Quantification of ferrous/ferric ratios in minerals:: new evaluation schemes of Fe L23 electron energy-loss near-edge spectra

Determination of Fe3-/SigmaFe in minerals at submicrometre scale has been a long-standing objective in analytical mineralogy. Detailed analysis of energy-loss near-edge structures (ELNES) of the Fe L-23 core-loss edges recorded in a transmission electron microscope (TEM) provides chemical information about the iron oxidation state. The valence-specific multiplet structures are used as valence fingerprints. Systematic investigations on the Fe L-23 ELNES of mono and mixed-valence Fe-bearing natural minerals and synthetic solid solutions of garnets (almandine-skiagite and andradite-skiagite), pyroxenes (acmite-hedenbergite) and spinels (magnetite-hercynite) are presented where the presence of multiple valence states is distinguished by a splitting of the Fe L-3 edge. We demonstrate the feasibility of quantification of the ferrous/ferric ratio in minerals by analyzing the Fe L-23 ELNES as a function of the ferric iron concentration resulting in three independent methods: (1) The method of the modified integral intensity ratio of the Fe L-23 white lines employs two 2-eV-wide integration windows centring around both the Fe L-3 maximum for Fe3+ and the Fe L-2 maximum for Fe2+. This refined routine. compared to the previously published quantification method of the ferrous/ferric ratio in minerals, leads to an improved universal curve with acceptable absolute errors of about +/-0.03 to +/-0.04 for Fe3+/SigmaFe ratios. (2) The second method uses a simple mathematical description of the valence-dependent splitting of Fe L-3 ELNES by fitting several Gaussian functions and an arctan function. The systematic analysis of the integral portions of the individual Gaussian curves for different mineral groups provides a further Fe3+/SigmaFe quantification method with an absolute error of about +/-0.02 to +/-0.03. (3) The Fe L-3 ELNES can also be modelled with the help of reference spectra, whereby the Fe3+/SigmaFe ratio can be determined with an absolute error of ca. +/-0.02.