Insights on the dissolution of water in an albite melt at high pressures and temperatures from a direct structural...

The water dissolution mechanism in silicate melts under high pressures is not well understood. Here we present the first direct structure investigation of a water-saturated albite melt to monitor the interactions between water and the network structure of silicate melt at the molecular level. In situ high-energy X-ray diffraction was carried out on the NaAlSi₃O₈-H₂O system at 800 °C and 300 MPa, at the Advanced Photon Source synchrotron facility. The analysis of the X-ray diffraction data was augmented with classical Molecular Dynamics simulations of a hydrous albite melt, incorporating accurate water-based interactions. The results show that metal-oxygen bond breaking at the bridging sites occurs overwhelmingly at the Si site upon reaction with H₂O, with subsequent Si-OH bond formation and negligible Al-OH formation. Furthermore, we see no evidence for the dissociation of the Al³⁺ ion from the network structure upon breaking of the Si-O bond in the hydrous albite melt. The results also indicate that the Na⁺ ion is an active participant in the modifications of the silicate network structure of the albite melt upon water dissolution at high P-T conditions. We do not find evidence for the Na⁺ ion dissociating from the network structure upon depolymerization and subsequent formation of NaOH complexes. Instead, our results show that the Na⁺ ion persists as a structure modifier with a shift away from Na-BO bonding to an increase in the extent of Na-NBO bonding, in parallel with pronounced depolymerization of the network. Our MD simulations show that the Si-O and Al-O bond lengths are expanded by about 6% in the hydrous albite melt compared to those of the dry melt at high P-T conditions. The changes in the network silicate structure of a hydrous albite melt at high pressure and temperature, as revealed in this study, must be considered in the advancement of water dissolution models of hydrous granitic (or alkali aluminosilicate) melts.