Achondrites
Achondrites are a grab-bag category of meteorites which are: 1. Rich in silicates 2. Lack chondrules 3. Have undergone relatively advanced geological processing on their parent bodies.
They are generally more coarse-grained than the chondrites and are generally closer to terrestrial igneous rocks in terms of chemistry, mineralogy, and structure than the chondrites. The near-absence of metal phases (Fe, Ni) in achondrites suggests that they are differentiated rocks; that is, that they were produced by melting, decantation and then cooling of a more primitive material (possibly chondritic originally). In this picture, achondrites would be rocks that have crystallized as part of near-surface magmas on their parent worlds, whereas iron meteorites would represent material that sank to their interior to form a core. Stony-irons would then be samples from the boundary between the achondritic upper layers and the core. Achondrites are classified into several groups, each believed to represent either a distinct parent body or, in some cases, a particular location on a common parent body. Some groups may be subdivided into subgroups.
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