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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.