The formation of ice, when and how water freezes, is still poorly understood, even though this is essential for understanding Earth’s climate. Knowing more about the molecular mechanisms underlying these processes can help to build atmospheric and climate models with higher confidence.

One of the key factors in ice nucleation is to understand how solid surfaces induce heterogeneous nucleation. Feldspar particles have been considered to be responsible of a large proportion of the ice nucleation particles present in clouds. In this work, using environmental scanning electron microscopy (E-SEM), we investigated ice nucleation on different faces of feldspar minerals exposed to water vapor below water saturation conditions.

Our experiments revealed that nucleation was dominated by natural micro-sized pores exposed at the surface of the mineral upon cleavage.  Sequences of E-SEM images showed that ice growth inside pores starts with the filling of the pore and then the growth of the crystal outside the pore, as it’s described by the pore condensation freezing theory to explain ice nucleation in porous materials exposed to humid air below saturation. Images also showed that in sequential nucleation events, crystals growth at the same site always appeared with the same orientation with respect to the feldspar surface. This finding indicates that the feldspar imposes structural constrains to the ice formation probably due to special arrangements of hydroxyls groups at the feldspar surface.