Effect of chemically induced fracturing on the ice nucleation activity of alkali feldspar

Abstract

Abstract. Feldspar is an important constituent of airborne mineral dust. Some alkali feldspars exhibit particularly high ice nucleation (IN) activity. This has been related to structural similarities of the ice (101‾0) prism planes and the (100) planes of alkali feldspar. Here the effect of generating feldspar surfaces with close to (100) orientation by means of chemically induced fracturing on the IN activity of alkali feldspar was investigated experimentally. To this end, gem-quality K-rich alkali feldspar was shifted towards more Na-rich compositions by cation exchange with an NaCl–KCl salt melt at 850 ∘C. By this procedure, a system of parallel cracks with an orientation close to the (100) plane of the feldspar was induced. Droplet-freezing assay experiments performed on grain mounts of the cation-exchanged alkali feldspars revealed an increase in the overall density of ice-nucleating active site (INAS) density with respect to the untreated feldspar. In addition, annealing at 550 ∘C subsequent to primary cation exchange further enhanced the INAS density and led to IN activity at exceptionally high temperatures. Although very efficient in experiment, fracturing by cation exchange with an alkali halide salt is unlikely to be of relevance in the conditioning of alkali feldspars in nature. However, parting planes with similar orientation as the chemically induced cracks may be generated in lamellar microstructures resulting from the exsolution of initially homogeneous alkali feldspar, a widespread phenomenon in natural alkali feldspar known as perthite formation. Perthitic alkali feldspars indeed show the highest IN activity. We tentatively ascribe this phenomenon to the preferential exposure of feldspar crystal surfaces oriented sub-parallel to (100).

Ice Nucleating Particle Connections to Regional Argentinian Land Surface Emissions and Weather During the Cloud, Aerosol, and Complex Terrain Interactions Experiment

“…A widely used metric for describing the ice nucleation activity of particles is the surface site density (hereafter n s (T) ), representing the number of ice nucleation active sites per unit of particle surface area at the processing temperature T (DeMott, Hill, et al., 2015; DeMott, Prenni, et al., 2015; Hoose & Möhler, 2012; Niemand et al., 2012). These active sites are conceptualized as specific locations on the INP surface that facilitate the formation of ice embryos by decreasing the activation energy necessary for the phase transition; their nature is still poorly understood but they may occur preferentially at surface defects (Holden et al., 2021; Kiselev et al., 2021).…”

Mechanism of ice nucleation in liquid water on alkali feldspars

“…The first glance at the freezing behaviour of both specimens reveals that droplets in contact with FS06 freeze at 10 K higher temperature than for the FS07, which we do not find surprising given the well-established empirical relationship between the feldspar microtexture and its IN efficacy. 7–9,13 What is indeed noteworthy is that the droplets are freezing at almost 5 K higher temperature on the surfaces of a thin section polished along the (100) plane as compared to other orientations. This result is somewhat unexpected given the absence of oriented ice growth in sucrose solution in contact with the same surfaces.…”

“…14 Indirectly, this hypothesis goes along with our recent observation that chemically induced cracks in the initially texture-free sanidine feldspar (similar to FS07) indeed increase its IN efficacy. 13 No such high-temperature IN active sites have been found in the samples prepared from the FS07 specimen.…”

“…The ice nucleating efficacy of the thin sections and powder suspensions of feldspar samples was measured in the DROFA setup as described in our earlier work. 8,13 Briey, a 10 Â 10 mm thin section was placed into a temperature-controlled cold stage setup consisting of a Linkam MDBCS-196 motorized cold stage, a piezo-driven drop-on-demand dispenser (GeSIM, model A010-006 SPIP) and a video camera with a wide-eld objective allowing for detection of individual freezing events with 0.125 s time resolution. In the experiments described in this work, 50 to 150 droplets of pure water each with a volume of approximately 1.4 nL were deposited onto the polished surface of a thin section in a square pattern with 400 mm centre-to-centre separation between the droplets.…”

“…This hypothesis has received experimental support in the more recent study demonstrating enhancement of the IN activity of K-rich feldspar caused by chemically-induced fracturing. 13 Preferential nucleation of ice in surface pits and cracks has been reported repeatedly for immersion 14,15 and deposition freezing, 10,11 however, no direct evidence that ice nucleation is associated with (100) surfaces in both freezing mechanisms has been provided so far.…”