Silicate minerals and their polymorphs make up a large percentage of the Earth’s crust and are therefore important for a large range of geochemical interactions. Quartz is a useful end-member of this class of minerals, however, its interaction with water is still poorly understood. Experiments show that quartz dissolution can increase by a magnitude of order or more with the introduction of ions to solution or an increase in pH, however, there is no change in the activation energy. This suggests that ions modify the structure of water such that reactants have greater access to the interface. To investigate this effect, we performed ab-initio molecular dynamics (AIMD) simulations of the quartz(101)-water interface in several different ionic solutions under different pH and temperature conditions. We found that both anions and cations in a near-surface configuration tend to increase the order of interfacial water, although at neutral pH this competes with a decrease in the H-bond lifetime induced by the presence of an anion-cation pair. We also found that neither the H or O atoms of H2O or OH- have greater access to the surface with the introduction of ions, but there is a greater incidence of intrasurface H-bonding in all systems studied. This suggests that intrasurface proton transfer could be an important component of the dissolution reaction, while nucleophilic attack of Si by H2O is unlikely to occur at the interface studied.

Structure of water at the quartz(101) surface: Effect of ions and pH

249th National Meeting (2015)

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