A mystery that has dogged materials science for 200 years has finally been solved. A mineral found in many ancient rock formations had stubbornly resisted the efforts of scientists to grow it in the lab, even though they could recreate the conditions they thought formed it in nature. Now, a team has cracked the problem, figuring out how to speedily grow dolomite crystals for the very first time.
Dolomite is a mineral so important, there’s a whole mountain range named after it. As well as these peaks in the Italian Alps, dolomite is abundant in the White Cliffs of Dover, the hoodoos of Utah, and other rocks dating back more than 100 million years. It actually accounts for almost 30 percent of minerals of its type – carbonates – in the Earth’s crust, but it’s notably absent in rocks that formed more recently.
Despite trying to carefully recreate its natural growing conditions, scientists have failed for two centuries to produce dolomite crystals in the lab. To solve the mystery, they had to get back to basics.
“If we understand how dolomite grows in nature, we might learn new strategies to promote the crystal growth of modern technological materials,” said corresponding author Wenhao Sun of the University of Michigan in a statement.
Professor Wenhao Sun shows off dolomite rocks from his personal collection.
Image credit: Marcin Szczepanski, Lead Multimedia Storyteller, Michigan Engineering
Dolomite crystals are formed over eons of geological time by the buildup of alternating layers of calcium and magnesium. Sounds simple enough, if time-consuming, but there’s a snag. When there’s water around, calcium and magnesium atoms can attach at random to the growth edge of the crystal, often in the wrong place. These defects prevent the alternating layers from forming correctly, which is why it takes so long – 10 million years – to create just one ordered layer of dolomite rock.
Since Sun and the team very much did not have 10 million years to wait, they turned to powerful software to simulate all the possible interactions going on between atoms in a growing dolomite crystal.
“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,” said first author Joonsoo Kim.
The team landed on a theory. Perhaps dolomite would grow faster if it were put through cycles where, periodically, there was a lower concentration of calcium and magnesium around. Most crystals will grow well in a supersaturated solution – that is, where their atomic components are present at very high levels. For dolomite, though, this just leads to more defects and slows everything down.
To test the theory, the team consulted with collaborators at Hokkaido University, and an ingenious experiment was devised using a transmission electron microscope.
“Electron microscopes usually use electron beams just to image samples,” explained Yuki Kimura, a professor of materials science at Hokkaido University. “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.”
A tiny crystal of dolomite in a solution of calcium and magnesium was exposed to the electron beam, which was pulsed 4,000 times over a period of two hours, to start to dissolve the crystal. When the beam is switched off, the surrounding solution quickly corrects itself to a more saturated state.
Only a tiny amount – about 2-billionths of a liter – of this calcium and magnesium solution was added to the sample holder for transmission electron microscope (pictured in background).
Image credit: Wenhao Sun, Dow Early Career Professor of Materials Science and Engineering, University of Michigan
It worked like a charm. After this treatment, the team was elated to observe that the crystal grew by approximately 100 nanometers. That may not sound like a lot, but it represents 300 newly formed layers of dolomite. The most that had ever been achieved in a lab before was five.
The findings also track with what is observed in nature. There are only a few locations where dolomite forms today, but they’re all places with cycles of flooding followed by drier conditions.
Solving the dolomite problem is a big milestone. “This discovery opens the door to investigating the geochemical process that influenced massive dolomite formation in the natural world,” wrote Juan Manuel García-Ruiz, who was not directly involved in the work, in a Perspective accompanying the study.
Not only that, learning how to grow defect-free crystals quickly could have important applications for the manufacture of many vital components of products like semiconductors, solar panels, and batteries.
“In the past, crystal growers who wanted to make materials without defects would try to grow them really slowly,” said Sun. “Our theory shows that you can grow defect-free materials quickly, if you periodically dissolve the defects away during growth.”
The study is published in Science.
Source Link: Crystals Grown With Help From Electron Beam Solve 200-Year-Old Geological Mystery
