Sillimanite. It has the same chemical composition (Al2O3SiO2) as andalusite and kyanite, but when heated, it is stable up to 1545 degrees Celsius but decomposes into mullite and silicate above this temperature. Upon further heating, it decomposes into corundum and glass at 1810 degrees Celsius. It is used as a raw material for mullitic high-grade refractories and specialty porcelains.
