Interaction Between FeOOH and NaCl at Extreme Conditions: Synthesis of Novel Na2FeCl4OHx Compound

Iron(III) oxide-hydroxide, FeOOH, is abundant in the banded iron formations (BIFs). Recent studies indicate that BIFs may carry water down to the lower mantle with subducting slabs. The previous experiments investigating the properties of FeOOH at extreme pressures (P) and temperatures (T) were performed in diamond anvil cells (DACs), where it was compressed inside alkali metal halide pressure-transmitting media (2). Alkali metal halides such as NaCl or KCl are expected to be chemically inert; therefore, they are widely used in DAC experiments. Here, we report the chemical interaction between FeOOH and NaCl pressure medium at 107(2) GPa and 2400(200) K. By means of single-crystal X-ray diffraction (SC-XRD) analysis applied to a multigrain sample, we demonstrate the formation of a Na2FeCl4OHx phase and provide its structural solution and refinement. Our results demonstrate that at high P-T conditions, the alkali metal halides could interact with hydrous phases and thus cannot be used as a pressure transmitting and thermal insulating medium in DAC experiments dedicated to studies of hydroxyl or water-bearing materials at high P-T.