Raman spectra and cation distribution in the lattice of olivines

We have obtained Raman spectra of olivines in the forsterite (Mg2SiO4)/fayalite (Fe2SiO4) series and thus the dependence of all totally symmetric modes on composition. These dependencies exhibit features at Fa40 (40 mol% of fayalite component) and Fa70, respectively. We have found that the distribution of cations over M1 and M2 positions is determined by the Mg/Fe ratio. Fe2+ cations occupy the M1 positions predominantly at small Fe concentration (up to Fa30) and are evenly distributed over the M1 and M2 sites at concentrations where the fayalite component content is greater than 45 mol%. At around Fa40, a redistribution of cations, accompanied by conformic-like transformation of SiO42− anions occurs.