High-Temperature XAS Study of Fe 2 SiO 4 Liquid: Reduced Coordination of Ferrous Iron

Abstract

X-ray absorption spectroscopy (XAS) of Fe²⁺ in Fe₂SiO₄ liquid at 1575 kelvin and 10^-4 gigapascal (1 bar) shows that the Fe²⁺ -O bond length is 1.98 ± 0.02 angstroms compared with ≈2.22 angstroms in crystalline Fe₂SiO₄ (fayalite) at the melting point (1478 kelvin), which indicates a decrease in average Fe²⁺ coordination number from six in fayalite to four in the liquid. Anharmonicity in the liquid was accounted for using a data analysis procedure. This reduction in coordination number is similar to that observed on the melting of certain ionic salts. These results are used to develop a model of the medium-range structural environment of Fe²⁺ in olivine-composition melts, which helps explain some of the properties of Fe₂SiO₄ liquid, including density, viscosity, and the partitioning of iron and nickel between silicate melts and crystalline olivines. Some of the implications of this model for silicate melts in the Earth's crust and mantle are discussed.