Cracking the code of dolomite crystallization
Horses graze with the Dolomites in the background in South Tyrol, Italy.
kallerna , CC BY-SA 4.0 , via Wikimedia Commons
The Dolomites, named for their pervasive calcium magnesium carbonate crystals, CaMg(CO3)2, are a towering mountain range in northeastern Italy. But according to two centuries of laboratory experiments, the mountains shouldn’t exist. Failed attempts to precipitate dolomites have posed a mysterious exception to the rule that crystals grow in supersaturated solutions—a phenomenon that’s been dubbed the dolomite problem.
Materials scientist Wenhao Sun, his doctoral student Joonsoo Kim (both at the University of Michigan), and their colleagues have now solved the mystery. Crystalline order in dolomites consists of alternating rows of Ca2+and Mg2+ ions, separated by carbonate clusters, CO32- , as shown in the diagram below. But the researchers’ Monte Carlo simulations demonstrated that dolomite crystals initially precipitate on a cation-disordered surface, on which the Ca2+ and Mg2+ cations often sit at each other’s sites—as a defect-ridden layer—and the high surface strain inhibits further growth beyond a few layers.
Step edge on the growth surface of the ordered dolomite crystal, in which alternating rows of calcium (blue) and magnesium (orange) ions are interspersed with carbonate clusters (gray).
J. Kim et al., Science 382, 915 (2023)
Crucially, that disordered structure is out of equilibrium on the surface, and the more-ordered, alternating Ca and Mg rows are more stable. The ordered surface thus has a lower solubility. The disordered regions therefore dissolve more quickly, and with occasional rains or floods, they wash away from the surface. New ordered layers may then precipitate on the remaining parts of the surface in a matter of years. The effect is one of fluctuations in saturation values that lead to repeated episodes of dissolution and recrystallization. On the basis of their simulations, Sun and his colleagues realized that cycling between undersaturated and supersaturated conditions makes crystal growth 10 million times as fast as it would be otherwise.
Even so, a yearslong experiment to test the theory wasn’t an option. To further speed up the process, Sun’s experimental colleagues—Yuki Kimura and his postdoctoral student Tomoya Yamazaki (both from Hokkaido University in Japan)—used electron pulses to mimic the fluctuation in saturation values. The pulses split the water molecules apart, render the solution more acidic, and speed up the dissolution of the disordered surfaces. With nearly 4000 pulses over two hours, Kimura and Yamazaki grew an ordered dolomite crystal 360 layers thick, a feat never accomplished before in a lab.
Understanding how dolomite grows might offer new strategies for quickly manufacturing defect-free semiconductors, solar panels, and other technologies. The trick in some cases may be to periodically dissolve defects away from the surface before growing the crystals. (J. Kim et al., Science 382, 915, 2023 .)
