The role of microfeatures, Al-Si-ordering and surface topography on the ice nucleation activity of alkali feldspars

Alkali feldspars have been identified as the most efficient ice-nucleating particles in airborne mineral dust [1, 2]. However, alkali feldspars exhibit large mineralogical variations which is also reflected in substantial differences in their ice-nucleating efficiency [2,3,4]. Identifying the mineralogical or surface characteristics responsible for the high ice-nucleation activity of certain alkali feldspars could advance our understanding of ice nucleation in mixed-phase clouds.For this study, seven natural alkali feldspars, ranging from homogeneous gem-quality sanidines to hydrothermally altered, micropore-rich perthitic microclines were characterized with petrographic microscopy, electron probe micro analysis (EPMA) and powder X-ray diffraction (pXRD). The ice nucleation efficiency was investigated by means of cooling ramp experiments conducted at a cooling rate of 2 K min-1 on a cold stage, using (001) and (010) cleavage plates as well as 1 wt% suspensions of 2–8 µm powder from each sample. For two gem-quality samples and one perthitic microcline, additional experiments were performed using 0.05 wt% suspensions of 2–8 µm powder as well as 0.5–2 µm powder to assess the influence of particle surface area and grain size. In these experiments, 7 nl droplets of the suspension were dispensed on Si-wafers, while for cleavage plate experiments, 7 nl droplets of nanopure water were dispensed onto the samples. Droplet freezing events were detected using an infrared camera.Hydrothermally altered perthitic microclines exhibit the highest ice nucleation activity in both cleavage plate and suspension experiments. The lowest ice-nucleation activity was observed for gem-quality sanidine in suspension experiments and for (001) cleavage plates of gem-quality orthoclase. For microcline, and more prominently for orthoclase (both perthitic and gem-quality), (010) cleavage plates showed higher freezing temperatures than (001) plates. The freezing sequence of droplets on cleavage plates was more strongly influenced by surface topography in gem-quality samples than in perthites, indicating that freezing in perthites is predominantly controlled by mineralogical features.The ice-nucleation activity of gem-quality samples was more sensitive to particle surface area than that of perthitic samples, showing a stronger decrease in freezing temperatures at lower suspension concentrations and a more pronounced increase with decreasing particle size.We conclude that features related to perthitic exsolution, a high degree of Al-Si-ordering and - for orthoclase and microcline - the crystallography of (010) surfaces are key factors for the high ice-nucleating activity of alkali feldspars. [1] Atkinson et al., Nature (2013) 498(7454), 355-358, doi:10.1038/nature12278[2] Whale et al. Phys. Chem. Chem. Phys. (2017) 19, 31186—31193, doi:10.1039/c7cp04898j[3] Harrison et al., Atmos. Chem. Phys. (2016), 16, 10927–10940, doi:10.5194/acp-16-10927-2016[4] Welti et al., Atmos. Chem. Phys. (2019), 19, 10901–10918, doi:10.5194/acp-19-10901-2019