce valley, Minas Gerais, Brazil Dimensions: 6 cm x 5.3 cm x 3.9 cm Photo Copyright © Rob Lavinsky & irocks
Brazilianite on Muscovite Corrego Frio, Minas Gerais, Brazil Miniature, 4.9 x 4.3 x 2 cm © irocks
Cassiterite on Muscovite Aracuai, Minas Gerais, Brazil Small Cabinet, 6.9 x 5.1 x 3.5 cm © irocks
Apatite-CaF and muscovite Zé Pinto prospect, Minas Gerais, Brazil Specimen weight:358 gr. Crystal size:32 mm Overall size: 130mm x 130 mm x 85 mm © minservice
Cassiterite with Muscovite Huya deposit, Mt Xuebaoding, Pingwu Co., Mianyang Prefecture, Sichuan Province, China Size: 11.5 x 9.5 x 8.5 cm (cabinet) © danweinrich
Arsenopyrite-Marcasite with Muscovite Minas da Panasqueira, Aldeia de São Francisco de Assis, Covilhã, Castelo Branco, Centro  Portugal (1987-2013) Specimen size: 5.5 × 5.4 × 3.7 cm = 2.2” × 2.1” × 1.5” Main crystal size: 1.5 × 1 cm = 0.6” × 0.4” © Fabre Minerals
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