Muscovite

Muscovite
Muscovite with albite from Doce valley, Minas Gerais, Brazil (dimensions: 6×5.3×3.9 cm)
General
Category	Phyllosilicate
Formula (repeating unit)	KAl2(AlSi3O10)(F,OH)2
Strunz classification	9.EC.15
Dana classification	71.02.02a.01
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	C2/c
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][4]

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.

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.

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Small specimen of Muscovite (fuchsite) from Brazil.

References[]

External links[]

• Media related to Muscovite at Wikimedia Commons