High-pressure and high-temperature equation of state of cobalt oxide: Implications for redox relations in Earth's mantle

Journal Article · Tue Jul 30 00:00:00 EDT 2013 · American Mineralogist

DOI:https://doi.org/10.2138/am.2013.4339· OSTI ID:1081356

Armentrout, Matthew M.; Rainey, Emma S.G.; Kavner, Abby ^[1]

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1. UCLA

The high-pressure and high-temperature equation of state of rock salt-structured cobalt oxide was measured up to 65 GPa and 2600 K using synchrotron X-ray diffraction in conjunction with the laser heated diamond-anvil cell. Fitting a Mie-Grüneisen-Debye model to the data we find best-fit parameters V₀ = 77.4 (fixed) ų, K₀ = 190 (1) GPa, K' = 3.49 (4), γ₀ = 1.54 (4), q = 2.87 (15), and θ₀ = 517.8 K (fixed). We use this newly determined equation of state in conjunction with existing measurements of the thermoelastic parameters of cobalt metal to calculate the Gibbs free-energy difference between the cobalt oxide and cobalt metal phases as a function of pressure and temperature. A comparison of the energetics of the Co/CoO system with the Ni/NiO system predicts that below 58 GPa CoO+Ni is stable relative to NiO+Co, while above 58 GPa the reverse is true. This tipping point in energy can be mapped as a crossing point in the electrochemical potential of the two metal ions, suggesting that cobalt becomes more siderophile than nickel with increasing pressure. This result is in qualitative agreement with existing measurements of nickel and cobalt partition coefficients between mantle and core materials.

• Research Organization: Argonne National Lab. (ANL), Argonne, IL (United States). Advanced Photon Source (APS)
• Sponsoring Organization: USDOE Office of Science (SC)
• OSTI ID: 1081356
• Journal Information: American Mineralogist, Vol. 98, Issue (5-6) ; 05, 2013; ISSN 0003-004X
• Country of Publication: United States
• Language: ENGLISH