Matthew Ward Agius
Matthew Agius is a science writer for Cosmos Magazine.
Australian scientists have confirmed the existence of ‘lonsdaleite’ – a diamond-like material with a hexagonal structure – embedded in some rare meteorites.
These rare meteorites, called ureilite meteors, are believed to form within the interior of ancient celestial bodies – dwarf planets. It is theorised that the lonsdaleite they contain, upon their arrival to earth, forms during cosmic collisions with other bodies, such as asteroids.
In the case of ureilites studied by a combined team from Monash University, RMIT, the CSIRO, Australian Synchrotron and Plymouth University, the event which caused this to happen was believed to have been some 4.5 billion years ago.
Using electron microscopy, the researchers were able to discern a high level of carbon presence within the ureilites, and ultimately the occurrence of lonsdaleite.
But high impacts are not the only way this hexagonal diamond can be created.
Lonsdaleite: Harder than diamond
Lonsdaleite is named for British scientist Kathleen Lonsdale, the first woman fellow of the Royal Society who was a pioneer of crystallography.
The crystal has a similar appearance to terrestrial diamonds, but is unique in having a hexagonal crystalline structure. Diamonds typically have a cubic structure.
This structure seems to make lonsdaleite up to 60 percent harder than diamond. But as paper co-author Alan Salek from RMIT told Cosmos, it isn’t the material’s only unusual trait.
Lonsdaleites have a property whereby the diamond appears to “bend around” when forming its shape.
They’re also likely – at least in the case of the ureilites studied in this research – to have formed naturally in a process known as a supercritical chemical vapour deposition.
This process is often used to grow cubic diamonds in the lab.
Lab-grown diamonds are identical to those found in nature. But instead of high natural pressures transforming graphite into prized gemstones, chemical vapour deposition (CVD) causes carbon-containing gases like methane to deteriorate at high pressure and deposit in a crystalline state around a ‘seed’ diamond.
“Supercritical CVD involves a mixture of different elements like carbon and hydrogen as a supercritical fluid,” says Salek.
“This then forms this very strange diamond structure, and it formed when a dwarf planet that was around about four and a half billion years ago got hit by an asteroid.”
The ability to create Salek’s ‘bendy’ londsdaleite occurs when graphite – the precursor substance to a hexagonal diamond – interacts with supercritical fluid. This preserves the original hexagonal structure of the graphite in the new lonsdaleite crystal.
“If you have a bendy graphite crystal that’s quite large, with this supercritical fluid, it can actually preserve that shape,” he says.
“And so you’ve got this transition from graphite – which is quite soft – bending into this super hard material.”
Real world applications under consideration
Lonsdaleites can be formed in the laboratory, as well as occurring in nature, and recreating the conditions involved in forming the crystal is next on the cards for the research team.
They are also looking to mimic the formation of bendable crystals where supercritical fluids imprint themselves on graphite, for possible industrial uses.
“We’ve got a lot of applications for super hard materials, like, in industry, things like saw blades,” Salek says.
“If we had material 60% harder than diamond, you potentially have a blade that could last much longer, especially since practically nothing could damage it.
“The fact that you can bend these crystals, you can let your imagination fly and, and being able to grow diamonds in any shape or size will be a pretty cool thing to do eventually.”