Some Unique Meteorites Found in Antarctica and Their Relation to Asteroids

概要

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Mineralogical examination of the smaller sized Yamato achondrites identify two unique meteorites : Yamato-74130 is the most iron-rich ureilite with Na, Cr-rich augite instead of pigeonite; Yamato-74160 was extensively recrystallized but the composition and proportion of olivine (Fa_<30>), orthopyroxene (Ca_4Mg_<72>Fe_<24>), augite (Ca_<43>Mg_<46>Fe_<11>) and plagioclase (An_8 and An_<16>) is consistent with LL7 chondrites. ALHA77005 is a unique achondrite with olivine, possible three pyroxene assemblage and maskelynite. These meteorites provide evidence that there may be other "thermalized" asteroids than the howardite parent body. Yamato-74356 is different from eucritic polymict breccias common in the Antarctic meteorites and is only one example of common eucrite. Detailed petrologic description is given on unique achondrites, recrystallized diogenite Yamato-74013 and rapidly cooled eucrite Yamato-74450 with pyroxene phenocrysts. The bulk chemistry and the mineralogical reexamination of Yamato-75028 indicate that it is composed of the H5-type clasts and chondrule-rich H (L) 3-like matrix with the H5 fragments. The fact may imply close relationship in the collisional evolution of some asteroids with these materials.
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