Measurement report: Ice nucleation ability of perthite feldspar powder

Feldspars are among the most efficient mineral ice-nucleating particles (INPs) in the atmosphere, yet their ice nucleation behavior varies widely across natural samples. Here, we investigate six feldspar powders selected for their perthitic or anti-perthitic textures and spanning a broad range of K / Na compositions. All samples were characterized in terms of mineralogy, bulk and surface chemistry, and microstructure. Droplet-freezing assays revealed consistent onset temperatures between -2 and -4 °C in high-concentration suspensions, suggesting the presence of similar highly active nucleation sites across all tested feldspar types. On the other hand, cumulative and differential freezing spectra showed pronounced differences in the density and distribution of ice nucleation sites, which correlate with both feldspar composition and microtexture. Heterogeneous Underlying-Based (HUB) analysis identified distinct subpopulations of ice nucleation sites. Perthites exhibiting ordered microcline structures showed a continuous increase in nucleation site density with decreasing temperature, whereas samples lacking dominant microcline features displayed plateaus in cumulative spectra over specific temperature ranges, indicating decreased ice nucleation ability of the subpopulation. These results demonstrate that exsolution textures and crystallographic structure play a central role in controlling feldspar ice-nucleation efficiency, with important implications for atmospheric INP parameterizations and cloud microphysics.