Water channel structure of bassanite at high air humidity: crystal structure of CaSO4·0.625H2O.

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 (CaSO(4)·0.5H(2)O). 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 CaSO(4)·0.625H(2)O 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 CaSO(4)·0.625H(2)O to CaSO(4)·0.5H(2)O was derived. The monoclinic cell setting of CaSO(4)·0.5H(2)O and the trigonal cell setting of CaSO(4)·0.625H(2)O were confirmed by powder diffraction.