What are the main types of meteorite?
There are three broad types: stony meteorites, iron meteorites and stony-iron meteorites. Stony meteorites are further divided into chondrites, which never melted, and achondrites, which did.
The simplest way to sort meteorites is by what they are made of, and that gives us three families. Stony meteorites are made mostly of silicate minerals and are by far the most common, making up around 94 per cent of meteorites seen to fall. They split into two groups. Chondrites are stones that were never melted; most contain chondrules, tiny round grains that formed as molten droplets in the early Solar System. Chelyabinsk, which exploded over Russia in 2013, is a chondrite. Achondrites are stones that did melt, inside asteroids or planets, so they resemble volcanic or deep-seated rocks on Earth. They include the howardites, eucrites and diogenites thought to come from asteroid Vesta, along with all the Martian and lunar meteorites.
Iron meteorites are made largely of iron-nickel metal and are thought mostly to be pieces of the cores of asteroids that melted and separated into layers. They form about 5 per cent of observed falls but are heavy, distinctive and resist weathering, so they are well represented in collections. The world's largest known meteorite, Hoba in Namibia, is an iron.
Stony-iron meteorites contain roughly equal amounts of metal and silicate. They account for only about 1 per cent of falls and come in two main kinds: pallasites, with gem-like olivine crystals set in metal, and mesosiderites, a jumbled mix of metal and broken rock.
Within each family, scientists recognise many more detailed groups, defined by chemistry, minerals and oxygen isotopes. Those groups help link meteorites to particular parent bodies and tell us how each one formed, so the broad three-way split is only the first step in a much richer classification.