Muscovite

n.
A native or resident of Moscow or Muscovy.

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

For more information on muscovite, visit Britannica.com.

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.

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KAl
Monoclinic -- prismatic

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.

Crystal description

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.

Physical properties

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).

Composition

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).

Tests

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.

Distinguishing characteristics

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.

Occurrences

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.

Remarks

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.

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