Effect of pressure on the Raman vibrational modes of layered aluminosilicate compounds

We have conducted extensive high-pressure Raman-scattering studies in order to determine the effects of structure on the vibrational properties of layered aluminosilicate materials. The basal oxygen torsional mode is found to shift up with pressure at a rate that agrees reasonably with that expected from a simple van der Waals model. We identify a mode at 50 cm^-1 in margarite as the interlayer shear mode. The Raman band near 700 cm^-1 in all clays is linked to an oxygen-layer breathing mode. The pressure coefficient of the hydroxyl stretching mode near 3600 cm^-1 is found to have a structure-dependent sign; it is positive for trioctahedral structures and negative for dioctahedral silicates. This is found to be associated with hydroxyl orientation and Coulombic repulsion between the hydroxyl proton and the interlayer cation.