Editing Chondrite (section)

== Chondrite classification  ==

Chondrites are divided into about 15 distinct groups ''(see [[Meteorites classification]])'' on the basis of their mineralogy,<ref name="Van Schmus">{{cite journal |last1=Van Schmus |first1=W. R. |last2=Wood |first2=J. A. |year=1967 |title=A chemical-petrologic classification for the chondritic meteorites |journal=Geochimica et Cosmochimica Acta |volume=31 |pages=747–765 |doi=10.1016/S0016-7037(67)80030-9 |bibcode=1967GeCoA..31..747V |issue=5}}</ref> bulk chemical composition, and oxygen isotope compositions<ref>{{citation |title=Oxygen Isotope Classification of Carbonaceous Chondrites |volume=20 |pages=169 |year=1989 |journal=Abstracts of the Lunar and Planetary Science Conference |last1=Clayton |first1=R. N. |last2=Mayeda |first2=T. K. |bibcode=1989LPI....20..169C }}</ref> ''(see below)''. The various chondrite groups likely originated on separate asteroids or groups of related asteroids. Each chondrite group has a distinctive mixture of chondrules, refractory inclusions, matrix (dust), and other components and a characteristic grain size. Other ways of classifying chondrites include weathering<ref>{{citation |last=Wlotzka |first=F. |title=A Weathering Scale for the Ordinary Chondrites |bibcode=1993Metic..28Q.460W |date=Jul 1993 |journal=Meteoritics |volume=28 |issue=3 |pages=460}}</ref> and shock.<ref>{{cite journal |doi=10.1016/0016-7037(91)90078-J |date=Dec 1991 |volume=55 |pages=3845–3867 |first1=Dieter|last1= Stöffler |first2=Klaus |last2=Keil |first3=Scott |last3=Edward R.D |bibcode=1991GeCoA..55.3845S |journal=Geochimica et Cosmochimica Acta |issue=12 |title=Shock metamorphism of ordinary chondrites|doi-access=free }}</ref>

Chondrites can also be categorized according to their petrologic type, which is the degree to which they were thermally metamorphosed or aqueously altered (they are assigned a number between 1 and 7). The chondrules in a chondrite that is assigned a "3" have not been altered. Larger numbers indicate an increase in thermal metamorphosis up to a maximum of 7, where the chondrules have been destroyed. Numbers lower than 3 are given to chondrites whose chondrules have been changed by the presence of water, down to 1, where the chondrules have been obliterated by this alteration.

A synthesis of the various classification schemes is provided in the table below.<ref name="clas">{{cite web |url= http://www.meteoritemarket.com/type.htm |title=Types of Meteorites |access-date=2009-04-18 |website=The Meteorite Market |archive-date=9 March 2021 |archive-url=https://web.archive.org/web/20210309172347/http://www.meteoritemarket.com/type.htm |url-status=live}}</ref>

{| class="wikitable"
|-----
! style="background:#efefef;" | Type
! style="background:#efefef;" | Subtype
! style="background:#efefef;" | Distinguishing features/Chondrule character
! style="background:#efefef;" | Letter designation{{efn|The E stands for Enstatite, H indicates a high metallic iron content of approximately 30%, and L low. The number refers to alteration.}}
|-----
| rowspan="5" style="background:#FFFF00" | Enstatite chondrites 
| rowspan="5" | 
| style="background:#FFFFB3" | Abundant || E3, EH3, EL3
|-----
| style="background:#FFFFB3" | Distinct || E4, EH4, EL4
|-----
| style="background:#FFFFB3" | Less distinct || E5, EH5, EL5
|-----
| style="background:#FFFFB3" | Indistinct || E6, EH6, EL6
|-----
| style="background:#FFFFB3" | Melted || E7, EH7, EL7
|-----
| rowspan="15" style="background:#FFFF00" | [[Ordinary chondrite]]s
| rowspan="5"  style="background:#FFDDB3" | H
| style="background:#FFFFB3" | Abundant || H3–H3,9
|-----
| style="background:#FFFFB3" | Distinct || H4
|-----
| style="background:#FFFFB3" | Less distinct || H5
|-----
| style="background:#FFFFB3" | Indistinct || H6
|-----
| style="background:#FFFFB3" | Melted || H7
|-----
| rowspan="5" style="background:#FFDDB3" | L
| style="background:#FFFFB3" | Abundant || L3–L3,9
|-----
| style="background:#FFFFB3" | Distinct || L4
|-----
| style="background:#FFFFB3" | Less distinct || L5
|-----
| style="background:#FFFFB3" | Indistinct || L6
|-----
| style="background:#FFFFB3" | Melted || L7
|-----
| rowspan="5" style="background:#FFDDB3" | LL
| style="background:#FFFFB3" | Abundant || LL3–LL3,9
|-----
| style="background:#FFFFB3" | Distinct || LL4
|-----
| style="background:#FFFFB3" | Less distinct || LL5
|-----
| style="background:#FFFFB3" | Indistinct || LL6
|-----
| style="background:#FFFFB3" | Melted || LL7
|-----
| rowspan="10" style="background:#FFFF00" | [[Carbonaceous chondrites]]
| style="background:#FFDDB3" |'''I'''vuna || [[Phyllosilicate]]s, [[Magnetite]] || CI
|-----
| style="background:#FFDDB3" | '''M'''ighei || Phyllosilicates, [[Olivine]] || CM1–CM2
|-----
| style="background:#FFDDB3" | [[Vigarano Mainarda|'''V'''igarano]]|| Olivines rich in Fe, [[Calcium|Ca]] minerals and [[Aluminium|Al]] || CV2–CV3.3
|-----
| style="background:#FFDDB3" | [[Renazzo di Cento|'''R'''enazzo]]|| Phyllosilicates, Olivine, [[Pyroxene]], metals || CR
|-----
| style="background:#FFDDB3" | '''O'''rnans || Olivine, Pyroxene, metals, Ca minerals and Al || CO3–CO3.7
|-----
| style="background:#FFDDB3" | '''K'''aroonda || Olivine, Ca minerals and Al || CK
|-----
| style="background:#FFDDB3" | '''B'''encubbin || Pyroxene, metals || CB
|-----
| style="background:#FFDDB3" | '''L'''oongana || Chondrules and CAIs, metals || CL
|-----
| style="background:#FFDDB3" | '''H'''igh Iron{{efn|Except for the High Iron, all the other carbonaceous chondrites are named after a characteristic meteorite.}} || Pyroxene, metals, Olivine || CH
|-----
| style="background:#FFDDB3" | [[Tagish_Lake_(meteorite)|Tagish Lake]]{{efn|This is a unique meteorite that has been suggested to be the only known sample of the D asteroid family.}} || Phyllosilicates, Magnetite, Ca-Mg-Fe carbonates || TAG
|-----
| style="background:#FFFF00" | [[Kakangari]]-type || &nbsp; || &nbsp; || K
|-----
| style="background:#FFFF00" | [[Rumurutiites]] || &nbsp; ||Olivine, Pyroxenes, [[Plagioclase]], [[Sulfide]]s || R
|}

=== Enstatite chondrites ===

[[Image:Météorite de Saint Sauveur MHNT2.jpg|thumb|225px|The ''Saint Sauveur'' [[enstatite chondrite]] (EH5)]]

{{main|Enstatite chondrite}}

Enstatite chondrites (also known as E-type chondrites) are a rare form of meteorite thought to comprise only about 2% of the chondrites that fall to Earth.<ref name=Norton>Norton, O.R. and Chitwood, L.A. Field Guide to Meteors and Meteorites, Springer-Verlag, London 2008</ref> Only about 200 E-Type chondrites are currently known.<ref name=Norton/> The majority of enstatite chondrites have either been recovered in [[Antarctica]] or have been collected by the American [[National Weather Association]]. They tend to be high in the mineral [[enstatite]] (MgSiO<sub>3</sub>), from which they derive their name.<ref name=Norton/>

E-type chondrites are among the most chemically [[redox|reduced]] rocks known, with most of their iron taking the form of metal or sulfide rather than as an oxide. This suggests that they were formed in an area that lacked [[oxygen]], probably within the [[orbit]] of [[Mercury (planet)|Mercury]].<ref name="new">{{cite web|url = http://www.meteorlab.com/METEORLAB2001dev/Open1.htm|title = Meteorlab|access-date = 22 April 2009|author = New England Meteoritical Services|archive-date = 21 February 2009|archive-url = https://web.archive.org/web/20090221114126/http://meteorlab.com/METEORLAB2001dev/Open1.htm|url-status = dead}}</ref>

=== Ordinary chondrites ===
{{Main|Ordinary chondrite}}
{{multiple image
| direction         = horizontal
| align             = center
| width             = 200
| image1            = NWA 778 El Mahbes meteorite - 200705.jpg
| image2            = Météorite de Phnom Penh MHNT.MIN.2011.0.1.jpg
| image3            = El_Menia_Meteorite.jpg
| caption1          = [[Ordinary chondrite]] LL6
| caption2          = [[Phnom Penh]] Chondrite L6 – 1868
| caption3          = El Menia [[Ordinary chondrite]] L5 - 2023
| total_width       = 800
}}

[[Ordinary chondrite]]s are by far the most common type of meteorite to fall to Earth: about 80% of all meteorites and over 90% of chondrites are ordinary chondrites.<ref name="lunar"/> They contain abundant chondrules, sparse matrix (10–15% of the rock), few refractory inclusions, and variable amounts of Fe–Ni metal and [[troilite]] (FeS). Their chondrules are generally in the range of 0.5 to 1&nbsp;mm in diameter. Ordinary chondrites are distinguished chemically by their depletions in [[refractory]] [[Goldschmidt classification|lithophile]] elements, such as Ca, Al, Ti, and [[rare earth elements|rare earths]], relative to Si, and isotopically by their unusually high <sup>17</sup>O/<sup>16</sup>O ratios relative to <sup>18</sup>O/<sup>16</sup>O compared to Earth rocks.

Most, but not all, ordinary chondrites have experienced significant degrees of metamorphism, having reached temperatures well above 500&nbsp;°C on the parent asteroids. They are divided into three groups, which have different amounts of metal and different amounts of total iron: 
*[[H chondrite]] have high total iron and high metallic Fe (15–20% Fe–Ni metal by mass<ref>{{cite web |last=Korotev |first=Randy |title=metal, iron, & nickel in meteorites 1 |url=http://meteorites.wustl.edu/id/metal.htm |url-status=dead |archive-url=https://web.archive.org/web/20190702084637/http://meteorites.wustl.edu/id/metal.htm |archive-date=2 July 2019 |access-date=1 July 2010 |website=meteorites.wustl.edu |publisher=[[Washington University in St. Louis]]}}</ref>), and smaller chondrules than L and LL chondrites. They are formed of bronzite, olivine, pyroxene, plagioclase, metals and sulfides and ~42% of ordinary chondrite falls belong to this group ''(see [[Meteorite fall statistics]])''.
*[[L chondrite]]s have low total iron contents (including 7–11% Fe–Ni metal by mass). ~46% of ordinary chondrite falls belong to this group, which makes them the most common type of meteorite to fall on Earth.
*[[LL chondrite]]s have low total iron and low metal contents (3–5% Fe–Ni metal by mass of which 2% is metallic Fe and they also contain bronzite, [[oligoclase]] and olivine).<ref name="clas"/> Only 1 in 10 ordinary chondrite falls belong to this group.

An example of this group is the [[NWA 869]] meteorite.

=== Carbonaceous chondrites ===
{{Main|Carbonaceous chondrite}}
{{multiple image
 | image1     =AllendeMeteorite.jpg
 | image2     =Privately owned meteorite - NWA 13887.jpg
 |caption1    =  [[Carbonaceous chondrite]] CV3 that fell in Mexico in 1969
 | caption2   = NWA 13887 [[Carbonaceous chondrite]] CO3
}}

[[Carbonaceous chondrite]]s (also known as C-type chondrites) make up less than 5% of the chondrites that fall on Earth.<ref name="enciclopedia">{{cite web | url = http://www.daviddarling.info/encyclopedia/C/carbchon.html | title = carbonaceous chondrite | access-date = 26 April 2009 | author = The Internet Encyclopedia of Science | archive-date = 8 February 2006 | archive-url = https://web.archive.org/web/20060208230602/http://www.daviddarling.info/encyclopedia/C/carbchon.html | url-status = live }}</ref> They are characterized by the presence of [[carbon]] compounds, including [[amino acid]]s.<ref>{{cite journal| title = Extra-terrestrial amino acids identified in metal-rich CH and CB carbonaceous chondrites from Antarctica |author1=Aaron S. Burton |author2=Jamie E. Elsila |author3=Jason E. Hein |author4=Daniel P. Glavin |author5=Jason P. Dworkin |date=March 2013 | journal = Meteoritics & Planetary Science | volume = 48 | issue = 3 | pages = 390–402 | bibcode=2013M&PS...48..390B |doi=10.1111/maps.12063 |hdl=2060/20130014351 |s2cid=59928474 |hdl-access=free }}</ref> They are thought to have been formed the farthest from the sun of any of the chondrites as they have the highest proportion of volatile compounds.<ref name="meteoroide"/> Another of their main characteristics is the presence of water or of minerals that have been altered by the presence of water.

There are many groups of carbonaceous chondrites, but most of them are distinguished chemically by enrichments in refractory lithophile elements relative to Si and isotopically by unusually low ratios of <sup>17</sup>O/<sup>16</sup>O relative to <sup>18</sup>O/<sup>16</sup>O, when compared to Earth rocks. All groups of carbonaceous chondrites except the CH group are named for a characteristic type specimen:
*[[CI chondrite|CI]] (Ivuna type) chondrites entirely lack chondrules and refractory inclusions; they are composed almost exclusively of fine-grained material that has experienced a high degree of aqueous alteration on the parent asteroid. CI chondrites are highly [[Redox|oxidized]], brecciated rocks, containing abundant [[magnetite]] and [[sulfate mineral]]s, and lacking metallic Fe. It is a matter of some controversy whether they once had chondrules and refractory inclusions that were later destroyed during formation of hydrous minerals, or they never had chondrules in the first place{{Citation needed|date=July 2008}}. CI chondrites are notable because their chemical compositions closely resemble that of the solar photosphere, neglecting the hydrogen and helium. Thus, they have the most "primitive" compositions of any meteorites and are often used as a standard for assessing the degree of chemical fractionation experienced by materials formed throughout the Solar System.
*CO ([[Ornans (meteorite)|Ornans type]]) and CM (Mighei type) chondrites are two related groups that contain very small chondrules, mostly 0.1 to 0.3&nbsp;mm in diameter; refractory inclusions are quite abundant and have similar sizes to chondrules.

**CM chondrites are composed of about 70% fine-grained material (matrix), and most have experienced extensive aqueous alteration. The much studied [[Murchison meteorite]], which fell in Australia in 1969, is the best-known member of this group.
**CO chondrites have only about 30% matrix and have experienced very little aqueous alteration. Most have experienced small degrees of thermal metamorphism.
*CR ([[Renazzo di Cento|Renazzo]] type), CB (Bencubbin type), and CH (high metal) carbonaceous chondrites are three groups that seem to be related by their chemical and oxygen isotopic compositions. All are rich in metallic Fe–Ni, with CH and especially CB chondrites having a higher proportion of metal than all other chondrite groups. Although CR chondrites are clearly similar in most ways to other chondrite groups, the origins of CH and CB chondrites are somewhat controversial. Some workers conclude that many of the chondrules and metal grains in these chondrites may have formed by impact processes after "normal" chondrules had already formed, and thus they may not be "true" chondrites.


**CR chondrites have chondrules that are similar in size to those in ordinary chondrites (near 1&nbsp;mm), few refractory inclusions, and matrix comprises nearly half the rock. Many CR chondrites have experienced extensive aqueous alteration, but some have mostly escaped this process.
**CH chondrites are remarkable for their very tiny chondrules, typically only about 0.02&nbsp;mm (20 micrometres) in diameter. They have a small proportion of equally tiny refractory inclusions. Dusty material occurs as discrete clasts, rather than as a true matrix. CH chondrites are also distinguished by extreme depletions in [[Volatility (chemistry)|volatile]] elements.
**CB chondrites occur in two types, both of which are similar to CH chondrites in that they are very depleted in volatile elements and rich in metal. CB<sub>a</sub> (subgroup a) chondrites are coarse grained, with large, often cm-sized chondrules and metal grains and almost no refractory inclusions. Chondrules have unusual textures compared to most other chondrites. As in CH chondrites, dusty material only occurs in discrete clasts, and there is no fine-grained matrix. CB<sub>b</sub> (subgroup b) chondrites contain much smaller (mm-sized) chondrules and do contain refractory inclusions.
*CV ([[Vigarano Mainarda|Vigarano]] type) chondrites are characterized by mm-sized chondrules and abundant refractory inclusions set in a dark matrix that comprises about half the rock. CV chondrites are noted for spectacular refractory inclusions, some of which reach centimetre sizes, and they are the only group to contain a distinctive type of large, once-molten inclusions. Chemically, CV chondrites have the highest abundances of refractory lithophile elements of any chondrite group. The CV group includes the remarkable [[Allende Meteorite|Allende]] fall in Mexico in 1969, which became one of the most widely distributed and, certainly, the best-studied meteorite in history.
*CK ([[Karoonda meteorite|Karoonda]] type) chondrites are chemically and texturally similar to CV chondrites. However, they contain far fewer refractory inclusions than CV, they are much more oxidized rocks, and most of them have experienced considerable amounts of thermal metamorphism (compared to CV and all other groups of carbonaceous chondrites).
*CL (Loongana type) chondrites are largely chondrules and CAIs, correspondingly low in matrix and volatiles, with trace elements resembling CR. [[Isotopes of oxygen|Triple oxygen position]] near the CV-CK area.
*Ungrouped carbonaceous chondrites: A number of chondrites are clearly members of the carbonaceous chondrite class, but do not fit into any of the groups. These include: the [[Tagish Lake (meteorite)|Tagish Lake]] meteorite, which fell in Canada in 2000 and is intermediate between CI and CM chondrites; and Acfer 094, an extremely primitive chondrite that shares properties with both CM and CO groups.

=== Kakangari chondrites ===

Three chondrites form what is known as the K (Kakangari type) grouplet: Kakangari, LEW 87232, and Lea Co. 002.<ref name="nature">{{cite journal|author1=Andrew M. Davis |author2=Lawrence Grossman |author3=R. Ganapathy | title = Yes, Kakangari is a unique chondrite| year = 1977| journal = Nature| volume = 265| id = 0028-0836, 230–232| issue=5591| doi=10.1038/265230a0| pages=230–232
|bibcode = 1977Natur.265..230D |s2cid=4295051 }}</ref> They are characterized by large amounts of dusty matrix and oxygen isotope compositions similar to carbonaceous chondrites, highly reduced mineral compositions and high metal abundances (6% to 10% by volume) that are most like enstatite chondrites, and concentrations of [[refractory]] lithophile elements that are most like ordinary chondrites.

Many of their other characteristics are similar to the O, E and C chondrites.<ref name="acta">{{cite journal|author1=Michael K. Weisberga |author2=Martin Prinza |author3=Robert N. Claytonb |author4=Toshiko K. Mayedab |author5=Monica M. Gradyc |author6=Ian Franchid |author7=Colin T. Pillingerd |author8=Gregory W. Kallemeyne | title = The K (Kakangari) chondrite grouplet | year = 1996| journal = [[Geochimica et Cosmochimica Acta]]| volume = 60| number = 21| id = 0016-7037, 4253–4263| doi=10.1016/S0016-7037(96)00233-5| bibcode=1996GeCoA..60.4253W| pages=4253–4263}}</ref>

=== Rumuruti chondrites ===

Rumuruti (R) type chondrites are a very rare group, with only one documented fall out of almost 900 documented chondrite falls. They have a number of properties in common with ordinary chondrites, including similar types of chondrules, few refractory inclusions, similar chemical composition for most elements, and the fact that <sup>17</sup>O/<sup>16</sup>O ratios are anomalously high compared to Earth rocks. However, there are significant differences between R chondrites and ordinary chondrites: R chondrites have much more dusty matrix material (about 50% of the rock); they are much more oxidized, containing little metallic Fe–Ni; and their enrichments in <sup>17</sup>O are higher than those of ordinary chondrites. Nearly all the metal they contain is oxidized or in the form of sulfides. They contain fewer chondrules than the E chondrites and appear to come from an asteroid's [[regolith]].<ref name="rumu">{{cite web
 |url         = http://www.meteorites.tv/index.html?lang=en-us&target=d140.html
 |title       = R Group (Rumurutiites)
 |access-date  = 28 April 2009
 |website     = Meteorites.tv. Meteorites for Science, Education & Collectors
 |url-status     = dead
 |archive-url  = https://archive.today/20130418201850/http://www.meteorites.tv/index.html?lang=en-us&target=d140.html
 |archive-date = 18 April 2013
}}</ref>