How ice grows on calcite

Ice nucleation is ubiquitous in nature and technology and decisive for a wide range of fields, including biology, geochemistry and environmental science. In most cases, ice nucleation occurs heterogeneously due to the presence of an ice nucleating material. Despite its importance, we still largely fail to reliably predict the ice nucleation efficacy of a given material. A particularly puzzling example is calcite, a major constituent of rocks in the Earth's crust. Although calcite possesses a high hydrophilicity, it is basically inactive in ice nucleation. Here, we combine non-contact atomic force microscopy with temperature programmed desorption measurements to investigate water multilayer formation and ice growth on calcite's (10.4) surface in ultrahigh vacuum. Adsorption of water results in the formation of up to four water layers. Interestingly, the fourth layer is observed to be metastable, as it shrinks as soon as crystalline ice nucleates. The molecular structure of the water layers is governed by the underlying calcite lattice, highlighting the strong impact of the surface. This strong templating effect of the substrate prevents the water from adopting an ice-like structure even in the fourth layer. Our results, thus, provide an explanation for the poor ice-nucleating efficacy of calcite and underscore that ice nucleation efficacy cannot be predicted based on a single descriptor.