Abstract

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 (fO2) at 1400°C. The spectra vary systematically with fO2 (log fO2 ~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+/∑Cr. The resulting Cr2+/∑Cr values are in agreement with the theoretical dependence on log fO2 suggesting that the 1s → 4s feature is diagnostic of Cr2+ in a silicate glass and σ(Cr2+/∑Cr) = ± 0.015. The Cr2+/∑Cr ratio for a given log fO2 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+/∑Cr varies between ~0.3 and 0.8 over the log fO2 range bounded by the Ni/NiO and Fe/FeO fO2 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+/∑Cr from XANES spectra of quenched glasses is an essential precursor to in situ experiments on Fe-bearing silicate melts at high temperature.

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