  Natural minerals contain ions that become hydrated when they come into
contact with water in vapor and liquid forms. Muscovite mica -- a common
phyllosilicate with perfect cleavage planes -- is an ideal system to
investigate the details of ion hydration. The cleaved mica surface is decorated
by an array of K$^+$ ions that can be easily exchanged with other ions or
protons when immersed in an aqueous solution. Despite the vast interest in the
atomic-scale hydration processes of these K$^+$ ions, experimental data under
controlled conditions have remained elusive. Here, atomically resolved
non-contact atomic force microscopy (nc-AFM) is combined with X-ray
photoelectron spectroscopy (XPS) to investigate the cation hydration upon
dosing water vapor at 100 K in ultra-high vacuum (UHV). The cleaved surface is
further exposed to ultra-clean liquid water at room temperature, which promotes
ion mobility and partial ion-to-proton substitution. The results offer the
first direct experimental views of the interaction of water with muscovite mica
in UHV. The findings are in line with previous theoretical predictions.
