| Mullite | |
|---|---|
 White, filamentous mullite in front of thickerosumilite platelets (Photo width 1.5 mm) Found in Wannenköpfe,Ochtendung,Eifel, Germany | |
| General | |
| Category | Nesosilicate |
| Formula | 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 orporcelainite[6] is a raresilicate mineral formed duringcontact metamorphism ofclay minerals. It can form twostoichiometric forms: 3Al2O32SiO2 or 2Al2O3 SiO2. Unusually, mullite has no charge-balancingcations present. As a result, there are three different aluminiumsites: two distortedtetrahedral and oneoctahedral.
Mullite was first described in 1924 for an occurrence on theIsle of Mull,Scotland.[5] It occurs asargillaceous inclusions involcanic rocks in the Isle of Mull, inclusions insillimanite within atonalite atVal Sissone,Italy and withemerylike rocks inArgyllshire, Scotland.[3]
Mullite (porcelainite) can be found as a constituent mineral in a type ofthermally-metamorphosed rock calledporcellanite.[7]
Mullite is present in the form of needles inporcelain.[8]
It is produced during various melting and firing processes, and is used as arefractory material,[9] because of its high melting point of 1840 °C.[10]
In 2006 researchers atUniversity College London andCardiff University discovered that potters in theHesse region of Germany since the lateMiddle Ages had used mullite in the manufacture of a type ofcrucible (known asHessian crucibles), that were renowned for enabling alchemists to heat their crucibles to very high temperatures.[11][12] The formula finally replicated in the above studies (usingkaolinitic clay and then firing it at temperatures above 1100 °C) was kept a closely guarded secret by those crucible makers since the 15th century.
Mullite morphology is also 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 duringsintering, 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 thesilica andalumina 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 contributes to the high mechanical strength of porcelain.[13][14]
Further recent research indicates that a synthetic analogue of mullite can be an effective replacement for platinum in diesel engines for exhaust management.[15]
Authority control databases: National |
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