Mascagnite
Mascagnite is the name given to native ammonium sulfate. It occurs in fumaroles, as at Mount Vesuvius. It is soluble in water; therefore it is rare. It is soft (not higher than 2.5 on the Mohs Scale) and has good cleavage. Optical properties are variable; the purest form is transparent and colorless, but opaque gray or yellow deposits are also known.