Growth and Equilibrium Morphology of Hydrohalite (NaCl·2H2O) and Its Epitaxy with Hexagonal Ice Crystals

The experimental growth morphology of NaCl·2H2O, mineral hydrohalite (HH), is compared with that calculated at equilibrium, at 0 K. The HH surface structure is theoretically investigated through the Hartman-Perdok method: the growth characters of the different {hkl} forms are determined along with their possible surface profiles in order to calculate, at the DFT level, their specific surface energies in the presence of vacuum. The striking difference between the calculated equilibrium shape (ES) and the observed growth shape (GS) is interpreted on the ground of a privileged epi-adsorption of {00.1}, the most important form of hexagonal ice (Ih) on the observed dominant {010} form of HH. The sharp pseudo-hexagonality of the {010} HH-form (substrate) seems to be the keystone to understand the very good epitaxy with the {00.1} pinacoid (deposit) of Ice (Ih).