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

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 NaAlSiO-HO 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 HO, with subsequent Si-OH bond formation and negligible Al-OH formation. Furthermore, we see no evidence for the dissociation of the Al 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.