Atomic structure of pristine and water-covered microcline (001) – A prerequisite for understanding the ice nucleation mechanism on feldspar mineral dust particles

Abstract

The aggregate state of water in clouds has a fundamental impact on the clouds’ properties such as reflectivity and lifetime. Consequently, it is crucial for the development and improvement of climate models to understand the mechanism of ice nucleation under atmospheric conditions. Most atmospheric ice nucleation is heterogeneous caused by the interaction between water droplets and ice nucleating particles. Under mixed-phase cloud conditions, one of the most important ice nucleating particles are feldspar minerals. Recent scanning electron microscopy studies have shown that ice nucleation on cleavage planes of K-rich feldspars predominantly takes place at step edges and pores (Kiselev, 2017). This has also been confirmed by video and atomic force microscopy on the micrometer scale (Holden, 2019). However, experimental insights into the atomic-scale structure of the most ice-nucleation active K-feldspar microcline are still missing, and, thus, the mechanism behind ice nucleation on feldspar minerals remains elusive. Here, we present high-resolution atomic force microscopy (AFM) data revealing the atomic structure of the microcline (001) surface in its pristine state and in contact with water (Dickbreder, 2024). AFM images of the pristine microcline (001) surface kept under ultrahigh-vacuum conditions, reveal features consistent with a hydroxyl-terminated surface. This finding suggests that water in the residual gas readily reacts with the surface highlighting the high reactivity of the as-cleaved surface. Indeed, corresponding density functional theory calculations confirm a dissociative water adsorption. Three-dimensional AFM measurements performed at the mineral-water interface unravel a layered hydration structure with two features per surface unit cell. Comparison with MD calculations suggest that the structure observed in AFM corresponds to the second hydration layer rather than the first water layer. We are convinced that the combination of structural information of the pristine and water-covered microcline (001) surface will contribute to uncovering the atomic-scale mechanism behind the exceptional ice-nucleation activity of feldspar minerals.