A close look at the hydration layer and at the premelting layer of K-feldspar

In this work, we perform molecular dynamics simulations to investigate the structural properties of water at the interface with K-feldspar. We aim at understanding the modifications induced on water structure by the presence of this mineral. We analysed both the hydration layer of K-feldspar in supercooled water and the premelting layer at the interface between K-feldspar and hexagonal ice. We studied the density profile, the orientational tetrahedral order parameter, the distribution of the cos(gamma) function, the hydrogen bond distribution, the number of water molecules belonging to different phases close to the surface of the feldspar. We focused on the interplay between high-density water and low-density water to inquire on local modifications of water structure with respect to the bulk phase. We investigated four different pressures where the structure of bulk water spans from high-density liquid (locally more disordered) to low-density liquid (locally more ordered). The investigation showed that both the hydration and the premelting layers present a low-density liquid water component and similarities as a function of pressure with the corresponding bulk. We also find that the premelting layer shows structural features intermediate between the liquid and the crystalline state acting as a matching layer.