RAMAN-SPECTROSCOPIC STUDY OF PHASE-TRANSITIONS IN NATURAL GYPSUM

The gypsum-bassanit-anhydrite phase transition sequence was followed u p to 550 K at ambient pressure in a naturally occurring gypsum (CaSO4 . 2H2O) using Raman spectroscopy. The spectral variations of the inter nal modes of sulphate tetrahedra (v(1) and v(2)) were used to probe th e structural phase transitions. A new Raman mode emerged at 1026 cm(-1 ), in the v(1) mode region, at around 388 +/- 5 K, indicating the onse t of the bassanite (CaSO4 . 0.5H(2)O) phase. This mode became weaker a fter showing an initial increase. The anhydrite (CaSO4) phase, with an onset temperature of around 448 +/- 5 K, was also characterized by th e 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 we re estimated to be 92.25 and 32.94 kJ mol(-1), respectively. The obser ved spectral anomalies in the v(2) mode clearly corroborate the transi tion sequence. (C) 1998 John Wiley & Sons, Ltd.