Mullite

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Mullite
Osumilite, thick tabulars with Mullite - Ochtendung, Eifel, Germany.jpg
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 09.AF.20
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
Identification
Color Colorless to pale pink or grey
Crystal habit Prismatic to acicular crystals
Crystal system Orthorhombic
Cleavage Good on [010]
Tenacity 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. 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.[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]

Use in porcelain[edit]

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

It is produced during various melting and firing processes, and is used as a refractory material,[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] [8]

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.[citation needed]

Use as a catalyst[edit]

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.[9]

Notes[edit]

  1. ^ a b http://rruff.geo.arizona.edu/doclib/hom/mullite.pdf Handbook of Mineralogy
  2. ^ http://www.mindat.org/min-2806.html Mindat
  3. ^ a b http://webmineral.com/data/Mullite.shtml Webmineral
  4. ^ Kingery, W.D., ed. (1960). Ceramic Fabrication Processes. New York City: John Wiley & Sons, Inc. 
  5. ^ H. Schneider & S. Komarneni (2005) Mullite. Wiley, VCH, 509 pp, ISBN 3-527-30974-8
  6. ^ Kyanite Mining Corporation (2009-10-25). "Virginia Mullite". 
  7. ^ University College London (2006-11-23). "21st Century Technology Cracks Alchemists' Secret Recipe". Science Daily. ScienceDaily LLC. Retrieved 2008-01-12. 
  8. ^ Martinon-Torres M.; Freestone I.C.; Hunt, A.; Rehren, T. (2005). "Mass-produced mullite crucibles in medieval Europe: Manufacture and material properties". Journal of the American Ceramic Society 91 (6): 2071–2074. doi:10.1111/j.1551-2916.2008.02383.x. 
  9. ^ Wang, W.; G. McCool, N. Kapur, G. Yuan, B. Shan, M. Nguyen, U. M. Graham, B. H. Davis, G. Jacobs, K. Cho, X. Hao (17 August 2012). "Mixed-Phase Oxide Catalyst Based on Mn-Mullite (Sm, Gd)Mn2O5 for NO Oxidation in Diesel Exhaust". Science 337 (6096): 832–835. Bibcode:2012Sci...337..832W. doi:10.1126/science.1225091. Retrieved 2012-08-16.