Raman spectroscopic study of phase transitions in natural gypsum

The gypsum–bassanite–anhydrite phase transition sequence was followed up to 550 K at ambient pressure in a naturally occurring gypsum (CaSO4·2H2O) using Raman spectroscopy. The spectral variations of the internal modes of sulphate tetrahedra (ν1 and ν2) were used to probe the structural phase transitions. A new Raman mode emerged at 1026 cm-1, in the ν1 mode region, at around 388±5 K, indicating the onset of the bassanite (CaSO4·0.5H2O) phase. This mode became weaker after showing an initial increase. The anhydrite (CaSO4) phase, with an onset temperature of around 448±5 K, was also characterized by the appearance of the Raman mode at 1016 cm-1. From the Arrhenius-type changes in the reduced intensity, the activation energies associated with the gypsum to bassanite and bassanite to anhydrite transitions were estimated to be 92.25 and 32.94 kJ mol-1, respectively. The observed spectral anomalies in the ν2 mode clearly corroborate the transition sequence. © 1998 John Wiley & Sons, Ltd.