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As part of our ongoing research into the stability of platinum group metal-bearing oxide phases (such as MgRh₂³⁺O₄ and geologically important solid solutions containing this endmember), we conducted experiments to determine the mixing behaviour of MgRh₂³⁺O₄-MgAl₂O₄ solid solutions. In order to overcome the limitations imposed by sluggish reaction kinetics at lower temperatures, we constructed diffusion couples between MgAl₂O₄ single crystals and disks of Rh metal. The samples were reacted at 1117 °C in mixtures of oxygen and Ar for 14 days, which resulted in the metal and spinel being sintered together tightly. Examination with microbeam techniques of sections cut perpendicular to the metal-spinel interface showed the presence of ß-Rh₂O₃ in all samples, indicating that the samples were not in equilibrium with the gas mixtures, but were likely buffered by ß-Rh₂O₃ instead.

We expected to find the same equilibrium spinel composition in all samples, given that they were all likely buffered by the same reaction. No reaction was observed to have occurred between metal and spinel at the resolution of the electron microprobe. However, after repolishing the samples followed by examination using a scanning electron microscope, we found that a layer of Mg(Rh,Al)₂O₄ spinel had formed as a separate phase at the interface between metal and spinel. As expected, the composition of the spinel layer was the same in all samples.

The composition of the Mg(Rh,Al)₂O₄ spinel in equilibrium with Rh and ß-Rh₂O₃ may be used to constrain the interaction coefficient of MgRh₂³⁺O₄ in MgRh₂³⁺O₄ - MgAl₂O₄ solid solutions. In Fig. 3.3-5 we plot the chemical potential of oxygen as a function of the Rh/(Rh + Mg) ratio at 1100 °C in the system Mg-Rh-O. The most important feature of the diagram is the considerable extent of solid solution between MgRh₂³⁺O₄ and Mg₂Rh⁴⁺O₄. In fact, pure MgRh₂³⁺O₄ is only found in equilibrium with ß-Rh₂O₃. At the oxygen fugacities below the Rh-ß-Rh₂O₃ buffer, MgRh₂O₄ breaks down according to the reaction 2 MgRh₂³⁺O₄ = Mg₂Rh⁴⁺O₄ + 3 Rh + 2 O₂. In our experiments the rhodate spinel is in equilibrium with both Rh and ß-Rh₂O₃, corresponding to point A in Fig. 3.3-5. Assuming a regular solution model and that the activity of Mg₂RhO₄ in our alumina-bearing system is the same as in an alumina-free system, we calculate a large positive deviation from ideality for the MgRh₂³⁺O₄-MgAl₂O₄ solid solution. Such a large positive deviation may be partly responsible for the reluctance of rhodium to diffuse into MgAl₂O₄. However additional measurements in which the compositional dependence of the interaction coefficient of MgRh₂O₄ must be evaluated are needed.
 

Fig. 3.3-5: Chemical potential of oxygen as a function of the Rh/(Rh + Mg) ratio in the system Mg-Rh-O at 1100 °C. At point "A" spinel coexisting with Rh and ß-Rh2O3 contains virtually no Mg2Rh4+O4 in solid solution.