Molecular Dynamics Simulation Study on the SiO₂–H₂O System: New Insights on Water Dissolution Mechanisms in Melts

The reactive force field (ReaxFF) molecular dynamics (MD) simulations were performed to mimic the two-phase equilibrium between silica melts and bulk water at conditions of 1473 K and 100–700 MPa. The water solubilities, H-bearing species, and dissolution-exsolution reactions in the melts are investigated. The main findings include: (1) The major H-bearing species in melts are −Si–O–H and molecular H2O; this agrees with previous views. In addition, we found that the content of H2O increases faster than that of −Si–O–H with increasing pressure. We also observed many minor H-bearing species at less than 1% in total, such as H, O–H, −Si–O(−H)–Si–, −Si–O–H–O–Si–, −Si–OH2, −Si–O–H–O–H, H3O, H–O–H–O–H, −Si–O–H–OH2, and (H2O)2. (2) The dissolving process of water has three steps: water molecules contact melts via H2O + −Si ⇔ −Si–OH2, the −H2O groups dissociate subsequently via −Si–OH2 ⇔ −Si–O–H + H, and then various H-bearing species reach equilibrium mainly via −Si–O–H + −Si ⇔ −Si–O(−H)–Si–. Water exsolution from melts is exactly the reversal of the dissolving process. (3) The calculated water solubility increases from 0.82 wt % at 100 MPa to 4.37 wt % at 700 MPa. The values are on average 2.27 wt % (47%) lower than the experimental values [Holtz et al. Am. Mineral. 2000, 85, 682] but are consistent with a recent calculation using the classical MD simulations for hydrous rhyolite melts [Dufils et al. Chem. Geol. 2020, 533, 119300]. The present findings reveal reaction pathways for water dissolving in these melts and are helpful for the simulations of compositionally more complicated magmas and magmas containing various volatiles, including H2O, S, Cl, F, B, and P, for holistic understanding of volcanic and magmatic–hydrothermal processes.