Monash University Professor Andrew Tomkins (left) and Royal Melbourne Institute of Technology (RMIT) Ph.D. scholar Alan Salek examine a ureilite meteorite sample at the RMIT Microscopy and Microanalysis Facility.
Monash University professor Andrew Tomkins (left) and Royal Melbourne Institute of Technology (RMIT) Ph.D. scholar Alan Salek examine a ureilite meteorite sample at the RMIT Microscopy and Microanalysis Facility. Credit: RMIT University

Meteorites are prized as relics of the early solar system, and some are also treasured for the rare minerals they contain. Researchers have now analyzed diamond-bearing meteorites to shed light on how lonsdaleite, diamond’s more durable cousin, forms in nature. The team’s discovery of how graphite transforms into lonsdaleite paves the way for producing ultrahard versions of tools like drills and blades for industrial applications, the researchers have suggested.

Harder Than Diamond

Diamond is commonly touted as the hardest known material, but a different form of carbon may actually hold that honor: Last year, researchers showed using modeling that a mineral sometimes referred to as “hexagonal diamond” is significantly harder. Lonsdaleite’s nickname derives from its crystalline structure: Whereas the carbon atoms in diamond are arranged in a cubic lattice, lonsdaleite’s carbon atoms are locked in a hexagonal structure.

But lonsdaleite is notoriously difficult to find in nature—it’s been spotted only in very small quantities in a few meteorites and one diamond deposit in Kazakhstan. With the goal of better understanding how lonsdaleite forms, Andrew Tomkins, a geoscientist at Monash University in Melbourne, Australia, and his colleagues analyzed carbon-rich meteorites known as ureilites. These meteorites are believed to be the remnants of a dwarf planet at least several hundred kilometers in diameter—the so-called ureilite parent body—that was destroyed by a collision with an asteroid early in the solar system’s history.

Tomkins and his collaborators used microscopy, spectroscopy, and chemical analysis to study 18 ureilites. All of the meteorites in the researchers’ sample contained graphite, some contained diamond, and a few contained both diamond and lonsdaleite, the team determined. (But no one would mistake these meteorites for extraterrestrial bling—diamond and lonsdaleite were present in only trace amounts, invisible to the human eye.)

Flipping Open the Lid

“It’s like taking the lid off a Coke bottle.”

On the basis of the spatial arrangement of the graphite, diamond, and lonsdaleite grains, the team hypothesized how the two types of diamond might have formed. They proposed that the catastrophic asteroid impact believed to have destroyed the ureilite parent body played a key role—that event resulted in rapid depressurization of rocks that had once been located below the world’s magma ocean. “It’s like taking the lid off a Coke bottle,” said Tomkins.

Abrupt changes in pressure and temperature would have triggered the transformation of graphite into lonsdaleite, Tomkins and his colleagues suggested. The reactions would have been further catalyzed by a gaseous mixture of carbon, hydrogen, sulfur, and oxygen, the team proposed. Finally, the shape of the original graphite grains would have been preserved in the final lonsdaleite, they concluded. “It just replaced it in the same shape,” said Tomkins. That’s an intriguing finding, the researchers suggested, because it means that laboratory efforts to create lonsdaleite from graphite, if successful, could yield specific, known shapes of the ultrahard mineral.

The researchers additionally proposed that diamond formed from lonsdaleite as the temperature dropped. The tip-off was finding veins of diamond running through lonsdaleite, said Tomkins. “We see crosscutting relationships like those as evidence of sequential processes.”

These results may rewrite our understanding of how diamond minerals form. It’s long been thought that diamonds are created in the crushing pressures of an event like an asteroid impact. However, it’s been difficult to explain their presence in ureilite meteorites that show little evidence of being shocked, said Tomkins. “We’re coming up with a different solution.”

These results were published in September in the Proceedings of the National Academy of Sciences of the United States of America.

Going to the Lab

Some researchers believe there’s more work to do, however. Lonsdaleite is notoriously tricky to identify, and some scientists have suggested that, in fact, it’s just diamond characterized by internal defects. Oliver Tschauner, a mineralogist at the University of Nevada, Las Vegas not involved in the research, said that additional follow-up analyses are necessary to prove there’s lonsdaleite in these meteorites at all. “I think the proof of lonsdaleite in the sample is still missing.”

“You can make machine components that are very hard wearing.”

The real test will be creating lonsdaleite in the laboratory, said Tomkins. “The next step is to try to make some of this stuff.” The researchers estimated that the necessary conditions—about 10 times Earth’s atmospheric pressure and roughly 1,000°C—are readily attainable. And although it’s unknown whether macroscopic pieces of lonsdaleite can be made, in principle, it should be possible to make tools like drills and blades for industrial use. “You can make machine components that are very hard wearing,” said Tomkins. “They’ll last a long time.”

—Katherine Kornei (@KatherineKornei), Science Writer

Citation: Kornei, K. (2022), Rare meteorites shed light on diamond formation, Eos, 103, Published on 18 October 2022.
Text © 2022. The authors. CC BY-NC-ND 3.0
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