# Ice nucleation activity of silicates and aluminosilicates in pure water and aqueous solutions – Part 3: Aluminosilicates

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, inline-formulaNH3 or inline-formula $M2inlinescrollmathmlchem{\mathrm{normal NH}}_{normal 4}^{+}$ 24pt15ptsvg-formulamathimg607f7ebf9a4fde3320a23a055c7bd38e acp-19-6059-2019-ie00001.svg24pt15ptacp-19-6059-2019-ie00001.png 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 inline-formulaNH3, inline-formula(NH4)2SO4, inline-formulaNH4Cl and inline-formulaNa2SO4, with solute concentrations corresponding to water activities inline-formulaaw equal to 0.88–1.0. Using differential scanning calorimetry (DSC) on emulsified micron-sized droplets, we derived onset temperatures of heterogeneous (inline-formulaThet) and homogeneous (inline-formulaThom) freezing as well as heterogeneously frozen water volume fractions (inline-formulaFhet). Suspensions in pure water of andesine, sanidine and kaolinite yield inline-formulaThet 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., inline-formula $M12inlinescrollmathml{T}_{\text{het}}\approx {T}_{\text{hom}}\approx normal 237.0$ 93pt12ptsvg-formulamathimgb7242778ace13e550edd9012179c750f acp-19-6059-2019-ie00002.svg93pt12ptacp-19-6059-2019-ie00002.png  K). The presence of inline-formulaNH3 and/or inline-formula $M14inlinescrollmathmlchem{\mathrm{normal NH}}_{normal 4}^{+}$ 24pt15ptsvg-formulamathimgcb88f58b3b25b473f7c5a29ace587a7f acp-19-6059-2019-ie00003.svg24pt15ptacp-19-6059-2019-ie00003.png 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 inline-formulaNH3 or inline-formula $M16inlinescrollmathmlchem{\mathrm{normal NH}}_{normal 4}^{+}$ 24pt15ptsvg-formulamathimg97c709e7ff43e05afce356dc3f53b497 acp-19-6059-2019-ie00004.svg24pt15ptacp-19-6059-2019-ie00004.png salts, inline-formulaThet 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 inline-formulaNa2SO4 cause the inline-formulaThet of feldspars to clearly fall below the water-activity-based immersion freezing description (inline-formulaΔaw= const.) even in very dilute inline-formulaNa2SO4 solutions, while inline-formulaThet of kaolinite follows the inline-formulaΔaw= 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 (inline-formulaK+, inline-formulaNa+, inline-formulaCa2+) 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 (inline-formulaΔaw= const.). We hypothesize that adsorption of inline-formulaNH3 and inline-formula $M29inlinescrollmathmlchem{\mathrm{normal NH}}_{normal 4}^{+}$ 24pt15ptsvg-formulamathimg8926cb25afd85746c56ef60531d71c8b acp-19-6059-2019-ie00005.svg24pt15ptacp-19-6059-2019-ie00005.png on the feldspar surface rather than ion exchange is the main reason for the anomalous increased inline-formulaThet in dilute solutions of inline-formulaNH3 or inline-formula $M32inlinescrollmathmlchem{\mathrm{normal NH}}_{normal 4}^{+}$ 24pt15ptsvg-formulamathimg280ce1d95365aa75ffcfe05a73d4ed2c acp-19-6059-2019-ie00006.svg24pt15ptacp-19-6059-2019-ie00006.png salts. This is supported by the response of kaolinite to inline-formulaNH3 or inline-formula $M34inlinescrollmathmlchem{\mathrm{normal NH}}_{normal 4}^{+}$ 24pt15ptsvg-formulamathimg5acffb7624bc47d59d396460984849a8 acp-19-6059-2019-ie00007.svg24pt15ptacp-19-6059-2019-ie00007.png , 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.

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Kumar, Anand / Marcolli, Claudia / Peter, Thomas: Ice nucleation activity of silicates and aluminosilicates in pure water and aqueous solutions – Part 3: Aluminosilicates. 2019. Copernicus Publications.

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