achondrite
American Heritage Dictionary:
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a·chon·drite
(ā-kŏn'drīt') 
n.
A stony meteorite that contains no chondrules.
achondritic a'chon·drit'ic (-drĭt'ĭk) adj.
n.
A stony meteorite that contains no chondrules.
achondritic a'chon·drit'ic (-drĭt'ĭk) adj.
Wiley Book of Astronomy:
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achondrite
The rarer of the two main types of stony meteorite, accounting for about 9% of all meteorite falls. Achondrites are made of rock that has crystallized from a molten state. They contain mostly one or more of the minerals plagioclase feldspar, pyroxene, and olivine, and generally, but not always, lack the small rounded inclusions known as chondrules that are typical of chondrites. Most achondrites are chemically similar to basalts and are thought to be the product of melting on large asteroids, moons, and planets. Soon after these bodies formed, they were heated from within and partially melted. Although this process is still active on Earth, it ended about 4.4 billion years ago on asteroids, 2.9 billion years ago on the Moon, and perhaps 1 billion years ago on Mars. Heating the primordial mixture of stony minerals, metals, and sulfides (of which chondrites are made) produced liquids, the densest of which sank to become planetary or asteroidal cores. Lighter stony minerals rose and solidified to become basaltic rocks, fragments of which were subsequently broken off by impacts and hurled into space. More than 200 of these evolved achondrites have been found, covering a wide range of compositions and origins. The so-called HED group includes the howardites, eucrites, and diogenites, which appear to share the same parent body, believed to be the asteroid Vesta. Other evolved achondrites that seem to have come from partially differentiated asteroids other than Vesta have been mostly assigned to two distinct groups known as the angrites and the aubrites. Although the majority of achondrites are of asteroidal origin, some are known to have come from the highland regions of the Moon's farside (see lunar meteorites) and from Mars (see Mars meteorites). NWA011, a meteorite found in the Sahara in 1999, is suspected of having originated on Mercury. As well as the evolved achondrites, there is an entire group of primitive achondrites whose members all seem to have derived from small chondritic parent bodies that only partially melted and differentiated through accretion processes or from impact events, and then rapidly cooled. Primitive achondrites vary widely in composition and fall into the following main subgroups: acapulcoites, lodranites, brachinites, winonaites, and ureilites.
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Achondrite
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This article may be expanded with text translated from the corresponding article in the Spanish Wikipedia. (December 2009) Don't speak Spanish? Click here to read a machine-translated version of the Spanish article. Click [show] on the right to review important translation instructions before translating.
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An achondrite[1] is a stony meteorite that does not contain chondrules. 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.[2][3] As a result, achondrites have distinct textures and mineralogies indicative of igneous processes.[4]
Achondrites account for about 8% of meteorites overall, and the majority (about two thirds) of them are HED meteorites, originating from the crust of asteroid 4 Vesta. Other types include Martian, Lunar, and several types thought to originate from as-yet unidentified asteroids other than Vesta. 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.[5]
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Classification
Achondrites are classified into the following groups:[6]
Primitive achondrites
Primitive achondrites, also called PAC group, are called in this way because their chemical composition is primitive in the sense that it is similar to the composition of chondrites, but their texture is igneous, indicative of melting processes. To this group belong:[6]
- Acapulcoites (after the meteorite Acapulco, Mexico)
- Lodranites (after the meteorite Lodran)
- Winonaites (after the meteorite Winona)
Asteroidal achondrites
Asteroidal achondrites, also called evolved achondrites, are called in this way because have been differentiated on a parent body. This means that their mineralogical and chemical composition was changed by melting and crytallization processes. They are divided several groups:[6]
- HED meteorites (Vesta). They originated most probably on the asteroid 4 Vesta, because their reflection spectra are very similar.[7] They are named after the initial letters of the three subgroups:
- Angrites
- Aubrites
- Ureilites (after the meteorite Novy Ureii, Russia)
- Brachinites (after the meteorite Brachina)
Lunar meteorites
Lunar meteorites, also called Lunanites, are meteorites that originated from the Moon.
Martian meteorite
Martian meteorites[8] are meteorites that originated from Mars. The are divided in four groups:
- Shergottites
- Nakhlites
- Chassignites
- OPX martian meteorites (ALH 84001)
References
- ^ Etymology: from the prefix a- (privative a) and the word chondrite.
- ^ Sahijpal, S.; Soni, P.;Gagan, G. (2007). "Numerical simulations of the differentiation of accreting planetesimals with 26Al and 60Fe as the heat sources". Meteoritics & Planetary Science 42 (9): 1529–1548. Bibcode 2007M&PS...42.1529S. doi:10.1111/j.1945-5100.2007.tb00589.x.
- ^ Gupta, G.; Sahijpal, S. (2010). "Differentiation of Vesta and the parent bodies of other achondrites". J. Geophys. Res. (Planets) 115. Bibcode 2010JGRE..11508001G. doi:10.1029/2009JE003525.
- ^ Mason, B. (1962). Meteorites. New York: John Wiley.
- ^ Mittlefehldt, David W.; McCoy, Timothy J.; Goodrich, Cyrena Anne; Kracher, Alfred (1998). "Non-chondritic Meteorites from Asteroidal Bodies". Reviews in Mineralogy and Geochemistry 36 (1): 4.1–4.195. http://rimg.geoscienceworld.org/cgi/content/abstract/36/1/4.1.
- ^ a b c O. Richard Norton. The Cambridge encyclopedia of meteorites. UK, Cambridge University Press, 2002. ISBN 0-521-62143-7.
- ^ Drake, M. J. (2001). "The eucrite/Vesta story". Meteoritics and Planetary Science 36 (4): 501–513. Bibcode 2001M&PS...36..501D. doi:10.1111/j.1945-5100.2001.tb01892.x.
- ^ Treiman, A. H. (2000). "The SNC meteorites are from Mars". Planetary and Space Science 48 (12–14): 1213–1230. Bibcode 2000P&SS...48.1213T. doi:10.1016/S0032-0633(00)00105-7.
External links
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- Achondrite Images from Meteorites Australia
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The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.
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