|Crystal symmetry||Orthorhombic (2/m 2/m 2/m) dipyramidal|
|Unit cell||a = 7.5785(6) Å, b = 7.6817(7) Å, c = 2.8864(3) Å; Z = 1|
|Color||Colorless to pale pink or grey|
|Crystal habit||Prismatic to acicular crystals|
|Cleavage||Good on |
|Mohs scale hardness||6 - 7|
|Diaphaneity||Transparent to translucent|
|Specific gravity||3.11 - 3.26|
|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°|
Mullite or porcelainite is a rare silicate mineral of post-clay genesis. 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 Al sites: two distorted tetrahedral Al sites and one Al other site which adopts a higher co-ordinate octahedral state.
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 Sithean Sluaigh, Scotland.
Use in porcelain
In 2006 researchers at University College London and Cardiff University discovered that potters in the Hesse region of Germany since the late Middle Ages had used mullite in the manufacture of a type of crucible (known as Hessian crucibles), that were renowned for enabling alchemists to heat their crucibles to very high temperatures. 
The formula for making it (using kaolinitic clay and then firing it at temperatures above 1100 °C) was kept a closely guarded secret.
Mullite morphology also is important for its application. In this case, there are two common morphologies for mullite. One is a platelet shape with low aspect ratio and the second is a needle shape with high aspect ratio. If the needle shape mullite can form in a ceramic body during sintering, it has an effect on both the mechanical and physical properties by increasing the mechanical strength and thermal shock resistance. The most important condition relates to ceramic chemical composition. If the silica and alumina ratio with low basic materials such as sodium and calcium is adjusted, the needle shape mullite forms at about 1400 °C and the needles will interlock. This mechanical interlocking causes the porcelain to have high mechanical strength.
Use as a catalyst
Recent research indicates that a synthetic analogue of mullite can be an effective replacement for platinum in reducing the amount of pollution generated by diesel engines.
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- Martinón-Torres, Marcos; Thilo Rehren and Ian C. Freestone (November 2006). "Mullite and the mystery of Hessian wares" (PDF online reproduction). Nature (London: Nature Publishing Group) 444 (7718): pp.437–438. Bibcode:2006Natur.444..437M. doi:10.1038/444437a. PMID 17122847.
- Schneider, Hartmut (2005). "General Introduction". In Hartmut Schneider and Sridhar Komarneri (eds.). Mullite. Weinheim, Germany: Wiley-VCH. pp. xv–xx. ISBN 978-3-527-30974-0. OCLC 58051400.