Order-disorder in Olivine Minerals by Synchrotron X-ray Absorption Near-edge Structure (XANES) Spectroscopy at the Mg, Fe and Ca K Edges

Synchrotron Mg and Fe X-ray absorption near-edge K XANES spectra of the Pbnm olivine series endmembers forsterite (Mg2SiO_4), fayalite (Fe_2SiO_4), monticellite (CaMgSiO_4) and kirschsteinite (CaFeSiO_4), as well as of the intermediate member of the Fe-Mg olivine solid solution series 'hortonolite' (MgFeSiO_4), have been recorded experimentally and calculated by the multiple-scattering theory using clusters of different size and the Xa exchange-correlation potential. Comparison of experimental and theoretical spectra shows that all transition features, energy positions and relative intensities can be reproduced in the energy range up to 60 eV above threshold when using clusters containing at least 80 atoms and extending over a sphere at least 0.6 nm in radius away from the absorber. It also indicates that, at the XANES sensitivity level, Mg and Fe are located entirely in the expected structural sites in all four endmembers, and distribute in 'hortonolite' following a regular order, however with no clear preference for either octahedral site. Thus, atomic order-disorder on a short- to medium-range scale can be detected by the XANES method, hence contributing to understanding the behavior of these materials. The Ca K-edge spectra of monticellite and kirschsteinite have been recorded and calculated too; they are consistent with the assignment of all Ca to the M2 site and confirm that all endmembers of the Pbnm olivine family match the octahedral cation distributions indicated by the ideal crystal structure.