The structural role of Fe 3+, Ga 3+, Al 3+ and homogeneous iron redox equilibria in K 2OAl 2O 3Ga 2O 3SiO 2Fe 2O 3FeO melts

Abstract

The compositional dependence of the iron redox ratio (FeO/FeO 1.5) has been experimentally determined in K 2OGa 2O 3SiO 2Fe 2O 3FeO silicate melts. Compositions were equilibrated at 1450°C in air, with 78 mol% SiO 2 and 1 mol% initial Fe 2O 3. The ratio K 2O/(K 2O + Ga 2 OO 3) was varied from 0.4 to 0.7. Peralkaline K 2O > Ga 2O 3) melts have low and constant values for the redox ratio, while this ratio increases with decreasing K 2O/(K 2O + Ga 2O 3) in pergallic (K 2O < Ga 2O 3) compositions. The results are compared and contrasted to a similar set of experiments in the system K 2OAl 2O 3SiO 2Fe 2O 3FeO. The analysis indicates that in peralkaline melts Fe 3+ and Ga 3+ are tetrahedrally coordinated with K + providing local charge balance. Pergallic melts also stabilize these species; however, the competition between Fe 3+ and Ga 3+ for charge balancing K + results in reduction of Fe 3+ to Fe 2+ and the partitioning of non-charge-balanced Ga 3+ into a tricluster structure. The coordination of Ga 3+ in the tricluster is suggested to be tetrahedral.