Summary: The existence and relationships of the polymorphs of CaSO₄ have been studied by X-ray methods. Only two distinct phases are found: no crystallographic evidence is available to support the suggestion that the γ-CaSO₄ structure can have two forms, and it is shown that most previous data on the existence of the high temperature α-CaSO₄ polymorphs are likely to be due to decomposition of the sulphate.

Some observations on the γ to β-CaSO₄ transformation are reported. The structural implications of these results and their relevance to the nature of the transformation are discussed.