Formation of iron hydride and high-magnetite at high pressure and temperature

Iron hydride (FeHx) is considered as suitable storage for hydrogen in the earth’s interior and possibly in the core [J. Geophys. Res. 91 (B 9) (1986) 9222]. Most experimental data on its stability pertain to low pressures (<10 GPa) and temperatures. We studied the reaction of iron with brucite (water) at pressures 75 GPa and temperatures of ∼2000 K using the double-side laser-heated diamond-anvil cell. A high-pressure phase of magnetite (Fe3O4) (orthorhombic) and iron hydride (double hcp) were found to exist stably under these conditions. The results indicate that at pressures corresponding to the earth’s lower mantle, the hydride phase is stable, and that orthorhombic high-magnetite (h-Fe3O4) may also be stabilized in lieu of or in addition to magnesiowuestite. The stability of these phases open up the possibility that water (as a component of a fluid phase or hydrous solids) may be present not only in the mantle but also in the core (as dissolved hydride and oxide), which helps melting and dynamic movements. The core may have been the reservoir of oceans of fluid. A percent of water (by weight) in the core is equivalent to about 10 times the water in all the oceans. The dissolved water components in the core would depress the melting temperature of iron (or iron–nickel alloy) significantly, reduce the density and effectively promote convection.