• journal_title：American Mineralogist
• Contributor：Kasthuri D. Jayasuriya ; Hugh St.C. O’Neill ; Andrew J. Berry ; Stewart J. Campbell
• Publisher：Mineralogical Society of America
• Date：2004-
• Format：text/html
• Language：en
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：89
• issue：11-12
• firstpage：1597
• section：Articles

Fe³⁺/∑Fe ratios were determined from Mössbauer spectra recorded for a series of 17 anorthite-diopside eutectic glasses containing 1 wt% ⁵⁷Fe₂O₃ quenched from melts equilibrated over a range of oxygen fugacities from f_O2~ 10⁵ bars (Fe³⁺/∑Fe = 1) to 10⁻¹³ bars (Fe³⁺/∑Fe = 0) at 1682 K. Fe³⁺/Fe²⁺ was found to be proportional to f_O2 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⁺O_1.5. The uncertainty in the Fe³⁺/∑Fe 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⁺/∑Fe ratios were then determined for samples of the anorthite-diopside eutectic composition equilibrated at selected values of f_O2, to which up to 30 wt% Fe₂O₃ had been added. Fe³⁺/∑Fe was found to vary with ∑Fe (or FeO_T), but both the 1 wt% and high FeO_T data could be satisfactorily fit assuming the ideal stoichiometry (i.e., Fe³⁺/Fe²⁺ ∞f_O2 ^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⁺/∑Fe 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⁺/∑Fe values of glasses with high FeO_T reasonably well, they describe the data for 1 wt% FeO_T poorly. The non-ideal values of the exponent describing the dependence of Fe³⁺/∑Fe on f_O2 at high FeO_T 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⁺O_1.5, and the non-integral valence species Fe^2.6+O_1.3, 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.