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 (fo(2)) at 1400 degreesC. The spectra vary systematically with fo(2) (logfo(2) similar to 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+/SigmaCr. The resulting Cr2+/SigmaCr values are in agreement with the theoretical dependence on logfo(2) suggesting that the 1s --> 4s feature is diagnostic of Cr2+ in a silicate glass and sigma(Cr2+/SigmaCr) = +/- 0.015. The Cr2+/SigmaCr ratio for a given logfo(2) 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+/SigmaCr varies between similar to 0.3 and 0.8 over the logfo(2) range bounded by the Ni/NiO and Fe/FeO fo(2) 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+/SigmaCr from XANES spectra of quenched glasses is an essential precursor to in situ experiments on Fe-bearing silicate melts at high temperature.