Atomic force microscopy showed ice nucleating on the common (001) face of microcline feldspar not only at rare defects such as steps and cracks but also on flat terraces. Simulations point to denser, ordered aluminol groups and a lattice match with ice as the likely mechanism, although the experimental conditions do not directly represent the atmosphere.
The researchers exposed microcline and chemically similar sanidine samples to water vapour at 145 kelvin under ultrahigh vacuum and followed their surfaces with atomic force microscopy. On microcline, ice clusters appeared both at step edges and on flat (001) terraces, and repeated cycles did not return to the same special terrace sites. On sanidine, nucleation at lower pressures remained concentrated mainly at step edges.[1], [2]
Although the two minerals have similar unit-cell dimensions, microcline's (001) surface carries two regularly placed aluminol groups per unit cell, while sanidine averages one randomly placed group. Molecular-dynamics simulations showed that a lattice match between ice's higher-index (10.4) plane and microcline, together with ice-like hydrogen bonds, could stabilize the cluster.[1], [2]
The study observed a small surface area at 145 kelvin and high supersaturation. Those conditions made nanometre-scale ice clusters visible but did not directly reproduce the atmospheric range of temperature and humidity. The result therefore offers a mechanism for why microcline can be effective; it does not measure how much nucleation accelerates in real clouds.[2]
