Post-Spinel Phases in the Earth’s Mantle

Abstract

—The post-spinel phases include compounds with stoichiometry {{A}^{{2 + }}}B_{2}^{{3 + }}{{{text{O}}}_{4}}left( {A_{2}^{{2 + }}{{B}^{{4 + }}}{{{text{O}}}_{4}}} right) and structures of the calcium ferrite CaFe2O4, calcium titanate CaTi2O4, and marokite CaMn2O4 types. The structures with a centered Cmcm (Bbmm) and primitive Pnma (Pmcn) and Pbcm (Pmab) cells are distinguished in this family of topologically related compounds with a “marokite” channel formed by six octahedra. The sites A and B are occupied by various cations, in particular, Cr, Al, Mg, Fe, Ca, Ti, Fe, Na, and Si, which implies the formation of solid solutions of a wide compositional range. In nature, such high-pressure phases were found in meteorites, as inclusions in diamond crystals, and in rocks from some metamorphic complexes. This review provides a characterization of natural mineralogical finds, the results of an experimental study of post-spinel phases of various compositions and their solid solutions, as well as crystal chemical simulation and assessment of likely compositions and the areas of stability of compounds with a “marokite” channel. The discrepancy between the results of individual studies indicates the necessity to clarify the stability parameters and probable isostructural transitions, and, ultimately, to improve the classification of post-spinel phases.