mullites

Mullite is an aluminosilicate mineral with the chemical formula 3Al2O3·2SiO2, commonly written as Al6Si2O13. It is a rare natural mineral that forms in aluminous rocks under high-temperature metamorphic conditions and is also produced synthetically for industrial use. Mullite was first described in 1924 from crystals found in the Isle of Mull, Scotland, and was named after this locality.

Natural mullite occurs as small crystals or as masses in aluminous metamorphic rocks, and may form during dehydration and melting of clay-rich rocks at high temperatures. In rocks, it is commonly associated with corundum, sillimanite or kyanite, quartz, and other aluminosilicates. Pure natural mullite is rare; most material in nature is intergrown with other minerals.

Physically, mullite is typically colorless to white and has a vitreous to pearly luster. It occurs in the orthorhombic crystal system and has a hardness around 6.5 to 7 on the Mohs scale, with a specific gravity near 3.0. It is renowned for high-temperature stability, resisting melting and chemical attack up to about 1,400–1,600 °C in many environments.

Industrial significance: Mullite is a major constituent in fired clays, porcelain, and refractory ceramics because of its high melting point, low thermal expansion, and chemical inertness. It is widely synthesized for use in refractory bricks and kiln linings, ceramic matrices, and advanced ceramic composites, as well as in thermal barrier coatings for high-temperature applications.