Muscovite (also known ascommon mica,isinglass, orpotash mica^[6]) is a hydratedphyllosilicate mineral ofaluminium andpotassium with formula KAl₂(AlSi₃O₁₀)(F,OH)₂, or (KF)₂(Al₂O₃)₃(SiO₂)₆(H₂O). It has a highly perfectbasal cleavage yielding remarkably thin laminae (sheets) which are often highlyelastic. Sheets of muscovite 5 by 3 meters (16.4 ft × 9.8 ft) have been found inNellore,India.^[7]

Muscovite
Muscovite withalbite fromDoce valley,Minas Gerais, Brazil (dimensions: 6 × 5.3 × 3.9 cm)
General
Category	Phyllosilicateminerals,micagroup
Formula	KAl2(AlSi3O10)(F,OH)2
IMA symbol	Ms[1]
Strunz classification	9.EC.15
Dana classification	71.02.02a.01
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (sameH-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
Ultravioletfluorescence	None
References	[2][3][4][5]

Muscovite has aMohs hardness of 2–2.25 parallel to the [001] face, 4 perpendicular to the [001] and aspecific gravity of 2.76–3. It can be colorless or tinted through grays, violet or red, and can be transparent or translucent. It isanisotropic and has highbirefringence. Its crystal system ismonoclinic. The green,chromium-rich variety is calledfuchsite;mariposite is also a chromium-rich type of muscovite.

Muscovite is the most commonmica, found ingranites,pegmatites,gneisses, andschists, and as a contactmetamorphic rock or as a secondarymineral resulting from the alteration oftopaz,feldspar,kyanite, etc. It is characteristic ofperaluminous rock, in which the content of aluminum is relatively high.^[8] In pegmatites, it is often found in immense sheets that are commercially valuable. Muscovite is in demand for the manufacture offireproofing and insulating materials and to some extent as alubricant.

The name muscovite comes fromMuscovy-glass, a name given to the mineral inElizabethan England due to its use in medievalRussia (Muscovy) as a cheaper alternative toglass in windows. This usage became widely known inEngland during the sixteenth century with its first mention appearing in letters byGeorge Turberville, the secretary of England's ambassador to theRussiantsarIvan the Terrible, in 1568.

Micas are distinguished from other minerals by their pseudohexagonal crystal shape and their perfect cleavage, which allows the crystals to be pulled apart into very thin elastic sheets.Pyrophyllite, andtalc are softer than micas and have a greasy feel, whilechlorite is green in color and its cleavage sheets are inelastic. The other common mica mineral,biotite, is almost always much darker in color than muscovite.Paragonite can be difficult to distinguish from muscovite but is much less common, though it is likely mistaken for muscovite often enough that it may be more common that is generally appreciated.^[9] Muscovite mica from Brazil is red due to manganese(3+).^[10]

Like allmica minerals, muscovite is aphyllosilicate (sheet silicate) mineral with aTOT-c structure. In other words, a crystal of muscovite consists of layers (TOT) bonded to each other bypotassiumcations (c).^[9]

Each layer is composed of three sheets. The outer sheets ('T' or tetrahedral sheets) consist ofsilicon-oxygen tetrahedra andaluminium-oxygen tetrahedra, with three of the oxygen anions of each tetrahedron shared with neighboring tetrahedra to form a hexagonal sheet. The fourth oxygen anion in each tetrahedral sheet is called anapical oxygen anion.^[9] There are three silicon cations for each aluminium cation but the arrangement of aluminium and silicon cations is largely disordered.^[11]

The middle octahedral (O) sheet consists of aluminium cations that are each surrounded by six oxygen orhydroxide anions forming an octahedron, with the octahedrons sharing anions to form a hexagonal sheet similar to the tetrahedral sheets. The apical oxygen anions of the outerT sheets face inwards and are shared by the octahedral sheet, binding the sheets firmly together. The relatively strong binding between oxygen anions and aluminium and silicon cations within a layer, compared with the weaker binding of potassium cations between layers, gives muscovite its perfect basal cleavage.^[9]

In muscovite, alternate layers are slightly offset from each other, so that the structure repeats every two layers. This is called the 1M polytype of the general mica structure.^[9]

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  View of tetrahedral sheet of muscovite. The apical oxygen ions are tinted pink.
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  View of octahedral sheet of muscovite. The binding sites for apical oxygen are shown as white spheres.
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  View of octahedral sheet of muscovite with ion sizes altered to emphasizing octahedral sites
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  Structure of muscovite, viewed in the [100] direction (along the layers of the crystal)

The formula for muscovite is typically given asKAl₂(AlSi₃O₁₀)(OH)₂, but it is common for small amounts of other elements to substitute for the main constituents.Alkali metals such assodium,rubidium, andcaesium substitute for potassium;magnesium,iron,lithium,chromium,titanium, orvanadium can substitute for aluminium in the octahedral sheet;fluorine orchlorine can substitute for hydroxide; and the ratio of aluminium to silicon in the tetrahedral sheets can change to maintain charge balance where necessary (as when magnesium cations, with a charge of +2, substitute for aluminium ions, with a charge of +3).^[12]

Up to 10% of the potassium may be replaced by sodium, and up to 20% of the hydroxide by fluorine. Chlorine rarely replaces more than 1% of the hydroxide. Muscovite in which the mole fraction of silicon is greater than aluminium, and magnesium or iron replaces some of the aluminium to maintain charge balance, is calledphengite.^[12]

Chromium-rich and vanadium-rich muscovite are known respectively asfuchsite androscoelite.

Muscovite can be cleaved into very thin transparent sheets that can substitute for glass, particularly for high-temperature applications such as industrial furnace or oven windows. It is also used in the manufacture of a wide variety of electronics and as afiller in paints, plastic, andwallboard. It lends a silky luster towallpaper. It is also used intire manufacture as amold release agent, indrilling mud, and in variouscosmetics for its luster.^[13]

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  Muscovite withberyl (var. morganite) from Paprok, Afghanistan
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  Muscovite (var. alurgite), from Prabornaz Mine, Aosta Valley, Italy
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  Tabular muscovite crystals in a gneiss inthin section viewed undercross-polarized light at 2x magnification.
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  Astrolite polytype of Muscovite

•   Media related toMuscovite at Wikimedia Commons