Mullite

Mullite or porcelainite is a rare silicate mineral of post-clay genesis. It can form two stoichiometric forms 3Al₂O₃2SiO₂ 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.