Thermodynamic mixing properties of corundum–eskolaite, α–(Al, Cr⁺³)₂O₃, crystalline solutions at high temperatures and pressures

Corundum–eskolaite, α–(Al,Cr)₂O₃, crystalline solutions with compositions in the range O < X_Cr2O3 < 1 have been synthesized at 25 kbar P_H2O and 1070° C. Homogeneity of the crystals was checked and chemical compositions established by electron-probe microanalysis. Calculations of cell volumes based on 20 to 27 powder diffraction peaks, including those from the back reflection region, indicate a positive $Vmex$ throughout the range of composition. Structure refinement of one synthetic crystal of (Al_0.49Cr_0.51)₂O₃ composition confirmed a 1:1 Al–Cr statistical occupancy of the A1 positions.

with W_G,Cr2O3 = 37484 + 0.0386 P + 4.334 T, and W_G,Al2O3 = 31729 + 0.0006 P + 4.719 T,

P in bars, T in K and $Gmex$ in joules.

Solvi for the Al₂O₃–Cr₂O₃ system have been computed using the $Gmex$ expression given above. At a pressure of 1 bar, the critical temperature, T_c, was found to be 945° C, the critical composition, X_c, being 0.45 X_Cr2O3 whereas at 50 kbar T_c = 989° C and X_c = 0.44 X_Cr2O3. Thus, the pressure dependence of the solvus is very slight, and it may be regarded as an excellent geothermometer. Application of this geothermometer to exsolved chromian corundum and aluminous eskolaite from a grosspydite xenolith of the Zagadochnaya kimberlite pipe in Yakutia indicates reequilibration down to 910° C during the cooling episode.