Influence of oxygen fugacity and temperature on the redox state of iron in natural silicic aluminosilicate melts

The influence of oxygen fugacity ( f O 2 ) and temperature on the valence and structural state of iron was experimentally studied in glasses quenched from natural aluminosilicate melts of granite and pantellerite compositions exposed to various T - f O 2 conditions (1100–1420°C and 10 −12 –10 −0.68 bar) at a total pressure of 1 atm. The quenched glasses were investigated by Mössbauer spectroscopy. It was shown that the effect of oxygen fugacity on the redox state of iron at 1320–1420°C can be described by the equation log(Fe 3+ /Fe 2+ ) = k log( f O 2 ) + q , where k and q are constants depending on melt composition and temperature. The Fe 3+ /Fe 2+ ratio decreases with decreasing f O 2 ( T = const) and increasing temperature ( f O 2 = const). The structural state of Fe 3+ depends on the degree of iron oxidation. With increasing Fe 3+ /Fe 2+ ≥ 1, the dominant coordination of Fe 3+ changes from octahedral to tetrahedral. Ferrous iron ions occur in octahedral (and/or five-coordinated) sites independent of Fe 3+ /Fe 2+ .