Meteorites are prized as relics of the early solar system, and some are also prized 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 ultra-hard versions of tools such as drills and blades for industrial applications, the researchers have suggested.
Harder than diamond
Diamond is commonly billed as the hardest material known, but a different form of carbon may hold that honor: Last year, researchers showed through modeling that a mineral sometimes called “hexagonal diamond” is significantly harder. Lonsdaleite’s nickname derives from its crystal structure: while the carbon atoms in diamond are arranged in a cubic lattice, the carbon atoms in lonsdaleite are locked in a hexagonal structure.
But lonsdaleite is notoriously difficult to find in nature: it has only been detected in very small amounts in a few meteorites and a diamond deposit in Kazakhstan. In order to better understand 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 remains of a dwarf planet at least a few hundred kilometers in diameter, the so-called ureilite parent body, which was destroyed by a collision with an asteroid early in the history of the solar system.
Tomkins and his collaborators used microscopy, spectroscopy and chemical analysis to study 18 ureilites. The team determined that all of the meteorites in the researchers’ sample contained graphite, some contained diamonds, and a few contained both diamonds and lonsdaleite. (But no one would mistake these meteorites for extraterrestrial bling: diamond and lonsdaleite were only present in small amounts, invisible to the human eye.)
Opening the lid
“It’s like taking the cap off a Coca-Cola bottle.”
Based on 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 impact of the asteroid believed to have destroyed the ureilite parent body played a key role: this event resulted in the rapid depressurization of rocks that had previously been located beneath the magma ocean of the world. “It’s like taking the cap off a Coke bottle,” Tomkins said.
Abrupt changes in pressure and temperature would have triggered the transformation of graphite into lonsdaleite, Tomkins and his colleagues suggested. The team proposed that the reactions would have been further catalyzed by a gaseous mixture of carbon, hydrogen, sulfur and oxygen. Finally, the shape of the original graphite grains would have been preserved in the final lonsdaleite, they concluded. “I just replaced it with the same shape,” Tomkins said. This is an intriguing finding, the researchers suggested, because it means that the lab’s efforts to create lonsdaleite from graphite, if successful, could yield specific, known forms of the ultrahard mineral.
The researchers also proposed that diamond formed from lonsdaleite as the temperature dropped. The tip was to find diamond veins running through lonsdaleite, Tomkins said. “We see cross-cutting relationships like these as evidence of sequential processes.”
These results may rewrite our understanding of how diamond minerals form. Diamonds have long been thought to be created under the crushing pressures of an event such as an asteroid impact. However, it has been difficult to explain their presence in ureilite meteorites that show little evidence of shock, Tomkins said. “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.
Go to the lab
Some researchers believe there is more work to be done, however. Lonsdaleite is notoriously difficult to identify, and some scientists have suggested that it is actually just a diamond characterized by internal flaws. Oliver Tschauner, a mineralogist at the University of Nevada, Las Vegas not involved in the research, said additional follow-up analyzes are needed to show that lonsdaleite is present in these meteorites. “I think the evidence for lonsdaleite is still missing from the sample.”
“You can make very tough machine components.”
The real test will be creating lonsdaleite in the lab, Tomkins said. “The next step is to try to do some of these things.” The researchers estimated that the necessary conditions—about 10 times Earth’s atmospheric pressure and approximately 1,000°C—are easily achievable. And while it is 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 very hard machine components,” Tomkins said. “They will last a long time.”
—Katherine Kornei (@KatherineKornei), science writer
Citation: Kornei, K. (2022), Rare meteorites shed light on diamond formation, Eos, 103, published 18 October 2022. Text © 2022. The authors. CC BY-NC-ND 3.0 Except where otherwise noted, images are copyrighted. Any reuse without the express permission of the copyright owner is prohibited.