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Analytical Chemistry

Spying On Crystal Formation

Microscopy: Study reveals complex, multipath nature of nucleation and growth

by Mitch Jacoby
September 8, 2014 | APPEARED IN VOLUME 92, ISSUE 36

METAMORPHOSIS
Credit: Science
A dropletlike nanosized particle of amorphous calcium carbonate (round) nucleates and grows. Suddenly it transforms into aragonite (bundles of wheat), which grow by consuming the amorphous particle.

Crystal nucleation and transformation events often happen suddenly and spontaneously, leaving researchers with little chance to study these microscopic processes in detail. But under the right conditions and discerning eye of a powerful probe, these everyday events can reveal their secrets.

MULTIPATHING
Credit: Science
Diamond-shaped nanocrystals of calcite and a second unidentified CaCO3 phase nucleate spontaneously from solution and grow independently.

For a team of scientists using a customized electron microscope, conditions were just right to watch crystals of calcium carbonate nucleate, grow, and undergo change (Science 2014, DOI: 10.1126/science.1254051).

The study shows that traditional theories indicating that crystals form via nucleation followed by orderly growth are incomplete. It also shows that one of the most abundant materials on Earth forms via multiple, often simultaneous mineralization pathways.

“For a decade, we’ve been studying formation pathways of carbonates using high-powered microscopes, but we hadn’t had the tools to watch the crystals form in real time,” says team leader James J. De Yoreo of Pacific Northwest National Laboratory. Now, the team has such a tool—a microscope flow cell that enables scientists to image liquid-phase reactions at the nanoscale.

[+]Enlarge
Credit: Science
These video stills show crystallization events, including an amorphous CaCO3 particle (top) suddenly sprouting crystals of aragonite (bottom).
09236-notw4-aragonite.jpg
Credit: Science
These video stills show crystallization events, including an amorphous CaCO3 particle (top) suddenly sprouting crystals of aragonite (bottom).

The group, which includes Michael H. Nielsen and Shaul Aloni of Lawrence Berkeley National Laboratory, loaded tiny quantities of solutions of sodium bicarbonate and calcium chloride into the cell. Under select flow-rate and concentration conditions, crystals suddenly began to grow and transform, revealing previously unseen events.

In some cases, amorphous dropletlike particles of CaCO3 formed in solution. Suddenly, crystals of the carbonate minerals aragonite and vaterite appeared on the surface of the particles and grew at the particles’ expense. In other cases, the group observed particles of calcite and other CaCO3 phases suddenly nucleating near one another in solution and growing independently.

“We are at an exciting moment in time in which the detailed in situ imaging of common liquid-phase reactions such as crystallization of calcium carbonate has come into reach,” says microscopist Nico A. J. M. Sommerdijk of Eindhoven University of Technology, in the Netherlands. He adds that the study confirms the existence of multiple and sometimes simultaneous crystallization pathways that some researchers suspected were present but could not verify on the basis of ex situ experiments.

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Comments
Alan B. White (September 10, 2014 3:52 PM)
What a fantastic tool! As a chemist (in my previous life) who often performed quantitative Solubility Analyses (equilibrium dependent for a valid analysis) this would be a great help in validating an analysis scheme during development, as there was always certain assumptions such as equilibrium between solid and liquid phases which was often unknown. Also I see many industrial applications for studying preferential precipitation.

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