Rare form of diamond discovered in meteorites

Scientists from Monash University, RMIT University, CSIRO, the Australian Synchrotron and the University of Plymouth have confirmed the existence of lonsdaleite, a rare hexagonal form of diamond, in ureilite meteorites from inside an ancient dwarf planet.

Published in Proceedings of the National Academy of Sciences (PNAS), their study provides evidence of lonsdaleite’s formation in nature, offering clues to synthetic production that could make more durable machine parts.

Lonsdaleite was named in honour of pioneering British crystallographer Dame Kathleen Lonsdale, though its existence has been a controversial topic. The new study, using a range of cutting-edge science techniques on the largest sample of ureleite meteorites to date, provides clear evidence of its existence.

At CSIRO, an electron probe microanalyser (EPMA) was used to quickly map the relative distribution of graphite, diamond and lonsdaleite in the samples. This flagship instrument, together with high-resolution transmission electron microscopy (TEM) at RMIT, helped identify the largest lonsdaleite crystallites to date — up to one micron in size. This collaboration of technology and expertise allowed the team to confirm the lonsdaleite with confidence.

Ureleite meteorite cross-section, captured with CSIRO’s electron probe microanalyser (EPMA). Iron is in red, magnesium in green, silicon in blue, lonsdaleite in yellow and diamond in pink.

The study was led by geologist Professor Andy Tomkins from Monash University, who discovered the lonsdaleite crystallites when looking at ureilite meteorites in his lab. He said the team’s findings reveal a novel process in which the lonsdaleite is created, replacing graphite crystals in the dwarf planet’s mantle facilitated by a super-hot fluid as it cools and decompresses.

“We propose that lonsdaleite in the meteorites formed from a supercritical fluid at high temperature and moderate pressures, almost perfectly preserving the textures of the pre-existing graphite,” Tomkins said. “Later, lonsdaleite was partially replaced by diamond as the environment cooled and the pressure decreased.”

Typically containing larger abundances of diamond than any known rock, ureilite meteorites are arguably the only major suite of samples available from the mantle of a dwarf planet. The parent asteroid would have been catastrophically disrupted by a giant impact while the mantle was still very hot, creating the ideal conditions for lonsdaleite then diamond growth as the pressure and temperature decreased in a fluid- and gas-rich environment.

“These findings help address a longstanding mystery regarding the formation of the carbon phases in ureilites that has been the subject of much speculation,” Tomkins said. “And they offer a novel model for diamond formation in ureilites that settles contradictions in the existing concepts.”

“There’s strong evidence that there’s a newly discovered formation process for the lonsdaleite and regular diamond, which is like a supercritical chemical vapour deposition process that has taken place in these space rocks, probably in the dwarf planet shortly after a catastrophic collision,” added senior researcher Professor Dougal McCulloch from RMIT. “Chemical vapour deposition is one of the ways that people make diamonds in the lab, essentially by growing them in a specialised chamber.”

Professor Dougal McCulloch and PhD scholar Alan Salek from RMIT with Professor Andy Tomkins from Monash University at the RMIT Microscopy and Microanalysis Facility. Image credit: RMIT University.

McCulloch said the hexagonal structure of lonsdaleite’s atoms makes it potentially harder than regular diamonds, which have a cubic structure. The unusual structure of lonsdaleite could thus help inform new manufacturing techniques for ultra-hard materials in mining applications.

“Nature has … provided us with a process to try and replicate in industry,” Tomkins concluded. “We think that lonsdaleite could be used to make tiny, ultra-hard machine parts if we can develop an industrial process that promotes replacement of pre-shaped graphite parts by lonsdaleite.”

Top image caption: Monash Professor Andy Tomkins with RMIT PhD scholar Alan Salek, holding a ureilite meteor sample at the RMIT Microscopy and Microanalysis Facility. Image credit: RMIT University.

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