The infra-red spectrum and crystal structure of gypsum

Infra-red spectra of single crystals of gypsum (CaSO 4 .2H 2 O) have been obtained between 450 and 3800 cm -1 by measurement of transmission and reflexion of plane-polarized radiation on three different crystal sections. Analysis of these observations, when combined with previous results on the Raman spectrum of gypsum, makes it possible to assign sixteen out of the eighteen internal fundamental modes of the two sulphate ions, and ten of the twelve internal fundamental modes of the four water molecules in the unit cell. Comparison of the spectra of the sulphate ions and water molecules in gypsum with those given by sulphate ions in solution and water molecules in the gaseous phase provides some information on the nature of the crystalline field. If the intensities and the dichroism of the water bands are used to verify the orientations of the water molecules in the crystal (as determined by nuclear magnetic resonance) the results obtained are anomalous. The agreement between prediction and observation is satisfactory for the deformation mode of vibration but quite unsatisfactory for the two stretching modes. Possible causes of this anomaly are discussed.