Mascagnite
Mascagnite is a rare ammonium sulfate mineral 2SO4. It crystallizes in the orthorhombic system typically forming as stalactitic masses exhibiting good cleavage. It is soft and water-soluble. Optical properties are variable; the purest form is transparent and colorless, but opaque gray or yellow deposits are also known.
It occurs in fumaroles, as at Mount Vesuvius and associated with coal [seam fire]s. It was named for Italian anatomist Paolo Mascagni who first described the mineral.