Mysterious hexagonal diamonds not found naturally on Earth have been identified in four meteorites from Northwest Africa. Investigating their formation, Australian researchers found that they were forged during a violent cataclysm on an ancient dwarf planet. Details of the study are published in the journal PNAS.
Lonsdaleite in meteorites
Diamonds are not particularly known for their flexibility. The discovery of many of these “folded” structures within four African meteorites quickly intrigued researchers at the Commonwealth Scientific and Industrial Research Organization (CSIRO). These small rocks belonged to a class known as ureilites. These stony, carbon-rich meteorites often contain small diamonds, but never in this form.
To find out more, the researchers mapped the carbon distribution in different samples. In doing so, they found evidence that these misshapen diamonds were actually i lonsdaleite. It’s kind of a hexagonal diamond which, like normal diamonds, is made of carbon. The difference is that its atoms are arranged in a hexagonal rather than a cubic structure.
High-resolution transmission electron microscopic (TEM) analysis then confirmed that these meteorites did indeed contain lonsdaleite. These diamonds were also i largest crystals ever found of this type of mineral (up to one micrometer in length). Previous samples collected from several meteorites collected decades ago in the United States and India were in fact much smaller, at the nanoscale.
But that still didn’t explain how those hex diamonds were folded. Further clues emerged when the team noticed that some of the lonsdaleite had been converted into graphite and cubic diamond. The team then compared the distribution of all these crystals in eighteen different ureilite samples in an attempt to reconstruct their probable origin.
Relics of the Solar System
When they first folded, these crystals weren’t diamonds yet, just graphite. This material would end up in mantle of a dwarf planet about 4.5 billion years ago, when the solar system was still forming. Over time, the high temperatures and pressure of surrounding materials would have deformed this graphite into the folded shape seen today.
Subsequently, the researchers believe that this dwarf planet is collision with another massive object. Analysis suggests that the crystals were created from a reaction between graphite – which is composed of carbon atoms layered in sheets – and a supercritical fluid of hydrogen, methane, oxygen and sulfur that formed during impact.
Recall that we speak of supercritical fluid when a fluid is heated beyond its critical temperature and when it is compressed beyond its critical pressure. ” When the planet collapsed, it was like taking the lid off a bottle of Coke. This released the pressure and this pressure drop combined with the high temperatures led to the release of this supercritical fluid“Summarizes Andy Tomkins, of Monash University in Melbourne.
The cataclysm would also have destroyed most of the original planetoid, scattering its materials into space. Some would end up on Earth eventually.
This natural formation process is quite similar to what ordinary diamonds are made with in the laboratory. This suggests that some adjustments could eventually produce lonsdaleite instead. In theory, these hex diamonds should be about 60% harder compared to ordinary diamonds due to their structure. This extra hardness could therefore have important industrial applications if they could be produced synthetically.