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

High-temperature (867-1534 degrees C) density measurements were performed in air oil 10 liquids in the Na2O-Fe2O3-FeO-SiO2 (NFS) system and 5 liquids ill the K2O-Fe2O3-FeO-SiO2 (KFS) system using Pt double-bob Archimedean method. Replicate measurements indicate all average reproducibility of 0.22%. Compositions (in mo1%) range from 4 to 18 Fe2O3, 0 to 3 FeO2 18 to 39 Na2O, 25 to 37 K2O, and 43 to 67 SiO2. Errors in the gram formula weight are similar to 0.4%. The Molar volumes were fitted to a linear compositional model, which give, a compositionally independent partial molar volume ( +/- 2 sigma) for the Fe2O3 component (V-Fe2O3) of 41.52 +/- 0.34 cm(3)/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 V-Fe2O3 in silicate liquids when Fe3+ is in fourfold vs. sixfold coordination is estimated to be similar to 45.5 +/- 1 vs. similar to 34 +/- 1 cm(3)/mol, respectively. Thus, the filled value of 41.5 cm(3)/mol appears to reflect all average Fell coordination [lumber 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, Fe-V(3+) is inferred to be present in trigonal bipyramidal sites, in contrast to the square pyramidal sites for Ti-V(4+). The lack of a strong compositional or temperature dependence for V-Fe2O3 in these alkaline silicate liquids, ill contrast to what is observed in the literature for V-TiO2, in similar melts, may reflect the different geometries for VFe3+ and Ti-V(4+).