Abstract

Corundum–eskolaite, α–(Al,Cr)2O3, crystalline solutions with compositions in the range O < XCr2O3 < 1 have been synthesized at 25 kbar PH2O 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 (Al0.49Cr0.51)2O3 composition confirmed a 1:1 Al–Cr statistical occupancy of the A1 positions.

Information on Hmex and Smex of corundum–eskolaite crystalline solutions at 1 bar was obtained by reevaluating Jacob’s (1978) emf data. By coupling these data with those for Vmex, a polybaric, polythermal equation of state was formulated. The Gmex can be expressed at any P and T as a function of XCr2O3:  
Gmex=(1XCr2O3)XCr2O3[WG,Cr2O3+(WG,Al2O3WG,Cr2O3)XCr2O3],

with WG,Cr2O3 = 37484 + 0.0386 P + 4.334 T, and WG,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, Tc, was found to be 945° C, the critical composition, Xc, being 0.45 XCr2O3 whereas at 50 kbar Tc = 989° C and Xc = 0.44 XCr2O3. 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.

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