Mullite

Mullite
Mullite
	White, filamentous mullite in front of thicker osumilite platelets; (Photo width 1.5 mm); Found in Wannenköpfe, Ochtendung, Eifel, Germany
General
Category	Nesosilicate
Formula; (repeating unit)	Al6Si2O13
IMA symbol	Mul
Strunz classification	9.AF.20
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) ; H-M symbol: (2/m 2/m 2/m)
Space group	Pbnm, Pnnm
Unit cell	a = 7.5785(6) Å, ; b = 7.6817(7) Å, ; c = 2.8864(3) Å; Z = 1
Identification
Color	Colorless to pale pink or grey
Crystal habit	Prismatic to acicular crystals
Cleavage	Good on [010]
Optical properties	Biaxial (+)
Refractive index	nα = 1.642 - 1.653 nβ = 1.644 - 1.655 nγ = 1.654 - 1.679
Birefringence	δ = 0.012 - 0.026
2V angle	Measured: 20° to 50°
References

Mullite or porcelainite^[6] is a rare silicate mineral formed during contact metamorphism of clay minerals. It can form two stoichiometric forms: 3Al₂O₃2SiO₂ or 2Al₂O₃ SiO₂. Unusually, mullite has no charge-balancing cations present. As a result, there are three different aluminium sites: two distorted tetrahedral and one octahedral.

Mullite was first described in 1924 for an occurrence on the Isle of Mull, Scotland.^[5] It occurs as argillaceous inclusions in volcanic rocks in the Isle of Mull, inclusions in sillimanite within a tonalite at Val Sissone, Italy and with emerylike rocks in Argyllshire, Scotland.^[3]

^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
^ Mineralienatlas
^ ^a ^b http://rruff.geo.arizona.edu/doclib/hom/mullite.pdf Handbook of Mineralogy
^ http://www.mindat.org/min-2806.html Mindat
^ ^a ^b http://webmineral.com/data/Mullite.shtml Webmineral
^ Neuendorf, K.K.E.; Mehl, J.P. Jr.; Jackson, J.A., eds. (2005). Glossary of Geology (5th ed.). Alexandria, Virginia: American Geological Institute. p. 428. ISBN 978-0922152896.