The determination of the oxidation state of iron by the electron microprobe

Abstract

A method is described which allows the determination of Fe 2+ /Fe 3+ ratios with the electron microprobe by analyzing the FeLα and FeLβ X-ray emission spectra. We use the ratios of intensities measured on the high energy flank of the Lα and the low energy flank of the Lβ peak. The sensitivity achieved with this «flank method» is about 3-4 times higher than with the conventional peak shift method or peak intensity ratio method. The method was calibrated with the iron oxides wustite, magnetite and hematite and was then applied to a synthetic spinel solid solution series magnetite-hercynite (Fe 3 O 4 -FeAl 2 O 4 ) and to several natural olivines (Mg 2 SiO 4 -Fe 2 SiO 4 solid solutions). The results obtained by the flank method vary not only with Fe 3+ , but also with total iron content. A quantitative method for determining the oxidation state of Fe must therefore not only be based on a method with a high resolution but must also include corrections for self-absorption and matrix effects. Our method can be applied, if the bulk composition and crystal structure of the samples are known and working curves are established with similar materials