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

Raman spectra were collected for synthetic gypsum (CaSO4 · · 2H2O) 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 SO4 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-CaSO4 and the subsequent rehydration of γ-CaSO4 into hemihydrate are clearly identified in the Raman spectra by the observed variation in Raman shifts of the OH and ν1(SO4) 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-CaSO4), which can be used as a fingerprint for the remote detection of these minerals on planetary surfaces.