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]
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

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 aluminium sites: two distorted tetrahedral and one octahedral.

Mullite was first described in 1924 for an occurrence on the Isle of Mull, Scotland. 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.

Mullite is present in the form of needles in porcelain.

It is produced during various melting and firing processes, and is used as a refractory material, due to its high melting point of 1840°C.

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.