Enhancement of ice nucleation efficiency of K-rich feldspar induced by K-Na cation exchange

Alkali feldspars have been demonstrated to be the most efficient ice nucleating components of airborne mineral dust particles. Despite recent efforts to explain this experimental phenomena, the molecular mechanism of ice nucleation on feldspar is far from being understood. Recently, the surface topography and crystalline structure have been recognized as the two major factors defining the ice nucleation activity of K-feldspar. However, complex morphology and strong variability of chemical composition of K-feldspar hinder the quantification of individual factors contributing to its ice nucleating properties. One of the major unanswered questions is how the spatial distribution of Na-rich and K-rich regions of alkali feldspar is related to its IN efficiency and what is the mechanism behind this relationship.

In this contribution, we report the results of a droplet freezing experiment conducted on the thin sections of feldspar artificially modified with respect to its chemical composition. Single crystals of gem-quality K-feldspar (sanidine) have been shifted towards more Na-rich composition by cation exchange in a NaCl-KCl salt melt at a temperature of 850°C, followed by tempering at 550°C for a time period of up to 6 months. This chemical shift produces highly anisotropic contraction of the crystal lattice, inducing a tensile stress which results in a system of microscopic cracks. The exsolution of Na-rich and K-rich phases during the tempering also results in formation of exsolution lamellae very similar to those often found in the natural microcline, which has been shown to be the most IN active of all alkali feldspars. By conducting freezing experiments with nL-volume droplets deposited on the surface of feldspar crystals in a cold-stage setup, we demonstrate that the gradual change of crystalline structure and appearance of cracks and exsolution lamellae is correlated with the pronounced increase of ice nucleation efficiency of modified feldspar. Finally, using scanning electron microscopy, X-Ray spectroscopy and electron diffraction microanalysis, we discuss the possible molecular mechanisms responsible for the observed behavior.