Quantification of Fe-oxidation state in mixed valence minerals: a geochemical application of EELS revisited

The use of TEM and electron energy loss spectroscopy to determine levels of Fe3+ in solids at nanometre spatial resolution is well established. Here we assess this technique at the energy resolution of a monochromated electron source (0.3 eV) and with the capability of simultaneously acquiring core-and low-loss spectra for absolute energy calibration of core-loss chemical shifts. Fe L-2,L-3-edges of four oxide standards give a characteristic energy loss near edge structure (ELNES) and the L-3-peak maxima of the spectra exhibit an energy shift of similar to 1.8 eV between ferrous (709 eV) and ferric (710.8 eV) iron. We show that diagenetic chlorites present in iron oxide loaded 'red beds' contain predominately Fe3+, based on the absolute energy position of the low signal Fe L-3-edges (centred around 710.8 eV). We also fit our standard-spectra to Fe L-2,L-3-edges from magnetite and maghemite nanoparticles to investigate iron site occupancy. An additional chemical shift needs to be applied to the octahedral ferrous iron site component to achieve a plausible, linear fit to the magnetite spectrum which we speculate may be due to electron hopping between the mixed valence octahedral iron sites in magnetite.