Mullite
Mullite or porcelainite 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 | |
|---|---|
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 was first described in 1924 for an occurrence on the Isle of Mull, Scotland. 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.
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