Largest Extraterrestrial Diamonds Ever Discovered: Cosmic Diamonds Formed During Gigantic Planetary Collisions

Artist’s impression of the collision of two protoplanets.
Credit: NASA/SOFIA/Lynette Cook

International research team solves theory of how diamonds formed inside protoplanets.

Geoscientists from Goethe University have found the largest extraterrestrial diamonds ever discovered – a few tenths of a millimeter in size nevertheless – inside meteorites. Together with an international team of researchers, they have now been able to prove that these diamonds formed in the early period of our solar system when minor planets collided together or with large asteroids. These new data disprove the theory that they originated deep inside planets – similar to diamonds formed on Earth – at least the size of Mercury.

It is estimated that over 10 million asteroids are circling the Earth in the asteroid belt. They are relics from the early days of our solar system, when our planets formed out of a large cloud of gas and dust rotating around the sun. When asteroids are cast out of orbit, they sometimes plummet towards Earth as meteoroids. If they are big enough, they do not burn up completely when entering the atmosphere and can be found as meteorites. The geoscientific study of such meteorites makes it possible to draw conclusions not only about the evolution and development of planets in the solar system but also their extinction.

A special type of meteorites are ureilites. These are fragments of a larger celestial body — probably a minor planet — which was smashed to pieces through violent collisions with other minor planets or large asteroids. Ureilites often contain large quantities of carbon, among others in the form of graphite or nanodiamonds. The diamonds on the scale of over 0.1 and more millimeters now discovered cannot have formed when the meteoroids hit the Earth. Impact events with such vast energies would make the meteoroids evaporate completely. That is why it was so far assumed that these larger diamonds — similar to those in the Earth’s interior — must have been formed by continuous pressure in the interior of planetary precursors the size of Mars or Mercury.

Together with scientists from Italy, the USA, Russia, Saudi Arabia, Switzerland, and the Sudan, researchers from Goethe University have now found the largest diamonds ever discovered in ureilites from Morocco and the Sudan and analyzed them in detail. Apart from the diamonds of up to several 100 micrometers in size, numerous nests of diamonds on just nanometre scale as well as nanographite were found in the ureilites. Closer analyses showed that what are known as londsdalite layers exist in the nanodiamonds, a modification of diamonds that only occurs through sudden, very high pressure. Moreover, other minerals (silicates) in the ureilite rocks under examination displayed typical signs of shock pressure. In the end, it was the presence of these larger diamonds together with nanodiamonds and nanographite that led to the breakthrough.

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