Wiktionary
bassanite
n. (context mineral English) A saline evaporite, consisting of calcium sulphate, found at Vesuvius.
Wikipedia
Bassanite
Bassanite is a calcium sulfate mineral with formula CaSO·0.5(HO) or 2CaSO·HO. In other words it has half a water per CaSO unit hence its synonym hemihydrate.
Bassanite was first described in 1910 for an occurrence on Mount Vesuvius. It was named for Italian paleontologist Francesco Bassani (1853–1916).
At Vesuvius it occurs as alterations from gypsum within leucite tephrite and as fumarole deposits. It occurs in dry lake beds in California and Australia. It also occurs interlayered with gypsum in caves.