New crystal form of lime discovered

Calcium carbonate forms a previously unknown hydrous crystal variant

The newly discovered calcium carbonate hemihydrate forms tiny acicular crystals. © Max Planck Institute for Colloids and Interfaces / Zhaoyong Zou
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Surprise at the Allerwelts Mineral: Researchers have discovered a previously unknown crystal form of lime. The so-called calcium carbonate hemihydrate (CCHH) is formed when amorphous calcium carbonate crystallizes in the presence of magnesium. Accordingly, calcium carbonate has not only five, but even six different crystal structures, as the researchers report in the journal "Science". Now the chemistry textbooks have to be rewritten.

Calcium carbonate (CaCO3), popularly known as lime, is one of the largest reservoirs of carbon on our planet. It forms rocks such as chalk, limestone and marble, shapes many caves with its bizarre stalactite formations and forms the karst landscapes of the earth. Calcium carbonate is also found in the shells and skeletons of many terrestrial and aquatic animals.

So far, there have been five

So far, five crystal forms of calcium carbonate have been known: the three pure crystal polymorphs calcite, aragonite and vaterite, as well as two crystal phases containing bound water - monohydrocalcite and ikaite. In addition to these crystallized forms, lime may also exist in various amorphous states. They play a crucial role as preforms for the lime shells of many creatures - from them, for example, sea urchins produce their skeletons.

Zhaoyong Zou from the Max Planck Institute for Colloids and Interfaces in Potsdam wanted to understand this process when they experimented with the transition from amorphous calcium carbonate to crystalline forms in the laboratory. At various temperatures and solution concentrations, they tested when and how crystals formed.

Tiny needle-shaped crystals

But Zhou noticed something out of the ordinary: If the solution contained a certain amount of magnesium ions in addition to the calcium carbonate, something quite different from the expected calcite was produced. It formed tiny needle-like crystals of up to one micron in length and about 200 nanometers thick. display

"We thought that was the result of pollution, " says Zhou's colleague Peter Fratzl. But experiments under the same conditions repeatedly led to these needle crystals. Further analysis by infrared and Raman spectroscopy revealed that the spectra of these crystals differed markedly from those of all previously known calcium carbonate variants, as the researchers report. Also in their X-ray scattering, these crystals did not match any known form.

A completely new crystal form

It was clear: Zhou and his team had come up with something completely new. "We report on the discovery and characterization of a previously unknown hydrated crystal form of caclium carbonate, " the researchers write. According to her analysis, this compound is a calcium carbonate hemihydrate with the formula CaCO 3 H 2 O. "It does not happen so often in a researcher's life that something completely unexpected is discovered, after which one actually did not look for it, "says Fratzl.

The scientists found that the new amorphous calcium carbonate crystal forms in solution whenever five to nine percent magnesium ions are present. "We assume that the dissolved magnesium inhibits crystallization of calcite in this system and promotes the formation of CCHH, " Zhou and his colleagues explain. Apparently, CCHH does not seem to be permanently stable: it will absorb more water over time and convert to the hydrated form monohydrocalcite.

Textbooks have to be adapted

"The discovery of CCHH, its pathways and structure greatly extend our understanding of the calcium carbonate family, " the researchers note. "This could have significant implications for biomineralization, geology, and industrial processes based on the hydration of calcium carbonate." It seems clear that the relevant textbooks and lexicon entry In addition to a further crystalline appearance of the lime must be added.

However, it is still unclear whether the newly discovered lime variant also occurs in nature. However, according to the researchers, CCHH might play an intermediate role in biomineralization processes, such as the transition from amorphous calcium carbonate to calcite. Zhou and his team want to further explore the new crystal form and related processes. (Science, 2019; doi: 10.1126 / science.aav0210)

Source: Max Planck Institute for Colloids and Interfaces

- Nadja Podbregar