The role of phase separation and related topography in the exceptional ice-nucleating ability of alkali feldspars

Abstract

Our understanding of crystal nucleation is a limiting factor in many fields, not least in the atmospheric sciences. It was recently found that feldspar, a component of airborne desert dust, plays a dominant role in triggering ice formation in clouds, but the origin of this effect was unclear. By investigating the structure/property relationships of a wide range of feldspars, we demonstrate that alkali feldspars with certain microtextures, related to phase separation into Na and K-rich regions, show exceptional ice-nucleating abilities in supercooled water. We found no correlation between ice-nucleating efficiency and the crystal structures or the chemical compositions of these active feldspars, which suggests that specific topographical features associated with these microtextures are key in the activity of these feldspars. That topography likely acts to promote ice nucleation, improves our understanding of ice formation in clouds, and may also enable the design and manufacture of bespoke nucleating materials for uses such as cloud seeding and cryopreservation.

Atmospheric Ice‐Nucleating Particles in the Dusty Tropical Atlantic

“…sources have been shown to nucleate ice with comparable efficiency (Boose, Welti, et al, 2016;Kaufmann et al, 2016;Niemand et al, 2012), which may indicate that there is a common component of desert dusts that controls its ice-nucleating ability (Atkinson et al, 2013). Mineral dust from deserts is made up of a variety of minerals (Glaccum & Prospero, 1980;Murray et al, 2012), and experiments with specific minerals have shown that the potassium feldspars (K-feldspars) are much more effective at nucleating ice than other tested minerals in desert dusts across the mixed phase-cloud temperature regime (Atkinson et al, 2013;Augustin-Bauditz et al, 2014;Harrison et al, 2016;Kaufmann et al, 2016;Niedermeier et al, 2015;Wex et al, 2014;Whale et al, 2017;Zolles et al, 2015).…”

Droplet Microfluidics XRD Identifies Effective Nucleating Agents for Calcium Carbonate

“…This also suggests the ability of montmorillonite to nucleate calcite is not due to structural matching alone. Again, it is the field of ice nucleation which has looked in most detail at the properties of individual mineral samples, where it was shown that of the mineral dusts examined, only alkali feldspars with nanoscale topographical features related to K‐ and Na‐rich domains show exceptional ice‐nucleating abilities . That topographical features such as pits and cracks promote ice nucleation has been seen in a range of experimental studies, and is also consistent with the formation of protein crystals on contaminants such as dust particles, fibers or hairs …”

“…Indeed, many crystals with good lattice matches to ice are ineffective nucleants, while crystalline steroids can effectively nucleate ice, despite no obvious structural relationship . The potential role of surface topography in promoting nucleation is also attracting increasing recognition, but offers a considerable challenge to study experimentally; it is extremely difficult to identify and characterize the precise site of nucleation, where this may often be part of a larger‐scale structure. The methods presented here therefore offer a highly valuable means of evaluating a nucleant's efficiency, where this ultimately enables us to build a comprehensive understanding of the mechanisms by which they operate.…”

“…To fully profit from these capabilities, it is also essential that crystallization reactions are conducted in well‐defined environments that can be analyzed using a range of techniques. This can be achieved by employing small reaction volumes that overcome the problems with impurities, solution inhomogeneities and convection that occur in bulk solution, where this has traditionally been achieved using droplet levitation or by creating arrays of droplets . Thanks to the increasing accessibility of microfabrication techniques, however, microfluidic devices are now seen as a versatile alternative.…”

Enhanced ice nucleation efficiency of microcline immersed in dilute NH<sub>3</sub> and NH<sub>4</sub><sup>+</sup>-containing solutions

“…If this were the case, the (100) surface plane would need to be present in almost all particles since a large fraction of submicron microcline particles have been found to show IN activity (Niedermeier et al, 2015;Kaufmann et al, 2016). Whale et al (2017) explain the exceptional ice-nucleating ability of alkali feldspars to microtextures related to phase separation into Na and K-rich regions.…”