Abstract
We determined activity-composition relationships in Pt-Cr and Pt-Fe-Cr alloys at 1300°C experimentally and used the results to constrain the thermodynamic properties of chromite-picrochromite spinels. The Pt-Cr binary is characterized by strong negative deviations from ideality throughout the investigated composition range and the activity-composition relationship can be fit by a four-suffix asymmetric regular solution with three binary interaction parameters. The ternary alloy was modeled as a four-suffix asymmetric regular solution; the three ternary interaction parameters in this model were constrained by combining interaction parameters for the three bounding binaries taken from this and previous work with results for a set of experiments in which the activity of Cr in Pt-Fe-Cr-alloys was fixed by coexisting Cr2O3 at known f O2. The free energy of formation of FeCr2O4 at 1300°C was determined using the activities of Fe and Cr in Pt-alloys in equilibrium with oxide mixes of FeCr2O4 and Cr2O3. The free energy of formation of chromite from Fe+Cr2O3+O2 is -202.7 ± 0.4 kJ/mol (1σ), indistinguishable from literature values. The corresponding free energy of formation of FeCr2O4 from the elements is -923.5 ± 2.1 kJ/mol (1σ), and the enthalpy of formation at 298 K is -1438 kJ/mol. The activity-composition relationship for the chromite component in (Fe,Mg)Cr2O4 solid solutions was determined from a set of experiments in which Pt-alloys were equilibrated with spinel + Cr2O3. (Fe,Mg)Cr2O4 spinels are nearly ideal at 1300°C; modeling our data with a one-site symmetric regular solution yields an interaction parameter of +2.14 ± 0.62 kJ/mol (1σ), similar to values based on data from the literature.
| Original language | English |
|---|---|
| Pages (from-to) | 3033-3044 |
| Number of pages | 12 |
| Journal | Geochimica et Cosmochimica Acta |
| Volume | 67 |
| Issue number | 16 |
| DOIs | |
| State | Published - 15 Aug 2003 |
| Externally published | Yes |
Bibliographical note
Funding Information:This work was supported by NASA grant NAG-10423. Discussions with Mike Baker and a written communication with Joe Goldstein led to significant improvements in the quality of this study. Ma Chi is appreciated for his help with the analytical work. The authors thank John Jones and Hugh O’Neill for their constructive reviews. Division contribution #5714.