Vermiculite

Vermiculite
General
Category	Phyllosilicates
Formula (repeating unit)	(Mg,Fe2+,Fe3+)3[(Al,Si)4O10](OH)2·4H2O
IMA symbol	Vrm[1]
Strunz classification	9.EC.50
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	C2/m
Unit cell	a = 5.24 Å, b = 9.17 Å c = 28.6 Å; β = 94.6°; Z = 2
Identification
Color	Colorless, white, yellow, green, brown, black
Crystal habit	As large crystalline plates to clay-sized particles; lamellar to scaley
Cleavage	Perfect on {001}
Tenacity	Pliable
Mohs scale hardness	1.5–2
Luster	Greasy or vitreous (pearly at cleavage planes)
Streak	White or yellowish, translucent, shiny, light-brown or greenish in color, in some cases. For example, palabora vermuculite.
Diaphaneity	Translucent
Specific gravity	2.4–2.7 (0.065–0.130 when exfoliated)
Optical properties	Biaxial (−)
Refractive index	nα = 1.525 – 1.561 nβ = 1.545 – 1.581 nγ = 1.545 – 1.581
Birefringence	δ = 0.020
Pleochroism	X in paler shades than Y and Z
References	[2][3][4]

Vermiculite is a hydrous phyllosilicate mineral which undergoes significant expansion when heated. Exfoliation (other links: A, B) occurs when the mineral is heated sufficiently; commercial furnaces can routinely produce this effect. Vermiculite forms by the weathering or hydrothermal alteration of biotite or phlogopite.^[3] Large commercial vermiculite mines exist in the United States, Russia, South Africa, China, and Brazil.