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

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 groupC2) 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.