What is a ureilite?
A ureilite is an unusual achondrite made of olivine and pyroxene with carbon, including graphite and tiny diamonds, from a parent body shattered early in its history.
Ureilites are the second-largest group of achondrites, after the HED meteorites from Vesta. They are named after the Novo-Urei meteorite, which fell in Russia in 1886.
They are made mainly of coarse grains of olivine and pyroxene, with dark veins between the grains rich in carbon, including graphite and tiny diamonds, along with metal and sulfides. Ureilites contain up to several per cent carbon, unusually high for a melted rock.
Their story is complex. Their texture suggests they are the residue of a body that was partly melted: its basaltic melt and some metal drained away, leaving an olivine-rich mantle. Yet their oxygen isotopes vary from grain to grain much like carbonaceous chondrites, which is surprising for a melted rock. The best explanation is that their parent body began melting but was smashed apart by a collision while still hot, within about 5 to 10 million years of the Solar System's start. The pieces cooled very quickly, preserving unusual chemistry.
The diamonds in ureilites are debated. Some may have formed by shock during impacts, but in 2018 researchers found inclusions in large diamonds from the Almahata Sitta ureilite that suggested formation at high pressure inside a large parent body, perhaps the size of Mercury or Mars. Almahata Sitta, which fell in Sudan in 2008 after its asteroid was tracked in space, is the most famous ureilite. It showed that ureilite material can mix with many other meteorite types.