Isomorphism Features of Post-Spinel Phases: Computer Modeling of Inclusion Composition in Lower Mantle Diamonds

Mixing properties of solid solutions of the post-spinel phases with a composition of CaCr 2 O 4 –CaAl 2 O 4 , CaCr 2 O 4 –CaFe 2 O 4 , MgCr 2 O 4 –MgAl 2 O 4 , and MgCr 2 O 4 –MgFe 2 O 4 in the temperature range of 1873–2223 K and at a pressure of 18–25 GPa were studied by the semi-empirical modeling. These PT parameters were used to estimate a formation energy of impurity defects of trivalent metal ions (aluminum and iron) in isomorphic sites. The study results: (1) the studied binary solid solutions are characterized by a complete miscibility; (2) the incorporation of Fe 3+ impurity ion into the post-spinel phases of MgCr 2 O 4 and CaCr 2 O 4 is less favorable in terms of energy than that of Al 3+ ions over the entire pressure and temperature range under study. The results obtained were used to interpret compositions of the post-spinel phases forming inclusions in lower mantle diamonds.