Vibrational studies of olivine-type compounds-I. The i.r. and Raman spectra of the isotopic species of Mg2SiO4

A group theoretical analysis has been carried out for the olivine structure, and the isotopic species of the type compound Mg2SiO4 have been investigated by Raman and i.r. spectroscopy. New assignments have been deduced from 24Mg-26Mg and 28Si-30Si isotopic shifts. The 1000-475 cm-1 region of the i.r. spectrum is essentially related to internal (stretching and bending) modes of the SiO4 tetrahedron whereas most of the lower frequency bands must be assigned to external modes. The SiO4 totally symmetric, stretching frequency (v1) is observed at 842 cm-1 in the i.r. spectrum where it is unequivocally identified through the nearly complete lack of a 28Si-30Si mass effect, whereas this mass effect appears in the v3 components by a 10-15 cm-1 shift. It has also been possible to evidence a number of vibrational interactions, such as the mixing of the symmetric and antisymmetric stretching, Raman-active, modes, the simultaneous participation of both Mg and Si atoms to some translational modes, and the mixing of some translational and rotational modes. The significance of force constant calculations is also briefly discussed. © 1973.