A rare mineral of theoretical composition, a relatively stable phase in ceramics produced by the high temperature reaction of alumina and silica or by the thermal decomposition of alumina-silica minerals such as kyanite, sillimanite, andalusite, and various clay minerals.
Mullite, or porcelainite, is a rare clay mineral, aluminum silicate (Al6Si2O13). It is produced artificially during various melting and firing processes, and is used as a refractory.