What are primitive achondrites?
Primitive achondrites are meteorites that were heated enough to partly melt but kept much of their original chondritic chemistry, such as acapulcoites and lodranites.
Meteorites are often divided into chondrites, which were never melted, and achondrites, which were melted. Primitive achondrites sit in between.
What they are. These meteorites were heated enough for some melting to begin, perhaps to around 1,000 to 1,250 °C, but not enough to separate completely into core, mantle and crust. They lost some melt, but their overall chemistry still resembles chondrites. Their textures are recrystallised, with no visible chondrules, or only relics.
Groups. - Acapulcoites, named after Acapulco in Mexico, are fine-grained and lost only a little metal-sulfide melt. - Lodranites, named after Lodran in Pakistan, are coarser-grained and lost both metal-sulfide and silicate melts. Acapulcoites and lodranites come from the same parent body. - Winonaites, named after Winona in Arizona, are related to the silicate inclusions in IAB iron meteorites. - Brachinites, named after Brachina in Australia, are olivine-rich and may be residues from more extensive melting. - Ureilites are sometimes included, although they experienced more complex histories.
Why they matter. Primitive achondrites show the early stages of planetary differentiation, when asteroids began to melt and metal began to separate. They help scientists understand how cores, mantles and crusts formed.
Ages. Many formed within a few million years of the Solar System's beginning, heated by aluminium-26.
Primitive achondrites are relatively rare, and many examples come from Antarctica and hot deserts.