For the past two centuries, scientists have struggled to replicate a common mineral in the laboratory under conditions believed to mimic its natural formation. However, a breakthrough has been achieved by a team of researchers from the University of Michigan and Hokkaido University in Sapporo, Japan. This success unravels an age-old geological mystery known as the “Dolomite Problem.” Dolomite, a key mineral found in the Dolomite Mountains of Italy, Niagara Falls, White Cliffs of Dover, and Utah’s Hoodoos, is abundant in rocks over 100 million years old but conspicuously absent in younger formations.

The key to finally growing dolomite in the lab lay in eliminating defects in the mineral’s structure as it formed. When minerals develop in water, atoms typically deposit in an orderly fashion on one edge of the growing crystal’s surface. However, the dolomite’s growth edge consists of alternating rows of calcium and magnesium. In water, calcium and magnesium randomly adhere to the growing dolomite crystal, often settling in the wrong places and creating defects that hinder the formation of additional dolomite layers. This disorder slows down dolomite growth, meaning it would take 10 million years to form a single ordered dolomite layer.

Fortunately, these defects are not permanent. Since disordered atoms are less stable than atoms in the correct position, they are the first to dissolve when the mineral is washed with water. Repeated washing of these defects, such as through rain or tidal cycles, allows a dolomite layer to form in a matter of years. Over geological time, these layers can accumulate to form dolomite mountains.

Professor Wenhao Sun shows off dolomite from his personal rock collection. Sun studies crystal growth of minerals from a materials science perspective. By understanding how atoms come together to form natural minerals, he believes we can reveal fundamental mechanisms of crystal growth, which can be used to make functional materials more quickly and efficiently | photo Marcin Szczepanski

To accurately simulate dolomite growth, researchers needed to calculate how strongly atoms would adhere to an existing dolomite surface. Precise simulations required determining the energy of each interaction between electrons and atoms in the growing crystal. While such calculations typically demand massive computing power, the software developed at the University of Michigan’s Predictive Structure of Materials (PRISMS) Material Science Center offered a shortcut.

Our software calculates the energy of some atomic arrangements and then extrapolates to predict the energies of other arrangements based on the symmetry of the crystal structure, explains Brian Puchala, one of the main developers of the software and a research scientist in the Department of Materials Science and Engineering at UM.

This shortcut allowed researchers to simulate dolomite growth on geological timescales. Each atomic step would normally take over 5,000 CPU hours on a supercomputer. Now, we can perform the same calculation in 2 milliseconds on a desktop computer, explains Joonsoo Kim, a doctoral student in Materials Science and Engineering and the study’s lead author.

The structure of a dolomite crystal edge. Rows of magnesium (orange spheres) alternate with rows of calcium (blue spheres), and are interspersed with carbonate (black structures). The pink arrows show the directions of crystal growth. Calcium and magnesium often attach to the growth edge improperly, which stops dolomite growth | photo Joonsoo Kim, University of Michigan

Current dolomite-forming areas are periodically flooded and then dried, aligning with the theory of Sun and Kim. However, these observations alone were not enough for complete conviction. Yuki Kimura, a professor of Materials Science at Hokkaido University, and Tomoya Yamazaki, a postdoctoral researcher in Kimura’s lab, put the new theory to the test using a peculiarity of transmission electron microscopes.

Electron microscopes usually use electron beams only to obtain images of samples, explains Kimura. However, the beam can also split water, producing an acid that can dissolve crystals. Normally, this is bad for imaging, but in this case, dissolution is exactly what we wanted.

After placing a tiny dolomite crystal in a solution of calcium and magnesium, Kimura and Yamazaki gently pulsed the electron beam 4,000 times over two hours, dissolving the defects.

Following the pulses, it was observed that the dolomite grew approximately 100 nanometers, about 250,000 times smaller than an inch. Although it was only 300 layers of dolomite, never before had more than five layers of dolomite been cultivated in the laboratory.

The lessons learned from the Dolomite Problem can assist engineers in manufacturing higher-quality materials for semiconductors, solar panels, batteries, and other technologies.

In the past, crystal growers aiming to produce defect-free materials tried to grow them very slowly, explains Sun. Our theory demonstrates that defect-free materials can be grown rapidly if defects are periodically dissolved during growth.

Sources

University of Michigan | Joonsoo Kim et al., Dissolution enables dolomite crystal growth near ambient conditions. Science 382,915-920(2023). DOI:10.1126/science.adi3690

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