Muscovite

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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	n_α = 1.552 - 1.576 n_β = 1.582 - 1.615 n_γ = 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: K Al₂(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 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.