Why do some meteorites contain diamonds?
Diamonds in meteorites form by shock during impacts, by high pressure inside large parent bodies, or as nanodiamonds from gas, some formed before the Sun.
Diamonds, the high-pressure form of carbon, appear in several kinds of meteorites, formed in different ways.
Nanodiamonds. The most abundant diamonds in meteorites are nanodiamonds, only a few nanometres across, found in primitive chondrites in amounts up to about 0.1 per cent. A small fraction of them carry unusual xenon isotopes, the Xe-HL component, suggesting an origin in supernovae. Most may have formed in the Solar System or in interstellar space by condensation from gas at low pressure, like synthetic diamond films.
Shock diamonds. When an impact squeezes carbon-rich material, such as graphite, diamond can form in an instant. Diamonds and lonsdaleite, a related form, are found in some iron meteorites, such as Canyon Diablo, and in ureilites, which contain abundant graphite. Impact craters on Earth, such as Popigai in Siberia, contain shock diamonds formed from graphite in the target rocks.
Diamonds from inside a large body. In 2018, researchers reported that diamonds up to about 100 micrometres in the Almahata Sitta ureilite contained inclusions of chromite, phosphate and iron sulfide that would form only at high pressure, above about 20 gigapascals. They argued these diamonds grew inside a large parent body, perhaps the size of Mercury or Mars, that was later destroyed. Others suggest shock could explain them.
Meteoritic diamonds are too small for jewellery, but they are valuable to scientists. They record impacts, conditions inside lost planetary bodies, and even the deaths of stars before the Sun was born.