Mineral dusts originating from Earth's crust are known to be important atmospheric ice nuclei. In agreement with earlier studies feldspar was found as the most active of the tested natural mineral dusts. Here we investigated in closer details the reasons for its activity and the difference in the activity of the different feldspars. Conclusions are drawn from scanning electron microscopy, X-ray powder diffraction, infrared spectroscopy, and oil-immersion freezing experiments. K-feldspar showed by far the highest ice nucleation activity. Finally, we give a potential explanation of this effect finding alkali-ions having different hydration shells and thus an influence on the ice nucleation activity of feldspar surfaces.