AbstractA microscope study has been made of the development of crystals of calcium sulphate hemihydrate produced on, or in, sheets of the dihydrate in the form of selenite by calcining under controlled conditions. The crystal forms were shown to be strongly dependent upon both temperature and ambient water vapour pressure. A mechanism for the nucleation and growth of these crystals is propounded in terms of the diffusion of molecular vacancies. The relevant properties of the so‐called α‐and β‐hemihydrates are discussed, and it is argued that these forms are due to incoherent and coherent growth, respectively, from the parent selenite; it is explained how it is possible to obtain incoherent growth in this solid transformation. The overall shape of the crystals is examined.