Mullite

Mullite
White, filamentous mullite in front of thicker osumilite platelets
(Photo width 1.5 mm)
Found in Wannenköpfe, Ochtendung, Eifel, Germany
General
CategoryNesosilicate
Formula
(repeating unit)
Al6Si2O13
IMA symbolMul[1]
Strunz classification9.AF.20
Crystal systemOrthorhombic
Crystal classDipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)
Space groupPbnm, Pnnm
Unit cella = 7.5785(6) Å,
b = 7.6817(7) Å,
c = 2.8864(3) Å; Z = 1
Identification
ColorColorless to pale pink or grey
Crystal habitPrismatic to acicular crystals
CleavageGood on [010]
Optical propertiesBiaxial (+)
Refractive indexnα = 1.642 - 1.653 nβ = 1.644 - 1.655 nγ = 1.654 - 1.679
Birefringenceδ = 0.012 - 0.026
2V angleMeasured: 20° to 50°
References[2][3][4][5]

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]

  1. ^ 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.
  2. ^ Mineralienatlas
  3. ^ a b http://rruff.geo.arizona.edu/doclib/hom/mullite.pdf Handbook of Mineralogy
  4. ^ http://www.mindat.org/min-2806.html Mindat
  5. ^ a b http://webmineral.com/data/Mullite.shtml Webmineral
  6. ^ 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.