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Thermodynamic Stabilities of U(VI) Minerals: Estimated and Observed Relationships

  • Robert J. Finch (a1)

Abstract

Gibbs free energies of formation (ΔG°ƒ) for several structurally related U(VI) minerals are estimated by summing the Gibbs energy contributions from component oxides. The estimated ΔG° f values are used to construct activity-activity (stability) diagrams, and the predicted stability fields are compared with observed mineral occurrences and reaction pathways. With some exceptions, natural occurrences agree well with the mineral stability fields estimated for the systems Sio2-Cao-Uo3-UOH2O and Co2-caO-UO3-H2O providing confidence in the estimated thermodynamic values. Activity-activity diagrams are sensitive to small differences in ΔG° f values, and mineral compositions must be known accurately, including structurally bound H2O. The estimated ΔG° f values are not considered reliable for a few minerals for two major reasons: (1) the structures of the minerals in question are not closely similar to those used to estimate the ΔG° f * values of the component oxides, and/or (2) the minerals in question are exceptionally fine grained, leading to large surface energies that increase the effective mineral solubilities.

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Thermodynamic Stabilities of U(VI) Minerals: Estimated and Observed Relationships

  • Robert J. Finch (a1)

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