4 mins read 23 Sep 2022

Folded Diamonds Found In A Meteorite

Researchers have found a super-hard type of diamond that appears folded inside a rare type of meteorite.

Professor Dougal McCulloch (left) and PhD scholar Alan Salek from RMIT, with Professor Andy Tomkins from Monash (right). Credit: RMIT Unviersity.

Scientists from Monash, RMIT, CSIRO, the Australian Synchrotron, and Plymouth University have found a super-hard type of diamond inside a 4.56 billion-year-old meteorite. This type of diamond - lonsdaleite - has only been theorised to be naturally occurring, until now.   

Unusually, the lonsdaleite the team found was folded, which shouldn’t be possible for a harder-than-diamond diamond. Folding happens when rocks or minerals experience pressure and deform, bending rather than breaking. In geology, large-scale folds are seen commonly and can tell us about Earth’s processes. For an individual crystal of a super-hard mineral to fold is highly unusual as it should be more resistant to the pressures that would cause it to fold. Regular diamonds have a cubic structure, whereas lonsdaleite has a hexagonal structure. It’s because of this structure that it was theorised to be harder than regular diamonds. 

“This is exactly the sort of curiosity-piquing observation that sends scientists diving down rabbit holes for months on end,” said Professor Andy Tomkins, the lead author of the study and ARC Future Fellow at Monash University’s School of Earth, Atmosphere and Environment.

The folded lonsdaleite was found in a rare type of stony meteorite called a ureilite, which gives some clues into how this super-hard material was folded. Ureilites are formed when a dwarf planet or giant asteroid is hit by an impactor. The ureilite meteorites come from the mantle of the dwarf planet, and so are typically rich in minerals associated with mantle magma such as olivine and pyroxene. Ureilites are also higher in Carbon than other types of stony meteorites - about 3% - which exists in the forms of graphite and nanodiamonds.  

Folded lonsdaleite seen under different lights. Credit: Tomkins et al. 2022, published in PNAS.

In the mantle of a cooling dwarf planet, billions of years ago, olivine and pyroxene were crystalising. As the crystals grew they pushed against the graphite causing it to bend and fold. When an impactor smashed into this dwarf planet supercritical fluids were created. Supercritical fluids are those at a temperature and pressure where no distinct liquid and gas phases exist. The lonsdaleite formed from the supercritical fluid, almost perfectly replacing the folded graphite and therefore preserving its texture. 

“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. Later, lonsdaleite was partially replaced by diamond as the environment cooled and the pressure decreased,” said Professor Tomkins.

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

The formation of lonsdaleite in a ureilite meteor. Credit: Tomkins et al. 2022, published in PNAS.

The process by which the lonsdaleite was formed is quite similar to how lab-grown diamonds are made. Lab-grown diamonds are made using chemical vapour deposition, as opposed to the supercritical fluid deposition theorised to have produced this folded lonsdaleite. Understanding how this super-hard type of diamond formed could allow scientists to try and recreate this process in labs, creating diamonds for industrial uses.

This research is a testament to the power of collaboration and allowing researchers to follow interesting findings. Each institute provided different expertise and equipment to analyse the meteorite.

“Individually, each of these techniques gives us a good idea of what this material is, but taken together – that’s really the gold standard,” said CSIRO’s Dr Nick Wilson.

“This is exactly the sort of curiosity-piquing observation that sends scientists diving down rabbit holes for months on end.” said Professor Tomkins.  

The article is available in the journal Proceedings of the National Academy of Sciences.