MULLITE

any of a type of rare mineral consisting of aluminum silicate (3Al2O32SiO2). It is formed upon firing aluminosilicate raw materials and is the most important constituent of ceramic whiteware, porcelains, and high-temperature insulating and refractory materials. Compositions, such as mullite, having an alumina-silica ratio of at least 3:2 will not melt below 1,810 C (3,290 F), whereas those with a lower ratio partially melt at temperatures as low as 1,545 C (2,813 F). Natural mullite was discovered as white, elongated crystals on the Island of Mull, Inner Hebrides, Scot. It has been recognized only in fused argillaceous (clayey) enclosures in intrusive igneous rocks, a circumstance that suggests very high temperatures of formation. For mineralogic properties, see silicate mineral (table).