Does the Si/Al-order in K-feldspars influence their hydration structure?

Abstract

Cloud properties affecting weather and climate, such as albedo or likelihood to precipitate, strongly depend on whether cloud droplets are liquid or frozen. Heterogeneous ice nucleation in the presence of ice nucleating particles (INP) predominates in ice cloud formation. An important example of INPs are K-feldspar mineral dust particles. The term K-feldspar describes a group of minerals with the same chemical composition (KSi3AlO8), but with differences in crystallographic structure, Si/Al-order and (surface) microstructure. K-feldspars with a high Si/Al order (microcline) were found to have a higher IN efficiency than K-feldspars with a lower Si/Al-order (orthoclase and sanidine). However, the origin of this difference in ice nucleation efficiency is not yet clarified. Here, we present an experimental and theoretical comparison between the hydration layer structures of the orthoclase- and microcline-water interfaces. The hydration layer structure strongly affects the heterogeneous ice nucleation mechanism in immersion freezing. The two- and three-dimensional atomic force microscopy experiments and the molecular dynamic simulations at the orthoclase (001)- and (010)-water interfaces both reveal a similar checkerboard-like structure 0.5 nm above the surfaces with a spacing of (0.27 ± 0.03) nm between the water layers. We observe no significant impact of the Si/Al-order on the hydration structures on orthoclase and microcline, but simulations indicate changes in the hydration layer dynamics.