X-ray absorption near-edge structure at the Mg and Fe K-edges in olivine minerals

Mg and Fe K-edge x-ray absorption near-edge spectroscopy spectra of the Pbnm olivine series end mem- bers forsterite, fayalite, monticellite, and kirschsteinite, as well as of a synthetic Fe-Mg olivine of intermediate composition Fo50, have been recorded and compared with multiple-scattering simulations. A 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. Mg and Fe are entirely in the expected structural sites in the four investigated end members. None of the models of Mg and Fe ordering in the octahedral site for the Fe50 olivine compound proposed so far is verified; thus Fe-Mg ordering still remains an open problem for olivines of intermediate compositions.