Single crystal Raman spectra of forsterite, fayalite, and monticellite

Polarized single crystal Raman spectra of the fundamental modes of forsterite (Mg₂SiO₄), olivine (Mg_0.88,Fe_0.12), fayalite (Fe₂SiO₄), and monticellite (CaMgSiO₄) are presented. Seven Iow energy modes (<450 cm⁻¹) in forsterite differ from those of all previous studies. All modes predicted by symmetry for forsterite and monticellite were observed; 34 out of 36 modes were observed for fayalite. Assignment of the modes was determined by systematics in frequency changes resulting from cation substitution. Although the lattice modes are generally mixed, likely mode assignments are made using the following observations. The lowest energy modes are assigned to SiO₄ translations and appear to be mixed with the cation translations. The lattice modes between 330 and 435 cm⁻¹ in forsterite that changed the least with composition were assigned as SiO₄ rotations. The modes from 300 to 390 cm⁻¹ that showed the greatest variation with composition or showed signs of two-mode behavior in the olivine Mg₈₈ were assigned as M2 translations. The SiOo internal stretching and bending modes were assigned to the highest frequencies; they vary little from forsterite to monticellite to fayalite and appear to depend more on cation mass than volume. These assignments are consistent with previous single-crystal infrared studies of forsterite and fayalite and with mode Griineisen parameters γ_i, measured vs. pressure in that the highest γ_i values should be associated with the M2 cations and the lowest with the SiO₄ internal modes.