Low Mantle Perovskite: Solid Solution, Spin State of Iron and Water Solubility.

Silicate perovskites ((Mg, Fe)SiO3 and CaSiO3) are believed to be the major constituent minerals in the lower mantle. The phase relation, solid solution, spin state of iron and water solubility related to the lower mantle perovskite are of great effect on the geodynamics of the Earth's interior and on ore mineralization. Previous studies indicate that a large amount of iron coupled with aluminum can incorporate into magnesium perovskite, but this is discordant with the disproportionation of (Mg,Fe)SiO3 perovskite into iron-free MgSiO3 perovskite and hexagonal phase (Mg0.6Fe0.4)SiO3 in the Earth's lower mantle. MnSiO3 is the first chemical component confirmed to form wide range solid solution with CaSiO3 perovskite and complete solid solution with MgSiO3 perovskite at the P-T conditions in the lower mantle, and addition of MnSiO3 will strongly affects the mutual solubility between MgSiO3 and CaSiO3. The spin state of iron is deeply depends on the site occupation of the Fe3+ or Fe2+, the synthesis and the annealing conditions of the sample. It seems that the spin state of Fe2+ in the lower mantle perovskite can be settled as high spin, however, the existence of intermediate spin or low spin state of Fe2+ in perovskite has not been clarified. Moreover, different results have also been reported for the spin state of Fe3+ in perovskite. The water solubility of the lower mantle perovskite is related with its composition. In pure MgSiO3 perovskite, only less than 500 ppm water was reported. Al-MgSiO3 perovskite or Al-Fe-MgSiO3 perovskite in the lower mantle accommodates water of 1100 to 1800 ppm. Further experiments are necessary to clarify the detailed conditions for perovskite solid solution, to reliably analyze the valence and spin states of iron in the coexisting iron-bearing phases, and to compare the water solubility of different phases at different layers for deeply understanding the geodynamics of the Earth's interior and ore mineralization. [ABSTRACT FROM AUTHOR]