Raman spectroscopic study of olivine-group minerals

Synthetic end-members Mg2SiO4, Fe2SiO4, MnSiO4, and Co2SiO4 and natural samples of forsterite-fayalite series, tephroite and monticellite, are investigated using Raman spectroscopy. The Raman spectra of the olivine-group minerals have a characteristic set of two intense lines of the Si-O asymmetric stretching band (wavenumber kappa(1)) and Si-O symmetric stretching band (kappa(2)). The kappa(1) and kappa(2) values of the Mg2SiO4-Ca(Mg, Fe)SiO4 series, in which the M2 site is occupied by nontransition elements, vary from 847 cm(-1) to 857 cm(-1) and from 815 cm(-1) to 825 cm(-1), respectively, and their omega (= kappa(1) - kappa(2)) value is fairly constant around 32 cm(-1). On the other hand, fayalite, tephroite, and other olivine-group minerals in which transition elements exist in the M2 site have a fairly constant kappa(1) (836-839 cm(-1)) value and variable kappa(2) (808-819 cm(-1)) and omega (20-32 cm(-1)) values. The omega value of the forsterite-fayalite series systematically increases with Mg#[= Mg/(Mg + Fe)] and is useful for determining their chemical compositions.