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A stable crystalline form of Aluminum silicate. Mullite is formed by heating aluminum silicate compounds, such as cyanite, Sillimanite and Andalusite, to high temperatures. It begins forming at 1000 C and the conversion is nearly complete by 1340 C. Mullite has needle-shaped crystals which provide a felting effect for strengthening stoneware and porcelain (Fournier 1996).

Synonyms and Related Terms

none; Mullit (Deut.); mulliet (Ned.)

Physical and Chemical Properties

  • Insoluble in water.
  • Orthorhombic system with needle-shaped crystals.
  • Good cleavage in one direction
Composition 3Al2O3 - 2SiO2
Melting Point 1810 C
Density 3.15 g/ml
Refractive Index 1.642 -1.679

Resources and Citations

  • R. Fournier, Illustrated Dictionary of Practical Pottery, Chilton Book Co., Radnor, PA, 1996.

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