Lattice vibrations of montmorillonite: an FT Raman and X-ray diffraction study

The lattice vibrations of a series of dioctahedral montmorillonitic clay minerals were studied using FT Raman spectroscopy. The lattice vibrations are assigned in terms of the molecular vibrations of (a) the distorted octahedron MO6 with symmetry S6 in the region 50–200 cm−1, (b) the OHO isosceles triangle of C2v symmetry in the 200 to 270 cm−1 and (c) distorted SiO4 tetrahedra of symmetry C3v in the region 300 to 1200 cm−1. The application of FT Raman spectroscopy to the analysis of montmorillonitic clay structure is elucidated. A series of montmorillonites with different weathering profiles from the Cressfield deposit in Australia were analysed by both FT Raman and X-ray diffraction. The FT Raman spectra of the Cressfield bentonites are shown to be a function of clay crystallinity. X-ray diffraction identified the changes in the montmorillonitic lattice as weathering occurs. The variation in the 001 d spacing changed from 17.53 to 17.36 Å and the CDS value increased from 126 to 148 Å, indicating an increase in both the order and crystallinity of the montmorillonite on ageing. FT Raman spectroscopy identified changes in the lattice vibrational modes as this weathering is taking place.