Redox equilibria and the structural states of ferric and ferrous iron in melts in the system CaO–MgO–Al2O3–SiO2–Fe–O;: relationships between redox equilibria, melt structure and liquidus phase equilibria

Abstract

Relationships between melt structure and redox equilibria of iron in CaO-AlrOr-SiOr-Fe-O and MgO-AlrOr-SiOr-Fe-O melts with CalAl and Mg/Al > 0.5 have been determined with M 0.5, and undergoes a gradual coordination transformation in the Fe3*pFe range between 0.5 add 0.3. In this Fe3+pFe-range, tetrahedrally- and octahedrally-coordinated ferric iron may coexist. The temperature-dependence of the Mcissbauer hyperfine parameters and the temperatureindependence of the intensity ofthe absorption envelope are consistent with a local structural unit that may be stoichiometrically similar to Fe.On. The Fe2+/Fe3+ is linearly correlated with polymerization (nonbridging oxygens per tetrahedrally coordinated cations, NBOru) and Al/(Al + Si) of the melt. There are linear relationships between log (Fe2*7Fe3+) and log fo, and between log (Fe2+/Fe3*) and 1/T (absolute temperature). The standard-state free energy of reduction of ferric to ferrous iron, calculated from these lines, decreases with increasing Zlrz (ionization potential) of the alkaline earth metal cation, with decreasing bulk melt NBO/T (more polymerized melts) and with decreasing AV(AI + Si) of the melt. In magnesium aluminosilicate melts with NBO/[:0.6 and AV(AI + Si):0.2 (typical values for quartz tholeiite and basaltic andesite) with 10 wt.% iron oxide added as FerO, the liquidus phase is tridymite when equilibrated with air. Calculations indicate that at fo between 10-2 and 10-3 atm the liquidus phase changes to protoenstatite, and then to forsterite at even lower oxygen fugacities. Substitution of Ca (or Na) for Mg results in expansion of the metasilicate (pyroxene) liquidus field and contraction of that of tridymite. Fractional crystallization trends of magmatic liquids are, therefore, significantly dependent on oxygen fugacity, degree of polymerization of the magma (NBOA), Al/(Al + Si) and the relative abundance of alkali metals and alkaline earths.