Activities of NiO, FeO, and O 2− in silicate melts

Abstract

We have measured activity coefficients for NiO and FeO in a variety of silicate melts (SiO 2-CaO-MgO-Al 2O 3) using electrochemical methods similar to square wave voltametry. We report the activity of the oxide ion (a O 2−) in one composition. Based on these measurements, we have constructed a model that predicts the variations in activity we observe, and also variations in NiO activity reported in the literature. Activity of metal-oxide components such as NiO and FeO in silicate melts can be understood by considering contributions from both the activity of the oxide ion and the activity of the cation through expressions of the type: a NiO = ( a O 2- ) ( a Ni 2+ ) and a FeO = ( a O 2- ) ( a Fe 2+ ) Based on measurements of a O 2−, we find that a O 2− is effectively modeled by the expression a O 2− = 0.75 · NBO 2/BO (relative to a standard state of melt with the composition of diopside). Once the effects of variations in a O 2− on the metal-oxide activity coefficient are accounted for, cation activities (a Ni 2+, a Fe 2+) can be modeled by an ideal-solution-like model in which the activity of Ni 2+ or Fe 2+ is related to its concentration among a subset of other cations with which it mixes (primarily other divalent cations). This model is successful in predicting NiO activities over the range of compositions 0.3<X MgO + X CaO<0.55. Modeling contributions to the NiO activity from both a O 2− and a Ni 2+ accounts for virtually all of the variation in measured activity coefficients not arising from experimental uncertainty.