Dynamic bonding analysis of the water diffusion mechanism in silicate melts

Water diffusion in silicate melt is an essential process for understanding the transport properties and dehydration kinetics of silicate melt. However, the fast liquid dynamics and interconversion of hydrous species pose a substantial challenge to both experimental and theoretical studies of water diffusion. We developed a bonding analysis method to unambiguously identify stable and transition-state hydrous species, and obtained their characteristic diffusion coefficients from restrained ab initio molecular dynamics simulations. We find that the stable species include a hydroxyl group, molecular water, and a trace amount of hydronium. Proton is only a transition-state species, but proton hopping dominates the overall water diffusion with a contribution over 60%, primarily through triggering the rotational diffusion of the O--H bond. Namely, the coupling of proton hopping and O--H bond rotation leads to fast water diffusion. The uncovered mechanism enables the construction of a water diffusion model based purely on thermodynamics, and extrapolating this model to temperatures below 2000 K aligns well with experimental observations.