ICE-NUCLEATION ACTIVITY OF ALKALI FELDSPAR IN AEROSOL

In this project, the role of microscopic particles of alkali feldspar, which are suspended in the air as so-called aerosols, in the formation of ice crystals in clouds is investigated. Alkali feldspar is one of the most abundant minerals in the Earth`s crust and represents about 5 to 10% of natural mineral aerosol particles. These particles play a critical role in cloud formation and in the glaciation of clouds. Finely dispersed water droplets that we perceive as clouds typically have diameters in the range of tenths and hundredths of a millimeter. As long as these droplets are nowhere in contact with solid aerosol particles, they can be cooled to -36C without freezing. When in contact with solid aerosol particles, the freezing point of the droplets is shifted to much higher temperatures, and the degree of freezing point increase depends, among other things, on the type of aerosol particles. Aerosol particles of alkali feldspar make a special contribution to the formation of ice in clouds by allowing the freezing of microscopic water droplets at exceptionally high temperatures as high as -3C. The physical properties of a cloud, such as its albedo, the degree to which it reflects sunlight, depend to a large extent on whether the cloud consists of water droplets or ice crystals. As the water droplets in the clouds freeze, heat of crystallisation is released, which can drive weather dynamics. Knowledge of the factors that control the freezing temperature in clouds is thus of great importance to any weather and climate models and also has practical significance. In this project, an interdisciplinary approach of mineralogy, aerosol physics and computer simulation is used to investigate the particular ice nucleation activity of alkali feldspar aerosol particles. Literature data and preliminary results from pilot studies suggest that the ice nucleation activity of feldspar is related to the degree of complexity of its surface structure. To test this hypothesis, largely defect-free gem-quality alkali feldspars are prepared, on the one hand, and ordinary feldspars with complex internal grain structures, on the other hand. Untreated cleavage pieces and surface-treated specimens are prepared, the latter by means of hydrofluoric acid etching, chemically induced fracturing or by means of solution-precipitation reactions in an aqueous solution. Subsequently, the conditioned feldspars are used for ice nucleation experiments under controlled conditions in the laboratory. The goal is to assign the exceptional ice nucleation activity of alkali feldspar to specific surface structures and to shed light on the mechanisms of ice nucleation and growth on alkali feldspar. Applications in the targeted icing of microscopic water droplets in clouds or in technical applications are conceivable.