First-principles investigations on the formation of H2O defects in lizardite with influence on the elastic property

Dehydration of hydrous minerals is the key for understanding the partial melting and earthquakes taking place at subduction zones. It has been reported that a large amount of H2O defects (HD) can be formed during the dehydration process. However, the HD effects on the properties of hydrous minerals have never been considered previously. In this work, density functional theory (DFT) calculations were carried out to study the formation enthalpies of HD in lizardite. The calculated formation enthalpies of lizardite with 0.8125 wt% HD are below 0.5 eV at pressures from 0 to 7 GPa, which are low enough for the formation of HD, especially at high temperature. The presence of HD exerts significant influence on the elastic property of lizardite, resulting in lower seismic velocities and obviously higher seismic velocity anisotropy. We also calculated the migration barrier energy of H+, Mg2+, and Si4+ in lizardite. H+ migration barrier energy is 1.16 eV (vertical with ‹001› direction) and 3.40 eV (along ‹001› direction) at the pressure of 3 GPa, while Mg and Si present much higher values of more than 5 and 9 eV, respectively. Further simulations of first-principles molecular dynamics (FPMD) on lizardite with and without HD indicate that the crystal structure of HD-bearing lizardite is thermodynamically stable, and proton conduction is not obvious in lizardite and HD-bearing lizardite at temperatures below 900 K and pressures below 2 GPa.