For two centuries, scientists have been unable to grow a common mineral in the laboratory under conditions where it occurs naturally. Now, a team of researchers from the University of Michigan and Hokkaido University in Sapporo, Japan, has finally succeeded, thanks to a new theory developed from atomic simulations.

Professor Sun Wenhao shows his personal collection of dolomite. Professor Sun studies the crystal growth of minerals from the perspective of materials science. By understanding how atoms join together to form natural minerals, he believes we can uncover the fundamental mechanisms of crystal growth, allowing us to make functional materials faster and more efficiently. Source: Marcin Szczepanski, multimedia storyteller at Michigan Engineering.

Their success solved a long-standing geological mystery known as the "dolomite problem." Dolomite - the dominant mineral in the Italian Dolomites, Niagara Falls, the White Cliffs of Dover and Utah's Hoodoos Mountains - is very abundant in rocks over 100 million years old but almost absent in younger formations.

Wenhao Sun, Dow Early Career Assistant Professor in the Department of Materials Science and Engineering at the University of Michigan, and Junxiu Jin, a doctoral student in materials science and engineering in Professor Sun's research group, display dolomite rocks collected in their laboratory. The two scientists have proposed a theory that could finally explain a two-century-old mystery about the abundance of dolomite on Earth. Image credit: Marcin Szczepanski, multimedia storyteller at Michigan Engineering.

Understanding the importance of dolomite growth

"If we understand how dolomite grows in nature, we may learn new strategies to promote crystal growth in modern technological materials," said Wenhao Sun, Dow Early Career Professor in the Department of Materials Science and Engineering at the University of Michigan and corresponding author of a paper recently published in Science.

The secret to finally growing dolomite in the lab is to eliminate defects in the mineral's structure during its growth. When minerals form in water, atoms are often deposited neatly at the edges of the crystal's growing surface. However, the growth edges of dolomite consist of alternating arrangements of calcium and magnesium. In water, calcium and magnesium randomly attach to growing dolomite crystals, often landing in the wrong places and creating defects that prevent more dolomite layers from forming. This disordered state will slow down the growth of dolomite, which means that it would take 10 million years to form just one layer of ordered dolomite.

Structure of the edges of dolomite crystals. Rows of magnesium (orange spheres) alternate with rows of calcium (blue spheres), interspersed with carbonates (black structures). Pink arrows indicate the direction of crystal growth. Calcium and magnesium often adhere improperly to the growth edges, preventing dolomite growth. Source: Junsu Kim, PhD student in materials science and engineering at the University of Michigan.

Fortunately, these flaws are not locked. Because disordered atoms are less stable than atoms in their correct positions, they dissolve first when minerals are rinsed with water. Repeated flushing of these defects - for example, by rain or tidal cycles - can form dolomite layers in just a few years. Over geological time, dolomite accumulates into mountains.

Advanced simulation technology

To accurately model the growth of dolomite, the researchers needed to calculate how strongly or loosely the atoms were attached to the existing dolomite surface. The most accurate simulations require calculating the energy of every interaction between electrons and atoms as the crystal grows. Such exhaustive calculations typically require enormous computing power, but software developed by MIT's Predictive Structural Materials Science (PRISMS) Center provides a shortcut.

"Our software first calculates the energy of certain atomic arrangements and then predicts the energy of other arrangements based on the symmetry of the crystal structure," said Brian Puchala, one of the software's lead developers and an associate researcher in MIT's Department of Materials Science and Engineering.

This shortcut makes it feasible to model dolomite growth over geological time scales.

Dolomite is a mineral very common in ancient rocks that formed mountains like this one in northern Italy. But dolomite is rare in younger rocks and cannot be made in the laboratory under the conditions in which it occurs naturally. A new theory helped scientists grow the mineral in the laboratory at ordinary temperatures and pressures for the first time, and helps explain the scarcity of dolomite in young rocks. Image source: Francesca.z73viaWikimediaCommons.

"Each atomic step usually takes more than 5,000 CPU hours on a supercomputer. Now, we can complete the same calculation in just 2 milliseconds on a desktop computer," said Junsu Kim, the study's first author and a doctoral student in materials science and engineering.

Practical application and testing of theory

The few areas where dolomites form today were intermittently flooded and then dried up, which is consistent with Sun and Kim's theory. But this evidence alone is not enough to be completely convincing. Yuki Kimura, a professor of materials science at Hokkaido University, and Tomoya Yamazaki, a postdoctoral researcher in Kimura's laboratory, got into action. They tested the new theory by exploiting a quirk of transmission electron microscopy.

"Electron microscopy typically uses only electron beams to image samples," Kimura said. "However, electron beams can also split water, creating acids that cause the crystals to dissolve. Normally, this is not good for imaging, but in this case, dissolution is exactly what we want."

After placing tiny dolomite crystals into a calcium-magnesium solution, Kimura and Yamasaki gently pulsed the electron beam 4,000 times over two hours, dissolving the crystal defects. After the pulse, the dolomite was seen to have grown by about 100 nanometers, about 250,000 times smaller than an inch. Although this is only 300 layers of dolomite, more than five layers of dolomite have never been grown in the laboratory before.

Lessons learned from the "dolomite problem" could help engineers create higher-quality semiconductors, solar panels, batteries and other technical materials.

"In the past, crystal growers would try to grow them very slowly if they wanted to make materials without defects," Sun said. "Our theory shows that defect-free materials can be grown quickly by simply dissolving defects regularly during the growth process.

Compiled source: ScitechDaily