An achondrite is a stony meteorite that does not contain chondrules. It consists of material similar to terrestrial basalts or plutonic rocks and has been differentiated and reprocessed to a lesser or greater degree due to melting and recrystallization on or within meteorite parent bodies. As a result, achondrites have distinct textures and mineralogies indicative of igneous processes.
|— Type —|
Cumberland Falls, achondrite (aubrite)
Achondrites account for about 8% of meteorites overall, and the majority (about two thirds) of them are HED meteorites, possibly originating from the crust of asteroid 4 Vesta. Other types include Martian, Lunar, and several types thought to originate from as-yet unidentified asteroids. These groups have been determined on the basis of e.g. the Fe/Mn chemical ratio and the 17O/18O oxygen isotope ratios, thought to be characteristic "fingerprints" for each parent body.
Achondrites are classified into the following groups:
Primitive achondrites, also called PAC group, are called in this way because their chemical composition is primitive in the sense that it is similar to the composition of chondrites, but their texture is igneous, indicative of melting processes. To this group belong:
- Acapulcoites (after the meteorite Acapulco, Mexico)
- Lodranites (after the meteorite Lodran)
- Winonaites (after the meteorite Winona)
- Ureilites (after the meteorite Novy Ureii, Russia)
- Brachinites (after the meteorite Brachina)
Asteroidal achondrites, also called evolved achondrites, are called in this way because have been differentiated on a parent body. This means that their mineralogical and chemical composition was changed by melting and crystallization processes. They are divided several groups:
- HED meteorites (Vesta). They may have originated on the asteroid 4 Vesta, because their reflection spectra are very similar. They are named after the initial letters of the three subgroups:
- Etymology: from the prefix a- (privative a) and the word chondrite.
- Recommended classifications: Eucrite-pmict
- Achondrite, Encyclopædia Britannica
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