Surface topography and ice nucleation activity of alkali feldspar

Alkali feldspar undergoes a variety of phase transformations during cooling from magmatic crystallization leading to increasing ordering of Al and Si on the tetrahedrally coordinated lattice sites and to grain-internal microstructures such as twins and exsolution lamellae and associated surface topography. Both, Al-Si ordering as well as the specific surface topography may contribute to the extraordinary ice nucleation activity of alkali feldspar. We studied seven natural alkali feldspars ranging from homogeneous and featureless gem-quality sanidine with disordered Al-Si to hydrothermally altered microcline with ordered Al-Si, several generations of exsolution lamellae and micropore-rich regions associated with domains of hydrothermal albitization. (010) and (001) cleavage plates were produced from each feldspar sample and mounted in a cooling stage. Then an array of 7nl droplets of ultra-pure water was applied and cooled at 2 K/min. Droplet freezing events were recorded with an infrared camera.The highest freezing temperatures are observed on (010) cleavage plates of K-rich (XK=0.94) microcline that exhibits 1-8 µm wide albite exsolution lamellae and 20-100 µm wide microporous regions along cracks related to hydrothermal albitization. In contrast, featureless (001) plates of gem-quality sanidines show freezing at over 10 K lower temperatures. The enhanced ice nucleation activity is tentatively ascribed to Si-Al ordering [1] and to heterogeneous ice nucleation on the surface features related to the grain-internal microstructures [2]. Which one of the two factors is more important is still unresolved. [1] Franceschi G., Conti A., Lezuo L., Abart R.,  Mittendorfer F., Schmid M., Diebold U. (2023) How Water Binds to Microcline Feldspar (001), J. Phys. Chem. Lett., Vol. 15, 1, 15–22, https://doi.org/10.1021/acs.jpclett.3c03235[2] Kiselev, A., Keinert, A., Gaedeke, T., Leisner, T., Sutter, C., Petrishcheva, E., Abart, R. (2021) Effect of chemically induced fracturing on the ice nucleation activity of alkali feldspar. Atmospheric Chemistry and Physics, 21 (15): 11801-11814, DOI 10.5194/acp-21-11801-202