High-pressure phase of brucite stable at Earth’s mantle transition zone and lower mantle conditions

We investigate the high-pressure phase diagram of the hydrous mineral brucite, Mg(OH)2, using structure search algorithms and ab initio simulations. We predict a new high-pressure phase stable at pressure and temperature conditions found in cold subducting slabs in Earth’s mantle transition zone and lower mantle. This implies that brucite can play a much more important role in water transport and storage in Earth’s interior than hitherto thought. The predicted high-pressure phase, stable in calculations between 20-35 GPa and up to 800 K, features MgO6 octahedral units arranged in the anatase-TiO2 structure. This suggests that brucite will transform from a layered to a compact three-dimensional network structure before eventual decomposition into periclase and ice. We show that the new phase has unique spectroscopic fingerprints that should allow for straightforward detection in experiments. The new phase also has distinct elastic properties that might make its direct detection in the deep Earth with geophysical methods possible.