CI chondrite

File:Météorite Orgueil, exposition Météorites, Muséum national d'histoire naturelle de Paris.jpg

The abbreviation CI is derived from the C for carbonaceous and in the name scheme of Wasson,{{cite book |last1=Wasson |first1=J. T. |title=Meteorites-classification and properties |date=1974 |publisher=Springer |location=Berlin |isbn=978-3-642-65865-5}} the I from Ivuna, the type locality in Tanzania. The 1 in C1 stands for the type 1 meteorites in the older classification scheme of Van Schmus-Wood,{{cite journal |last1=Van Schmus |first1=W. R. |last2=Wood |first2=J.A. |title=A chemical-petrologic classification for the chondritic meteorites |journal=Geochim. Cosmochim. Acta |date=1967 |volume=31 |issue=5 |pages=74765 |doi=10.1016/S0016-7037(67)80030-9|bibcode=1967GeCoA..31..747V }} still used for petrography. Petrographic type-1 meteorites, by definition, have no fully-visible chondrules.

CI chondrites contain significant amounts of carbon, ranging from approximately 3-5 wt%, primarily in organic form.{{Cite journal |last1=Pearson |first1=V. K. |last2=Sephton |first2=M. A. |last3=Franchi |first3=I. A. |last4=Gibson |first4=J. M. |last5=Gilmour |first5=I. |date=26 January 2010 |title=Carbon and nitrogen in carbonaceous chondrites: Elemental abundances and stable isotopic compositions |url=https://doi.org/10.1111/j.1945-5100.2006.tb00459.x |journal=Meteoritics & Planetary Science |volume=41 |issue=12 |pages=1899–1918 |doi=10.1111/j.1945-5100.2006.tb00459.x |issn=1086-9379}} Analysis of the Ivuna meteorite revealed a total carbon concentration of 3.31 wt%, with about 90% being organic carbon.{{Cite journal |last1=Yokoyama |first1=Tetsuya |last2=Nagashima |first2=Kazuhide |last3=Nakai |first3=Izumi |last4=Young |first4=Edward D. |last5=Abe |first5=Yoshinari |last6=Aléon |first6=Jérôme |last7=Alexander |first7=Conel M. O’D. |last8=Amari |first8=Sachiko |last9=Amelin |first9=Yuri |last10=Bajo |first10=Ken-ichi |last11=Bizzarro |first11=Martin |last12=Bouvier |first12=Audrey |last13=Carlson |first13=Richard W. |last14=Chaussidon |first14=Marc |last15=Choi |first15=Byeon-Gak |date=2023-02-24 |title=Samples returned from the asteroid Ryugu are similar to Ivuna-type carbonaceous meteorites |url=https://www.science.org/doi/10.1126/science.abn7850 |journal=Science |language=en |volume=379 |issue=6634 |pages=eabn7850 |doi=10.1126/science.abn7850 |pmid=35679354 |bibcode=2023Sci...379.7850Y |issn=0036-8075|hdl=2115/90313 |hdl-access=free }} While this represents the highest carbon content among carbonaceous chondrites, it is surpassed by some Ureilites, which can contain even greater carbon concentrations.{{Citation |last1=Glavin |first1=Daniel P. |title=The Origin and Evolution of Organic Matter in Carbonaceous Chondrites and Links to Their Parent Bodies |date=2018 |work=Primitive Meteorites and Asteroids |pages=205–271 |url=https://doi.org/10.1016/b978-0-12-813325-5.00003-3 |access-date=2025-04-06 |publisher=Elsevier |isbn=978-0-12-813325-5 |last2=Alexander |first2=Conel M.O'D. |last3=Aponte |first3=José C. |last4=Dworkin |first4=Jason P. |last5=Elsila |first5=Jamie E. |last6=Yabuta |first6=Hikaru|doi=10.1016/b978-0-12-813325-5.00003-3 |hdl=2060/20180004493 |hdl-access=free }}

Oxygen is the most abundant element in CI chondrites (46 wt%.),{{Cite book |last=Mason |first=B |title=Handbook of elemental abundances in meteorites |date=1 January 1971 |publisher=Gordon and Breach, Science Publishers, Inc., New York |bibcode=1971heam.book.....M |osti=4366427 }} with a distinctive isotopic composition that serves as a crucial identifier. CI chondrites contain three stable oxygen isotopes (¹⁶O, ¹⁷O, and ¹⁸O) that, when plotted on a three-isotope diagram, occupy a specific field clearly distinguishable from other meteorite groups.{{Cite journal |last1=Doh |first1=Seong-Jae |last2=Yu |first2=Yong-Jae |date=2010-12-31 |title=Meteorites: Rocks from the Outer Space |url=https://doi.org/10.5303/jkas.2010.43.6.183 |journal=Journal of the Korean Astronomical Society |volume=43 |issue=6 |pages=183–190 |doi=10.5303/jkas.2010.43.6.183 |bibcode=2010JKAS...43..183D |issn=1225-4614}}{{Cite journal |last1=Rowe |first1=Marvin W. |last2=Clayton |first2=Robert N. |last3=Mayeda |first3=Toshiko K. |date=22 August 1994 |title=Oxygen isotopes in separated components of CI and CM meteorites |url=https://linkinghub.elsevier.com/retrieve/pii/0016703794903174 |journal=Geochimica et Cosmochimica Acta |language=en |volume=58 |issue=23 |pages=5341–5347 |doi=10.1016/0016-7037(94)90317-4|bibcode=1994GeCoA..58.5341R }} They show significant enrichment in ¹⁸O and moderate enrichment in ¹⁷O compared to petrologically similar CM chondrites, with no overlap between these groups. Antarctic CI-like meteorites exhibit even greater ¹⁸O enrichment, representing the macroscopic samples with the heaviest oxygen isotopic composition in the Solar System—a signature that provides essential insights into their unique formation conditions.

Iron is present with 18-20 wt%.{{Cite journal |last1=Palme |first1=H. |last2=Zipfel |first2=J. |date=30 July 2021 |title=The composition of CI chondrites and their contents of chlorine and bromine: Results from instrumental neutron activation analysis |url=https://onlinelibrary.wiley.com/doi/10.1111/maps.13720 |journal=Meteoritics & Planetary Science |language=en |volume=57 |issue=2 |pages=317–333 |doi=10.1111/maps.13720 |issn=1086-9379}} This is a marginally higher level than CM chondrites, as iron is somewhat cooler-forming than magnesium. The siderophiles nickel and cobalt follow iron as well.{{Cite journal |last1=Kallemeyn |first1=Gregory W. |last2=Wasson |first2=John T. |date=12 March 1981 |title=The compositional classification of chondrites—I. The carbonaceous chondrite groups |url=https://linkinghub.elsevier.com/retrieve/pii/0016703781901459 |journal=Geochimica et Cosmochimica Acta |language=en |volume=45 |issue=7 |pages=1217–1230 |doi=10.1016/0016-7037(81)90145-9|bibcode=1981GeCoA..45.1217K }} The majority of the iron is in the form of cations in the phyllosilicates and iron bound as magnetite. Some appears as ferrihydrite,{{Cite journal |last1=Tomeoka |first1=Kazushige |last2=Buseck |first2=Peter R. |date=11 March 1988 |title=Matrix mineralogy of the Orgueil CI carbonaceous chondrite |url=https://linkinghub.elsevier.com/retrieve/pii/0016703788902311 |journal=Geochimica et Cosmochimica Acta |language=en |volume=52 |issue=6 |pages=1627–1640 |doi=10.1016/0016-7037(88)90231-1|bibcode=1988GeCoA..52.1627T }} but not in Ivuna.{{cite conference |last1=Brearley |first1=A. J. |date=1992 |title=Mineralogy of fine grained matrix in the Ivuna CI carbonaceous chondrite |conference=LPS XXIII |page=153}}

CI chondrites are primarily composed of fine-grained phyllosilicates (>90% by volume) with a dark and fine-grained clay-like matrix rich in carbonaceous material.{{Cite journal |last1=Larimer |first1=J.W |last2=Anders |first2=Edward |date=March 1970 |title=Chemical fractionations in meteorites—III. Major element fractionations in chondrites |url=https://linkinghub.elsevier.com/retrieve/pii/0016703770901122 |journal=Geochimica et Cosmochimica Acta |language=en |volume=34 |issue=3 |pages=367–387 |doi=10.1016/0016-7037(70)90112-2|bibcode=1970GeCoA..34..367L }}{{Cite journal |last1=Brearley |first1=Adrian J. |last2=Prinz |first2=martin |date=March 1992 |title=CI chondrite-like clasts in the Nilpena polymict ureilite: Implications for aqueous alteration processes in CI chondrites |url=https://linkinghub.elsevier.com/retrieve/pii/001670379290068T |journal=Geochimica et Cosmochimica Acta |language=en |volume=56 |issue=3 |pages=1373–1386 |doi=10.1016/0016-7037(92)90068-T|bibcode=1992GeCoA..56.1373B }} Their matrix contains magnetite (~10%), iron sulfides like pyrrhotite (~7%), carbonates (~5%), and ferrihydrite (~5%), with smaller amounts of pentlandite and other minerals. The dominant components are serpentine-saponite intergrowths (~65% by weight).{{Citation |last1=Scott |first1=E. R. D. |title=1.07 - Chondrites and Their Components |date=2007-01-01 |work=Treatise on Geochemistry |pages=1–72 |editor-last=Holland |editor-first=Heinrich D. |url=https://linkinghub.elsevier.com/retrieve/pii/B0080437516011452 |access-date=2025-04-07 |place=Oxford |publisher=Pergamon |doi=10.1016/b0-08-043751-6/01145-2 |isbn=978-0-08-043751-4 |last2=Krot |first2=A. N. |editor2-last=Turekian |editor2-first=Karl K.}}{{cite journal |last1=Zolensky |first1=M. |last2=Barrett |first2=R. |last3=Browning |first3=L. |date=1993 |title=Mineralogy and composition of matrix and chondrule rims in carbonaceous chondrites |journal=Geochim. |volume=57 |issue=13 |pages=3123−48 |bibcode=1993GeCoA..57.3123Z |doi=10.1016/0016-7037(93)90298-B}}{{cite journal |last1=Bass |first1=M. N. |date=1971 |title=Montmorillonite and serpentine in Orgueil meteorite |journal=Geochim. Cosmochim. Acta |volume=35 |issue=2 |pages=139−47 |bibcode=1971GeCoA..35..139B |doi=10.1016/0016-7037(71)90053-6}} Framboidal magnetite occurs within the matrix and may have formed through precipitation from a gel-like phase.{{Cite journal |last1=Kerridge |first1=John F. |last2=Mackay |first2=Alan L. |last3=Boynton |first3=William V. |date=1979-07-27 |title=Magnetite in CI Carbonaceous Meteorites: Origin by Aqueous Activity on a Planetesimal Surface |url=https://www.science.org/doi/10.1126/science.205.4404.395 |journal=Science |language=en |volume=205 |issue=4404 |pages=395–397 |doi=10.1126/science.205.4404.395 |pmid=17790849 |bibcode=1979Sci...205..395K |issn=0036-8075}} While most phyllosilicates in the CI chondrites are fine-grained and poorly crystalline, in Alais and Ivuna well-crystallized phyllosilicates often occur as coarse (10s μm in size) fragments and clusters that are not commonly found in Orgueil.

Magnetite is the second most abundant mineral in CI chondrites.{{cite journal |last1=Rahmdor |first1=P. |date=1963 |title=The Opaque Minerals in Stony Meteorites |journal=J. Geophys. Res. |volume=68 |issue=7 |page=2011 |bibcode=1963JGR....68.2011R |doi=10.1029/JZ068i007p02011 |s2cid=129294262 |quote="very common" "characteristic"}}{{cite journal |last1=Alfing |first1=J. |last2=Patzek |first2=M. |last3=Bischoff |first3=A. |date=2019 |title=Modal Abundances of coarse-grained (>5um) components within CI-chondrites and their individual clasts − Mixing of various lithologies on the CI parent body(ies) |journal=Geochemistry |volume=79 |issue=4 |page=125532 |bibcode=2019ChEG...79l5532A |doi=10.1016/j.chemer.2019.08.004 |s2cid=202041205 |doi-access=free}} It occurs in various morphologies,{{cite journal |last1=Jedwab |first1=J. |date=1967 |title=La Magnetite en Plaquettes des Meteorites carbonees D'Alais, Ivuna et Orgueil |journal=Earth Planet. Sci. Lett. |volume=2 |issue=5 |pages=440–444 |bibcode=1967E&PSL...2..440J |doi=10.1016/0012-821X(67)90186-0}}{{cite journal |last1=Jedwab |first1=J. |date=1971 |title=La Magnetite de la Meteorite D'Orgueil Vue au Microscope Electronique a Balayage |journal=Icarus |volume=15 |issue=2 |pages=319–45 |bibcode=1971Icar...15..319J |doi=10.1016/0019-1035(71)90083-2}}{{cite journal |last1=Kerridge |first1=J. F. |last2=Chatterji S. |date=1968 |title=Magnetite Content of a Type I Carbonaceous Meteorite |journal=Nature |volume=220 |issue=5169 |pages=775–76 |bibcode=1968Natur.220R.775K |doi=10.1038/220775b0 |s2cid=4192603}} including crystals, spheres,{{cite journal |last1=Kerridge J. F. |date=1970 |title=Some observations on the nature of magnetite in the Orgueil meteorite |journal=Earth Planet. Sci. Lett. |volume=9 |issue=4 |pages=229–306 |bibcode=1970E&PSL...9..299K |doi=10.1016/0012-821X(70)90122-6}} framboids (raspberry-like clusters),{{cite journal |last1=Hua |first1=X. |last2=Buseck P. R. |date=1998 |title=Unusual forms of magnetite in the Orgueil carbonaceous chondrite |journal=Meteorit. Planet. Sci. |volume=33 |pages=A215-20 |doi=10.1111/j.1945-5100.1998.tb01335.x |s2cid=126546072 |doi-access=free}} and plaquettes (stacked or beehive-like structures), which are distinctive to CIs. The mineral formed through the oxidation of sulfides, primarily pyrrhotite and its nickel-rich variants,{{cite journal |last1=Larson E .E. |last2=Watson D. E. |last3=Herndon J. M. |last4=Rowe M. W. |date=1974 |title=Thermomagnetic analysis of meteorites, 1. C1 chondrites |journal=Earth and Planetary Science Letters |volume=21 |issue=4 |pages=345–50 |bibcode=1974E&PSL..21..345L |doi=10.1016/0012-821X(74)90172-1 |s2cid=33501632 |quote="presumably FeS" |hdl-access=free |hdl=2060/19740018171}}{{cite journal |last1=Watson D. E. |last2=Larson E. E. |last3=Herndon J. M. |last4=Rowe M. W. |date=1975 |title=Thermomag anal of meteorites, 2. C2 chondrites |journal=Earth Planet. Sci. Lett. |volume=27 |pages=101–07 |doi=10.1016/0012-821X(75)90167-3 |hdl-access=free |hdl=2060/19740018171}} likely occurring in multiple generations.{{cite journal |last1=Hyman M. |last2=Rowe M. W. |date=1983 |title=Magnetite in CI chondrites |journal=J. Geophys. Res. |volume=88 |pages=A736-40 |bibcode=1983LPSC...13..736H |doi=10.1029/JB088iS02p0A736}}{{cite journal |last1=Gounelle |first1=M. |last2=Zolensky M. E. |date=2014 |title=The Orgueil meteorite: 150 years of history |journal=Meteoritics & Planetary Sci |volume=49 |issue=10 |pages=1769−94 |bibcode=2014M&PS...49.1769G |doi=10.1111/maps.12351 |s2cid=128753934}} Other minerals found include iron sulfides like pyrrhotite, pentlandite, troilite and cubanite.Mason, B.: Meteorites. John Wiley and Son Inc., New York 1962. The matrix also hosts isolated ferromagnesian silicates, such as olivine (forsterite with fayalite Fa10–20), clinopyroxene, and orthopyroxene, which crystallized at high temperatures and remain unaltered.Dodd, R. T.: Meteorites: A Petrologic-Chemical Synthesis. Cambridge University Press, New York 1981 Water-bearing, clay-rich phyllosilicates, including montmorillonite and serpentine-like minerals, are among the main constituents.{{cite journal |last1=Beck |first1=P. |last2=Quirico |first2=E. |last3=Montes-Hernandez |first3=G. |last4=Bonal |first4=L. |last5=Bollard |first5=J. |last6=Orthous-Daunay F.-R. |last7=Howard K. T. |last8=Schmitt B. |last9=Brissaud O. |last10=Deschamps F. |last11=Wunder B. |last12=Guillot S. |date=2010 |title=Hydrous mineralogy of CM and CI chondrites from infrared spectroscopy and their relationship with low albedo asteroids |journal=Geochim. Cosmochim. Acta |volume=74 |issue=16 |pages=4881−92 |bibcode=2010GeCoA..74.4881B |doi=10.1016/j.gca.2010.05.020}} Additionally, alteration minerals such as epsomite (found in microscopic veins), vaterite, carbonates, and sulfates are present.{{Cite journal |last=Richardson |first=Steven M. |date=March 1978 |title=Vein Formation in the C1 Carbonaceous Chondrites |url=https://onlinelibrary.wiley.com/doi/10.1111/j.1945-5100.1978.tb00803.x |journal=Meteoritics |language=en |volume=13 |issue=1 |pages=141–159 |doi=10.1111/j.1945-5100.1978.tb00803.x |bibcode=1978Metic..13..141R |issn=0026-1114}}

Furthermore, these meteorites lack intact chondrules, calcium-aluminum-rich inclusions (CAIs), and amoeboid olivine aggregates (AOAs) due to extensive aqueous alteration.{{Citation |last=King |first=Ashley J. |title=An introduction to meteorites |date=2024 |work=Reference Module in Materials Science and Materials Engineering |url=https://linkinghub.elsevier.com/retrieve/pii/B9780443214394000390 |access-date=2025-04-07 |publisher=Elsevier |language=en |doi=10.1016/b978-0-443-21439-4.00039-0 |isbn=978-0-12-803581-8}}

CI chondrites contain between 18-20 wt% water{{Cite journal |last=Wiik |first=H.B. |date=June 1956 |title=The chemical composition of some stony meteorites |url=https://linkinghub.elsevier.com/retrieve/pii/001670375690028X |journal=Geochimica et Cosmochimica Acta |language=en |volume=9 |issue=5–6 |pages=279–289 |doi=10.1016/0016-7037(56)90028-X|bibcode=1956GeCoA...9..279W }}{{cite journal |last1=King |first1=A.J. |last2=Solomon J.R. |last3=Schofield P.F. |date=2015 |title=Characterising the CI and CI-like carbonaceous chondrites using thermogravimetric analysis and infrared spectroscopy |journal=Earth, Planets and Space |volume=67 |page=198 |bibcode=2015EP&S...67..198K |doi=10.1186/s40623-015-0370-4 |s2cid=2148318 |doi-access=free |hdl-access=free |hdl=10141/622224}} (a greater level than Comet 67P/Churyumov-Gerasimenko{{cite journal |last1=Fulle |first1=M. N. |last2=Altobelli B. |last3=Buratti B. |last4=Choukroun M. |last5=Fulchignoni M. |last6=Grün E. |last7=Taylor M. G. G. T. |last8=Weissman P. |title=Unexpected and significant findings in comet 67P/Churyumov–Gerasimenko: an interdisciplinary view |journal=Mon. Not. R. Astron. Soc. |date=2016 |volume=462 |issue=Suppl_1 |pages=S2−8 |doi=10.1093/mnras/stw1663|doi-access=free }}{{cite journal |last1=Fulle |first1=M. |last2=Della Corte V. |last3=Rotundi A. |last4=Green S. F. |last5=Accolla M. |last6=Colangeli L. |last7=Ferrari M. |last8=Ivanovski S. |last9=Sordini R. |last10=Zakharov V. |title=The dust-to-ices ratio in comets and Kuiper belt objects |journal=Mon. Not. R. Astron. Soc. |date=2017 |volume=469 |issue=Suppl_2 |pages=S45−49 |doi=10.1093/mnras/stx983|doi-access=free }}{{cite book |last1=Rickman |first1=H. |title=Origin and Evolution of Comets: Ten Years after the Nice Model and One Year after Rosetta |date=2017 |publisher=World Scientific |isbn=978-981-3222-57-1}}{{cite journal |last1=Fulle |first1=M. |last2=Blum J. |last3=Green S. F. |last4=Gundlach B. |last5=Herique A. |last6=Moreno F. |last7=Mottola S. |last8=Rotundi A. |last9=Snodgrass C. |title=The refractory-to-ice mass ratio in comets |journal=Mon. Not. R. Astron. Soc. |date=2019 |volume=482 |issue=3 |pages=3326–40|doi=10.1093/mnras/sty2926 |doi-access=free |hdl=10261/189497 |hdl-access=free }}) with porosities reaching up to ~25-30%,{{Cite journal |last1=Consolmagno S.J. |first1=G. J. |last2=Britt |first2=D. T. |date=November 1998 |title=The density and porosity of meteorites from the Vatican collection |url=https://onlinelibrary.wiley.com/doi/10.1111/j.1945-5100.1998.tb01308.x |journal=Meteoritics & Planetary Science |language=en |volume=33 |issue=6 |pages=1231–1241 |doi=10.1111/j.1945-5100.1998.tb01308.x |bibcode=1998M&PS...33.1231C |issn=1086-9379}} which appears correlated to their high water content. The water is primarily bound within water-bearing silicates and present in the form of hydroxyl (-OH) groups in phyllosilicates (e.g., montmorillonite and serpentine-like minerals). Analysis of the Ivuna meteorite revealed 12.73 wt% total water, divided between interlayer water (6.58 wt%) and structural OH/H₂O in phyllosilicates (6.15 wt%). Extensive aqueous alteration is evidenced by the presence of crosscutting veins filled with Na-, Ca-, and Mg-sulfates (epsomite, hexahydrite, gypsum, and blodite).{{Cite journal |last1=Bostrom |first1=Kurt |last2=Fredriksson |first2=Kurt |date=1966-07-27 |title=SURFACE CONDITIONS OF THE ORGUEIL METEORITE PARENT BODY AS INDICATED BY MINERAL ASSOCIATIONS |url=https://biostor.org/reference/285944 |journal=Smithsonian Miscellaneous Collections |volume=151 |issue=3 |pages=1–36}}{{Cite journal |last1=Fredriksson |first1=Kurt |last2=Kerridge |first2=John F. |date=March 1988 |title=Carbonates and Sulfates in CI Chondrites: Formation by Aqueous Activity on the Parent Body |url=https://onlinelibrary.wiley.com/doi/10.1111/j.1945-5100.1988.tb00894.x |journal=Meteoritics |language=en |volume=23 |issue=1 |pages=35–44 |doi=10.1111/j.1945-5100.1988.tb00894.x |pmid=11538410 |bibcode=1988Metic..23...35F |issn=0026-1114}} Liquid water must have penetrated the parent body through cracks and fissures, depositing these hydrated phases. Interestingly, fluid inclusions—intact crystal voids containing ancient liquids—have been identified in Ivuna and Orgueil,{{cite conference |last1=Saylor |first1=J. |last2=Zolensky M. |last3=Bodnar R. |last4=Le L. |last5=Schwandt C. |date=2001 |title=Fluid Inclusions in Carbonaceous Chondrites |conference=LPS XXXII |page=1875}}{{cite journal |last1=Zolensky |first1=M. E. |last2=Bodnar R. J. |last3=Yurimoto H. |last4=Itoh S. |last5=Fries M. |last6=Steele A. |last7=Chan Q. H.-S. |last8=Tsuchiyama A. |last9=Kebukawa Y. |last10=Ito M. |date=2017 |title=The search for and analysis of direct samples of early Solar System aqueous fluids |journal=Phil. Trans. R. Soc. A |volume=375 |issue=2094 |page=20150386 |bibcode=2017RSPTA.37550386Z |doi=10.1098/rsta.2015.0386 |pmc=5394253 |pmid=28416725}} representing the only surviving direct samples of brines from the early Solar System.

The majority of the carbon in CI chondrites (> 70%) exists as insoluble organic matter (IOM), a kerogen-like macromolecule consisting of a highly cross-linked aromatic network with aliphatic linkages, heterocyclic compounds, and various functional groups.{{Cite journal |last1=Alexander |first1=C.M.O’D. |last2=Fogel |first2=M. |last3=Yabuta |first3=H. |last4=Cody |first4=G.D. |date=September 2007 |title=The origin and evolution of chondrites recorded in the elemental and isotopic compositions of their macromolecular organic matter |url=https://linkinghub.elsevier.com/retrieve/pii/S001670370700395X |journal=Geochimica et Cosmochimica Acta |language=en |volume=71 |issue=17 |pages=4380–4403 |doi=10.1016/j.gca.2007.06.052|bibcode=2007GeCoA..71.4380A }} The soluble organic matter (the remaining < 30% portion) includes various compounds such as aliphatic hydrocarbons, polycyclic aromatic hydrocarbons (PAHs), alcohols, and carbonyl compounds.{{Citation |last1=Glavin |first1=Daniel P. |title=The Origin and Evolution of Organic Matter in Carbonaceous Chondrites and Links to Their Parent Bodies |date=2018 |work=Primitive Meteorites and Asteroids |pages=205–271 |url=https://doi.org/10.1016/b978-0-12-813325-5.00003-3 |access-date=2025-04-07 |publisher=Elsevier |isbn=978-0-12-813325-5 |last2=Alexander |first2=Conel M.O'D. |last3=Aponte |first3=José C. |last4=Dworkin |first4=Jason P. |last5=Elsila |first5=Jamie E. |last6=Yabuta |first6=Hikaru|doi=10.1016/b978-0-12-813325-5.00003-3 |hdl=2060/20180004493 |hdl-access=free }}{{cite journal |last1=Yang |first1=J. |last2=Epstein |first2=S. |date=1983 |title=Interstellar organic matter in meteorites |journal=Geochim. Cosmochim. Acta |volume=47 |issue=12 |pages=21992216 |bibcode=1983GeCoA..47.2199Y |doi=10.1016/0016-7037(83)90043-1}}{{cite journal |last1=Grady |first1=M. M. |last2=Wright |first2=I. A. |date=2003 |title=Elemental and Isotopic Abundances of Carbon and Nitrogen in Meteorites |journal=Space Sci. Rev. |volume=106 |issue=1 |pages=231−48 |bibcode=2003SSRv..106..231G |doi=10.1023/A:1024645906350 |s2cid=189792188}}{{cite journal |last1=Tartèse |first1=R. |last2=Chaussidon M. |last3=Gurenko A. |last4=Delarue F. |last5=Robert F. |date=2018 |title=Insights into the origin of carbonaceous chondrite organics from their triple oxygen isotope composition |journal=PNAS |volume=115 |issue=34 |pages=8535−40 |bibcode=2018PNAS..115.8535T |doi=10.1073/pnas.1808101115 |pmc=6112742 |pmid=30082400 |doi-access=free}}

Phenanthrene and anthracene, which are three-ring PAHs, are the most prevalent PAHs and thought to be the result of IOM fraction during aqueous and thermal processing.{{Cite journal |last1=Aponte |first1=José C. |last2=Dworkin |first2=Jason P. |last3=Glavin |first3=Daniel P. |last4=Elsila |first4=Jamie E. |last5=Parker |first5=Eric T. |last6=McLain |first6=Hannah L. |last7=Naraoka |first7=Hiroshi |last8=Okazaki |first8=Ryuji |last9=Takano |first9=Yoshinori |last10=Tachibana |first10=Shogo |last11=Dong |first11=Guannan |last12=Zeichner |first12=Sarah S. |last13=Eiler |first13=John M. |last14=Yurimoto |first14=Hisayoshi |last15=Nakamura |first15=Tomoki |date=2023-02-27 |title=PAHs, hydrocarbons, and dimethylsulfides in Asteroid Ryugu samples A0106 and C0107 and the Orgueil (CI1) meteorite |journal=Earth, Planets and Space |language=en |volume=75 |issue=1 |page=28 |doi=10.1186/s40623-022-01758-4 |doi-access=free |bibcode=2023EP&S...75...28A |issn=1880-5981}} Diverse molecular distributions of polycyclic PAHs have been observed between the Ivuna and Orgueil meteorites, revealing significant compositional heterogeneity within the CI parent body. Furthermore, this variation has been attributed to a process termed "asteroidal chromatography," whereby organic compounds are differentially separated and distributed throughout the asteroid during fluid migration. Several biologically relevant molecules have been identified in the Orgueil meteorite, including purines such as adenine and guanine,{{Cite journal |last=Hayatsu |first=Ryoichi |date=1964-12-04 |title=Orgueil Meteorite: Organic Nitrogen Contents |url=https://www.science.org/doi/10.1126/science.146.3649.1291 |journal=Science |language=en |volume=146 |issue=3649 |pages=1291–1293 |doi=10.1126/science.146.3649.1291 |pmid=17810143 |bibcode=1964Sci...146.1291H |issn=0036-8075}}{{Cite journal |last1=Stoks |first1=Peter G. |last2=Schwartz |first2=Alan W. |date=December 1979 |title=Uracil in carbonaceous meteorites |url=https://www.nature.com/articles/282709a0 |journal=Nature |language=en |volume=282 |issue=5740 |pages=709–710 |doi=10.1038/282709a0 |bibcode=1979Natur.282..709S |issn=0028-0836}}{{Cite journal |last1=Callahan |first1=Michael P. |last2=Smith |first2=Karen E. |last3=Cleaves |first3=H. James |last4=Ruzicka |first4=Josef |last5=Stern |first5=Jennifer C. |last6=Glavin |first6=Daniel P. |last7=House |first7=Christopher H. |last8=Dworkin |first8=Jason P. |date=2011-08-23 |title=Carbonaceous meteorites contain a wide range of extraterrestrial nucleobases |journal=Proceedings of the National Academy of Sciences |language=en |volume=108 |issue=34 |pages=13995–13998 |doi=10.1073/pnas.1106493108 |doi-access=free |issn=0027-8424 |pmc=3161613 |pmid=21836052|bibcode=2011PNAS..10813995C }} and the pyrimidine uracil,{{Cite journal |last1=Oba |first1=Yasuhiro |last2=Koga |first2=Toshiki |last3=Takano |first3=Yoshinori |last4=Ogawa |first4=Nanako O. |last5=Ohkouchi |first5=Naohiko |last6=Sasaki |first6=Kazunori |last7=Sato |first7=Hajime |last8=Glavin |first8=Daniel P. |last9=Dworkin |first9=Jason P. |last10=Naraoka |first10=Hiroshi |last11=Tachibana |first11=Shogo |last12=Yurimoto |first12=Hisayoshi |last13=Nakamura |first13=Tomoki |last14=Noguchi |first14=Takaaki |last15=Okazaki |first15=Ryuji |date=2023-03-21 |title=Uracil in the carbonaceous asteroid (162173) Ryugu |journal=Nature Communications |language=en |volume=14 |issue=1 |page=1292 |doi=10.1038/s41467-023-36904-3 |issn=2041-1723 |pmc=10030641 |pmid=36944653|bibcode=2023NatCo..14.1292O }} alongside non-biological compounds like trizines.

Amino acids are present in CI chondrites at concentrations of approximately 70-75 nmol/g, with a relatively simple distribution dominated by beta-alanine.{{Cite journal |last1=Glavin |first1=Daniel P. |last2=Callahan |first2=Michael P. |last3=Dworkin |first3=Jason P. |last4=Elsila |first4=Jamie E. |date=December 2010 |title=The effects of parent body processes on amino acids in carbonaceous chondrites: Amino acids in carbonaceous chondrites |url=https://onlinelibrary.wiley.com/doi/10.1111/j.1945-5100.2010.01132.x |journal=Meteoritics & Planetary Science |language=en |volume=45 |issue=12 |pages=1948–1972 |doi=10.1111/j.1945-5100.2010.01132.x|hdl=2060/20100032396 |hdl-access=free }}{{cite journal |last1=Ehrenfreund |first1=P. |last2=Glavin D. P. |last3=Botta O. |last4=Cooper G. |last5=Bada J. L. |date=2001 |title=Extraterrestrial amino acids in Orgueil and Ivuna: Tracing the parent body of CI type carbonaceous chondrites |journal=PNAS |volume=98 |issue=5 |pages=2138–2141 |doi=10.1073/pnas.051502898 |pmc=30105 |pmid=11226205 |doi-access=free}} This contrasts with other carbonaceous chondrite groups and may result from extensive aqueous alteration rather than inherent chemical differences. Orgueil displays a notable L-isovaline enantiomeric excess of about 19%, likely amplified by aqueous processes. Additionally, CI chondrites contain carbonates (approximately 5% by volume){{Cite journal |last1=Dufresne |first1=E.R. |last2=Anders |first2=Edward |date=November 1962 |title=On the chemical evolution of the carbonaceous chondrites |url=https://linkinghub.elsevier.com/retrieve/pii/0016703762900479 |journal=Geochimica et Cosmochimica Acta |language=en |volume=26 |issue=11 |pages=1085–1114 |doi=10.1016/0016-7037(62)90047-9|bibcode=1962GeCoA..26.1085D }} including dolomite, calcite, and breunnerite,{{Cite journal |last1=Endress |first1=Magnus |last2=Zinner |first2=Ernst |last3=Bischoff |first3=Adolf |date=February 1996 |title=Early aqueous activity on primitive meteorite parent bodies |url=https://www.nature.com/articles/379701a0 |journal=Nature |language=en |volume=379 |issue=6567 |pages=701–703 |doi=10.1038/379701a0 |pmid=8602215 |bibcode=1996Natur.379..701E |issn=0028-0836}}{{Cite journal |last1=Johnson |first1=Craig A. |last2=Prinz |first2=Martin |date=June 1993 |title=Carbonate compositions in CM and CI chondrites and implications for aqueous alteration |url=https://linkinghub.elsevier.com/retrieve/pii/001670379390393B |journal=Geochimica et Cosmochimica Acta |language=en |volume=57 |issue=12 |pages=2843–2852 |doi=10.1016/0016-7037(93)90393-B|bibcode=1993GeCoA..57.2843J }} as well as various sulfur compounds such as alkyl and aromatic disulfides,{{Cite journal |last1=Orthous-Daunay |first1=F.-R. |last2=Quirico |first2=E. |last3=Lemelle |first3=L. |last4=Beck |first4=P. |last5=deAndrade |first5=V. |last6=Simionovici |first6=A. |last7=Derenne |first7=S. |date=December 2010 |title=Speciation of sulfur in the insoluble organic matter from carbonaceous chondrites by XANES spectroscopy |url=https://linkinghub.elsevier.com/retrieve/pii/S0012821X10006461 |journal=Earth and Planetary Science Letters |language=en |volume=300 |issue=3–4 |pages=321–328 |doi=10.1016/j.epsl.2010.10.012|bibcode=2010E&PSL.300..321O }} though some sulfur content may result from terrestrial weathering oxidative processes.{{Cite journal |last1=Gounelle |first1=Matthieu |last2=Zolensky |first2=Michael E. |date=October 2001 |title=A terrestrial origin for sulfate veins in CI1 chondrites |url=https://onlinelibrary.wiley.com/doi/10.1111/j.1945-5100.2001.tb01827.x |journal=Meteoritics & Planetary Science |language=en |volume=36 |issue=10 |pages=1321–1329 |doi=10.1111/j.1945-5100.2001.tb01827.x |bibcode=2001M&PS...36.1321G |issn=1086-9379}}

CI chondrites stand apart from all other meteorite groups due to their extensive aqueous alteration, with minimal (< 0.1wt%){{Cite journal |last1=Kimura |first1=Makoto |last2=Ito |first2=Motoo |last3=Monoi |first3=Akira |last4=Yamaguchi |first4=Akira |last5=Greenwood |first5=Richard C. |date=August 2024 |title=The primary abundance of chondrules in CI chondrites |url=https://linkinghub.elsevier.com/retrieve/pii/S0016703724002874 |journal=Geochimica et Cosmochimica Acta |language=en |volume=378 |pages=36–44 |doi=10.1016/j.gca.2024.06.002|bibcode=2024GeCoA.378...36K }} visible chondrules{{Cite journal |last1=Leshin |first1=Laurie A. |last2=Rubin |first2=Alan E. |last3=McKeegan |first3=Kevin D. |date=February 1997 |title=The oxygen isotopic composition of olivine and pyroxene from CI chondrites |url=https://linkinghub.elsevier.com/retrieve/pii/S0016703796003742 |journal=Geochimica et Cosmochimica Acta |language=en |volume=61 |issue=4 |pages=835–845 |doi=10.1016/S0016-7037(96)00374-2|bibcode=1997GeCoA..61..835L }}{{Cite journal |last1=Morin |first1=Gatien L.F. |last2=Marrocchi |first2=Yves |last3=Villeneuve |first3=Johan |last4=Jacquet |first4=Emmanuel |date=September 2022 |title=16O-rich anhydrous silicates in CI chondrites: Implications for the nature and dynamics of dust in the solar accretion disk |url=https://linkinghub.elsevier.com/retrieve/pii/S0016703722003015 |journal=Geochimica et Cosmochimica Acta |language=en |volume=332 |pages=203–219 |doi=10.1016/j.gca.2022.06.017}} and calcium-aluminum-rich inclusions (CAIs),{{cite conference |last1=Frank |first1=D. |last2=Zolensky |first2=M. |last3=Martinez J. |last4=Mikouchi T. |last5=Ohsumi K. |last6=Hagiya K. |last7=Satake W. |last8=Le L. |last9=Ross D. |date=2011 |title=A CAI in the Ivuna CI1 Chondrite |conference=42nd LPSC |page=2785 |last10=Peslier A.}} and no reported amoeboid olivine aggregates (AOAs). Despite this alteration, they paradoxically maintain the closest match to solar abundances for non-volatile elements while containing higher volatile concentrations than most meteorites.

This unique composition is reflected in their elemental ratios—CI chondrites exhibit a relatively high Mg/Si ratio (1.07),Von Michaelis, H., Ahrens, I. H. & Willis, J.P.: The compositions of stony meteorites – II. The analytical data and an assessment of their quality. In: Earth and Planetary Scientific Letters. 5, 1969. exceeded only by CV chondrites, alongside the lowest Ca/Si ratio (0.057) among all carbonaceous chondrites.Van Schmus, W. R. & Hayes, J. M.: Chemical and petrographic correlations among carbonaceous chondrites. In: Geochimica Cosmochimica Acta. 38, 1974. Their oxygen isotope values reach the highest levels in the carbonaceous chondrite family, with ratios comparable to terrestrial values.

When compared to CM chondrites, CI chondrites show evidence of more extensive aqueous alteration. CM chondrites preserve some original chondrules and calcium-aluminum-rich inclusions despite containing up to 70% phyllosilicates. CI chondrites, by contrast, consist of over 95% phyllosilicate matrix with virtually no recognizable primordial features. The mineral assemblages in these groups are distinctly different: CM chondrites contain abundant tochilinite-cronstedtite intergrowths with Fe-Ni sulfides, while CI chondrites are characterized by magnesium-rich serpentine and saponite (smectite) minerals, along with significant amounts of magnetite, carbonates, and sulfates.{{cite journal |last1=Keller L. P. |last2=Thomas K. L. |last3=McKay D. S. |date=1992 |title=An interplanetary dust particle with links to CI chondrites |journal=Geochim. Cosmochim. Acta |volume=56 |issue=3 |pages=1409–12 |bibcode=1992GeCoA..56.1409K |doi=10.1016/0016-7037(92)90072-Q}} These mineralogical differences reflect varying water-to-rock ratios and alteration temperatures during parent body processing.

CI chondrites formed forming within the first few million years of the Solar System history in volatile-rich regions of the solar nebula, likely beyond the snow line (> 4 AU from the Sun) where temperatures around 160K allowed water ice preservation. This formation location explains their higher concentrations of carbonaceous and volatile-rich materials compared to other chondrite groups. This is supported by the similarity of CI chondrites with the icy moons of the outer Solar System. Furthermore, there seems to exist a connection to comets: like the comets, CI chondrites accreted silicates, ice and other volatiles, as well as organic compounds (example: Comet Halley).

Although classified as Type 1 chondrites (lacking recognizable chondrules), CIs do contain rare chondrule fragments, anhydrous minerals, and CAIs (less than 1% by volume). Oxygen isotopic compositions of these minerals support their origin as relics of chondrules and refractory inclusions. Before aqueous alteration, CIs likely consisted primarily of chondrules, refractory inclusions, opaque minerals, and anhydrous matrix.

After formation, CI parent bodies experienced heating that melted ice to create liquid water. This water reacted with primary minerals at temperatures of 50-150 °C, converting them to hydrated phyllosilicates over approximately 15 million years.Zolensky, M. E. & Thomas, K. L. (1995). GCA, 59, p. 4707–4712. The alteration occurred in environments with high water/rock ratios (> 0.6-1.2) and neutral to alkaline pH (7-10).

Liquid water must have penetrated the parent body through cracks and fissures and then deposited the water-bearing phases. This process transformed nearly all anhydrous precursor materials into secondary phases. Different CI chondrites show varying alteration levels: Orgueil (containing fine-grained phyllosilicates, ferrihydrite, and corroded magnetite/sulfide grains) represents the most altered, while Ivuna (lacking ferrihydrite) shows less alteration.{{Cite journal |last1=King |first1=A.J. |last2=Schofield |first2=P.F. |last3=Howard |first3=K.T. |last4=Russell |first4=S.S. |date=September 2015 |title=Modal mineralogy of CI and CI-like chondrites by X-ray diffraction |url=https://linkinghub.elsevier.com/retrieve/pii/S0016703715003634 |journal=Geochimica et Cosmochimica Acta |language=en |volume=165 |pages=148–160 |doi=10.1016/j.gca.2015.05.038|bibcode=2015GeCoA.165..148K |hdl=10141/622204 |hdl-access=free }}

Despite this extensive alteration, CI chondrites paradoxically retain the most primitive element abundances.{{cite journal |last1=McSween |first1=H. Y. |last2=Richardson |first2=S. M. |date=1977 |title=The composition of carbonaceous chondrite matrix |journal=Geochim. Cosmochim. Acta |volume=41 |issue=8 |pages=1145–61 |bibcode=1977GeCoA..41.1145M |doi=10.1016/0016-7037(77)90110-7}} This suggests that either mineral transport during alteration remained limited to mm- to cm-scales, or that the parent body was so thoroughly fluidized that its materials were homogenized—creating a closed system.{{cite journal |last1=Piralla |first1=M. |last2=Marrocchi |first2=Y. |last3=Verdier-Paoletti M. J. |last4=Vacher L. |last5=Villeneuve J. |last6=Piani L. |last7=Bekaert D. V. |last8=Gounelle M. |title=Primitive water and dust of the Solar System: Insights from in situ oxygen measurements of CI chondrites |journal=Geochim. Cosmochim. Acta |volume=269 |pages=451−64 |doi=10.1016/j.gca.2019.10.041 |s2cid=209722141 |doi-access=free}}{{cite journal |last1=Bland P. A. |last2=Travis B. J. |date=2017 |title=Giant convecting mud balls of the early solar system |journal=Science Advances |volume=3 |issue=7 |page=e1602514 |bibcode=2017SciA....3E2514B |doi=10.1126/sciadv.1602514 |pmc=5510966 |pmid=28740862}}{{cite journal |last1=Tonui |first1=E. K. |last2=Zolensky |first2=M. E. |last3=Lipschutz |first3=M. E. |last4=Wang |first4=M.-S. |last5=Nakamura |first5=T. |date=2003 |title=Yamato 86029: Aqueously altered and thermally metamorphosed CI-like chondrite with unusual textures |journal=Meteorit. Planet. Sci. |volume=38 |issue=2 |pages=269−92 |bibcode=2003M&PS...38..269T |doi=10.1111/j.1945-5100.2003.tb00264.x |s2cid=56238044 |doi-access=free}}{{cite journal |last1=Morlok |first1=A. |last2=Bischoff A. Stephan T. Floss C. Zinner E. Jessberger E. K. |date=2006 |title=Brecciation and chemical heterogeneities of CI chondrites |journal=Geochim. Cosmochim. Acta |volume=70 |issue=21 |pages=5371–94 |bibcode=2006GeCoA..70.5371M |doi=10.1016/j.gca.2006.08.007}} The debate continues over whether this alteration occurred in free-floating particles before accretion (the nebular hypothesis){{cite journal |last1=Bischoff |first1=A. |date=1998 |title=Aqueous Alteration of Carbonaceous Chondrites: Evidence for Preaccretionary Alteration |journal=Meteorit. Planet. Sci. |volume=33 |issue=5 |pages=1113−22 |bibcode=1998M&PS...33.1113B |doi=10.1111/J.1945-5100.1998.TB01716.X |s2cid=129091212 |doi-access=free}} or within the parent asteroid (the parent body hypothesis),{{cite journal |last1=Tomeoka |first1=K. |date=1990 |title=Phyllosilicate veins in a CI meteorite: evidence for aqueous alteration on the parent body |journal=Nature |volume=345 |issue=6271 |pages=138−40 |bibcode=1990Natur.345..138T |doi=10.1038/345138a0 |s2cid=4326128}} with the presence of veins and diverse magnetite morphologies suggesting multiple episodes of aqueous activity.{{cite journal |last1=Ikeda Y. |last2=Prinz M. |date=1993 |title=Petrologic study of the Belgica 7904 carbonaceous chondrite: Hydrous alteration, thermal metamorphism, and relation to CM and CI chondrites |journal=Geochim. Cosmochim. Acta |volume=57 |pages=439–52 |doi=10.1016/0016-7037(93)90442-Y}}

CI chondrites are strongly linked to dark, primitive C-type asteroids in the outer asteroid belt based on spectral matches.{{Cite journal |last1=Amano |first1=Kana |last2=Matsuoka |first2=Moe |last3=Nakamura |first3=Tomoki |last4=Kagawa |first4=Eiichi |last5=Fujioka |first5=Yuri |last6=Potin |first6=Sandra M. |last7=Hiroi |first7=Takahiro |last8=Tatsumi |first8=Eri |last9=Milliken |first9=Ralph E. |last10=Quirico |first10=Eric |last11=Beck |first11=Pierre |last12=Brunetto |first12=Rosario |last13=Uesugi |first13=Masayuki |last14=Takahashi |first14=Yoshio |last15=Kawai |first15=Takahiro |date=2023-12-08 |title=Reassigning CI chondrite parent bodies based on reflectance spectroscopy of samples from carbonaceous asteroid Ryugu and meteorites |journal=Science Advances |language=en |volume=9 |issue=49 |pages=eadi3789 |doi=10.1126/sciadv.adi3789 |issn=2375-2548 |pmc=10699774 |pmid=38055820|bibcode=2023SciA....9I3789A }}{{Cite journal |last1=Sugita |first1=S. |last2=Honda |first2=R. |last3=Morota |first3=T. |last4=Kameda |first4=S. |last5=Sawada |first5=H. |last6=Tatsumi |first6=E. |last7=Yamada |first7=M. |last8=Honda |first8=C. |last9=Yokota |first9=Y. |last10=Kouyama |first10=T. |last11=Sakatani |first11=N. |last12=Ogawa |first12=K. |last13=Suzuki |first13=H. |last14=Okada |first14=T. |last15=Namiki |first15=N. |date=2019-04-19 |title=The geomorphology, color, and thermal properties of Ryugu: Implications for parent-body processes |journal=Science |language=en |volume=364 |issue=6437 |page=252 |doi=10.1126/science.aaw0422 |issn=0036-8075 |pmc=7370239 |pmid=30890587|bibcode=2019Sci...364..252S }}{{Cite journal |last1=Kitazato |first1=K. |last2=Milliken |first2=R. E. |last3=Iwata |first3=T. |last4=Abe |first4=M. |last5=Ohtake |first5=M. |last6=Matsuura |first6=S. |last7=Arai |first7=T. |last8=Nakauchi |first8=Y. |last9=Nakamura |first9=T. |last10=Matsuoka |first10=M. |last11=Senshu |first11=H. |last12=Hirata |first12=N. |last13=Hiroi |first13=T. |last14=Pilorget |first14=C. |last15=Brunetto |first15=R. |date=2019-04-19 |title=The surface composition of asteroid 162173 Ryugu from Hayabusa2 near-infrared spectroscopy |url=https://www.science.org/doi/10.1126/science.aav7432 |journal=Science |language=en |volume=364 |issue=6437 |pages=272–275 |doi=10.1126/science.aav7432 |pmid=30890589 |bibcode=2019Sci...364..272K |issn=0036-8075}} Recent research has expanded this connection, revealing that some C-complex asteroids without UV-drop-off features and certain X-complex asteroids may also be CI parent bodies. Notably, a significant fraction of C-type asteroids display dehydrated surfaces with spectral features resembling thermally metamorphosed CI-like chondrites.

The asteroids Ryugu and Bennu have provided crucial evidence in this relationship. Initially, reflected spectra from Ryugu acquired by spacecraft appeared most similar to laboratory spectra of heated and partially dehydrated CI chondrites. However, analysis of samples returned from Ryugu revealed mineralogical and chemical properties more closely matching unheated CI chondrites.{{Cite journal |last1=Nakamura |first1=T. |last2=Matsumoto |first2=M. |last3=Amano |first3=K. |last4=Enokido |first4=Y. |last5=Zolensky |first5=M. E. |last6=Mikouchi |first6=T. |last7=Genda |first7=H. |last8=Tanaka |first8=S. |last9=Zolotov |first9=M. Y. |last10=Kurosawa |first10=K. |last11=Wakita |first11=S. |last12=Hyodo |first12=R. |last13=Nagano |first13=H. |last14=Nakashima |first14=D. |last15=Takahashi |first15=Y. |date=2023-02-24 |title=Formation and evolution of carbonaceous asteroid Ryugu: Direct evidence from returned samples |url=https://www.science.org/doi/10.1126/science.abn8671 |journal=Science |language=en |volume=379 |issue=6634 |pages=eabn8671 |doi=10.1126/science.abn8671 |pmid=36137011 |bibcode=2023Sci...379.8671N |hdl=10141/623149 |issn=0036-8075}}{{cite journal |last1=Yokoyama |first1=T. Nagashima, K. Nakai, I. Young, E. D. Abe, Y. Aléon, J. O'D. Alexander, C. M. Amari, S. Amelin, Y. Bajo, K. Bizzarro, M. Bouvier, A. Carlson, R. W. Chaussidon, M. and 135 others |date=10 June 2022 |title=Samples returned from the asteroid Ryugu are similar to Ivuna-type carbonaceous meteorites |url=https://hal.science/hal-03825124v1/file/Yokoyama-et-al_2022_science.abn7850.pdf |journal=Science |volume=379 |issue=6634 |page=785 |doi=10.1126/science.abn7850 |pmid=35679354 |bibcode=2023Sci...379.7850Y |hdl-access=free |hdl=2115/90313}}{{cite journal |last1=Goodrich |first1=C. A. |date=Mar 2023 |title=Ryugu and the Quest for Unaltered CI-Like Materials from the Early Solar System |journal=LPSC 2023 |volume=2806 |page=1446 |bibcode=2023LPICo2806.1446G}} This discrepancy between remote sensing and direct sampling highlights the complexity of identifying meteorite parent bodies through spectroscopy alone.

Furthermore, several lines of evidence suggest that the Orgueil meteorite, the most studied CI chondrite, may have originated from a comet fragment or extinct cometary nucleus. This hypothesis is supported by Orgueil's high water-to-rock ratio, abundance of hydrated minerals, distinctive oxygen isotopes, and deuterium/hydrogen ratios similar to those measured in Comet Hartley 2. Further evidence comes from reconstructed orbital and atmospheric trajectory analyses of the Orgueil fall.{{cite journal |last1=Gounelle |first1=M. |last2=Spurny P. Bland P. A. |date=2006 |title=The atmospheric trajectory and orbit of the Orgueil meteorite |journal=Meteoritics & Planetary Sci |volume=41 |pages=13550 |doi=10.1111/j.1945-5100.2006.tb00198.x |s2cid=59461463 |doi-access=free}} The dwarf planet Ceres has also been proposed as a possible CI parent body, though definitive evidence remains elusive.{{cite journal |last1=McSween |first1=H. Y. |last2=Emery J. P. Rivkin A. S. Toplis M. J. Castillo-Rogez J. C. Prettyman T. H. De Sanctis M. C. Pieters C. M. Raymond C. A. Russell C. T. |date=2018 |title=Carbonaceous chondrites as analogs for the composition and alteration of Ceres |journal=Meteorit. Planet. Sci. |volume=53 |issue=9 |pages=1793−1804 |bibcode=2018M&PS...53.1793M |doi=10.1111/maps.12947 |s2cid=42146213 |doi-access=free}}{{cite journal |last1=Chan |first1=Q. H.-S. |last2=Zolensky M. E. |date=2018 |title=Organic matter in extraterrestrial water-bearing salt crystals |journal=Science Advances |volume=4 |issue=1 |pages=eaao3521 |bibcode=2018SciA....4.3521C |doi=10.1126/sciadv.aao3521 |pmc=5770164 |pmid=29349297}}

While some researchers argue against cometary origins for CI chondrites, these arguments are often based on philosophical positions or circumstantial evidence.{{cite book |last1=Anders |first1=E. |title=Physical studies of minor planets |date=1971 |publisher=NASA |editor=Gehrels, T. |page=429 |chapter=Interrelations of meteorites, asteroids, and comets}}{{cite journal |last1=Anders |first1=E. |date=1975 |title=Do stony meteorites come from comets |journal=Icarus |volume=24 |issue=3 |pages=363−71 |bibcode=1975Icar...24..363A |doi=10.1016/0019-1035(75)90132-3}} Space missions have significantly altered our understanding of comets, particularly the Stardust mission to Comet Wild 2, which returned material with surprisingly asteroidal characteristics.{{cite journal |last1=Zolensky |first1=M. |author2=Nakamura-Messenger K. |author3=Rietmeijer F. |author4=Leroux H. |author5=Mikouchi T. |author6=Ohsumi K. |display-authors=etal |date=2008 |title=Comparing Wild 2 particles to chondrites and IDPs |journal=Meteorit. Planet. Sci. |volume=43 |issue=1 |pages=261−72 |bibcode=2008M&PS...43..261Z |doi=10.1111/j.1945-5100.2008.tb00621.x |s2cid=55294679 |hdl-access=free |hdl=2060/20080013409}} This finding suggests that the boundary between asteroids and comets may be less distinct than previously thought, with considerable mixing between these populations in the early solar system.{{cite journal |last1=Zolensky |first1=M. E. |last2=Bodnar R. J. Gibson E. Nyquist |date=1999 |title=Asteroidal Water Within Fluid Inclusion–Bearing Halite in an H5 Chondrite, Monahans (1998) |journal=Science |volume=285 |issue=5432 |pages=1377−9 |bibcode=1999Sci...285.1377Z |doi=10.1126/science.285.5432.1377 |pmid=10464091}}{{cite journal |last1=Yurimoto |first1=H. |last2=Itoh S. Zolensky M. Kusakabe M. Karen A. Bodnar R. |date=2014 |title=Isotopic compositions of asteroidal liquid water trapped in fluid inclusions of chondrites |journal=Geochemical Journal |volume=48 |issue=6 |pages=549−60 |bibcode=2014GeocJ..48..549Y |doi=10.2343/geochemj.2.0335 |doi-access=free |hdl-access=free |hdl=2115/57641}}{{cite journal |last1=Bouquet |first1=A. |last2=Miller K. E. Glein C. R. Mousis O. |date=2021 |title=Limits on the contribution of early endogenous radiolysis to oxidation in carbonaceous chondrites' parent bodies |journal=Astron. Astrophys. |volume=653 |page=A59 |bibcode=2021A&A...653A..59B |doi=10.1051/0004-6361/202140798 |s2cid=237913967 |doi-access=free}} The possibility that CI chondrites are comet samples is still being postulated.{{cite journal |last1=Campins |first1=H. |last2=Swindle T. D. |date=1998 |title=Expected characteristics of cometary meteorites |journal=Meteorit. Planet. Sci. |volume=33 |issue=6 |pages=1201−11 |bibcode=1998M&PS...33.1201C |doi=10.1111/j.1945-5100.1998.tb01305.x |s2cid=129019797 |doi-access=free}}{{cite journal |last1=Lodders |first1=K. |author1-link=Katharina Lodders |last2=Osborne R. |date=1999 |title=Perspectives on the Comet-Asteroid-Meteorite Link |journal=Space Science Reviews |volume=90 |pages=289−97 |bibcode=1999SSRv...90..289L |doi=10.1023/A:1005226921031 |s2cid=189789172}}{{cite book |last1=Gounelle |first1=M. |title=The Solar System Beyond Neptune |last2=Morbidelli A. Bland P. A. Spurny P. Young E. D. Sephton M. A. |date=2008 |publisher=University of Arizona Press |isbn=978-0-8165-2755-7 |editor=Barucci M. A. Boehnhardt H. Cruikshank D. P. Morbidelli A. |location=Tucson |pages=525−41 |chapter=Meteorites from the Outer Solar System?}}

Micrometeorites and interplanetary dust particles provide additional perspectives on CI chondrite origins. The Earth receives significantly more extraterrestrial material as micrometeorites and dust (by at least one to two orders of magnitude) than as macroscopic meteorites. These smaller particles can better survive atmospheric entry due to their high surface-area-to-volume ratio, overcoming the "fragility filter" that limits CI chondrite recoveries. While most micrometeorites show CM-like compositions, a significant portion display CI-like characteristics.{{cite journal |last1=Brownlee |first1=D. E. |date=1985 |title=Cosmic Dust: Collection and Research |journal=Annual Review of Earth and Planetary Sciences |volume=13 |pages=147−73 |bibcode=1985AREPS..13..147B |doi=10.1146/annurev.ea.13.050185.001051}} The most primitive dust particles that have survived since the formation of the solar system without significant parent body processing may have compositions even closer to protosolar abundances,{{cite journal |last1=Ebel |first1=D. S. |last2=Grossman L. |date=2000 |title=Condensation in dust-enriched systems |journal=Geochim. Cosmochim. Acta |volume=64 |issue=2 |pages=339−66 |arxiv=2307.00641 |bibcode=2000GeCoA..64..339E |doi=10.1016/S0016-7037(99)00284-7}} including higher volatile content as seen in ultracarbonaceous Antarctic micrometeorites (UCAMMs).

There are very few finds of CI chondrites, with five confirmed specimens and CI-like specimens (see CI-like meteorites). Orgueil in particular has been distributed among collections around the world. Revelstoke, and to a lesser extent Tonk, are small and difficult to study, let alone disperse.{{cite book |last1=Grady |first1=M. M. |title=Catalogue of Meteorites |date=2000 |publisher=Cambridge University Press |isbn=0-521-66303-2 |edition=5th |location=Cambridge}}

Alais, which fell near what is now Alès, France on March 15, 1806, holds historical significance as one of the first carbonaceous chondrites recognized as extraterrestrial and oldest CI find. Consequently, pieces weighing 6 kilograms were discovered at Saint-Étienne-de-l'Olm and Castelnau-Valence, small villages southeast of Alès. Alais contains well-crystallized phyllosilicates occurring as coarse fragments and clusters. However, it more closely resembles Orgueil in containing ferrihydrite (suggesting later-stage alteration) and assaying to higher gas levels than typical meteorites.

The Orgueil meteorite, which fell near its namesake town in France on May 14, 1864, represents the largest and most extensively studied CI chondrite. This significant fall disintegrated into approximately 20 pieces during atmospheric entry, yielding a total recovered mass of about 14 kg.

Generally considered the most altered CI chondrite, Orgueil became controversial in the 1960s when researchers reported "organized elements" initially proposed as possible microfossils, though later work revealed these were likely mineral structures or terrestrial contamination.

Orgueil displays several distinct chemical signatures, including a high L-isovaline enantiomeric excess (approximately 19%)—significantly higher than in unaltered chondrites. Its amino acid concentration (71 nmol/g) and distribution (predominantly beta-alanine) differ markedly from the complex alpha-amino acids found in CM2 meteorites.

Tonk fell in Rajasthan, India in 1911. It is one of the less-studied CI chondrites due to limited available material. Total known weight is about 7.7 grams, making it difficult to study in depth or distribute widely to researchers.{{cite journal |last=Christie |first=W. A. K. |year=1914 |title=The composition of the Tonk Meteorite |journal=The Journal of the Astronomical Society of India |volume=4 |pages=71–72 |number=2}} Like other CI chondrites, Tonk assays to higher gas levels than typical meteorites. It shares the characteristic features of CI chondrites, including extensive aqueous alteration, though detailed studies are limited by the small available sample size

Ivuna, which fell in Tanzania on December 16, 1938, serves as the type specimen for the entire CI group. With a total recovered mass of approximately 705 grams, this meteorite is distinguished by well-crystallized phyllosilicates that often appear as coarse fragments and clusters.

Among CI chondrites, Ivuna represents the least altered specimen, lacking the ferrihydrite found in Alais and Orgueil. Its composition includes 3.31 wt% total carbon (90% organic), 1.59 wt% hydrogen (89% inorganic), and 12.73 wt% total water. Recent oxygen isotope studies of its dolomite and magnetite grains suggest these minerals may have precipitated from the same fluid as similar components in samples from asteroid Ryugu.

The Revelstoke CI chondrite fall was in 1965, notable for its very bright fall in Revelstoke, British Columbia. It yielded only two tiny fragments, totaling ~1 gram (>0.03 oz).

Antarctica has been a significant source of meteorites, including specimens that exhibit similarities to CI chondrites. The first such finds, Yamato 82042 and Y-82162, were discovered in the Yamato Mountains. These meteorites share many characteristics with CI chondrites but also exhibit notable differences. Y-82162 and Y-86029, for instance, contain less water and have bulk oxygen isotopic compositions shifted to higher values, suggesting significant water loss from phyllosilicates due to thermal metamorphism.

In 1992, Ikeda proposed that these Antarctic meteorites, which differ somewhat from non-Antarctic CI chondrites, should be classified as a distinct grouplet.{{cite journal |last1=Ikeda |first1=Y. |date=1992 |title=An overview of the research consortium, "Antarctic carbonaceous chondrites with CI affinities, Yamato-86720, Yamato-82162, and Belgica-7904" |journal=Proceedings, NIPR Symp. Antarctic Meteorites |volume=5 |pages=49–73 |bibcode=1992AMR.....5...49I}} By 2015, the list of CI-like specimens had expanded to include Yamato 86029 (11.8 g), Y-86720, Y-86737 (2.81 g), Y-86789, Y-980115 (772 g), Y-980134 (12.2 g), Belgica 7904, and the desert chondrite Dhofar 1988.{{cite book |last1=Wasson |first1=J. T. |title=Meteorites: Classification and Properties |date=1974 |publisher=Springer |isbn=978-3-642-65865-5}}{{cite book |last1=Weisberg |first1=M. K. |title=Systematics and Evaluation of Meteorite Classification |date=2006 |publisher=University of Arizona Press |isbn=9780816525621 |location=Tucson |page=19}}{{cite book |last1=Hutchison |first1=R. |title=Meteorites: A Petrologic, Chemical, and Isotopic Synthesis |date=2004 |publisher=Cambridge University Press |isbn=0-521-47010-2 |location=Cambridge}} King et al. later proposed a separate classification for these meteorites, naming them CY chondrites. In 2023, MacLennan Gravik claimed (using mid-infrared spectroscopy) that asteroid (3200) Phaethon is the parent body of the CY chondrites, further supporting their distinction from CI chondrites.{{undue weight inline|date=April 2025}}{{cite journal |last1=MacLennan |first1=Eric |last2=Granvik |first2=Mikael |date=2 November 2023 |title=Thermal decomposition as the activity driver of near-Earth asteroid (3200) Phaethon |journal=Nature Astronomy |volume=8 |pages=60–68 |arxiv=2207.08968 |doi=10.1038/s41550-023-02091-w}} This claim is countered by direct examination of the meteorites.{{cite journal |last1=Schrader |first1=D. L. |last2=Torrano |first2=Z. A. |last3=Foustoukos |first3=D. I. |last4=Alexander |first4=C. M. O’D. |last5=Render |first5=J. |last6=Brennecka |first6=G. A. |title=Reassessing the proposed "CY chondrites": Evidence for multiple meteorite types and parent bodies from Cr-Ti-H-C-N isotopes and bulk elemental compositions |journal=Geochim. Cosmochim. Acta |date=2025 |volume=390 |pages=24–37 |doi=10.1016/j.gca.2024.12.021|bibcode=2025GeCoA.390...24S }}

A key difference between Antarctic CI-like meteorites and CI chondrites is the alteration of phyllosilicates. In many Antarctic specimens, these minerals have undergone dehydration and reversion to silicates, accompanied by an increase in sulfide content. Unlike typical CI chondrites, where magnetite is more abundant, sulfides dominate in CY chondrites. Additionally, these meteorites exhibit the highest recorded oxygen isotope compositions among all meteorites.

Organic analysis of the Yamato chondrites has revealed significantly lower concentrations of amino acids (~3 nmol/g), approximately 25 times lower than in other CI chondrites.{{cite journal |last1=Burton A. S. |last2=Grunsfeld S. |last3=Elsila J. E. |last4=Glavin D. P. |last5=Dworkin J. P. |date=2014 |title=The effects of parent-body hydrothermal heating on amino acid abundances in CI-like chondrites |journal=Polar Science |volume=8 |issue=3 |page=255 |bibcode=2014PolSc...8..255B |doi=10.1016/j.polar.2014.05.002 |doi-access=free}} The amino acid composition is dominated by proteinogenic amino acids, suggesting terrestrial contamination. Furthermore, thermal history varies between Antarctic CI-like meteorites and traditional CI chondrites. While Ivuna and Orgueil likely never experienced temperatures above 150 °C,{{cite conference |last1=Clayton |first1=R. N. |last2=Mayeda |first2=T. K. |date=2001 |title=Oxygen isotope composition of the Tagish Lake carbonaceous chondrite |conference=Lunar and Planetary Sciences Conf. 32 |page=1885}}{{Cite journal |last1=Bullock |first1=E.S. |last2=Gounelle |first2=M. |last3=Lauretta |first3=D.S. |last4=Grady |first4=M.M. |last5=Russell |first5=S.S. |date=May 2005 |title=Mineralogy and texture of Fe-Ni sulfides in CI1 chondrites: Clues to the extent of aqueous alteration on the CI1 parent body |url=https://linkinghub.elsevier.com/retrieve/pii/S0016703705000426 |journal=Geochimica et Cosmochimica Acta |language=en |volume=69 |issue=10 |pages=2687–2700 |doi=10.1016/j.gca.2005.01.003|bibcode=2005GeCoA..69.2687B }} Y-86029 and Y-980115 have undergone heating up to 600 °C.{{Cite journal |last=Nakamura |first=Tomoki |date=2005 |title=Post-hydration thermal metamorphism of carbonaceous chondrites |url=http://www.jstage.jst.go.jp/article/jmps/100/6/100_6_260/_article |journal=Journal of Mineralogical and Petrological Sciences |language=en |volume=100 |issue=6 |pages=260–272 |doi=10.2465/jmps.100.260 |bibcode=2005JMPeS.100..260N |issn=1345-6296}}{{Cite journal |last1=Tonui |first1=Eric K. |last2=Zolensky |first2=Michael E. |last3=Lipschutz |first3=Michael E. |last4=Wang |first4=Ming-Sheng |last5=Nakamura |first5=Tomoki |date=February 2003 |title=Yamato 86029: Aqueously altered and thermally metamorphosed CI-like chondrite with unusual textures |url=https://onlinelibrary.wiley.com/doi/10.1111/j.1945-5100.2003.tb00264.x |journal=Meteoritics & Planetary Science |language=en |volume=38 |issue=2 |pages=269–292 |doi=10.1111/j.1945-5100.2003.tb00264.x |bibcode=2003M&PS...38..269T |issn=1086-9379}} The low abundance of γ- and δ-amino acids in the Yamato meteorites suggests that either minimal amino acid synthesis occurred on their parent bodies or that prolonged heating led to near-complete amino acid destruction.

Lastly, the meteorite find Oued Chebeika 002,{{cite web |title=Oued Chebeika 002 |url=https://www.lpi.usra.edu/meteor/metbull.php?sea=CI1&sfor=types&ants=&nwas=&falls=&valids=&stype=exact&lrec=50&map=ge&browse=&country=All&srt=&categ=All&mblist=All&rect=&phot=&strewn=&snew=0&pnt=Normal%20table&code=84012 |website=Meteoritical Bulletin Database |publisher=Meteoritical Society |access-date=Mar 20, 2025}} recovered by locals in the Moroccan deserts, appears to be a CI chondrite. Although it was not an observed fall, the arid environment appears to have caused minimal alteration to the sample.{{cite journal |last1=Garvie |first1=Laurence A.J. |last2=Wittmann |first2=Axel |title=Mineralogical Observations Of The Oued Chebeika 002 (CI1) Meteorite. |journal=56th LPSC |date=Mar 10, 2025 |page=1802}}{{cite journal |last1=Mikouchi |first1=T. |title=Mineralogy Of Oued Chebeika 002: A True CI1 Chondrite. |journal=56th LPSC |date=Mar 10, 2025 |page=2021}}{{cite journal |last1=Sadaka |first1=C. Gattacceca, J. Gounelle, M. Barrat, J.-A. Devouard, B. Bonal, L. King, A. Maurel, C. Beck, P. Poch, O. Viennet, J.-C. Roskosz, M. Grauby, O. AuYang, D. Borschneck, D. Tikoo, S. Vidal, V. Rochette, P. |title=Oued Chebeika 002: expanding the CI chondrite inventory. |journal=56th LPSC |date=Mar 10, 2025 |page=2208}}

{{main|Hayabusa2}}

Samples of asteroid (162173) Ryugu, as selected by the Hayabusa2 mission, appear to be a match to CI meteorites.{{cite journal |last1=Yokoyama |first1=T. Nagashima, K. Nakai, I. Young, E. D. Abe, Y. Aléon, J. O'D. Alexander, C. M. Amari, S. Amelin, Y. Bajo, K. Bizzarro, M. Bouvier, A. Carlson, R. W. Chaussidon, M. and 135 others |title=Samples returned from the asteroid Ryugu are similar to Ivuna-type carbonaceous meteorites |journal=Science |date=Jun 10, 2022 |volume=379 |issue=6634 |page=785 |doi=10.1126/science.abn7850|pmid=35679354 |hdl=2115/90313 |url=https://hal.science/hal-03825124v1/file/Yokoyama-et-al_2022_science.abn7850.pdf |hdl-access=free }}{{cite journal |last1=Goodrich |first1=C. A. |title=Ryugu and the Quest for Unaltered CI-Like Materials from the Early Solar System |journal=LPSC 2023 |date=Mar 2023 |volume=2806 |page=1446|bibcode=2023LPICo2806.1446G }} As the sample was hermetically sealed, it has never been exposed to Earth biota and is claimed for use as a cosmochemical reference.{{cite web |title=Ryugu Reference Project |url=https://curation.isas.jaxa.jp/rrp/ |publisher=JAXA |access-date=Mar 20, 2025}}

The defining feature of CI meteorites is their chemical composition, rich in volatile elements- richer than any other meteorites. The element assay of CI meteorite is used as a geochemical standard, as it has "a remarkably close relationship"{{cite journal |last1=Holweger |first1=H. |date=1977 |title=The solar Na/Ca and S/Ca ratios: A close comparison with carbonaceous chondrites |journal=Earth and Planetary Science Letters |volume=34 |issue=1 |pages=152−54 |bibcode=1977E&PSL..34..152H |doi=10.1016/0012-821X(77)90116-9}} to the makeup of the Sun and greater Solar System.{{cite journal |last1=Asplund |first1=M. |last2=Grevesse |first2=N. |last3=Sauval |first3=A. J. |last4=Scott |first4=P. |date=2009 |title=The chemical composition of the Sun |journal=Annual Review of Astronomy and Astrophysics |volume=47 |issue=1 |pages=481−522 |arxiv=0909.0948 |bibcode=2009ARA&A..47..481A |doi=10.1146/annurev.astro.46.060407.145222 |s2cid=17921922}}{{cite book |last1=Palme |first1=H. |title=Treatise on Geochemistry |last2=Lodders |first2=K. |author2-link=Katharina Lodders |last3=Jones A. |date=2014 |publisher=Elsevier |editor=Davis, A. M. |pages=15−36 |chapter=Solar system abundances of the elements}} This abundance standard is the measure by which other meteorites,{{cite journal |last1=Arndt |first1=P. |last2=Bohsung |first2=J. |last3=Maetz |first3=M. |last4=Jessberger |first4=E. K. |date=1996 |title=The elemental abundances in interplanetary dust particles |journal=Meteoritics & Planetary Science |volume=31 |issue=6 |pages=817−33 |bibcode=1996M&PS...31..817A |doi=10.1111/j.1945-5100.1996.tb02116.x}}{{cite book |last1=Lodders |first1=K. |author1-link=Katharina Lodders |title=Chemistry of the Solar System |last2=Fegley |first2=B. Jr. |date=2011 |publisher=RSC Publishing |isbn=978-0-85404-128-2 |location=Cambridge}}{{cite journal |last1=Russell |first1=S. S. |author1-link=Sara Russell |last2=Suttle |first2=M. D. |last3=King |first3=A. J. |date=2021 |title=Abundance and importance of petrological type 1 chondritic material |url=https://onlinelibrary.wiley.com/doi/10.1111/maps.13753 |journal=Meteorit Planet Sci |volume=57 |issue=2 |pages=277–301 |doi=10.1111/maps.13753 |s2cid=243853829}} comets,{{cite journal |last1=Anders |first1=E. |last2=Grevesse |first2=N. |date=1989 |title=Abundances of the elements: Meteoritic and solar |journal=Geochim. Cosmochim. Acta |volume=53 |issue=1 |pages=197–214 |bibcode=1989GeCoA..53..197A |doi=10.1016/0016-7037(89)90286-X |s2cid=40797942}}{{cite book |last1=Lodders |first1=K. |author1-link=Katharina Lodders |title=The Planetary Scientist's Companion |last2=Fegley |first2=B. Jr. |date=1998 |publisher=Oxford University Press |isbn=9780195116946 |location=New York}}{{cite book |last1=Lewis |first1=J. S. |title=Comet and Asteroid Impact Hazards on a Populated Earth |date=2000 |publisher=Academic Press |isbn=0-12-446760-1 |location=San Diego |page=50}}{{cite journal |last1=Paquette |first1=J. A. |last2=Engrand |first2=C. |last3=Stenzel |first3=O. |last4=Hilchenbach |first4=M. |last5=Kissel |first5=J. |display-authors=etal |date=2016 |title=Searching for calcium–aluminum-rich inclusions in cometary particles with Rosetta/COSIMA |url=https://hal-insu.archives-ouvertes.fr/insu-01351380/file/maps.12669.pdf |journal=Meteorit Planet Sci |volume=51 |issue=7 |pages=1340−52 |bibcode=2016M&PS...51.1340P |doi=10.1111/maps.12669 |s2cid=132170692}} and in some cases the planets themselves{{cite journal |last1=Harkins |first1=W. D. |date=1917 |title=The Evolution of the Elements and the Stability of Complex atoms. I. A new periodic system which shows a relation between the abundance of the elements and the structure of the nuclei of atoms. |url=https://zenodo.org/record/1429060 |journal=J. Am. Chem. Soc. |volume=39 |issue=5 |page=856 |bibcode=1917JAChS..39..856H |doi=10.1021/ja02250a002}}{{cite journal |last1=Morgan |first1=J. W. |last2=Anders |first2=E. |date=1979 |title=Chemical composition of Mars |journal=Geochim. Cosmochim. Acta |volume=43 |issue=10 |pages=1601−10 |bibcode=1979GeCoA..43.1601M |doi=10.1016/0016-7037(79)90180-7}}{{cite journal |last1=Dreibus |first1=G. |last2=Wanke |first2=H. |date=1985 |title=Mars, a volatile-rich planet |journal=Meteoritics |volume=20 |issue=2 |pages=367−81 |bibcode=1985Metic..20..367D}} (since revised{{cite journal |last1=Warren |first1=P. H. |date=2011 |title=Stable-isotope anomalies and the accretionary assemblage of the Earth and Mars: A subordinate role for cabonaceous chondrites |journal=Earth Planet. Sci. Lett. |volume=311 |issue=1 |pages=93−100 |bibcode=2011E&PSL.311...93W |doi=10.1016/j.epsl.2011.08.047}}{{cite journal |last1=Palme |first1=H. |last2=Zipfel |first2=J. |date=2021 |title=The composition of CI chondrites and their contents of chlorine and bromine: Results from instrumental neutron activation analysis |journal=Meteorit. Planet. Sci. |volume=57 |issue=2 |pages=317–333 |doi=10.1111/maps.13720 |s2cid=238805417 |doi-access=free}}) are assayed.

Goldschmidt noted the primitive (pre-differentiated) compositions of some meteorites, calling it the "cosmic" abundance- he assumed meteorites had arrived from free space, not our Solar System.{{cite book |last1=Goldschmidt |first1=V. M. |title=Skrifter Norske Videnskaps: Geochemische Verteilungsgesetze der Elemente |date=1938 |publisher=Dybwad |location=Oslo}}{{cite book |last1=Goldschmidt |first1=V. M. |title=Geochemistry |date=1954 |publisher=Clarendon Press |location=Oxford}} In turn, the study of such abundances stimulated- then validated- work in nucleosynthesis and stellar physics.{{cite journal |last1=Grevesse |first1=N. |last2=Sauval |first2=J. |date=1998 |title=Standard Solar Composition |journal=Space Science Reviews |volume=85 |pages=161−74 |bibcode=1998SSRv...85..161G |doi=10.1023/A:1005161325181 |s2cid=117750710}} In a sense, Goldschmidt's choice of terms may have been borne out: both Solar and CI compositions appear similar to nearby stars as well,{{cite journal |last1=Anders |first1=E. |date=1971 |title=How well do we know "Cosmic" abundances? |journal=Geochim. Cosmochim. Acta |volume=35 |issue=5 |page=516 |bibcode=1971GeCoA..35..516A |doi=10.1016/0016-7037(71)90048-2}}{{cite journal |last1=Asplund |first1=M. |last2=Grevesse |first2=N. |last3=Sauval |first3=A. J. |last4=Scott |first4=P. |date=2009 |title=The Chemical Composition of the Sun |journal=Annual Review of Astronomy and Astrophysics |volume=47 |issue=1 |pages=481−522 |arxiv=0909.0948 |bibcode=2009ARA&A..47..481A |doi=10.1146/annurev.astro.46.060407.145222 |s2cid=17921922}} and presolar grains exist (though too small to be relevant here).

The CI abundance is more properly linked to the abundances in the solar photosphere. Small differences exist between the solar interior, photosphere, and corona/solar wind. Heavy elements may settle to the interiors of stars (for the Sun, this effect appears low); the corona and thus the solar wind are affected by plasma physics and high-energy mechanisms and are imperfect samples of the Sun. Other issues include the lack of spectral features- and thus, straightforward photospheric observation- of noble gases. Since the CI values are measured directly (first by assay, now by mass spectrometry, and when necessary, neutron activation analysis), they are more precise than solar values, which are subject to (besides the above field effects) spectrophotometric assumptions, including elements with conflicting spectral lines. In particular, when the iron abundances of CIs and the Sun did not match,{{cite journal |last1=Warner |first1=B. |date=1968 |title=The Abundance of Elements in the Solar Photosphere−IV The Iron Group |journal=Mon. Not. R. Astron. Soc. |volume=138 |pages=229−43 |doi=10.1093/mnras/138.2.229 |doi-access=free}}{{cite journal |last1=Kostik |first1=R. I. |last2=Shchukina |first2=N. G. |last3=Rutten |first3=R. J. |date=1996 |title=The solar iron abundance: not the last word |journal=Astron. Astrophys. |volume=305 |pages=325−42 |bibcode=1996A&A...305..325K}} it was the solar value that was questioned and corrected, not the meteorite number.{{cite journal |last1=Grevesse |first1=N. |last2=Sauval |first2=A. J. |date=1999 |title=The solar abundance of iron and the photospheric model |journal=Astron. Astrophys. |volume=347 |pages=348–54 |bibcode=1999A&A...347..348G}} Solar and CI abundances, for better and for worse, differ in that e. g., chondrites condensed ~4.5 billion years ago and represent some initial planetary states (i. e., the proto-solar abundance),{{cite journal |last1=Wieler |first1=R. |last2=Kehm |first2=K. |last3=Meshik |first3=A. P. |last4=Hohenberg |first4=C. M. |date=1996 |title=Secular changes in the xenon and krypton abundances in the solar wind recorded in single lunar grains |journal=Nature |volume=384 |issue=6604 |pages=46−49 |bibcode=1996Natur.384...46W |doi=10.1038/384046a0 |s2cid=4247877}}{{cite journal |last1=Burnett |first1=D. S. |last2=Jurewicz |first2=A. J. G. |last3=Woolum |first3=D. S. |date=2019 |title=The future of Genesis science |journal=Meteorit. Planet. Sci. |volume=54 |issue=5 |pages=1094−114 |bibcode=2019M&PS...54.1092B |doi=10.1111/maps.13266 |pmc=6519397 |pmid=31130804}} while the Sun continues burning lithium{{cite journal |last1=Anders |first1=E. |last2=Ebihara |first2=M. |date=1982 |title=Solar-system abundances of the elements |journal=Geochim. Cosmochim. Acta |volume=46 |issue=11 |pages=2363−80 |bibcode=1982GeCoA..46.2363A |doi=10.1016/0016-7037(82)90208-3}} and possibly other elements and continually creating helium from e. g., deuterium.

Issues with CI abundances include heterogeneity (local variation),{{cite journal |last1=Ebihara |first1=M. |last2=Wolf |first2=R. |last3=Anders |first3=E. |date=1982 |title=Are C1 chondrites chemically fractionated? A trace element study |journal=Geochim. Cosmochim. Acta |volume=46 |issue=10 |pages=1849−62 |bibcode=1982GeCoA..46.1849E |doi=10.1016/0016-7037(82)90123-5}}{{cite journal |last1=Barrat |first1=J. A. |last2=Zanda |first2=B. |last3=Moynier |first3=F. |last4=Bollinger |first4=C. |last5=Liorzou |first5=C. |last6=Bayon |first6=G. |date=2012 |title=Geochemistry of CI chondrites: Major and trace elements, and Cu and Zn isotopes |url=https://hal-insu.archives-ouvertes.fr/insu-00670053/file/GCA_W8592_manuscript_Tables_fig.pdf |journal=Geochim. Cosmochim. Acta |volume=83 |pages=79−92 |bibcode=2012GeCoA..83...79B |doi=10.1016/j.gca.2011.12.011 |s2cid=53528401}} and bromine and other halogens, which are water-soluble and thus labile.{{cite journal |last1=Burnett |first1=D. S. |last2=Woolum |first2=D. S. |last3=Benjamin |first3=T. M. |last4=Rogers |first4=P. S. Z. |last5=Duffy |first5=C. J. |last6=Maggiore |first6=C. |date=1989 |title=A Test of the Smoothness of the Elemental Abundances of Carbonaceous Chondrites |journal=Geochim. Cosmochim. Acta |volume=53 |issue=2 |page=471 |bibcode=1989GeCoA..53..471B |doi=10.1016/0016-7037(89)90398-0}} Volatiles, such as noble gases (though see below) and the atmophile elements carbon, nitrogen, oxygen, etc. are lost from minerals and not assumed to hold the Solar correspondence. However, in the modern era the Solar carbon and oxygen measurements have come down significantly.{{cite journal |last1=Allende Prieto |first1=C. |last2=Lambert |first2=D. L. |last3=Asplund |first3=M. |date=2001 |title=The Forbidden Abundance of Oxygen In The Sun |journal=Astrophys. J. |volume=556 |issue=1 |pages=L63−66 |arxiv=astro-ph/0106360 |bibcode=2001ApJ...556L..63A |doi=10.1086/322874 |s2cid=15194372}}{{cite journal |last1=Lodders |first1=K. |author1-link=Katharina Lodders |date=2003 |title=Solar system abundances and condensation temperatures of the elements |journal=Astrophys. J. |volume=591 |issue=2 |pages=1220−47 |bibcode=2003ApJ...591.1220L |doi=10.1086/375492 |s2cid=42498829 |doi-access=free}} As these are the two most abundant elements after hydrogen and helium, the Sun's metallicity is affected significantly.{{cite book |last1=Allende Prieto |first1=C. |title=14th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun |date=2008 |publisher=Astronomical Society of the Pacific |isbn=978-1-58381-331-7 |editor=van Belle, G. |chapter=The Abundances of Oxygen and Carbon in the Solar Photosphere}} It is possible that CI chondrites may hold too many volatiles, and the matrix of CM chondrites (excluding chondrules, calcium–aluminium-rich inclusions, etc.), or bulk Tagish Lake, may be a better proxy for the Solar abundance.{{cite journal |last1=Buseck |first1=P. Hua X. |date=1993 |title=Matrices Of Carbonaceous Chondrite Meteorites |journal=Annual Review of Earth and Planetary Sciences |volume=21 |pages=255–305 |bibcode=1993AREPS..21..255B |doi=10.1146/annurev.ea.21.050193.001351}}{{cite journal |last1=Asplund |first1=M. |last2=Amarsi |first2=A. M. |last3=Grevesse |first3=N. |date=2021 |title=The chemical make-up of the Sun: A 2020 vision |journal=Astron. Astrophys. |volume=653 |page=A141 |arxiv=2105.01661 |bibcode=2021A&A...653A.141A |doi=10.1051/0004-6361/202140445 |s2cid=233739900}}

{{See also|Advanced Composition Explorer}}

Due to their rarity and importance as geochemical references, there has been significant interest in classifying meteorites as CI chondrites. However, several meteorites once thought to be CI chondrites have later been reclassified.

Compared to all the meteorites found to date, CI chondrites possess the strongest similarity to the elemental distribution within the original solar nebula. For this reason they are also called primitive meteorites. Except for the volatile elements carbon, hydrogen, oxygen and nitrogen, as well as the noble gases, which are deficient in the CI chondrites, the elemental ratios are nearly identical. Lithium is another exception, it is enriched in the meteorites (lithium in the Sun is involved during nucleosynthesis and therefore diminished).

Because of this strong similarity, it has become customary in petrology to normalize rock samples versus CI chondrites for a specific element, i. e. the ratio rock/chondrite is used to compare a sample with the original solar matter. Ratios > 1 indicate an enrichment, ratios < 1 a depletion of the sample. The normalization process is used mainly in spider diagrams for the rare-earth elements.

CI chondrites also have a high carbon content. Besides inorganic carbon compounds like graphite, diamond and carbonates, organic carbon compounds are represented. For instance, amino acids have been detected. This is a very important fact in the ongoing search for the origin of life.

• Glossary of meteoritics
• Meteorite classification

{{Reflist}}

{{Meteorites}}

{{DEFAULTSORT:Ci Chondrite}}