Iron speciation in minerals and glasses probed by M_2/3 -edge X-ray Raman scattering spectroscopy

We present a spectroscopic study of the iron M_2/3 -edge for several minerals and compounds to reveal information about the oxidation state and the local coordination of iron. We describe a novel approach to probe the iron M_2/3 -edge bulk sensitively using X-ray Raman scattering. Significant changes in the onset and shape of the Fe M_2/3 -edge were observed on ferrous and ferric model compounds with Fe in octahedral and tetrahedral coordination. Simulation of the spectra is possible using an atomic multiplet code, which potentially allows determination of, e.g., crystal-field parameters in a quantitative manner. A protocol is discussed for determination of the Fe oxidation state in compounds by linear combination of spectra of ferric and ferrous end members. The presented results demonstrate the capabilities of Fe M_2/3 -edge spectroscopy by X-ray Raman scattering to extract information on the ratio of trivalent to total iron Fe^3+/∑Fe and local coordination. As X-ray Raman scattering is performed with hard X-rays, this approach is suitable for in situ experiments at high pressure and temperature. It thus may provide indispensable information on oxidation state, electronic structure and local structure of materials that are important for physical and chemical processes of the deep Earth.