Mullite | |
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![]() 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[1] |
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 | [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]
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