HP-PdF2-type FeCl2as a potential Cl-carrier in the deep Earth

Abstract

We report for the first time the formation of a HP-PdF2-type FeCl2 phase (space group Pa3), through high pressure-temperature (P-T) reactions in the hydrous systems (Mg0.6Fe0.4)SiO3-H2O-NaCl and FeO2H- NaCl in a laser-heated diamond-anvil cell up to 108 GPa and 2000 K. Applying single-crystal X-ray difraction (XRD) analysis to individual submicrometer-sized grains, we have successfully determined the crystal structure of the as-synthesized FeCl2 phase, in agreement with our theoretical structure search results. In situ high P-T XRD data revealed the substitution of Cl for OH(O) in such a cubic Pa3 structure, demonstrating that this topology is a potential host for both H and Cl in the deep Earth. The chemical analysis of the recovered sample showed that the post-perovskite phase contains considerable amounts of Na2O and Fe2O3. The coexistence of the cubic FeCl2 phase and post-perovskite suggests that the lowermost mantle could be a potential reservoir of Cl. The possible presence of volatiles such as H and Cl in the deep lower mantle would impact the composition and iron valence state of the post-perovskite phase.