The gypsum-bassanite-anhydrite phase transition sequence was followed up to 550 K at ambient pressure in a naturally occurring gypsum using Raman spectroscopy. The spectral variations of the internal (CaSO 4 AE 2H 2 O) modes of sulphate tetrahedra and were used to probe the structural phase transitions. A new Raman mode (m 1 m 2 ) emerged at 1026 cm-1, in the mode region, at around 388 » 5 K, indicating the onset of the bassanite m 1 phase. This mode became weaker after showing an initial increase. The anhydrite (CaSO 4 AE 0.5H 2 O) (CaSO 4 ) 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 mode clearly corroborate the transition sequence. m 2

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