Muscovy glass

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

Stereo image

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Right frame

Parallel view ()

Cross-eye view ()

Small specimen of Muscovite (fuchsite) from Brazil.

Muscovite (also known as common mica, isinglass, or potash mica) 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 m × 3 m have been found in Nellore, India.

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.