A Mossbauer study of the oxidation state of Fe in silicate melts

Fe3+/SigmaFe ratios were determined from Mossbauer spectra recorded for a series of 17 anorthited-iopside eutectic glasses containing 1 wt% (Fe2O3)-Fe-57 quenched from melts equilibrated over a range of oxygen fugacities from fO(2) similar to 10(5) bars (Fe3+/SigmaFe = 1) to 10 bars (Fe3+/SigmaFe = 0) at 1682 K. Fe3+/Fe2+ was found to be proportional to fo(2) to the power of 0.245 +/- 0.004, in excellent agreement with the theoretical value of 0.25 expected from the stoichiometry of the reaction Fe2+O + 0. 25 O-2 = Fe2+O1.5. The uncertainty in the Fe3+/SigmaFe ratios determined by Mossbauer spectroscopy was estimated as +/- 0.01 (1sigma) from the fit of the data to the theoretical expression, which is significantly less than that quoted for previous measurements on silicate glasses; this results from fitting the spectra of a large number of systematically varying samples, which allows many of the ambiguities associated with the fitting procedure to be minimized. Fe3+/SigmaFe ratios were then determined for samples of the anorthite-diopside eutectic composition equilibrated at selected values of fo(2), to which up to 30 wt% Fe2O3 had been added. Fe3+/SigmaFe was found to vary with SigmaFe (or FeOT), but both the 1 wt% and high FeOT, data could be satisfactorily fit assuming the ideal stoichiometry (i.e., Fe3+/Fe(proportional to)(2+)fo(2)(1/4)) by the inclusion of a Margules term describing Fe2+-Fe3+ interactions. The large negative value of this term indicates a tendency toward the formation of Fe2+-Fe3+ Complexes in the melt. The resulting expression, using the ideal exponent of 0.25, gave a fit to 289 Fe3+/SigmaFe values, compiled from various literature sources, of similar quality as previous empirical models which found an exponent of similar to0.20. Although the empirical models reproduce Fe3+/SigmaFe values of glasses with high FeOT reasonably well, they describe the data for 1 wt% FeOT poorly. The non-ideal values of the exponent describing the dependence of Fe3+/SigmaFe on fo(2), at high FeOT are an artifact of models that did not include a term explicitly to describe the Fe3+-Fe2+ interactions. An alternative model in which Fe in the silicate melt is described in terms of three species, Fe2+O, Fe3+O1.5, and the non-integral valence species Fe2.6+O1.3, was also tested with promising results. However, at present there is no model that fits the data within the assessed accuracy of the experimental measurements.