Determination of the Iron Oxidation State in Cr-Spinels by Electron Microprobe X-ray Emission Spectroscopy of L_α,β Lines

The oxidation state of Fe, Cu, Eu, Ce, etc. in minerals is an important indicator of redox conditions for their formation and evolution. In this work, the features of X-ray emission Fe L_α,β spectra (XES) obtained by Cameca SX100 microprobe, as well as the Mössbauer spectra of Cr-spinels of various compositions from a number of the Urals ultramafic massifs, have been studied. Using microprobe data on the composition and homogeneity of the grains, as well as the Fe³⁺/\({\text{Fe}}_{{{\text{tot}}}}^{{{\text{M\"o ssb}}}},\) 19 intralaboratory reference samples have been selected from a large number of the Urals Cr-spinels. Based on the analysis of BSE images and maps of the distribution of elements in Cr-spinel grains, the most homogeneous ones were identified. It was shown that, up to the determination error, Fe³⁺/\({\text{Fe}}_{{{\text{tot}}}}^{{{\text{M\"o ssb}}}}\) coincides with that obtained in the framework for calculation approach based on microprobe data on the composition and assumptions about the mineral stoichiometry. For determination of Fe oxidation state by XES data, it has been proposed to use as a calibration dependence for intralaboratory reference samples the position of the maximum of Fe L_α line on the content of Fe²⁺; the latter is satisfactorily approximated by a linear function (r_xy = 0.96); the relative error of Fe²⁺ determination is 2%. Approbation of the developed approach was carried out on a series of control samples of Cr-spinels; the obtained values of Fe²⁺ and Fe³⁺/Fe_tot agree satisfactorily with the Mössbauer and calculated data in the stoichiometric approximation.