Electrochemical determination of the Gibbs free energies of rock-forming minerals. Final report

This grant provided support for a series of measurements of thermodynamic data for rock-forming minerals using an electrochemical approach. The relative accuracy of electrochemical measurements and the fact that this technique is the only one that directly measures the Gibbs energy of a phase as a function of temperature makes data obtained in this manner ideal for many types of geochemical calculations. A laboratory for these measurements was completed, and data were acquired on a series of metal-oxide buffers. Data were obtained with precisions of approximately {plus_minus}20 J/mole O{sub 2} at a single temperature, and fitted precisions of {plus_minus}50 J/mole O{sub 2}. Tests of the accuracy of these data were completed by running relative to air, to air through an intermediate gas stage, and relative to a solid buffer, and temperatures were calibrated relative to a primary standard obtained from NIST. These tests suggested that precision of currently available electrochemical studies may not reflect the accuracy of these measurements. The chemical potential of oxygen measured at any given temperature for all solid buffers tested appears to be a direct function of the voltage across the electrolyte containing the solid sample. Further tests of this effect, and recalibration of most or all of these reactions are needed if truly accurate data for these basic reactions are to be available. Preliminary to a planned electrochemical measurements on pyroxenes, a thermodynamic model of the system diopside-enstatite was derived. These results suggest that the activity/composition relations derived from solvus data are strongly dependent on the thermodynamic formulation chosen. This appears to be especially true for ordered intermediate compositions like diopside