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The monoclinic I2 structure of bassanite, calcium sulphate hemihydrate (CaSO₄ · 0.5H₂O)

¹ Dipartimento di Scienze della Terra, Università di Roma "La Sapienza", P.le A. Moro 5, I-00185 Roma, Italy
² Dipartimento di Chimica, Istituto Nazionale per la Fisica della Materia, Università di Roma "La Sapienza", P.le A. Moro 5, I-00185 Roma, Italy

e-mail: paolo.ballirano{at}uniromal.it

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 -anhydrite, space group P6₂22, cell parameters a = 6.9691(2) Å, c = 6.3033(2) Å. The final product of the rehydration of -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.

Key-words: -anhydrite, CaSO₄ · 0.5H₂O, bassanite, gypsum, X-ray powder diffraction, Rietveld method.

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