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 (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 (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.
Andrew J. Berry* and Hugh St.C. O’Neill
Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

Correspondence: * E-mail: andrew.berry@anu.edu.au