The stability of bassanite and the mechanism of dehydration of natural gypsum were investigated by in situ micro-Raman spectroscopy in the temperature range 300-380 K and 300-450 K. From the thermal evolution of the sulphate (940-1200 cm(-1)) and water (3250-3750 cm(-1)) molecular stretching (v(1) and v(3)) modes, it was evident that the gypsum dehydrated irreversibly into bassanite at about 370 +/- 5 K, We report that this transformation was preceded by the soluble anhydrite phase, with a characteristic Raman shift for v(1)(SO4) to about 1026 cm(-1). From the Arrhenius-type variations in the reduced intensities of v(1)(SO4) for bassanite and anhydrite at 1014 cm(-1) and 1026 cm(-1), respectively, the activation energy for bassanite to anhydrite was estimated to be 32.5 +/- 6.6 kJ mol(-).
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