Discovery of a hexagonal ultradense hydrous phase in (Fe,Al)OOH.

A deep lower-mantle (DLM) water reservoir depends on availability of hydrous minerals which can store and transport water into the DLM without dehydration. Recent discoveries found hydrous phases AlOOH ( = 2) with a CaCl-type structure and FeOOH ( = 4) with a cubic pyrite-type structure stable under the high-pressure-temperature () conditions of the DLM. Our experiments at 107-136 GPa and 2,400 K have further demonstrated that (Fe,Al)OOH is stabilized in a hexagonal lattice. By combining powder X-ray-diffraction techniques with multigrain indexation, we are able to determine this hexagonal hydrous phase with = 10.5803(6) Å and = 2.5897(3) Å at 110 GPa. Hexagonal (Fe,Al)OOH can transform to the cubic pyrite structure at low with the same density. The hexagonal phase can be formed when δ-AlOOH incorporates FeOOH produced by reaction between water and Fe, which may store a substantial quantity of water in the DLM.