Understanding the mineralogy of the Earth’s interior is a prerequisite for unravelling the evolution and dynamics of our planet. Here, we conducted high pressure-temperature experiments mimicking the conditions of the deep lower mantle (DLM, 1800–2890 km in depth) and observed surprising mineralogical transformations in the presence of water. Ferropericlase, (Mg, Fe)O, which is the most abundant oxide mineral in Earth, reacts with H2O to form a previously unknown (Mg, Fe)O2Hx (x ≤ 1) phase. The (Mg, Fe)O2Hx has a pyrite structure and it coexists with the dominant silicate phases, bridgmanite and post-perovskite. Depending on Mg content and geotherm temperatures, the transformation may occur at 1800 km for (Mg0.6Fe0.4)O or beyond 2300 km for (Mg0.7Fe0.3)O. The (Mg, Fe)O2Hx is an oxygen excess phase that stores an excessive amount of oxygen beyond the charge balance of maximum cation valences (Mg2+, Fe3+ and H+). This important phase has a number of far-reaching implications including extreme redox inhomogeneity, deep-oxygen reservoirs in the DLM and an internal source for modulating oxygen in the atmosphere.

Qingyang Hu, Jin Liu, Jiuhua Chen, Bingmin Yan, Yue Meng, Vitali B Prakapenka, Wendy L Mao, Ho-kwang Mao, Mineralogy of the deep lower mantle in the presence of H2O, National Science Review, nwaa098, abstract

Mineralogy at the lower mantle. The lower mantle is divided roughly in the middle of the lower mantle, below which is the DLM. While H2O or dense hydrous phases (blue droplets) are carried down by plate subduction or form from primitive water in shallower lower mantle, they react with Fp to form the Py-phases in the DLM (solid patches). Color gradient in the patches indicates the content of Py-phases. The mineral composition within the wet pockets of the DLM may include Brg, Fp as well as Py-(Mg, Fe)O2Hx.