Lonsdaleite, a rare hexagonal form of diamond found in ureilite meteorites, formed shortly after an inner solar system dwarf planet collided with a large asteroid about 4.5 billion years ago.
Images of graphite, lonsdaleite, and diamond in ureilite meteorites: (A) reflected light image showing folded crystalline graphite, with fold morphology defined by graphite cleavage; different shading in the graphite is produced by axial planar kink bands; (B) reflected light image (stacked foci) showing an example of the inherited fold morphology preserved in lonsdaleite; (C) cathodoluminescence map of the same area as (B) indicating different phases of carbon, where the green regions are lonsdaleite and the red areas on the periphery (including the purple dashed circle) are cubic diamond (blue is the cathodoluminescence response from olivine); (D) scanning TEM image of a region cut out of the area indicated by yellow circle in (C), highlighting dark lonsdaleite crystals. Image credit: Tomkins et al., doi: 10.1073/pnas.2208814119.
“We predicted the hexagonal structure of lonsdaleite’s atoms made it potentially harder than regular diamonds, which had a cubic structure,” said RMIT University’s Professor Dougal McCulloch, senior author of the study.
“Our study proves categorically that lonsdaleite exists in nature.”
“We also discovered the largest lonsdaleite crystals known to date that are up to a micron in size — much, much thinner than a human hair.”
“The unusual structure of lonsdaleite could help inform new manufacturing techniques for ultra-hard materials in mining applications.”
In their study, Professor McCulloch and colleagues used advanced electron microscopy techniques to capture solid and intact slices from ureilite meteorites to create snapshots of how lonsdaleite and regular diamonds formed.
“There’s strong evidence that there’s a newly discovered formation process for the lonsdaleite and regular diamond, which is like a supercritical chemical vapor deposition process that has taken place in these space rocks, probably in the dwarf planet shortly after a catastrophic collision,” Professor McCulloch said.
“Chemical vapor deposition is one of the ways that people make diamonds in the lab, essentially by growing them in a specialized chamber.”
The authors propose that lonsdaleite in the meteorites formed from a supercritical fluid at high temperature and moderate pressures, almost perfectly preserving the shape and textures of the pre-existing graphite.
“Later, lonsdaleite was partially replaced by diamond as the environment cooled and the pressure decreased,” said Monash University’s Professor Andy Tomkins, first author of the study.
“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 replacement of pre-shaped graphite parts by lonsdaleite.”
The team’s results appear in the Proceedings of the National Academy of Sciences.
Andrew G. Tomkins et al. 2022. Sequential Lonsdaleite to Diamond Formation in Ureilite Meteorites via In Situ Chemical Fluid/Vapor Deposition. PNAS 119 (38): e2208814119; doi: 10.1073/pnas.2208814119