Achondrite

{{Short description|Stony meteorite that does not contain chondrules}} {{Infobox meteorite subdivision |Subdivision = Type |Name = Achondrite |Alternative_names = |Image = Cumberland Falls meteorite.jpg |Image_caption = Cumberland Falls, achondrite (aubrite) |Image_alt_text = |Compositional_type = Stony |Type = |Class = |Clan =  |Structural_classification = |Parent_body = |Composition = |Petrologic_type = |Number_of_specimens = |TKW =  |Image2 = MillbillillieMeteorite.jpg |Image2_caption = A eucrite achondrite from the Millbillillie meteorite shower. |Image2_alt_text = }} An '''achondrite'''<ref>Etymology: from the prefix a- (privative a) and the word chondrite.</ref> is a stony meteorite that does not contain chondrules.<ref>[http://www.lpi.usra.edu/meteor/metbullclass.php?sea=Eucrite-pmict Recommended classifications: Eucrite-pmict]</ref><ref>[http://www.britannica.com/EBchecked/topic/3630/achondrite Achondrite], Encyclopædia Britannica</ref> 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.<ref name="Sahijpal2007">{{cite journal |author=Sahijpal, S. |author2=Soni, P.|author3=Gagan, G. |title=Numerical simulations of the differentiation of accreting planetesimals with 26Al and 60Fe as the heat sources |journal=Meteoritics & Planetary Science |volume=42 |pages=1529–1548 |date=2007 |doi=10.1111/j.1945-5100.2007.tb00589.x |issue=9 |bibcode=2007M&PS...42.1529S|doi-access=free }}</ref><ref name="Gupta2010">{{cite journal |author=Gupta, G. |author2=Sahijpal, S. |title=Differentiation of Vesta and the parent bodies of other achondrites |journal=J. Geophys. Res. Planets |date=2010 |doi=10.1029/2009JE003525 |volume=115 |issue=E8 |bibcode=2010JGRE..115.8001G |s2cid=129905814 }}</ref> As a result, achondrites have distinct textures and mineralogies indicative of igneous processes.<ref>{{cite book |first=B. |last=Mason |title=Meteorites |url=https://archive.org/details/meteorites0000maso |url-access=registration |publisher=John Wiley |location=New York |date=1962 }}</ref>

Achondrites account for about 8% of meteorites overall, and the majority (about <math>2/3</math>) of them belong to the HED clan, possibly originating from the crust of asteroid 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.<ref>{{cite journal |last1=Mittlefehldt |first1=David W. |last2=McCoy |first2=Timothy J. |last3=Goodrich |first3=Cyrena Anne |last4=Kracher |first4=Alfred |title=Non-chondritic Meteorites from Asteroidal Bodies |journal=Reviews in Mineralogy and Geochemistry |volume=36 |issue=1 |date=1998 |pages=4.1–4.195 |url=http://rimg.geoscienceworld.org/cgi/content/abstract/36/1/4.1 }}</ref>

==Classification== Achondrites are classified into the following groups:<ref name=cambridgeencyclopedia>O. Richard Norton. The Cambridge encyclopedia of meteorites. UK, Cambridge University Press, 2002. {{ISBN|0-521-62143-7}}.</ref> *Primitive achondrites *Asteroidal achondrites *Lunar meteorites *Martian meteorites

===Primitive achondrites=== Primitive achondrites, also called PAC group, are so-called 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:<ref name=cambridgeencyclopedia /> * Acapulcoites (after the meteorite Acapulco, Mexico) * Lodranites (after the meteorite Lodran) * Winonaites (after the meteorite Winona) * Ureilites (after the meteorite Novy Ureii, Russia) * Brachinites (after the meteorite Brachina)

===Asteroidal achondrites=== Asteroidal achondrites, also called evolved achondrites, are so-called because they have been ''differentiated'' on a parent body. This means that their mineralogical and chemical composition was changed by melting and crystallization processes. They are divided into several groups:<ref name=cambridgeencyclopedia /> * HED meteorites (Vesta). They may have originated on the asteroid 4 Vesta, because their reflection spectra are very similar.<ref>{{cite journal |first=M. J. |last=Drake |title=The eucrite/Vesta story |journal=Meteoritics and Planetary Science |volume=36 |issue=4 |date=2001 |pages=501–513 |doi=10.1111/j.1945-5100.2001.tb01892.x |bibcode=2001M&PS...36..501D|s2cid=128394153 |doi-access= }}</ref> They are named after the initial letters of the three subgroups: ** Howardites ** Eucrites ** Diogenites * Angrites * Aubrites

===Lunar meteorites=== Lunar meteorites are meteorites that originated from the Moon.

===Martian meteorite=== Martian meteorites<ref>{{cite journal |first=A. H. |last=Treiman |title=The SNC meteorites are from Mars |journal=Planetary and Space Science |volume=48 |issue=12–14 |date=2000 |pages=1213–1230 |doi=10.1016/S0032-0633(00)00105-7 |bibcode=2000P&SS...48.1213T}}</ref> are meteorites that originated from Mars. They are divided into three main groups, with two exceptions (see last two entries): * Shergottites * Nakhlites * Chassignites * OPX martian meteorites (ALH 84001) * Regolith/Soil samples (NWA 7034 and pairings)

==See also== * Glossary of meteoritics

==References== {{Reflist|30em}}

==External links== {{Commons category}} *[http://www.meteorites.com.au/collection/achondrites.html Achondrite Images] from Meteorites Australia

Category:Planetary science Category:Meteorite types Category:Achondrite meteorites