Abstract
Kalsilite shows the domain structure corresponding to the disordered oxygen atoms at two mirror equivalent sites. The domain structure was considered to be due to the high-low inversion, and the structural model of the high-form was constructed. The heating experiment was carried out, which revealed that the X-ray powder pattern changes at 1138 K. It was confirmed that kalsilite undergoes a displacive transformation like quartz and tridymite. The high-form is hexagonal, probably P63mc, with a=5.30(1), c=8.65(1)Å. The high-form was also considered to be divided into two groups, high-kalsilite (P63mc) and disordered high-kalsilite (P63⁄mmc). The Zellengleichen subgroup relations are found among these three polymorphs.
