Bassanite is a calcium with formula CaSO4 or 2CaSO4·H2O. In other words it has half a water per CaSO4 unit hence its synonym calcium sulfate 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 deposits. It occurs in dry lake beds in California. It also occurs interlayered with gypsum in caves