Mullite

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
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	[1][2][3][4]

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.^[4] 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.^[2]

Porcellanite

Mullite (porcelainite) can be found as a constituent mineral in a type of thermally-metamorphosed rock called porcellanite.^[7]

Use in porcelain

Mullite is present in the form of needles in porcelain.^[8]

It is produced during various melting and firing processes, and is used as a refractory material,^[9] because of its high melting point of 1840 °C.^[10]

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.^[11][12]

The formula for making it (using kaolinitic clay and then firing it at temperatures above 1100 °C) was kept a closely guarded secret.^{[citation needed]}

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 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 contributes to the high mechanical strength of porcelain.^[13][14]

Use as a catalyst

Recent research indicates that a synthetic analogue of mullite can be an effective replacement for platinum in diesel engines for exhaust management.^[15]

Notes

• Martinón-Torres, Marcos; Thilo Rehren; Ian C. Freestone (November 2006). "Mullite and the mystery of Hessian wares" (PDF online reproduction). Nature (London: Nature Publishing Group) 444 (7718): 437–438. doi:10.1038/444437a. PMID 17122847. Bibcode: 2006Natur.444..437M. http://www.ucl.ac.uk/archaeology/staff/profiles/martinon/Martinon-Torres%20et%20al%202006%20Nature.pdf. 
• Schneider, Hartmut (2005). "General Introduction". in Hartmut Schneider. Mullite. Weinheim, Germany: Wiley-VCH. pp. xv–xx. ISBN 978-3-527-30974-0. OCLC 58051400. 
• "Sithean Sluaigh, Strathclyde, Scotland, UK". MinDat. http://www.mindat.org/loc-5100.html. Retrieved 6 November 2015. 

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