High-pressure phase transformation in CaSO₄

Raman spectroscopic study of anhydrite (CaSO₄) at P > 140 kbar (after being heated to ∼1000°C by laser irradiation, and then cooled to room temperature) revealed that anhydrite transformed to a new high-pressure modification. Depending on the rate of pressure release, the high-pressure modification can only be partially preserved at ambient condition, if the pressure is slowly released. X-ray diffraction study of the partially preserved high-pressure modification at ambient condition revealed 15 diffraction lines for the new phase. The d-spacings for these new lines can be reasonably fitted by an orthorhombic cell with a = 6.602 ± 0.005, b = 7.759 ± 0.007, c = 5.970 ± 0.007 Å. If Z = 4 is assumed, the calculated volume of the new phase is 0.2% greater than that of anhydrite at ambient condition. The fitting is non-unique, however. The new phase has also been fitted by other possible candidate structures, such as sheelite, monazite and barite, but without success.