Pure water only freezes at around minus 38 degrees Celsius. Tiny mineral dust particles act as so‑called ice nucleators—crystallization seeds on which ice crystals form. A research team at Bielefeld University and the University of Vienna in cooperation with researchers from Helsinki University has now demonstrated for the first time, at the molecular scale, why the mineral microcline is particularly effective at forming ice in clouds. Published in a renowned scientific journal, the study provides a new explanation for processes that influence climate and precipitation worldwide. An accompanying film from the research_tv series by Bielefeld University illustrates the discovery (available with English subtitles).
Key facts at a glance:
- The mineral microcline forms ice already on its most common, most stable surface.
- Spatially ordered aluminol groups on the surface bind water particularly well.
- The researchers directly visualized ice clusters at the nanometer scale.
The process under investigation helps determining when water in clouds freeze, how much precipitation falls, and how strongly clouds reflect sunlight. All of these factors play a crucial role in the climate.
‘We wanted to understand why microcline is so exceptionally good at forming ice, even though it differs only slightly in chemical composition from other feldspars, a group of common rock-forming minerals,’ says Dr. Florian Schneider from Bielefeld University, first author of the study. ‘Our results reveal for the first time at the molecular level what makes this mineral so special.’
Ice grows in an ordered way on a stable surface
Feldspars are among the most abundant minerals in atmospheric dust. Microcline—chemically potassium aluminum silicate—is considered an extremely effective ice nucleator. Until now, researchers assumed that ice forms mainly at rare surface features such as step edges or cracks, so‑called active sites.
The research team has now shown that, in the case of microcline, its most common and thermodynamically stable surface—the so‑called (001) cleavage plane—is sufficient. Ice grows there in an ordered manner, a process known as epitaxial growth. This means that the crystal lattice of ice aligns at a fixed angle with the crystal lattice of the mineral.
‘Interestingly, the (001) surface of microcline does not match with the common surfaces of hexagonal ice. Instead, the ice crystals grow with a less common surface, a so-called higher index plane, aligned to the microcline structure,’ says Dr. Tobias Dickbreder from the University of Vienna, last author of the study. ‘This finding provides a new perspective on the importance of uncommon ice faces for understanding ice nucleation.’
Nanometer-scale images provide crucial evidence
The discovery was made possible by high‑resolution atomic force microscopy under ultra‑high vacuum conditions. The researchers cooled the samples and directly observed where tiny ice clusters formed. Complementary computer simulations confirmed the experimental findings.
For comparison, the team also studied the closely related mineral sanidine. Although it has the same chemical composition as microcline, ice forms on sanidine mainly at step edges, as classical theory predicts. The difference appears to lie in the surface structure: the most stable surface of microcline carries roughly twice as many so‑called aluminol groups. These chemical groups can form strong hydrogen bonds with water molecules and stabilize the first ice clusters.
The study thus provides a long‑sought nanoscale explanation for microcline’s exceptional ice‑nucleating ability. The results show that rare surface features are not necessarily required; widespread, stable surfaces can also play a decisive role.
This is particularly relevant for climate research: even small differences in ice formation can alter cloud properties, precipitation patterns, and the reflection of sunlight. A better understanding of mineral surfaces in the atmosphere will help make climate models more accurate.
At Bielefeld University, research into ice formation on mineral surfaces falls within the strategic Focus Area ‘Architecture of Nature: Elementary Building Blocks and the Formation of New Structures’ (ANBauEn).
Assessment by Professor Dr Angelika Kühnle from Bielefeld University:
‘Our work closes a key knowledge gap. For the first time, we can explain at the molecular scale why microcline promotes ice formation so efficiently. These insights are not only fascinating from a fundamental science perspective, but also highly relevant for climate models and atmospheric research. Being able to directly visualize nanoscale ice cluster represents a methodological breakthrough.’
Further information