Achondrite
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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]
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. (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.
{{cite journal}}: Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ Gupta, G. (2010). "Differentiation of Vesta and the parent bodies of other achondrites". J. Geophys. Res. (Planets). 115. Bibcode:2010JGRE..11508001G. doi:10.1029/2009JE003525.
{{cite journal}}: Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ 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.
- ^ 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
- Achondrite Images from Meteorites Australia
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