The structural role and homogeneous redox equilibria of iron in peraluminous,metaluminous and peralkaline silicate melts

The compositional dependence of the redox ratio (FeO/FeO_1.5) has been experimentally determined in K₂O-Al₂O₃-SiO₂-Fe₂O₃-FeO (KASFF) and K₂O-CaO-Al₂O₃-SiO₂-Fe₂O₃-FeO (KCASFF) silicate melts. Compositions were equilibrated at 1,450° C in air, with 78 mol % SiO₂. KASFF melts have from 1 to 5 mol % Fe₂O₃ and include both peraluminous (K₂O2O₃) and peralkaline (K₂O>Al₂O₃) compositions. KCASFF melts have 1 mol % Fe₂O₃ encompassing peraluminous, metaluminous (CaO+K₂O>Al₂O₃) and peralkaline compositions. Peralkaline KASFF melts with 1 mol % Fe₂O₃ have low and constant values for the redox ratio, whereas in peraluminous melts the redox ratio increases with increasing (K₂O/Al₂O₃). Increasing total iron concentration increases the redox ratio in peraluminous melts and slightly decreases the redox ratio in peralkaline melts. Substituting CaO for K₂O at fixed total iron (1 mol %) increases the redox ratio in both peraluminous and metaluminous KCASFF melts; however, the redox ratio in peralkaline KCASFF melts is not affected by this exchange. These data indicate that Fe³⁺ is in four-fold coordination, with K⁺ or Ca²⁺ providing local charge balance. The tetrahedral ferric species is most stable in peralkaline melts and least stable in peraluminous melts, due to the competition between Al³⁺ and Fe³⁺ for charge balancing cations in the latter melt. Tetrahedral Fe³⁺ is also less stable when Ca²⁺ provides local charge balance. The data are consistent with a network modifying role for Fe²⁺ in the melt.The data are interpreted to reflect the effects of melt composition on the partitioning of K⁺ and Ca²⁺ and Fe³⁺ and Al³⁺ between various species in the melt. These relationships are discussed in terms of homogeneous equilibria between various iron-bearing and iron-free melt species. The results also reflect the effect of liquid composition on the exchange potentials Fe³⁺ Al_–1 and Ca_0.5K_–1. The exchange potentials are relatively constant in peralkaline melts, but decrease in metaluminous and peraluminous melts as both (CaO+K₂O)/(CaO+K₂O+Al₂O₃) and K₂O/CaO decrease. These qualitative observations imply that minerals exhibiting these exchanges will also be similarly affected as liquid composition changes.Present address: Department of Geological Sciences, Virginia Tech, Blacksburg, VA 24061, USA