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 + Al 2 O 3 + SiO 2 ± TiO 2 , quenched from melts equilibrated as a function of oxygen fugacity (f O₂ ) at 1400 °C. The spectra vary systematically with fO 2 (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 Cr 2+ /ΣCr. The resulting Cr 2+ /ΣCr values are in agreement with the theoretical dependence on logf O₂ suggesting that the 1s → 4s feature is diagnostic of Cr 2+ in a silicate glass and σ(Cr 2+ /ΣCr) = ± 0.015. The Cr 2+ /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 Cr 2+ . Cr 2+ /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 Cr 2+ 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 Cr 2+ /ΣCr from XANES spectra of quenched glasses is an essential precursor to in situ experiments on Fe-bearing silicate melts at high temperature