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 CaCr₂O₄–CaAl₂O₄, CaCr₂O₄–CaFe₂O₄, MgCr₂O₄–MgAl₂O₄, and MgCr₂O₄–MgFe₂O₄ 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 Fe³⁺ impurity ion into the post-spinel phases of MgCr₂O₄ and CaCr₂O₄ is less favorable in terms of energy than that of Al³⁺ 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.