Bassanite

Acta Cryst. (2011). B67, 467-475
Water channel structure of bassanite at high air humidity: crystal structure of CaSO4·0.625H2O

H. Schmidt, I. Paschke, D. Freyer and W. Voigt

Abstract: Structure analysis using single-crystal diffraction was carried out as a contribution to the dispute about the nature of the water channel structure of bassanite (CaSO₄·0.5H₂O). A recent result of Weiss & Bräu (2009) for the crystal structure of bassanite (monoclinic, space group C2) at ambient conditions of air humidity was confirmed. In the presence of high relative air humidity the crystal structure of bassanite transformed due to the incorporation of additional water of hydration. The crystal structure of CaSO₄·0.625H₂O was solved by single-crystal diffraction at 298 K and 75% relative air humidity. The experimental results provided an insight into both crystal structures. A model explaining the phase transition from CaSO₄·0.625H₂O to CaSO₄·0.5H₂O was derived. The monoclinic cell setting of CaSO₄·0.5H₂O and the trigonal cell setting of CaSO₄·0.625H₂O were confirmed by powder diffraction.

Technically, this is a different mineral.

Actually, after reading the pape, it is a synthetic.