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Eucrites are achondritic stony meteorites, many of which originate from the surface of the asteroid 4 Vesta and as such are part of the HED meteorite clan. They are the most common achondrite group with well over 100 distinct finds at present. Eucrites consist of basaltic rock from the crust of 4 Vesta or a similar parent body. They are mostly composed of Ca-poor pyroxene, pigeonite, and Ca-rich plagioclase (anorthite).〔B. Mason: Meteorites. John Wiley, New York 1962.〕 Based on differences of chemical composition and features of the component crystals, they are subdivided into several groups:〔Mittlefehldt, McCoy, Goodrich and Kracher: Non-chondritic Meteorites from Asteroidal Bodies, In: Reviews in Mineralogy Vol. 36, Planetary Materials, Mineralogical Society of America, 1998.〕 * Non-cumulate eucrites Are the most common variety and can be subdivided further: * * Main series eucrites formed near the surface and are mostly, though not exclusively, regolith breccias lithified under the pressure of overlying newer deposits. * * Stannern trend eucrites are a rare variety. * * Nuevo Laredo trend eucrites are thought to come from deeper layers of 4 Vesta's crust, and are a transition group towards the cumulate eucrites. * Cumulate eucrites are rare types with oriented crystals, thought to have solidified in magma chambers deep within Vesta's crust. * Polymict eucrites are regolith breccias consisting of mostly eucrite fragments and less than one part in ten of diogenite, an arbitrary dividing line from the howardites, which are related in structure. They are comparably rare. ==Etymology== Eucrites get their name from the Greek word ''eukritos'' meaning "easily distinguished". This refers to the silicate minerals in them, which can be easily distinguished because of their relatively large grain size. Eucrite is also a now obsolete term for bytownite-gabbro, an igneous rock formed in the Earth's crust. The term was used as a rock type name for some of the Paleogene igneous rocks of Scotland.〔Sutherland, D. S. (1982) ''Igneous Rocks of the British Isles'', Chichester, John Wiley, page 536.〕 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「Eucrite」の詳細全文を読む スポンサード リンク
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