  The interaction between ammonia (NH$_3$) and (alumino)silicates is of
fundamental and applied importance, yet the specifics of NH$_3$ adsorption on
silicate surfaces remain largely unexplored, mainly because of experimental
challenges related to their electrically insulating nature. An example of this
knowledge gap is evident in the context of ice nucleation on silicate dust,
wherein the role of NH$_3$ for ice nucleation remains debated. This study
explores the fundamentals of the interaction between NH$_3$ and microcline
feldspar (KAlSi$_3$O$_8$), a common aluminosilicate with outstanding ice
nucleation abilities. Atomically resolved non-contact atomic force microscopy,
x-ray photoelectron spectroscopy, and density functional theory-based
calculations elucidate the adsorption geometry of NH$_3$ on the lowest-energy
surface of microcline, the (001) facet, and its interplay with surface
hydroxyls and molecular water. NH$_3$ and H$_2$O are found to adsorb
molecularly in the same adsorption sites, creating H-bonds with the proximate
surface silanol (Si-OH) and aluminol (Al-OH) groups. Despite the closely
matched adsorption energies of the two molecules, NH$_3$ readily yields to
replacement by H$_2$O, challenging the notion that ice nucleation on microcline
proceeds via the creation of an ordered H$_2$O layer atop pre-adsorbed NH$_3$
molecules.
