Role of impurities in gypsum-bassanite phase transition: A comparative Raman study

A comparative study on two naturally-occurring gypsum crystals has been car ried out using Raman spectroscopy, in the temperature range 300-550 K, to u nderstand the effect of alkali and alkaline halide impurities such as Na an d Ba, on its transition sequence. New Raman modes emerged at around 388 +/- 5 K, at 1026, 1032 cm(-1), respectively, indicating the onset of bassanite phase in both the samples in which the relative concentration of sodium im purities varies by about 4%. The role of impurities is also reflected in th e thermal evolution of the characteristic v(1) (SO4) mode at 1008 cm(-1) du ring the gypsum-bassanite transition.