Ice nucleation on microcline (001) in the absence of active sites

Of the many different ice-nucleating mineral particles, dust from feldspar mineral microcline stands out as being particularly active. However, why microcline outperforms other particles remains a puzzle. Here, we use nanoscale imaging to show that the (001) surface of microcline can induce ice nucleation even in the absence of active sites. In contrast, for the closely related feldspar sanidine, ice nucleation is dominant at step edges as expected. Atomistic simulations suggest that the ice nucleation is induced by a lattice match between the higher-index (10.4) plane of hexagonal ice and microcline. Our findings provide a nanoscopic explanation for microcline's exceptional ice nucleation ability and demonstrate the importance of taking higher-index surfaces into account.