High pressure-temperature superionic states of P-3 brucite [Mg(OH)₂] promoting enhanced protonic conductivity

Brucite is an archetype dense hydroxide mineral, belonging to ternary MgO-SiO₂-H₂O system, which can host a significant amount of water in the form of OH^-. The consideration of brucite in H-circulation is thus of crucial importance as the dense hydrous silicates¹ cannot account for the observed mantle conductivity. Using ab initio molecular dynamics simulations, we investigate the diffusion of H⁺(proton) in high-pressure P-3 polymorph of brucite in a combined high pressure (p)–temperature (T) range of 10-85 GPa and 1250-2000K. We observe vigorous proton diffusion at elevated p-T resulting in a liquid state of the H-sublattice, within solid Mg-O sublattice, generating the superionic states in brucite. This study also reveals an unusual pressure-dependent proton migration characterized by maximum H⁺-diffusion in the pressure range of 72-76 GPa along the isotherms. The mobility of H⁺ is found to be strongly anisotropic, allowing no transport across the MgO₆-octahedral layers i.e. along c-axis. The liquid state of the protonic sublattice yields a quasi-2D layer of fast H⁺ causing a drastic enhancement in the electrical conductivity of P-3 brucite. Comparison of our calculated conductivity with ex-situ geophysical data^2,3 indicates the possible brucite abundance to explain high-conductive zones in the deep mantle of earth.

¹Hirschmann & Kohlstedt Phys. Today 65 40–5 2012

²Constable & Constable, Geochem. Geophys. Geosystems 5 1–15 2004

³Civet et al., Geophys. Res. Lett. 42 3338–46 2015