Achondrite meteorites are stony meteorites lacking chondritic texture; that is, they lack the spherical chondrules that are the defining characteristic of chondrites. As originally defined in the Rose—Tschermak—Brezina classification system, achondrites were also distinct from chondritic meteorites in composition. In particular, the original achondrite groups were poor in iron—nickel metal, and many had broadly basic compositions, rather than the ultrabasic compositions typical of chondrites. Hence, the chemical compositions of most achondrites are significantly different from that of material formed directly from the solar nebula. However, recent achondrite falls and finds have blurred the boundary between achondritic and chondritic compositions. A subset of the achondrites, often referred to as the primitive achondrites, are rocks with broadly chondritic compositions, but with achondritic textures. Almost all achondrites have ages of about 4.6 Ga, essentially identical to the ages...
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Mittlefehldt, D.W. (1997). Achondrite meteorites . In: Encyclopedia of Planetary Science. Encyclopedia of Earth Science. Springer, Dordrecht. https://doi.org/10.1007/1-4020-4520-4_3
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