We conducted a Mössbauer study of synthetic glasses with anorthite-diopside eutectic composition modified with either 5 or 1 wt% Fe2O3 and variable amounts of silica. Glasses were synthesized in air in the temperature range 1400–1570 °C. Room-temperature Mössbauer spectra showed the presence of magnetically ordered Fe3+, for which a systematic fitting procedure was developed to correctly estimate Fe3+/Fe2+. At constant T-fO2 we observed a minimum in the variation of Fe3+/Fe2+ ratio with silica that varied from 56 to 63 wt% SiO2, depending on temperature. It was also demonstrated that the temperature dependence of the ferric/ferrous ratio [log(Fe3+/Fe2+) vs. 1/T] is essentially similar for basic and silicic melts.

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