Ice nucleation activity of silicates and aluminosilicates in pure water and aqueous solutions – Part 3: Aluminosilicates
Authors: Kumar Anand , Marcolli Claudia , Peter Thomas 
Published: May 2019

Abstract. Aluminosilicates and quartz constitute the majority of airborne mineral dust.
Despite similarities in structures and surfaces they differ greatly in terms
of their ice nucleation (IN) efficiency. Here, we show that determining
factors for their IN activity include surface ion exchange, NH3 or
NH4+ adsorption, and surface degradation due to the slow dissolution
of the minerals. We performed immersion freezing experiments with the
(Na-Ca)-feldspar andesine, the K-feldspar sanidine, the clay mineral
kaolinite, the micas muscovite and biotite, and gibbsite and compare their IN
efficiencies with those of the previously characterized K-feldspar microcline
and quartz. Samples were suspended in pure water as well as in aqueous
solutions of NH3, (NH4)2SO4, NH4Cl and
Na2SO4, with solute concentrations corresponding to water activities
aw equal to 0.88–1.0. Using differential scanning calorimetry
(DSC) on emulsified micron-sized droplets, we derived onset temperatures of
heterogeneous (Thet) and homogeneous (Thom) freezing as
well as heterogeneously frozen water volume fractions (Fhet).
Suspensions in pure water of andesine, sanidine and kaolinite yield
Thet equal to 242.8, 241.2 and 240.3 K, respectively, while no
discernable heterogeneous freezing signal is present in the case of the micas
or gibbsite (i.e., Thet≈Thom≈237.0 K). The
presence of NH3 and/or NH4+ salts as solutes has distinct
effects on the IN efficiency of most of the investigated minerals. When
feldspars and kaolinite are suspended in very dilute solutions of NH3
or NH4+ salts, Thet shifts to higher temperatures (by
2.6–7.0 K compared to the pure water suspension). Even micas and gibbsite
develop weak heterogeneous freezing activities in ammonia solutions.
Conversely, suspensions containing Na2SO4 cause the Thet
of feldspars to clearly fall below the water-activity-based immersion
freezing description (Δaw= const.) even in very dilute
Na2SO4 solutions, while Thet of kaolinite follows the
Δaw= constant curve. The water activity determines how the
freezing temperature is affected by solute concentration alone, i.e., if the
surface properties of the ice nucleating particles are not affected by the
solute. Therefore, the complex behavior of the IN activities can only be
explained in terms of solute-surface-specific processes. We suggest that the
immediate exchange of the native cations (K+, Na+,
Ca2+) with protons, when feldspars are immersed in water, is a
prerequisite for their high IN efficiency. On the other hand, excess cations
from dissolved alkali salts prevent surface protonation, thus explaining the
decreased IN activity in such solutions. In kaolinite, the lack of
exchangeable cations in the crystal lattice explains why the IN activity is
insensitive to the presence of alkali salts (Δaw= const.).
We hypothesize that adsorption of NH3 and NH4+ on the
feldspar surface rather than ion exchange is the main reason for the
anomalous increased Thet in dilute solutions of NH3 or
NH4+ salts. This is supported by the response of kaolinite to
NH3 or NH4+, despite lacking exchangeable ions. Finally, the
dissolution of feldspars in water or solutions leads to depletion of Al and
formation of an amorphous layer enriched in Si. This hampers the IN activity
of andesine the most, followed by sanidine, then eventually microcline, the
least soluble feldspar.