Chondrite

Chondrite

Chondrite meteorites are the most common meteorite overall accounting for ~86% of falls. Chondrites are made mostly of Fe- and Mg-bearing silicate minerals and have remained little changed from the origin of the Solar System. Except for the lightest elements (e.g., H, He), chondrites have the same elemental composition as the original solar nebula because they come from asteroids that never melted or underwent differentiation. Chondrites are so named because they nearly all contain chondrules – small round droplets of olivine and pyroxene. Chondrules are one of the first solids to have condensed and crystallized in the solar nebula and all similar in bulk composition to the Sun’s photosphere. Variations in chemical composition among chondrites reflect formation of their parent bodies in different regions of the solar nebula. The main groups are carbonaceous, enstatite (E), ordinary (H, L, and LL) and rumurutiites (R). Other, rarer chondrite groups are F-chondrites, G-chondrites and kakangariites (K).

Each group is further subdivided into petrologic types 1 through 7. Types 1 and 2 show evidence of aqueous alteration to the extent that chondrules are either absent (Type 1) or rare (Type 2). Petrologic types 3 to 7 show evidence of varying degrees of thermal metamorphism, which is reflected by modification of the chondrules and chemical homogenization. Type 3 samples show plentiful, unaltered and distinct chondrules; whereas the chondrules become increasingly indistinct due to recrystallization in types 4 to 6. Type 7 chondrites, better termed metachondrites, in which chondrules are absent, are transitional between chondrites and primitive achondrites.

Content above used with permission from J. H. Wittke.

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