UW Today

August 3, 2015

Crystals form through a variety of paths, with implications for biological, materials and environmental research

Crystals play an important role in the formation of substances from skeletons and shells to soils and semiconductor materials. But many aspects of their formation are shrouded in mystery. Scientists have long worked to understand how crystals grow into complex shapes. Now, an international group of researchers has shown how nature uses a variety of pathways to grow crystals beyond the classical, one-piece-at-a-time route.

“Because crystallization is a ubiquitous phenomenon across a wide range of scientific disciplines, a shift in the picture of how this process occurs has far-reaching consequences,” said James De Yoreo, a materials scientist and physicist at the Department of Energy’s Pacific Northwest National Laboratory and affiliate UW professor of chemistry and materials science and engineering.

These conclusions, published July 31 in Science with De Yoreo as lead author, have implications for decades-old questions in crystal formation, such as how animals and plants form minerals into shapes that have no relation to their original crystal symmetry or why some contaminants are so difficult to remove from stream sediments and groundwater.

Artist's rendition of calcium carbonate crystals.

An artist’s rendition of the early crystallization process of calcium carbonate. Adam F. Wallace/University of Delaware/David J. Carey

Their findings crystalized during discussions among 15 scientists from diverse fields such as geochemistry, physics, biology and the earth and materials sciences. At their home institutions, these researchers conduct experiments, investigate animal skeletons, study soils and streams or use computer simulations to visualize how particles can form and attach. They met for a three-day workshop in Berkeley, California, that was sponsored by the Council on Geosciences from the Department of Energy’s Office of Basic Energy Sciences.

“Researchers across all disciplines have made observations of skeletons and laboratory-grown crystals that cannot be explained by traditional theories,” said senior author Patricia Dove, a professor of geosciences at Virginia Tech. “We show how these crystals can be built up into complex structures by attaching particles — as nanocrystals, clusters, or droplets — that become organized into complex shapes. Many scientists have contributed to identifying these particles and pathways to becoming a crystal — our challenge was to put together a framework to understand them.”

In animal and laboratory systems alike, the crystal formation process begins by constructing their constituent particles. These can be small molecules, clusters, droplets or nanocrystals. These particles are unstable and begin to combine with each other, nearby crystals and other surfaces. For example, nanocrystals prefer to orient themselves along the same direction as a larger crystal before attaching, much like adding Legos. In contrast, amorphous conglomerates can simply aggregate. Their atoms later become organized by “doing the wave” through the mass to rearrange into a single crystal.

“Because we largely show a community consensus on this topic, the study has the potential to define the directions of future research on crystallization,” said De Yoreo.

Aragonite crystals forming on calcium carbonate.

Aragonite crystals forming on calcium carbonate.Pacific Northwest National Laboratory/James De Yoreo

The authors say much work remains to understand the forces that cause these particles to move and combine. It is one of the driving forces behind new research.

“Particle pathways are tricky because they can form what appear to be crystals with the traditional faceted surfaces or they can have completely unexpected shapes and chemical compositions,” said Dove. “Our group synthesized the evidence to show these pathways to growing a crystal become possible because of interplays between of thermodynamic and kinetic factors.”

The implications of these discussions span diverse scientific fields. By understanding how animals form crystals into working structures such as shells, teeth and bones, scientists will have a bigger and better toolbox to interpret crystals formed in nature. These insights may also help design novel materials and explain unusual mineral patterns in rocks. In addition, knowledge of how pollutants are transported or trapped in the minerals of sediments has implications for environmental management of water and soil.

“How we think about the ways to crystallization impacts how we interpret natural crystallization processes in geochemical and biological environments, as well as how we design and control synthetic crystal growth processes,” said De Yoreo. “I was surprised at how widespread a phenomenon particle-mediated crystallization is and how easily one can create a unified picture that captures its many styles.”

The work was supported by the Council on Geosciences of the U.S. Department of Energy’s office of Science. All co-authors and their affiliations are listed on the paper.

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For more information, contact De Yoreo at James.DeYoreo@pnnl.gov.

Adapted from PNNL press release: “New insight on how crystals form may advance materials, health and basic science research

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