Aluminum sulfate is a salt with the formula Al2(SO4)3. It is soluble in water. The anhydrous form occurs naturally as a rare mineral millosevichite, found for example in volcanic environments and on burning coal-mining waste dumps. Aluminum sulfate is rarely, if ever, encountered as the anhydrous salt. It forms a number of different hydrates, of which the hex decahydrate Al2(SO4)3·16H2O and octa decahydrate 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.