Orthorhombic calcium sulphate: a thermoluminescence and electron-spin resonance study

Natural CaSO4 anhydrite gave a glow-curve with a single peak (Tm=500 K), which could be annealed out by heating to 580 K. Irradiation of the original anhydrite by ultraviolet light broadened and reduced the intensity of this 500 K peak slightly, but some apparent recovery of intensity occurred at longer irradiation times. Two other peaks appeared in the glow-curve upon irradiation, with Tm at 340 and 420 K. The annealed material, upon ultraviolet-irradiation, gave a glow-curve with the same three peaks. Analysis by the Chen method shows that the processes taking place in the natural anhydrite are different from those occurring in the annealed and irradiated materials. The coincidence of temperature for the two 500 K peaks in the natural and irradiated materials is due to the balance of activation energy and frequency factor for different processes. Derived parameters suggest that all of the processes are second order, normally taken to mean that the combining species are widely separated in the solid. ESR spectra were obtained from the original anhydrite and the same material annealed at 1000 K. The signals are clustered around a value of 2 x 10-4T. Comparison with literature values suggests that -S-O species are predominant. In the present case this is mainly SO4-, probably formed by natural gamma-irradiation, as SO42- = SO4- + e. The presence of hydroxyl radicals (weak doublet at g = 2.0) in annealed anhydrite, but not in the original material, has not been reported previously, and such a radical could arise from electron trapping by hydrogen sulphate groups. (Author)