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

Sofiane Schaack,Philippe Depondt,Simon Huppert,Fabio Finocchi

doi:10.1038/s41598-020-64813-8

Abstract

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.

Highlights

Transport of hydrogen in hydrous minerals under high pressure is a key step for the water cycle within the Earth interior
The trigonal brucite structure consists of alternating layers along the c axis that terminate with hydroxyls (Fig. 1)
We address the proton diffusion process occurring in brucite Mg(OH)[2] that appears to be a specific case as compared with portlandite Ca(OH)[2], which was the object of a recent theoretical study[28], and is discussed here

Summary

Introduction

Transport of hydrogen in hydrous minerals under high pressure is a key step for the water cycle within the Earth interior. X-ray diffraction of Mg(OH)[2] up to 78 GPa showed that the c/a ratio decreases steadily from ambient pressure up to about 25 GPa and stays almost constant[6] Those results suggest that the properties of brucite at very high pressure, and in particular the nature of the inter-layer bonding, could differ significantly from ambient conditions. The previous observations call for a dynamical treatment of the proton arrangement within the brucite structure at high pressure In such conditions nuclear quantum effects, that is, all the properties that go beyond a purely classical description of ion dynamics[14], such as zero-point. Institut des NanoSciences de Paris (INSP), Sorbonne Université, CNRS-UMR 7588, 75005, Paris, France. ✉e-mail: The size of atoms decreases with depth

Methods

Results

Conclusion