A Mössbauer study of ferric iron in olivine

Natural and synthetic olivines with ferrous and ferric iron have been studied by Mössbauer spectroscopy. Their spectra exhibit three superimposed quadrupole-split doublets corresponding to Fe²⁺ at M1 and M2 sites and Fe³⁺ at an unidentified position, which is probably M2. The hyperfine parameters of Fe³⁺ at temperatures between 300°C and 450°C are: Δ=0.67–1.23 mm/s, δ=0.04–0.23 mm/s relative to metallic iron, and the full width at half height, HW=0.15–0.43 mm/s. The Fe²⁺ populations of M2 decrease with increasing Fe³⁺ content. However, Fe²⁺ prefers M1 in synthetic olivines, even at high temperatures (800°–1,400°C). In kirschsteinite, Fe²⁺ and Fe³⁺ are exclusively in M1. Fe²⁺/Fe³⁺ ratios estimated from the peak areas are consistent with chemical analyses.