THERMODYNAMIC DATA FROM REDOX REACTIONS AT HIGH-TEMPERATURES .4. CALIBRATION OF THE RE-REO2 OXYGEN BUFFER FROM EMF AND NIO+NI-PD REDOX SENSOR MEASUREMENTS

The chemical potential of oxygen defined by the equilibrium: Re(solid) + O-2(gas) = ReO2(solid) has been measured between 850 and 1250 K via an electrochemical method using calcia-stabilized zirconia electrolytes and either Ni + NiO or Cu + Cu2O as the reference electrode. The results are: mu O-2(Re+ReO22)(+/-400) = -451020 + 297.595 T - 14.6585 T 1n T (850 < T ( 1250) where mu O-2 is in J.mol(-1), T in kelvins, and the reference pressure for O-2 is 1 bar (10(5) Pa). (Values in terms of log-f(o2) may be obtained from the above expression by dividing by RTIn10, where R = 8.31441 J.K-1.mol(-1)). The standard enthalpy of formation of ReO2 is - 444.350 +/- 0.400 (1 sigma) kJ.mol(-1), requiring a significant modification to previously published estimates. These results were checked in hydrothermal experiments using the double capsule method with NiO + Ni-Pd alloy as an oxygen sensor. Reversals at P = 1 kbar over the T range 823 to 1073 K are in good agreement with the electrochemical measurements. These latter results also serve to demonstrate: (1) the usefulness of the ''redox sensor'' method; (2) the viability of using Re + ReO2 as a buffer in hydrothermal experiments. Re + ReO2 lies nearly midway between the Ni + NiO and Fe3O4 + Fe2O3 buffers in mu O-2-T space, and thus fills a petrologically important gap in the range of mu O(2)s which can be covered by accurately calibrated oxygen buffers.