Dictionary: Muscovite   (mŭs'kə-vīt')

A native or resident of Moscow or Muscovy.

Of or relating to Moscow, Muscovy, or the Muscovites.

A mineral of the mica group with an ideal composition of KAl₂(AlSi₃)O₁₀(OH)₂. Sometimes it is referred to as a white mica or potash mica.

Physical properties include specific gravity 2.76–2.88, hardness on the Mohs scale 2–2.5, and luster vitreous to pearly. Thin sheets are flexible and may be colorless, with books (thick crystals) translucent, yellow, brown, reddish, or green. Muscovite occurs commonly in all the major rock types, in igneous rocks (granites, pegmatites, and hydrothermal alteration products), in metamorphic rocks (slates, phyllites, schists and gneisses), and in sedimentary rocks (sandstones and other clastic rocks). As larger flakes, muscovite is used as an electrical insulator, both for its dielectric properties and for its resistance to heat. Ground muscovite is used for fireproofing, as an additive to paint to provide a sheen and for durability, as a filler, and for many other applications. See also Mica; Silicate minerals.

For more information on muscovite, visit Britannica.com.

Environment

One of the common rock-forming minerals, an important constituent of granite and the main constituent of some schists, but best developed in pegmatite dikes.

Crystals are common in occurrences of mineralogical interest, though rare in relation to the abundance of rock-forming muscovite in general. Usually tabular parallel to the cleavage, often hexagonal in outline. Also fine-grained, sometimes so granular and compact as not to resemble a mica at all. May have inclusions between the sheets: flat, opaque black magnetite and/or hematite crystals that develop sets of crystallographically aligned cracks, red hematite, and flattened almandine garnets.

White, light yellow, colorless, amber, bright rose, green. Luster glassy to pearly; hardness 2-2Ɖ, specific gravity 2.8-3.0; cleavage perfect basal. Easily peeled plates flexible; translucent to transparent (thick crystals pearly and opaque on the cleavage face, but, through the sides, often very transparent, brown or emerald green).

Alkaline potassium, aluminum silicate, often with impurities of many other elements (11.8% K ₂ O 38.5% Al ₂ O ₃ , 45.2% SiO ₂ , 4.5% H ₂ O).

Variable fusibility, sometimes almost infusible, rounding and whitening a little on the edges of the flakes; sometimes actually melting and bubbling slightly. Insoluble in acid.

The thin, flexible, and elastic cleavage flakes distinguish it from most other minerals except other micas. Greater flexibility distinguishes it from margarite. Sharpness of the prism faces and elasticity of the basal plates distinguish even green crystals from the chlorites. Selenite gypsum has been mistaken for muscovite, but it cannot be split into the thin, flexible, elastic sheets obtainable from muscovite. Pink muscovite resembles lepidolite but is far less fusible and does not color the flame red. The fine-grained, compact type is nondescript, difficult or impossible to recognize without microscopic tests. Phlogopite is darker and is decomposed by sulfuric acid. Biotite is very dark to black.

Muscovite is found wherever igneous and metamorphic rocks are found. The best crystals are in pegmatites and may be free-growing or embedded. The largest crystals are always embedded, and are mined in pegmatite areas in New England, North Carolina, the Black Hills of South Dakota, and in Colorado. India and Brazil are commercial sources. Beautifully zoned green crystals have been found near Salt Lake City, Utah, and small, bright "green-edged" crystal aggregates are found in Lincoln Co., North Carolina. Rose muscovite has been found in Goshen, Massachusetts, Amelia, Virginia, and in abundance in Dixon, New Mexico.

Other minerals are often trapped between the plates of the muscovite and grow in characteristic patterns of flattened crystals. The flat garnets of Spruce Pine, North Carolina, are particularly fine in this respect.

It was once valued as window-making material, derived from Russia -- whence the name muscovite -- and is still used in iron-stove windows. More important as an insulator for electrical equipment (screw-in fuses with mica windows, older electric toasters). Large clear sheets were once of great commercial value. Scrap mica has many uses, from lubricant to Christmas-tree "snow." A fluorine-bearing muscovite is synthesized in medium-sized crystal plates and is used in a crushed ceramic aggregate form, not in sheets like natural micas.

This article is about the mineral. For the people, see Muscovy.

Muscovite
Muscovite from a pegmatite outcrop along the Patapsco River in Baltimore County, Maryland.
General
Category	Mineral Species
Chemical formula	KAl2(AlSi3O10)(F,OH)2
Identification
Molecular Weight	398.71 gm
Color	White, grey, silvery
Crystal habit	massive to platy Crystaline
Crystal system	Monoclinic - Prismatic
Twinning	common on the [310] less common on the {001}
Cleavage	Perfect on the {001}
Fracture	Micaeous
Tenacity	Elastic
Mohs Scale hardness	2-2.5, 4
Luster	Vitreous
Refractive index	nα = 1.552 - 1.576 nβ = 1.582 - 1.615 nγ = 1.587 - 1.618
Optical Properties	Biaxial
Birefringence	δ = 0.035 - 0.042
Dispersion	r > v weak
Pleochroism	weak when colored
Ultraviolet fluorescence	None
Streak	White
Specific gravity	2.76 - 3
Density	2.83 g/cm3
Melting point	approximately 1320°C
Diaphaneity	transparent or translucent
References	[1][2]

Muscovite (also known as Common mica, Isinglass, or Potash mica^[3]) is a phyllosilicate mineral of aluminium and potassium with formula: KAl₂(AlSi₃O₁₀)(F,OH)₂. It has a highly perfect basal cleavage yielding remarkably thin laminae (sheets), which are often highly elastic. Sheets of muscovite 5 metres by 3 metres have been found in Nellore, India.^[4]

Muscovite melts at approximately 1320°C, 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. The green chromium rich variety is called fuchsite.

Muscovite layer

Fuchsite from Slovakia

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. It is anisotropic, and has quite a high birefringence. Its crystal system is monoclinic.

See also

• List of minerals

References

• Mineral Handbook
• Mindat
• Webmineral

1. ^ http://www.mindat.org/min-2815.html
2. ^ http://www.webmineral.com/data/Muscovite.shtml
3. ^ Encyclopædia Britannica
4. ^ The Complete Encyclopedia of Minerals by P. Korbel and M. Novak

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