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INVESTIGATING ACTIVE SITES FOR IMMERSION MODE ICE NUCLEATION ON ALKALI
FELDSPARS
M.A. HOLDEN1,2, T.F. WHALE1, B.J. MURRAY1, A. BEJARANO-VILLAFUERTE2,3, A.N. KULAK2,
D. O’SULLIVAN1, F.C. MELDRUM2 and H.K. CHRISTENSON3
1
School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, United Kingdom.
2
School of Chemistry, University of Leeds, Leeds, LS2 9JT, United Kingdom
3
School of Physics and Astronomy, University of Leeds, Leeds, LS2 9JT, United Kingdom
Keywords: ICE, NUCLEATION, ACTIVE-SITES, IMMERSION, FELDSPAR.
INTRODUCTION
In the absence of foreign particles, water in clouds can supercool to below -33 °C. Particles immersed in
cloud droplets are able to catalyze ice formation at warmer temperatures; the exact freezing temperature is
related to the efficiency of the ice nucleating particle (INP). In mixed-phase clouds, mineral dusts are an
important class of INP (Hoose and Möhler 2012; Murray et al. 2012). In particular, feldspars have been
identified as the most important component, owing to their ability to catalyze freezing at warmer
temperatures than other mineral phases (Atkinson et al. 2013). Of the feldspars, those containing K+ and
Na+ are known as alkali feldspars, whilst those containing Na+ and Ca2+ are known as plagioclase
feldspars. Certain alkali feldspars have been shown to nucleate ice at warmer temperatures than
plagioclase feldspars (Harrison et al. 2016). However, the reason that some alkali feldspars are such
efficient INPs in the immersion mode is not fully understood.
In deposition mode experiments on alkali feldspars, ice nucleation was shown to originate in cracks in the
surface. The ice crystals that grew at higher temperatures were oriented. Through computational
modelling, it was shown that this orientation related to favourable nucleation on the (100) face (Kiselev et
al. 2016). For immersion mode nucleation, Whale et al. have found that ‘perthitic microtextures’ in alkali
feldspars give rise to their superior nucleating efficiency compared to plagioclase feldspars (Whale et al.
2017). These ‘microtextures’ are features and topographies that result from separation of the K+ and Na+
into discrete regions, and include the cracks from which ice nucleated in the deposition mode experiments
of Kiselev et al. (Parsons et al. 2015). Alkali feldspars that do not contain these ‘perthitic microtextures’
nucleate ice at colder temperatures, similar to plagioclase feldspars. Therefore, it may be that the
topographies formed during alkali feldspar cation separation, revealing the (100) face, are sites for
nucleation in immersion mode too.
Whilst nucleation sites related to a specific crystal face have been proposed, typical immersion mode
experiments are not able to investigate individual crystal faces (Slater et al. 2015). This is because samples
are ground or milled, meaning that a mixture of crystal faces are present on each particle and in each water
droplet. Here, we use flat sheets of alkali feldspars to investigate immersion freezing on specific crystal
faces. We investigate freezing efficiencies on the two cleavage planes, the (010) and (001), and compare
these to ground, immersed particles. Using freeze thaw experiments, we characterize the distribution of
active sites on the surface. We find that the ice nucleating efficiencies on the (010) and (001) faces of both
feldspars investigated are the same as those for the ground, immersed particles. We also find that the
freezing efficiency across the feldspar surface is not uniform, meaning that there are specific locations, or
sites, on the surface that control the freezing temperature.
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METHODS
Two alkali feldspars were selected for this study; one containing ‘perthitic mictotextures’, and one in
which the cations were not separated, known as ‘microtexturally pristine’ (Parsons et al., 2015; Whale et
al., 2017). Petrographic thin sections of the alkali feldspars were prepared. This involved cleaving the
samples along the (010) and (001) faces and mounting them on glass slides. Through sequential polishing,
the thickness of feldspar was reduced to 30 m. The final polish was performed with 0.25 m diamond
powder. Arrays of MilliQ water were pipetted onto the surface of the thin sections, and freezing
experiments were performed on the L-NIPI, described by Whale et al. (2015). The thin sections were
cooled at a constant 1 Kmin-1 ramp rate, recording the temperatures of freezing events. The data obtained
here were compared to gravimetric dilutions of suspensions of ground feldspar particles. Comparisons
between the two methods were performed by converting the fraction frozen to active site density, ns(T),
using:
n(T) / N = 1 - exp(-ns(T) A)
(1)
where n(T) is the number of droplets frozen at temperature T, N is the total number of droplets in the
experiment and A is the surface area per droplet (Connolly et al. 2009). The surface area per droplet, A,
was calculated using the BET surface area for immersed particles, whilst the contact area was used for thin
sections.
Freeze-thaw experiments were performed on thin sections to compare the freezing temperatures of
individual droplets, in a similar manner to Vali (2008). In these experiments, the feldspar was cooled at 1
Kmin-1 until all droplets were frozen. The droplets were then brought to 278 K, and then they were refrozen at 1 Kmin-1. The freezing temperatures of each droplet were recorded and compared between the
individual runs. Several cycles were performed for both feldspars.
RESULTS
Figure 1. Experimental results for (a-d) perthitic and (e-h) non-perthitic alkali feldspars. (a,e) crosspolarised light microscopy revealing the microtexture of the perthitic LD3 microcline in (a), and the lack
of microtexture in Eifel sanidine (e); (b,f) EDX mapping showing the locations of K+ (green) and Na+
(gold) in the feldspars tested. In (b), the two are separated into discrete ‘exsolved’ regions, in (f) the
cations are mixed in a ‘solid solution’; (c,g) Plot of active site density, ns(T), comparing freezing
temperatures on thin sections of selected crystal orientations and ground immersed particles for (c) LD3
microcline and (g) Eifel sanidine; (d,h) correlation plots showing the freezing temperatures of individual
droplets in a freeze thaw experiment, selecting cycles 1 and 2, for (d) LD3 microcline and (h) Eifel
sanidine.
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In Figure 1, the results from immersion mode experiments are shown, alongside micrographs
characterizing the feldspars investigated. Figures 1a-b, and Figures 1e-f, show the difference between a
perthitic and non-perthitic feldspar. In the light microscope images, the striking difference between a
feldspar containing microtexture and one without is shown. The EDX maps show that for LD3 microcline,
the cations have separated into discrete regions, whereas for Eifel sanidine they are mixed. This difference
in microtexture was shown by Whale et al. (2017) to be important in determining ice nucleation
efficiencies. In Figure 1c and 1g, active site densities, ns(T), are shown for these experiments. For
feldspars both with and without microtextures, there is good agreement between the active site densities of
ground, immersed particles and water droplets on a specific crystal face of feldspar. This demonstrates
that the sites that control freezing on ground particles are also present in similar concentrations on both
crystal faces tested. We also note that these results demonstrate that the active-site densities measured for
feldspar particles in ‘wet-suspension’ methods are not affected by aggregation, as suggested by Emersic et
al. (2015).
The results of freeze-thaw experiments are shown in Figures 1d and 1h. These show that droplets in
specific regions always freeze at higher temperatures than droplets in other regions. This means that there
is a distribution of sites across the surface, and that this distribution is of first-order importance in
determining the temperature at which a water droplet will freeze. This is in agreement with the freezethaw experiments on immersed feldspars by Peckhaus et al. (2016). The reason that some areas
consistently nucleate ice at warmer temperatures than others could be related to the amount of (100) face
exposed in each droplet, since this face is known to nucleate ice well in deposition mode (Kiselev et al.
2016). However, this would mean that concentrations of the (100) face present under different droplets
would have to vary significantly, which seems unlikely given the number of cracks covered by each mm
diameter droplet. Instead, we propose that other factors, in concert with the templating of the (100) face,
affect the nucleating efficiency of a particular site. These factors could be related to the topographies
introduced through the exsolution process, which may create geometries that stabilize the ice critical
nucleus (Whale et al. 2017).
CONCLUSIONS
We have investigated the immersion mode ice-nucleation efficiencies of two alkali feldspars; one with
perthitic microtexture, and one without. By using petrographic thin sections, we have demonstrated that
the two cleavage planes, (010) and (001), have similar ice active-site densities to ground, immersed
particles. From freeze-thaw experiments on arrays of droplets, we demonstrate that the freezing
temperature has a first-order dependence on the location of the droplet on the feldspar surface. This
heterogeneous ice nucleating activity demonstrates that the bulk framework is not responsible for feldspar
ice nucleation activity, but instead ice-nucleation active sites exist on feldspar surfaces.
ACKNOWLEDGEMENTS
We would like to thank the Engineering and Physical Sciences Research Council (EPSRC,
EP/M003027/1) and the European Research Council (648661 MarineIce) for funding.
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