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.