In situ X-ray diffraction during ice formation on alkali feldspar

Alkali feldspar is the most effective ice nucleating particle in airborne mineral dust and can initiate heterogeneous cloud ice formation at high temperatures [1]. It may thus influence precipitation formation and the Earth's radiation budget. The particularly high ice nucleation ability of microcline within the group of alkali feldspars was attributed to its complex perthitic microstructure in the form of Na-rich and K-rich exsolution lamellae [2], which naturally result from phase transformations during the cooling process after the magmatic and metamorphic crystallization, and defects like step edges, cracks, pores or cavities [2,3]. The lamellae and surface features are usually aligned with the non-rational Murchison plane with Miller indices between (-601) and (-801), subparallel to the direction where the elastic energy associated with exsolution is minimized [4]. Those features were hypothesized to expose small facets of particularly highly ice-nucleation active, but non-cleavable surfaces with the crystallographic (100) orientation [3]. This would explain the epitaxial relationship between feldspar (100) and the primary prismatic crystal planes of macroscopic ice crystals, observed in microscopic freezing experiments [3,5,6,7], but an understanding of this epitaxial relationship on the molecular level is still missing.

We study ice formation from the vapor phase on (001) and (010) cleavage plates of gem quality (featureless reference), gem quality with chemically induced fractures along the Murchison plane, and natural perthitic alkali feldspar under atmospheric pressure using a newly developed in situ X-ray diffraction setup and synchrotron radiation. The high-resolution information allows us to quantify the average ice crystal orientation with respect to the crystallographic domains of feldspar and complement previous electron microscope experiments. For the first time, we confirm the epitaxial relationship between ice and feldspar on defect-rich samples under atmospheric-relevant conditions, as observed in our experiments through a narrow orientation distribution in reciprocal space. The highest fraction of oriented ice crystals is found on natural perthite surfaces of (010) orientation, while ice grows rather randomly on the gem quality reference. In addition, we always detect the XRD-signal of oriented ice well before the XRD-signal of the ice fraction growing with random orientation.

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