The density and compressibility of CaO-FeO-SiO2 liquids at one bar: Evidence for four-coordinated Fe2+ in the CaFeO2 component

Double-bob density measurements using molybdenum bobs in a reducing atmosphere were made on hedenbergite (CaFeSi2O6) liquid and four other CaO-FeO-SiO2 (CFS) liquids that each contains 40 mol% FeO and variable CaO and SiO2 concentrations. In addition, relaxed sound speeds were measured with an acoustic interferometer at centered frequencies of 4.5 and 5.5 MHz between 1571 and 1885 K. An ideal mixing model using the oxide components (CaO-FeO-SiO2) for molar volume and isothermal compressibility cannot recover the experimental results within error. When each experimental liquid is fitted individually, the partial molar volume and isothermal compressibility of the FeO component (V‾T,FeO and β‾T,FeO, respectively) increase systematically with CaO concentration, consistent with a composition-induced decrease in the average Fe2+ coordination number. When the four CFS liquids with 40 mol% FeO are recast in terms of the CaFeO2-FeO-SiO2 components, an ideal mixing model for both volume and compressibility recovers the data for the four liquids within experimental error. At 1723 K, the fitted V‾FeO and β‾FeO values (± 1σ) are 12.1 (± 0.2) cm3/mol and 3.5 (± 0.3) × 10−2 GPa−1, respectively, which are postulated to reflect 6-coordinated Fe2+ given the similarity of V‾FeO in the melt to the molar volume of crystalline FeO (wüstite). In contrast, the derived values of V‾FeO and β‾FeO from the CaFeO2 component are 17.1 (± 0.2) cm3/mol and 7.1 (± 0.2) × 10−2 GPa−1, respectively, which are postulated to reflect Fe2+ in 4-fold given that crystalline CaFeO2 contains 4-coordinated Fe2+. These two end-member values for V‾FeO at 1723 K are used to develop a linear equation to calculate Fe2+ coordination as a function of V‾FeO, which leads to average Fe2+ coordination numbers in the CFS liquids that range from 5.2 to 4.6, including a value of 4.6 (± 0.2) for hedenbergite (CaFe2Si2O6) liquid. The key to developing a model equation for density and compressibility for FeO-bearing magmatic liquids is to determine how melt composition affects Fe2+ coordination.