Raman spectroscopic study of olivine-group minerals

Abstract

Synthetic end-members Mg₂SiO₄, Fe₂SiO₄, Mn₂SiO₄, 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 κ₁) and Si-O symmetric stretching band (κ₂). The κ₁ and κ₂ 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 ω (= κ₁ − κ₂) 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 κ₁ (836-839 cm⁻¹) value and variable κ₂ (808-819 cm⁻¹) and ω (20-32 cm⁻¹) values. The ω value of the forsterite-fayalite series systematically increases with Mg#[= Mg/(Mg + Fe)] and is useful for determining their chemical compositions.