Aluminium sulfate is sometimes referred to as a type of alum. Alums are double sulfate salts, with the formula AM(SO4)2·12H2O, where A is a monovalent cation such as potassium or ammonium and M is a trivalent metal ion such as aluminium.The anhydrous form occurs naturally as a rare mineral millosevichite, found e.g. in volcanic environments and on burning coal-mining waste dumps. Aluminium sulfate is rarely, if ever, encountered as the anhydrous salt. It forms a number of different hydrates, of which the hexadecahydrate Al2(SO4)3•16H2O and octadecahydrate Al2(SO4)3•18H2O are the most common. The heptadecahydrate, whose formula can be written as [Al(H2O)6]2(SO4)3•5H2O, occurs naturally as the mineral alunogen.