Quantification of ferrous/ferric ratios in minerals: new evaluation schemes of Fe L ₂₃ electron energy-loss near-edge spectra

Abstract

 Determination of Fe³⁺/ΣFe 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⁺/ΣFe 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⁺/ΣFe 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⁺/ΣFe ratio can be determined with an absolute error of ca. ±0.02.