Mullite or porcelainite is a rare silicate mineral formed during contact metamorphism of clay minerals.  It can form two stoichiometric forms:(₃Al₂O₃·₂SiO₂) or ₂Al₂O₃ SiO₂.  Unusually, mullite has no charge-balancing cations present.  As a result, there are three different aluminium sites: two distorted tetrahedral and one octahedral.