Iron redox analysis of silicate-based minerals and glasses using synchrotron X-ray absorption and laboratory X-ray emission spectroscopy

There is considerable need for robust, accessible, and non-destructive Fe redox analysis methods for silicate-based minerals and glasses. A popular method assigns the distribution of Fe ²⁺ and Fe ³⁺ atoms using pre-edge features in the Fe K-edge X-ray absorption near edge structure (XANES). We expand on this work in two ways. First, we use XANES to assign the Fe redox distribution in four silicate materials: two geological reference materials and two industrial glasses. This example highlights the effect of the Fe atomic environment on pre-edge features’ intensities and stresses the need for specifically developed calibration curves. We then investigate Fe Kβ X-ray emission spectroscopy as an independent complement for redox assignments. Here, the silicates do not show a systematic dependence on oxidation state. This is explored with electronic structure calculations, which support previous claims that the lack of sensitivity is due to charge transfers following creation of the 1s core hole.