Micro-Raman studies of gypsum in the temperature range between 9 K and 373 K

Raman spectra were collected for synthetic gypsum (CaSO 4 ⋅2H 2 O) powder between 9 and 373 K under atmospheric pressure with special emphasis on the temperature dependence of the OH-stretching modes. The stretching bands of the water molecules in gypsum were found to shift in opposite directions as a result of the different degree of intermolecular hydrogen-bonding between nonequivalent water H atoms and the O atoms of nearby SO 4 ions. The anharmonic parameters of the OH-stretching modes are calculated using the temperature derivatives measured from the present investigation and existing pressure derivatives. These parameters are -4.7 × 10 -6 K -1 and -0.6 × 10 -6 K -1 for the 3407 and 3494 cm -1 bands, respectively. The dehydration of gypsum into γ-CaSO 4 and the subsequent rehydration of γ-CaSO 4 into hemihydrate are clearly identified in the Raman spectra by the observed variation in Raman shifts of the OH and ν 1 (SO 4 ) bands. The latter increases as the mineral becomes increasingly anhydrous (1007 cm -1 in gypsum; 1014 cm -1 in hemihydrate; 1026 cm -1 in γ-CaSO 4 ), which can be used as a fingerprint for the remote detection of these minerals on planetary surfaces.