Researchers at the University of Michigan and Hokkaido University have succeeded in solving the long-standing ‘Dolomite Problem,’ which had impeded growing the mineral in the lab under the conditions believed to have formed it naturally.
In a paper published in the journal Science, the researchers explain that the secret to finally growing dolomite in the lab was removing defects in the mineral structure as it grows.
The scientists note that when minerals form in water, atoms usually deposit neatly onto the edge of the growing crystal surface. However, the growth edge of dolomite consists of alternating rows of calcium and magnesium.
In water, calcium and magnesium will randomly attach to the growing dolomite crystal, often lodging into the wrong spot and creating defects that prevent additional layers of dolomite from forming. This disorder slows dolomite growth to a crawl, meaning it would take 10 million years to make just one layer of ordered dolomite.
Luckily, these defects aren’t locked in place. Because the disordered atoms are less stable than atoms in the correct position, they are the first to dissolve when the mineral is washed with water. Repeatedly rinsing away these defects—for example, with rain or tidal cycles—allows a dolomite layer to form in only a matter of years. Over geologic time, mountains of dolomite can accumulate.
Simulating growth over geologic timescales
To simulate dolomite growth accurately, the researchers needed to calculate how strongly or loosely atoms would attach to an existing dolomite surface. The most accurate simulations require the energy of every single interaction between electrons and atoms in the growing crystal. Such exhaustive calculations usually require huge amounts of computing power, but software developed at U-M’s Predictive Structure Materials Science Center offered a shortcut.
“Our software calculates the energy for some atomic arrangements, then extrapolates to predict the energies for other arrangements based on the symmetry of the crystal structure,” Brian Puchala, one of the software’s lead developers, said in a media statement.
That shortcut made it feasible to simulate dolomite growth over geologic timescales.
“Each atomic step would normally take over 5,000 CPU hours on a supercomputer. Now, we can do the same calculation in 2 milliseconds on a desktop,” Joonsoo Kim, the study’s first author, said.
Important for semiconductors, solar panels and batteries
The few areas where dolomite forms today intermittently flood and later dry out, which aligns well with the researchers’ theory. But such evidence alone wasn’t enough to be fully convincing. Thus, the group tested the new theory with a quirk of transmission electron microscopes.
“Electron microscopes usually use electron beams just to image samples,” Yuki Kimura, co-author of the study, said. “However, the beam can also split water, which makes acid that can cause crystals to dissolve. Usually, 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, the team gently pulsed the electron beam 4,000 times over two hours, dissolving away the defects. After the pulses, dolomite was seen to grow approximately 100 nanometers—around 250,000 times smaller than an inch. Although this was only 300 layers of dolomite, never had more than five layers of dolomite been grown in the lab before.
In the scientists’ view, the lessons learned from the Dolomite Problem can help engineers manufacture higher-quality materials for semiconductors, solar panels, batteries and other tech.
“In the past, crystal growers who wanted to make materials without defects would try to grow them really slowly,” Wenhao Sun, corresponding author of the paper, said. “Our theory shows that you can grow defect-free materials quickly if you periodically dissolve the defects away during growth.”
