materials science
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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.
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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
ublished in Proceedings of the National
meteorites formed from a supercritical fluid at
Academy of Sciences (PNAS), their study provides
high temperature and moderate pressures, almost
evidence of lonsdaleite’s formation in nature,
perfectly preserving the textures of the pre-existing
offering clues to synthetic production that could
graphite,” Tomkins said. “Later, lonsdaleite was
make more durable machine parts.
partially replaced by diamond as the environment
Lonsdaleite was named in honour of pioneering
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. Image credit: CSIRO.
cooled and the pressure decreased.”
British crystallographer Dame Kathleen Lonsdale,
Typically containing larger abundances of
taken place in these space rocks, probably in
though its existence has been a controversial topic.
diamond than any known rock, ureilite meteorites
the dwarf planet shortly after a catastrophic
The new study, using a range of cutting-edge science
are arguably the only major suite of samples available
collision,” added senior researcher Professor
techniques on the largest sample of ureleite meteorites
from the mantle of a dwarf planet. The parent
Dougal McCulloch from RMIT. “Chemical
to date, provides clear evidence of its existence.
asteroid would have been catastrophically disrupted
vapour deposition is one of the ways that people
At CSIRO, an electron probe microanalyser
by a giant impact while the mantle was still very hot,
make diamonds in the lab, essentially by growing
(EPMA) was used to quickly map the relative
creating the ideal conditions for lonsdaleite then
them in a specialised chamber.”
distribution of graphite, diamond and lonsdaleite in
diamond growth as the pressure and temperature
the samples. This flagship instrument, together with
decreased in a fluid- and gas-rich environment.
McCulloch said the hexagonal structure of lonsdaleite’s atoms makes it potentially harder than
high-resolution transmission electron microscopy
“These findings help address a longstanding
regular diamonds, which have a cubic structure.
(TEM) at RMIT, helped identify the largest lonsdaleite
mystery regarding the formation of the carbon
The unusual structure of lonsdaleite could thus
crystallites to date — up to one micron in size. This
phases in ureilites that has been the subject of
help inform new manufacturing techniques for
collaboration of technology and expertise allowed
much speculation,” Tomkins said. “And they offer
ultra-hard materials in mining applications.
the team to confirm the lonsdaleite with confidence.
a novel model for diamond formation in ureilites
“Nature has … provided us with a process to
The study was led by geologist Professor Andy
that settles contradictions in the existing concepts.”
try and replicate in industry,” Tomkins concluded.
Tomkins from Monash University, who discovered
“There’s strong evidence that there’s a newly
“We think that lonsdaleite could be used to make
the lonsdaleite crystallites when looking at ureilite
discovered formation process for the lonsdaleite
tiny, ultra-hard machine parts if we can develop
meteorites in his lab. He said the team’s findings
and regular diamond, which is like a supercritical
an industrial process that promotes replacement of
reveal a novel process in which the lonsdaleite is
chemical vapour deposition process that has
pre-shaped graphite parts by lonsdaleite.”
26 | LAB+LIFE SCIENTIST - Oct/Nov 2022
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