A structural analysis of CaSO 4 · 0.5H 2 O, a dehydration product of gypsum, has been carried out through the Rietveld method on X-ray powder diffraction data. A dehydrated powder of synthetic gypsum has been charged inside a non-hermetically sealed capillary in order to allow a slow rehydration. The starting material has been identified as γ-anhydrite, space group P 6 2 22, cell parameters a = 6.9691(2) A, c = 6.3033(2) A. The final product of the rehydration of γ-anhydrite is CaSO 4 · 0.5H 2 O, space group I 2 (unique axis b ), cell parameters a = 12.0350(5) A, b = 6.9294(3) A, c = 12.6705(4) A, β = 90.266(3)°. The structure of the hemihydrate is strongly pseudo-trigonal, space group P 3 1 21. The symmetry lowering arises from water molecules ordering inside the channels.
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Ballirano et al. (2001) studied this question.
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