Solid solutions in the MgO-Al2O3-Cr2O3 system

Coexisting solid solutions with spinel and corundum structure were synthesized at 1773 K and two pressures, 1 bar and 25 kbar. Samples were analyzed by electron microprobe analysis and X-ray powder diffraction. Pressure and temperature were shown to affect the properties of the solid solutions in different ways. Pressure governs the composition of the defect spinel Mg1-xAl2O4, and temperature changes the cation distribution between coexisting phases. This allows one to separate the effects of cation exchange and magnetic contribution to the heat capacity in thermodynamic modeling. The defect spinel itself can form only because gamma-Al2O3 exists, polymorph with spinel structure. Thermodynamic considerations argue in favor of eskolaite-spinel assemblages prevailing over corundum-picrochromite ones at very high temperatures deep in the Earth.