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An achondrite〔Etymology: from the prefix a- (privative a) and the word chondrite.〕 is a stony meteorite that does not contain chondrules.〔(Recommended classifications: Eucrite-pmict )〕〔(Achondrite ), Encyclopedia Britannica〕 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. 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. ==Classification== Achondrites are classified into the following groups:〔O. Richard Norton. The Cambridge encyclopedia of meteorites. UK, Cambridge University Press, 2002. ISBN 0-521-62143-7.〕 *Primitive achondrites *Asteroidal achondrites *Lunar meteorites *Martian meteorites 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Achondrite」の詳細全文を読む スポンサード リンク
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