Mullite
General
Category	Silicate mineral
Chemical formula	Al6Si2O13
Identification
Color	Colorless to pale pink or grey
Crystal habit	Prismatic to acicular crystals
Crystal system	Orthorhombic
Cleavage	Good on [010]
Fracture	Brittle
Mohs scale hardness	6 - 7
Luster	Vitreous
Streak	White
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°
References	[1][2][3]

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 therefore 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.^[3] 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.^[1]

[edit] Use in porcelain

Mullite is present in the form of needles in porcelain.^[4]

It is produced artificially during various melting and firing processes, and is used as a refractory,^[5] due to its high melting point of 1840°C.^[6]

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.^[7] 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 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 mechanism, mechanical interlocking causes highly mechanical strength of the porcelain.^{[citation needed]}

[edit] Notes

• 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. doi:10.1038/444437a. http://www.ucl.ac.uk/archaeology/staff/profiles/martinon/Martinon-Torres%20et%20al%202006%20Nature.pdf. 
• 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. 

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