Role of Feldspar and Pyroxene Minerals in the Ice Nucleating Ability of Three Volcanic Ashes

The mineralogical and immersion-mode freezing properties of volcanic ashes from three volcanoes, Volcán de Fuego and Santiaguito in Guatemala, and Soufrière Hills Volcano, in Montserrat, were examined. All ashes (sieved to <37 μm) contained effective ice nuclei, typically freezing over the temperature range of ？12 to ？25 °C and possessing ice active site densities (ns) spanning ～10 1 to 10 5 cm–2 over this temperature range. The high freezing activity of the ashes was determined to likely originate from pyroxene minerals, and the ice nucleation properties of pyroxene minerals are also reported here for the first time for comparison. Ca- and Na-rich plagioclase feldspars also contributed to the observed freezing properties. Volcanic glass was present in all of the samples and is theorized to be a much weaker ice nucleant, effectively diluting the freezing ability of the crystalline mineral phases. Smaller particle size fractions of the Volcán de Fuego ash were observed to contain more active ice nucleating particles, attributed to an increase in the amount of pyroxene minerals with decreasing particle size fraction. The particle resuspension and size segregated collection process was also observed to increase the ice nucleating ability of all size fractions, likely due to mechanical ablation removing passivated surfaces and exposing fresher and more ice-active mineral surfaces