A Mössbauer study of the oxidation state of Fe in silicate melts
-
Kasthuri D. Jayasuriya
,
Andrew J. Berry
Abstract
Fe3+/ΣFe ratios were determined from Mössbauer spectra recorded for a series of 17 anorthitediopside eutectic glasses containing 1 wt% 57Fe2O3 quenched from melts equilibrated over a range of oxygen fugacities from fO₂ ~ 105 bars (Fe3+/ΣFe = 1) to 10-13 bars (Fe3+/ΣFe = 0) at 1682 K. Fe3+/Fe2+ was found to be proportional to fO₂ 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 O2 = Fe3+O1.5. The uncertainty in the Fe3+/ΣFe ratios determined by Mössbauer spectroscopy was estimated as ± 0.01 (1σ) 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+/ΣFe ratios were then determined for samples of the anorthitediopside eutectic composition equilibrated at selected values of fO₂, to which up to 30 wt% Fe2O3 had been added. Fe3+/ΣFe was found to vary with ΣFe (or FeOT), but both the 1 wt% and high FeOT data could be satisfactorily fit assuming the ideal stoichiometry (i.e., Fe3+/Fe2+ ∝fO₂1/4) 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+/ΣFe values, compiled from various literature sources, of similar quality as previous empirical models which found an exponent of ~0.20. Although the empirical models reproduce Fe3+/ΣFe 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+/ΣFe on fO₂ at high FeOT are an artifact of models that did not include a term explicitly to describe the Fe2+-Fe3+ 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.
© 2015 by Walter de Gruyter Berlin/Boston
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Artikel in diesem Heft
- First-principle study of polytype structures of 1:1 dioctahedral phyllosilicates
- Calorimetry of liquids in the system Na2O-Fe2O3-SiO2
- A Mössbauer study of the oxidation state of Fe in silicate melts
- Nanometer-scale measurements of Fe3+/ΣFe by electron energy-loss spectroscopy: A cautionary note
- The most reduced rock from the moon, Apollo 14 basalt 14053: Its unique features and their origin
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- Lizardite-chlorite structural relationships and an inferred high-pressure lizardite polytype
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