Articles | Volume 16, issue 17
Research article
05 Sep 2016
Research article |  | 05 Sep 2016

Not all feldspars are equal: a survey of ice nucleating properties across the feldspar group of minerals

Alexander D. Harrison, Thomas F. Whale, Michael A. Carpenter, Mark A. Holden, Lesley Neve, Daniel O'Sullivan, Jesus Vergara Temprado, and Benjamin J. Murray

Abstract. Mineral dust particles from wind-blown soils are known to act as effective ice nucleating particles in the atmosphere and are thought to play an important role in the glaciation of mixed phase clouds. Recent work suggests that feldspars are the most efficient nucleators of the minerals commonly present in atmospheric mineral dust. However, the feldspar group of minerals is complex, encompassing a range of chemical compositions and crystal structures. To further investigate the ice-nucleating properties of the feldspar group we measured the ice nucleation activities of 15 characterized feldspar samples. We show that alkali feldspars, in particular the potassium feldspars, generally nucleate ice more efficiently than feldspars in the plagioclase series which contain significant amounts of calcium. We also find that there is variability in ice nucleating ability within these groups. While five out of six potassium-rich feldspars have a similar ice nucleating ability, one potassium rich feldspar sample and one sodium-rich feldspar sample were significantly more active. The hyper-active Na-feldspar was found to lose activity with time suspended in water with a decrease in mean freezing temperature of about 16 °C over 16 months; the mean freezing temperature of the hyper-active K-feldspar decreased by 2 °C over 16 months, whereas the "standard" K-feldspar did not change activity within the uncertainty of the experiment. These results, in combination with a review of the available literature data, are consistent with the previous findings that potassium feldspars are important components of arid or fertile soil dusts for ice nucleation. However, we also show that there is the possibility that some alkali feldspars may have enhanced ice nucleating abilities, which could have implications for prediction of ice nucleating particle concentrations in the atmosphere.

Short summary
Mineral dust particles are known to act as effective ice nucleating particles in the atmosphere and to play a role in the glaciation of mixed phase clouds. We have measured the ice nucleation activity of 15 different feldspar samples using a droplet freezing experiment and shown that alkali feldspars nucleate ice much more efficiently than plagioclase feldspars. We have also shown that certain "hyper-active" alkali feldspars nucleate ice very efficiently compared to typical alkali feldspars.
Final-revised paper