Water mineral chemistry in Earth's lower mantle

Understanding the mineralogy of the Earth's interior is a prerequisite for unravelling the evolution and dynamics of our planet. We conducted high pressure-temperature experiments mimicking the conditions of equivalent to 1800-2890 km depth in Earth's lower mantle 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 release hydrogen and form a previously unknown (Mg,Fe)O₂Hx (x<1) phase with the pyrite structure coexisting with the dominant silicate phase bridgmanite or post-perovskite. Depending on the depth and Mg content, the transformation may occur at 1900 km for (Mg_0.6Fe_0.4)O or beyond 2300 km for (Mg_0.7Fe_0.3)O. The mineral chemistry with water and major rock-forming minerals has a number of far-reaching implications including the extreme redox inhomogeneity and oxygen reservoirs near Earth's core mantle bounary. It can also acts as a factor for modulating oxygen concentration in shallower depth of Earth's interiors.