a mineral from the silicate class, with a chemical composition that varies from Al₆Si₂O₁₃ to Al₄SiO₈ (or from 3A1₂O₃ • 2SiO₂iO₂ to 2Al₂O₃ • SiO₂). Its structure is similar to that of sillimanite; however, mullite differs significantly in the irregularity of the Si and Al atoms. Mullite occurs in small prisms of the orthorhombic system. It has a hardness of 6–7 on Mohs’ scale and a density of 3,030 kg/m³; it melts at a temperature of about 1,810°C. Pure mullite is colorless; it is pinkish or bluish as a result of admixtures of iron or titanium oxides and exhibits pleochroism. Mullite is the only compound of Al₂O₃ and SiO₂ that is stable at high temperatures.

Mullite is rarely found in nature. It was discovered in partially fused clayey enclosures in Quaternary lava on the island of Mull in Scotland. Mullite is an important component of such man-made industrial products as porcelain and fireclay (alumina-containing refractory). It is formed by heating kaolinite to 950°C and by heating such alumina silicates as andalusite, sillimanite, and kyanite at temperatures ranging from 1,300–1,550°C. Fused mullite refractory is obtained in electric furnaces from a mixture consisting of bauxite, alumina, kaolin, coke, and other substances.