Unusual hexagonal diamonds discovered in extraterrestrial meteorite

Diamonds found in four meteorites in north-west Africa probably came from an ancient dwarf planet, and they are expected to be harder than Earth diamonds

Electron microscopy has revealed hexagonal diamonds (the dark area near the middle of the picture) in meteorites found in Africa

Alan Salek/RMIT

Mysterious hexagonal diamonds that don’t occur naturally on Earth have been discovered in four meteorites in north-west Africa.

“It’s really exciting because there were some people in the field who doubted whether this material even existed,” says Alan Salek at RMIT University in Melbourne, Australia, who was part of the team that found them.

Hexagonal diamonds, like regular diamonds, are made of carbon, but their atoms are arranged in a hexagonal structure rather than a cubic one.

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Also known as lonsdaleite, hexagonal diamonds were first reported in meteorites in the US and India in the 1960s. However, the previously discovered crystals were so small – only nanometres in size – that it was hard to confirm whether they were truly hexagonal diamonds.

To hunt for larger crystals, Salek and his colleagues used a powerful electron microscope to peer into 18 meteorite samples. One was from Australia and the rest were from north-west Africa.

They found hexagonal diamonds in four of the African meteorites, with some crystals measuring up to a micrometre in size – about 1000 times bigger than previous discoveries. This allowed the team to confirm the unusual hexagonal structure.

“It’s an important discovery because now we have larger crystals, we can get a better idea of how they formed and maybe replicate that process in the lab,” says Salek.

Based on the chemical composition of the meteorites that brought them to Earth, the hexagonal diamonds appear to have formed inside dwarf planets, says Andy Tomkins at Monash University in Melbourne, who led the research.

The team’s analysis suggests the crystals were created by a reaction between graphite – which is made of carbon atoms layered in sheets – and a supercritical fluid of hydrogen, methane, oxygen and sulphur chemicals that probably formed when an asteroid crashed into the dwarf planet and broke it into fragments that eventually fell onto Earth.

“When the planet broke apart, it was like taking a lid off a Coke bottle – it released the pressure and that drop in pressure combined with high temperatures led to the release of this supercritical fluid,” says Tomkins.

This is similar to the process by which regular diamonds are made in labs, by heating graphite with gases like hydrogen and methane, suggesting that a few tweaks could produce lonsdaleite instead, says Salek.

Hexagonal diamonds are predicted to be about 60 per cent harder than regular diamonds based on their structure, and this extra hardness could have important industrial applications if they could be made synthetically. For example, they could potentially be used to make ultra-hard saw blades or other machine parts, says Salek.