Quantitative determination of iron oxidation states in minerals using Fe L _2,3 -edge electron energy-loss near-edge structure spectroscopy

Abstract 

The Fe L _2,3-edge spectra for a range of natural minerals and synthetic solid solutions have been measured using the technique of parallel electron energy-loss spectroscopy (PEELS) recorded in a transmission electron microscope (TEM). The Fe L _2,3 -edges of the minerals are characterised by two white-line features and exhibit electron energy-loss near-edge structure (ELNES) characteristic of Fe valence state. For divalent iron, the Fe L ₃ -edge spectra are dominated by a sharp peak (white-line) at ca. 707.8 eV, followed by a broader and less intense peak at ca. 710.5 eV. The ELNES on the Fe L ₃ -edge of trivalent iron consists of a white-line with its maximum at ca. 709.5 eV and a preceeding peak at ca. 708.0 eV. Mineral solid solutions that contain both Fe²⁺ and Fe³⁺ exhibit an Fe L ₃ -edge shape that is composed of Fe L ₃ -edges from the respective Fe²⁺- and Fe³⁺-bearing end members. The integral Fe L _2,3 -edge white-line intensity ratios I(L ₃ )/I(L ₂ ) show clear differences for Fe²⁺ and Fe³⁺. We demonstrate the feasibility of quantification of the ferrous/ferric ratio in minerals by determining the integral Fe L _2,3 -edge white-line intensity ratios I(L ₃ )/I(L ₂ ) as a function of the ferric iron concentration resulting in an universal curve within the experimental errors. The application of the universal curve combined with the high spatial resolution using the PEELS/TEM allows the quantification of the ferric iron concentration on a scale down to 10 nm, which is illustrated from a sample of ilmenite containing hematite exsolution lamellae that shows different Fe L _2,3 -edge shapes consistent with variations in the Fe²⁺-Fe³⁺ ratio over distances of ca. 100 nm.