What are achondrites?
Achondrites are stony meteorites that formed from melted rock, so they lack the chondrules found in chondrites. They come from asteroids, the Moon and Mars that were hot enough to melt and separate into layers.
The name simply means without chondrules. Chondrites are stones that never melted; achondrites are stones that did. Melting erased any original chondrules and produced rocks that resemble volcanic lavas or deep-seated crystal mushes on Earth. Achondrites make up only around 8 per cent of observed falls, but they are scientifically rich because each tells the story of a world that became hot enough to evolve. The largest family is the HED group: howardites, eucrites and diogenites, linked to the asteroid Vesta. Eucrites, such as Juvinas, which fell in France in 1821, are basalts; diogenites are rich in pyroxene crystals; and howardites are impact-mixed breccias of the two.
Other asteroid achondrites include the angrites, extremely ancient basalts named after Angra dos Reis in Brazil; the aubrites, pale rocks made largely of the mineral enstatite, such as Norton County, which fell in the United States in 1948; and the ureilites, carbon-rich rocks that can contain tiny diamonds, which include the Almahata Sitta fragments from asteroid 2008 TC3. Primitive achondrites, such as acapulcoites and lodranites, were only partly melted and sit between chondrites and fully melted rocks.
Then there are achondrites from planets and moons. Martian meteorites include the shergottites, nakhlites and chassignites, named after Shergotty in India, Nakhla in Egypt and Chassigny in France, plus the unusual ALH 84001. Lunar meteorites, many of them breccias from the Moon's highlands, provide samples from areas no mission has visited.
Some achondrites fit no existing group at all. Each new ungrouped stone can represent a previously unsampled parent body, which is why classification teams study unusual specimens so carefully.