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 - κ 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 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.