First described as a natural mineral in 1924 from thermally metamorphosed rocks on the Isle of Mull in Scotland, it forms stoichiometric 3Al2O3·2SiO2 or 2Al2O3·SiO2 phases.
When kaolin-rich clays are fired near 1,400 °C, interlocking acicular mullite needles crystallize within the melt, giving porcelain and refractory kiln furniture their high hot-strength.
Modern analysis revealed that 15th-century German Hessian crucibles owed their famous thermal-shock and chemical resistance to mullite needles formed by firing kaolinitic clay above 1,100 °C.