Microanalysis of Fe3+/ΣFe in oxide and silicate minerals by investigation of electron energy-loss near-edge structures (ELNES) at the Fe M2,3 edge

The Fe M-2,M-3-edge spectra of solid solutions of garnets (almandine-skiagite Fe-3(Al1-xFex)(2)[SiO4](3) and andradite-skiagite (Fe1-xCax)(3)Fe-2[SiO4](3)), pyroxenes (acmite-hedenbergite (Ca1-xNax)(Fe1-x2+Fex3+)Si2O6), and spinels (magnetite-hercynite Fe(Al1-xFex)(2)O-4) have been measured using the technique of parallel electron energy-loss spectroscopy (EELS) conducted in a transmission electron microscope (TEM). The Fe M-2,M-3 electron energy-loss near-edge structures (ELNES) of the minerals exhibit a characteristic peak located at 4.2 eV and 2.2 eV for trivalent and divalent iron, respectively, prior to the main maximum at about 57 eV. The intensity and energy of the pre-edge feature varies depending on Fe3+/Sigma Fe. We demonstrate a new quantitative method to extract the ferrous/ferric ratio in minerals. A systematic relationship between Fe3+/Sigma Fe and the integral intensity ratio of the main maximum and the pre-edge peak of the Fe M-2,M-3 edge is observed. Since the partial cross sections of the Fe M-2,M-3 edges are some orders of magnitude higher than those of the Fe L-2,L-3 edges, the Fe M-2,M-3 edges are interesting for valence-specific imaging of Fe. The possibility of iron valence-specific imaging is illustrated by Fe M-2,M-3-ELNES investigations with high lateral resolution from a sample of ilmenite containing hematite exsolution lamellae that shows different edge shapes consistent with variations in the Fe3+/Sigma Fe ratio over distances on the order of 100 nm.