Crystals usually form angular shapes – from cubes to the jagged stars of a snow crystal. But under certain conditions, spherical crystal shapes can also arise. A research team has now deciphered how such spherulites are formed and what conditions are necessary for this, using various sulfate solutions as an example. Accordingly, the polycrystalline spheres form when, in addition to a monovalent sulfate, second-valent ions also occur in the solution. They then form clusters, from which first loosely associated nanocrystals and then the spherulite emerge. The new findings not only clarify how spherical kidney stones or pancake-like crystal structures form, for example. They could also help create novel crystalline materials.
From common table salt to snowflakes to gemstones and diamonds – we encounter crystals everywhere in nature. They arise when water freezes, a melt cools or when a supersaturated salt solution stands for a longer period of time. Triggered by a crystallization nucleus, an ordered crystal forms from the liquid starting material – a solid lattice of regularly arranged atoms. The crystals formed usually have a characteristic angular geometric shape: they form cubes, trapezoids or six-armed stars like snowflakes. But there are crystalline structures that form complex, polycrystalline spheres, so-called spherulites. These also occur in nature in many different materials and contexts. “Mineralogists are researching meter-sized spherulites in rhyolitic lava; in medicine, spherulites are found in kidney stones or the deposits of amyloid proteins from Alzheimer’s and Parkinson’s,” explain Tess Heeremans from the University of Amsterdam and her colleagues. “But despite their widespread use, the dynamics of spherulite crystallization and the conditions for the growth of such crystals remain poorly understood.”
Crystallization tests with sulfates
In order to provide more clarity, Heeremans and her team have now examined the formation of these spherical crystal structures in the laboratory. Using various sulfate salt solutions, they tested when and how exactly the spherulites form. To do this, they mixed sodium sulfate (Na2SO4) with divalent iron or magnesium sulfate solutions and placed drops of this supersaturated solution on a surface. This showed that at certain concentration ratios of monovalent sodium sulfate to one of the divalent sulfates, not the usual polyhedral crystals were formed, but rather spherical spherulites. Under the microscope, these resembled tiny sea urchins or corals. “When we saw these balls developing from our salty mixtures under the microscope, we could hardly look away – it was just cool!” says Heeremans, describing her first impression.
Further experiments showed that the spherulites also arise when the iron and magnesium sulfates are replaced by other divalent salts, for example based on copper ions or tin. “This suggests that the mechanism of spherulite growth is not specific, but can take place in different mixtures of divalent sulfate salts,” the researchers write. However, mixing sodium sulfate with another monovalent sulfate salt was not successful. The presence of divalent ions in the salt solution is therefore crucial for the formation of sodium sulfate spherulites, according to the team.
Clusters, nanocrystals and then spherulite
Closer analyzes revealed the mechanism and process of this crystallization: “First, highly concentrated, dense liquid clusters are stabilized in a highly viscous medium because the divalent ions have the ability to form complexes,” write Heeremans and her colleagues. In the second step, nanocrystals crystallize from these clusters. They form an intermediate stage on the way to spherulite and, over time, self-organize to form loose, spherical groups. As they continue to grow, these nanocrystals fuse and develop into more stable forms. This means that the spherical spherulites are among the crystals that are formed through non-classical growth processes, as the team explains: They do not grow directly from the supersaturated solution, but rather form via a metastable transition state from intermediate products – in this case the nanocrystals. “This provides new insights into multi-step crystallization processes and demonstrates the metastable nature of spherulites,” write Heeremans and her colleagues.
The new findings could also have practical applications. Crystallization processes play an important role in many areas, from the chemical separation of active pharmaceutical ingredients to the cultivation of semiconductor crystals to new materials. With the help of specifically cultivated spherulites, it would be possible, for example, to develop materials with complex internal structures and an exceptionally large surface area. “By identifying the critical growth conditions, we can get crystalline salts to form these high surface-to-volume ratio structures, which can fundamentally change the properties of many materials,” says Heeremans.
Source: Tess Heeremans (Universiteit van Amsterdam) et al., Communications Chemistry, doi: 10.1038/s42004-026-01892-0