Mascagnite is a rare ammonium sulfate mineral (NH₄)₂SO₄. It crystallizes in the orthorhombic system typically forming as stalactitic masses exhibiting good cleavage. It is soft (not higher than 2.5 on the Mohs Scale) and water soluble. Optical properties are variable; the purest form is transparent and colorless, but opaque gray or yellow deposits are also known.