Redox equilibria of iron in alkaline earth silicate melts: relationships between melt structure, oxygen fugacity, temperature and properties of iron-bearing silicate liquids

Mössbauer and Raman spectroscopy have been used to relate redox equilibria of iron to melt structure in MO–SiO₂ systems (M = Ba²⁺, Sr²⁺, Ca²⁺, Mg²⁺). Log Fe²⁺/Fe³⁺ is a linear function of log f_O2, although the slope of the lines depends on bulk composition. The Fe²⁺/Fe³⁺ is a linear function of NBO/T (nonbridging oxygens per tetrahedral cation) for each type of divalent metal cation. The ferrous-ferric ratio is also proportional to Z/r² of the metal cation at constant M/Si (or NBO/Si) of the melts.

With Fe²⁺/Fe³⁺ < 1, ferric iron is in tetrahedral coordination. The spectroscopic data indicate that further reduction may result in a coordination transformation of the remaining Fe³⁺, probably to six-fold coordination. The Fe²⁺/Fe³⁺ of this transformation appears insensitive to the type of metal cation and the NBO/T of the melt. Its value increases somewhat as the total iron content of the system is increased.

Liquidus phase equilibria for compositions with Ca/Si = 0.7 and Mg/Si = 0.7 · with 10 wt.% iron oxide added were calculated as a function of Fe²⁺/Fe³⁺. An increase in Fe²⁺/Fe³⁺ results in a decrease of 35°C in the liquidus temperature in the system CaO–SiO₂–Fe₂O₃–Fe0 (at Fe²/Fe³⁺ ~ 0.25). In the analogous Mg-bearing system, the maximum temperature decrease is 200°C (at Fe²⁺/Fe³⁺ ~ 4.0). Silica polymorphs (and, over a limited ferric/ferrous range, two liquids in the Mg-bearing system) occur on the liquidus until the temperature minimum is reached. With additional increase in Fe²⁺/Fe³⁺, metasilicate minerals (pseudowollastonite and clinoenstatite, respectively) are stable. In the system MgO–SiO₂–Fe₂O₃–FeO, a silica polymorph (cristobalite) reappears on the liquidus with Fe²⁺/Fe³⁺ ~ 9.0, whereas in the analogous Ca system, pseudowollastonite remains on the liquidus until all ferric iron has been reduced to ferrous iron.