  Microcline feldspar (KAlSi$_3$O$_8$) is a common mineral with important roles
for Earth's ecological balance. It participates in the carbon, potassium, and
water cycles, contributing to CO$_2$ sequestration, soil formation, and
atmospheric ice nucleation. To understand the fundamentals of these processes,
it is essential to establish microcline's surface atomic structure and its
interaction with the omnipresent water molecules. This work presents
atomic-scale results on microcline's lowest-energy surface and its interaction
with water, combining ultrahigh vacuum investigations by non-contact atomic
force microscopy and X-ray photoelectron spectroscopy with density functional
theory calculations. An ordered array of hydroxyls bonded to silicon or
aluminum readily forms on the cleaved surface at room temperature. The distinct
proton affinities of these hydroxyls influence the arrangement and orientation
of the first water molecules binding to the surface, holding potential
implications for the subsequent condensation of water.
