Pressure-induced elongation of hydrogen-oxygen bond in sodium silicate melts

Abstract

Despite their importance in melting processes in the Earth’s interior, the structure and properties of hydrous silicate melts at high pressure remain poorly constrained due to experimental challenges. Here we explore the structures of dry and hydrated (H₂O and D₂O) Na₆Si₈O₁₉ melt at 0-6 GPa and 1000-1300 K and glasses recovered from high pressure and temperatures by in-situ neutron and X-ray diffraction. The structures of the melts at 0-10 GPa and 3000 K are also investigated by ab-initio molecular dynamics simulation. In-situ neutron experiments reveal that the D-O distance increases with compression due to the formation of -O-D-O- bridging species, which is reproduced by the molecular dynamics simulations. The pressure-induced -O-D-O- formation reflects a more rigid incorporation of hydrogen, which acts as a mechanism for the experimentally observed higher solubility of water in silicate melts. Together with shrinking modifier domains, this process dominates the compression behavior of hydrous Na₆Si₈O₁₉ melt, whereas the compression of dry Na₆Si₈O₁₉ at 0-10 GPa and 3000 K is governed largely by bending of the Si-O-Si angle. The molecular dynamics simulations on hydrous Na₆Si₈O₁₉ melts further suggest that the sodium ions are scavenged from its network-modifying role via 2·(^[4]Si-O⁻ + Na⁺) → ^[4]Si-(O-^[5]Si-O)²⁻ + 2·Na⁺ and Si-O⁻ + Na⁺ + Si-OH → Si-(O-H-O-Si)⁻ + Na⁺ with increasing pressure.