The effect of composition on Cr2+/Cr3+ in silicate melts

Andrew J. Berry, Hugh St C. O'Neill, Dean R. Scott, Garry J. Foran, J. M.G. Shelley

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Abstract

Chromium K-edge X-ray absorption near-edge structure (XANES) spectra were recorded at room temperature for 27 CaO-MgO- Al2O3-SiO2 (CMAS) glass compositions quenched from melts equilibrated at various oxygen fugacities (fo2 ) at 1400 °C. Values of Cr2+/∑Cr were determined from the intensity of a shoulder on the main absorption edge, attributed to the 1s → 4s transition, which is characteristic of Cr2+ in these glasses. For each composition, Cr2+/∑Cr could be quantified as a function of fo2, using a theoretical expression, from as few as three samples (Cr2+/∑Cr ≈ 0, 0.5, and 1). This allowed logK1, or the reduction potential of the Cr3+/2+ half-reaction, and hence the relative change in the ratio γCrmelt3 +O1.5meltCr2+O, to be determined for each composition. At constant fo2, log[Cr2+/Cr3+] was found to decrease linearly with increasing optical basicity. The variation in logK1 with composition is controlled by γCrmelt 3+O1.5, corresponding to the capacity of the melt to stabilize both the charge and the preferred solvation site of Cr3+. The method was then applied to spectra recorded in situ at 1400 °C for a synthetic mid-ocean ridge basalt (MORB) composition, allowing Cr2+/∑Cr to be quantified in a Fe-bearing melt for the first time. Cr2+/∑Cr was found to vary from ∼0.45 at the nickel-nickel oxide (NNO) fo2 buffer to ∼0.90 at iron-wüstite (IW). This indicates that Cr2+ is likely to be the dominant oxidation state in terrestrial basaltic melts.

Original languageEnglish
Pages (from-to)1901-1908
Number of pages8
JournalAmerican Mineralogist
Volume91
Issue number11-12
DOIs
Publication statusPublished - 2006
Externally publishedYes

Keywords

  • Chromium oxidation states
  • MORB
  • Silicate melts
  • XANES spectroscopy

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