The partial molar volume of Fe2O3 in alkali silicate melts: Evidence for an average Fe3+ coordination number near five

High-temperature (867-1534 ºC) density measurements were performed in air on 10 liquids in the Na2O-Fe2O3-FeO-SiO2 (NFS) system and 5 liquids in the K2O-Fe2O3-FeO-SiO2 (KFS) system using Pt double-bob Archimedean method. Replicate measurements indicate an average reproducibility of 0.22%. Compositions (in mol%) range from 4 to 18 Fe2O3, 0 to 3 FeO, 18 to 39 Na2O, 25 to 37 K2O, and 43 to 67 SiO2. Errors in the gram formula weight are ~0.4%. The molar volumes were fitted to a linear compositional model, which gives a compositionally independent partial molar volume (±2σ) for the Fe2O3 component (V̄Fe₂O₃) of 41.52 ± 0.34 cm3/mol and zero thermal expansivity. The average residual to the fit is ±0.36% for our 57 measurements on 15 liquids at various temperatures. The value for V̄Fe₂O₃ in silicate liquids when Fe3+ is in fourfold vs. sixfold coordination is estimated to be ~45.5 ± 1 vs. ~34 ± 1 cm3/mol, respectively. Thus, the fitted value of 41.5 cm3/mol appears to reflect an average Fe3+ coordination number between 4.5 and 5.0, which is consistent with recently published X-ray absorption fine structure (XAFS) spectroscopy and molecular dynamics (MD) simulations on Fe3+- bearing silicate glasses. In the literature, VFe3+ is inferred to be present in trigonal bipyramidal sites, in contrast to the square pyramidal sites for VTi4+. The lack of a strong compositional or temperature dependence for V̄Fe₂O₃ in these alkaline silicate liquids, in contrast to what is observed in the literature for V̄TiO₂ in similar melts, may reflect the different geometries for VFe3+ and VTi4+.