Some diamonds are found on Earth

Mysterious hexagonal diamonds that do not occur naturally on Earth have been identified in four meteorites from northwest Africa. Investigating their formation, Australian researchers discovered that they were forged during a violent catastrophe on an ancient dwarf planet. Details of the study are published in the journal PNAS.

Places called Lonsdaleite in meteorites

Diamonds are not particularly known for their flexibility. The discovery of several of these “folded” structures within four African meteorites therefore quickly intrigued researchers from the Commonwealth Scientific and Industrial Research Organization (CSIRO). These pebbles belong to a class known as ureilites. These rocky, carbon-rich meteorites usually contain small diamonds, but never in this shape.

To learn more, the researchers mapped the distribution of carbon in several samples. In doing so, they found evidence that these false diamonds were indeed lonsdaleite. It’s a kind of hexagonal diamond which, like ordinary diamonds, is made of carbon. The difference is that its atoms are arranged in a hexagonal structure rather than a cubic one.

High-resolution transmission electron microscopy (TEM) analysis then confirmed that these meteorites contained true lonsdaleite. These diamonds are also largest crystal of this type of mineral ever found (up to one micrometer in length). Previous samples collected from many meteorites collected decades ago in the United States and India were smaller, on the nanometer scale.

But that still doesn’t explain how hexagonal diamonds are bent. Further clues emerged when the team noticed that some of the lonsdaleite had converted to graphite and cubic diamond. The team then compared the distribution of all these crystals to eighteen different samples of ureilite in an effort to piece together their possible origins.

An electron probe microanalyzer image of a meteorite fragment. The different minerals present are: Iron (red), magnesium (green), silicon (blue), lonsdaleite (yellow) and diamond (pink). Source: PNAS

Relics of the Solar System

When they first bent, these crystals weren’t real diamonds, just graphite. This material should be finished in mantle of the dwarf planet about 4.5 billion years ago, when the solar system was still forming. Over time, high temperatures and pressure from the surrounding materials can transform this graphite into the bent shape seen today.

Later, researchers believe that this dwarf planet is collision along with another big thing. The analysis suggests that the crystals were formed by a reaction between graphite – made of carbon atoms stacked in sheets – and a supercritical fluid of hydrogen, methane, oxygen and sulfur chemicals formed. during the impact.

As a reminder, we talk about supercritical fluid when a fluid is heated beyond the critical temperature and when it is compressed above the critical pressure. ” When the planet collapsed, it was like taking the cap off a Coke bottle. This releases the pressure and this decrease in pressure combined with the high temperature leads to the release of this supercritical fluid.“, summarizes Andy Tomkins, from Monash University in Melbourne.

The cataclysm also destroyed much of the original planet, scattering its materials into space. Some would have ended up on Earth eventually.

This natural formation process is the same as that by which ordinary diamonds are made in the laboratory. This suggests that some changes may eventually produce lonsdaleite. In theory, these hexagonal diamonds should be about 60% harder than ordinary diamonds because of their structure. This extreme hardness may have important industrial applications if it can be synthesized synthetically.

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