An approach to modeling trace component activities in silicate melts: NiO

Abstract

This report presents a model predicting activities for NiO in a wide range of silicate melts that include the components SiO2, TiO2, Al2O3, MgO, FeO, CaO, Na2O, and K2O. The conceptual simplicity of this model, combined with its success in modeling complex variations in activity with melt composition, suggests that the approach may provide insight into the character of trace components in the melt. The model presented in this report considers NiO to exist as Ni2+ and O2− in the melt, and predicts the activity of NiO by modeling variations in both aNi2+ and aO2−. Activities of Ni2+ are modeled assuming that NiO mixes randomly with a hypothetical ‘mixing pool’ of cations dominated by cations of similar size and charge to Ni2+, mainly Fe2+, Mg2+, Ca2+, and Ni2+. aO2− is modeled as a function of total oxygen − 2·network-forming cations, with the understanding that O2− in silicate melts exists in equilibrium with bridging and non-bridging oxygens through reactions of the type Si–O–Si + O2− → 2 Si–O. For illustration, the model is applied to reduced mafic lunar samples that may have equilibrated with a Ni-bearing metal phase.