Quantum driven proton diffusion in brucite-like minerals under high pressure.

Transport of hydrogen in hydrous minerals under high pressure is a key step for the water cycle within the Earth interior. Brucite Mg(OH)2 is one of the simplest minerals containing hydroxyl groups and is believed to decompose under the geological condition of the deep Earth’s mantle. In the present study, we investigate the proton diffusion in brucite under high pressure, which results from a complex interplay between two processes: the O–H reorientations motion around the c axis and O–H covalent bond dissociations. First-principle path-integral molecular dynamics simulations reveal that the increasing pressure tends to lock the former motion, while, in contrast, it activates the latter which is mainly triggered by nuclear quantum effects. These two competing effects therefore give rise to a pressure sweet spot for proton diffusion within the mineral. In brucite Mg(OH)2, proton diffusion reaches a maximum for pressures close to 70GPa, while the structurally similar portlandite Ca(OH)2 never shows proton diffusion within the pressure range and time scale that we explored. We analyze the different behavior of brucite and portlandite, which might constitute two prototypes for other minerals with same structure.