What is a chondrite?
A chondrite is a stony meteorite that was never melted after it formed, so it preserves the original mixture of dust and grains from the early Solar System. Most contain chondrules, small round grains that give the group its name.
Chondrites are the most common meteorites that fall to Earth, accounting for roughly 85 per cent of observed falls. They come from asteroids that never grew hot enough to melt completely, which makes them some of the least changed rocks available to science.
Their name comes from chondrules, tiny spheres of once-molten rock, usually less than a few millimetres across, packed inside them. Around the chondrules lies a fine-grained matrix of dust, often with specks of iron-nickel metal and iron sulfide. Some chondrites also contain pale, irregular lumps called calcium-aluminium-rich inclusions, the oldest solids formed in the Solar System, about 4.567 billion years ago.
Scientists recognise several classes. Ordinary chondrites are the commonest and are split into H, L and LL groups according to how much iron and metal they contain; Chelyabinsk, which burst over Russia in 2013, is an LL chondrite. Carbonaceous chondrites, such as Allende from Mexico and Murchison from Australia, both of which fell in 1969, are rich in carbon compounds and water-bearing minerals. Enstatite chondrites, such as Abee from Canada, formed in conditions so short of oxygen that they contain unusual sulfide minerals.
Why do chondrites matter so much? Apart from the lightest gases, their chemistry is remarkably close to that of the Sun. They are effectively samples of the original building material of the rocky planets. By studying how much each one was later heated or altered by water on its parent asteroid, recorded in a number from 1 to 6 in its classification, researchers can reconstruct what happened in the first few million years of the Solar System's history.