vermiculite

{{Infobox mineral | name = Vermiculite | category = Phyllosilicates | image = VermiculiteUSGOV.jpg | imagesize = 260px | alt = | caption = | formula = (Mg,Fe2+,Fe3+)3[(Al,Si)4O10](OH)2·4H2O | IMAsymbol = Vrm | molweight = | strunz = 9.EC.50 | dana = | system = Monoclinic | class = Prismatic (2/m)(same H-M symbol) | symmetry = C2/m | unit cell = a = 5.24 Å, b = 9.17 Å c = 28.6 Å; β = 94.6°; Z = 2 | color = Colorless, white, yellow, green, brown, black | habit = As large crystalline plates to clay-sized particles; lamellar to scaly | twinning = | cleavage = Perfect on {001} | fracture = | tenacity = Pliable | mohs = 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 | gravity = 2.4–2.7 (0.065–0.130 when exfoliated) | opticalprop = Biaxial (−) | refractive = 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 = }} Vermiculite is a hydrous phyllosilicate mineral which undergoes significant expansion when heated. Exfoliation 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. Large commercial vermiculite mines exist in the United States, Russia, South Africa, China, and Brazil.

==Occurrence== Vermiculite was first described in 1824 for an occurrence in Millbury, Massachusetts. Its name is from the Latin , "to breed worms", for the manner in which it exfoliates when heated.