Mössbauer Spectroscopy of Synthetic Olivine across the Mg-Fe Solid Solution
Olivine is a major mineral component on terrestrial planetary surfaces and, therefore, it is important to understand its spectroscopic characteristics at all wavelengths. This study presents a detailed Mössbauer investigation of ten synthetic olivines in the fayalite-forsterite series (Fe2SiO4 Mg2SiO4). Two distributions of Fe in olivine were consistently resolved. However, these doublets cannot be unambiguously assigned to Fe site occupancy in the M1 and M2 crystallographic sites in the olivine structure because their areas do not match known site occupancies. Moreover, these two doublets can be fit using two distinct models that yield different doublet area ratios yet are statistically indistinguishable in the spectra acquired at 13-293K. In all cases, isomer shift of the doublets is relatively constant across the entire solid solution, but quadrupole splitting correlates with composition (± 20% absolute) as QS = 0.000135x + 2.999 for a sample with composition Fox. The magnetic ordering temperature in olivine is also roughly linearly correlated with composition. Mössbauer spectra of olivine can appear very similar to those of some ferrous sulfates at low resolution and in mineral mixtures. Finally, recoil-free fraction values are not indicative of composition at a given temperature using our current method for their calculation.