In other words it has half a water molecule per CaSO4 unit, hence its synonym calcium sulfate hemihydrate.

[2] 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.

[3] H. Schmidt and coinvestigators reported in 2011 that under dry conditions, the structure is monoclinic with space group C2, but at 75% humidity, the structure is trigonal with space group P3221.

This reflects the incorporation of additional water of hydration, such that the trigonal form has the formula CaSO4·0.625H2O.