A XANES determination of the oxidation state of chromium in silicate glasses

Cr K-edge X-ray absorption near-edge structure (XANES) spectra were recorded for silicate glasses of various compositions in the system CaO + MgO + Al₂O₃ + SiO₂ ± TiO₂, quenched from melts equilibrated as a function of oxygen fugacity (f_o2) at 1400 °C. The spectra vary systematically with f_o2 (logf_o2 ∼0 to -16) indicating changes in the Cr oxidation state. The intensity of a shoulder on the absorption edge (attributed to the 1s → 4s transition) was quantified using the corresponding peak in the XANES derivative spectrum and used to determine Cr²⁺/ΣCr. The resulting Cr²⁺/ΣCr values are in agreement with the theoretical dependence on logf_o2 suggesting that the 1s → 4s feature is diagnostic of Cr²⁺ in a silicate glass and σ(Cr²⁺/ΣCr) = ± 0.015. The Cr²⁺/ΣCr ratio for a given logf_o2 changes with the glass composition which may reflect the ability of the melt to accommodate the Jahn-Teller distorted coordination geometry which stabilizes Cr²⁺·Cr²⁺/ΣCr varies between ∼ 0.3 and 0.8 over the logf_o2 range bounded by the Ni/NiO and Fe/FeO f_o2 buffers, suggesting that Cr²⁺ may be important in natural melts even though this oxidation state has never been identified in a terrestrial material. The development of a methodology for determining Cr²⁺/ΣCr from XANES spectra of quenched glasses is an essential precursor to in situ experiments on Fe-bearing silicate melts at high temperature.