A structural analysis of CaSO4 · 0.5H2O, 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 P6222, cell parameters a = 6.9691(2) Å, c = 6.3033(2) Å. The final product of the rehydration of γ-anhydrite is CaSO4 · 0.5H2O, space group I2 (unique axis b), cell parameters a = 12.0350(5) Å, b = 6.9294(3) Å, c = 12.6705(4) Å, β = 90.266(3)°. The structure of the hemihydrate is strongly pseudo-trigonal, space group P3121. The symmetry lowering arises from water molecules ordering inside the channels.

You do not have access to this content, please speak to your institutional administrator if you feel you should have access.