CI chondrite

CI chondrites, also called C1 chondrites or Ivuna-type carbonaceous chondrites, are a group of rare carbonaceous chondrites, a type of stony meteorite. They are named after the Ivuna meteorite, the type specimen. They represent the most chemically primitive meteorites known, with elemental compositions closely matching the Sun.
These rare carbonaceous chondrites are defined by their lack of visible chondrules due to extensive aqueous alteration. Despite this alteration, they preserved the solar system's original elemental composition, making them the standard reference material for cosmic abundances in planetary science. The Orgueil, Alais, Ivuna, Tonk, and Revelstoke meteorites, along with CI-like Antarctic specimens, provide windows into the early solar system's chemistry, the formation of volatiles, and possibly the origins of life's building blocks.

Designation

The abbreviation CI is derived from the C for carbonaceous and in the name scheme of Wasson, 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, still used for petrography. Petrographic type-1 meteorites, by definition, have no fully visible chondrules.

Physical and Chemical Characteristics

Elemental composition

Carbon

CI chondrites contain significant amounts of carbon, ranging from approximately 3-5 wt%, primarily in organic form. Analysis of the Ivuna meteorite revealed a total carbon concentration of 3.31 wt%, with about 90% being organic carbon. While this represents the highest carbon content among carbonaceous chondrites, it is surpassed by some Ureilites, which can contain even greater carbon concentrations.

Oxygen

Oxygen is the most abundant element in CI chondrites, with a distinctive isotopic composition that serves as a crucial identifier. CI chondrites contain three stable oxygen isotopes that, when plotted on a three-isotope diagram, occupy a specific field clearly distinguishable from other meteorite groups. 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

Iron is present with 18-20 wt%. 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. The majority of the iron is in the form of cations in the phyllosilicates and iron bound as magnetite. Some appears as ferrihydrite, but not in Ivuna.

Mineralogical composition and matrix

CI chondrites are primarily composed of fine-grained phyllosilicates with a dark and fine-grained clay-like matrix rich in carbonaceous material. Their matrix contains magnetite, iron sulfides like pyrrhotite, carbonates, and ferrihydrite, with smaller amounts of pentlandite and other minerals. The dominant components are serpentine-saponite intergrowths. Framboidal magnetite occurs within the matrix and may have formed through precipitation from a gel-like phase. While most phyllosilicates in the CI chondrites are fine-grained and poorly crystalline, in Alais and Ivuna well-crystallized phyllosilicates often occur as coarse fragments and clusters that are not commonly found in Orgueil.
Magnetite is the second most abundant mineral in CI chondrites. It occurs in various morphologies, including crystals, spheres, framboids, and plaquettes, which are distinctive to CIs. The mineral formed through the oxidation of sulfides, primarily pyrrhotite and its nickel-rich variants, likely occurring in multiple generations. Other minerals found include iron sulfides like pyrrhotite, pentlandite, troilite and cubanite. The matrix also hosts isolated ferromagnesian silicates, such as olivine, clinopyroxene, and orthopyroxene, which crystallized at high temperatures and remain unaltered. Water-bearing, clay-rich phyllosilicates, including montmorillonite and serpentine-like minerals, are among the main constituents. Additionally, alteration minerals such as epsomite, vaterite, carbonates, and sulfates are present.
Furthermore, these meteorites lack intact chondrules, calcium-aluminum-rich inclusions, and amoeboid olivine aggregates due to extensive aqueous alteration.

Water-bearing minerals

CI chondrites contain between 18-20 wt% water with porosities reaching up to ~25-30%, which appears correlated to their high water content. The water is primarily bound within water-bearing silicates and present in the form of hydroxyl groups in phyllosilicates. Analysis of the Ivuna meteorite revealed 12.73 wt% total water, divided between interlayer water and structural OH/H₂O in phyllosilicates. Extensive aqueous alteration is evidenced by the presence of crosscutting veins filled with Na-, Ca-, and Mg-sulfates. 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, representing the only surviving direct samples of brines from the early Solar System.

Carbon compounds

The majority of the carbon in CI chondrites exists as insoluble organic matter, a kerogen-like macromolecule consisting of a highly cross-linked aromatic network with aliphatic linkages, heterocyclic compounds, and various functional groups. The soluble organic matter includes various compounds such as aliphatic hydrocarbons, polycyclic aromatic hydrocarbons, alcohols, and carbonyl compounds.
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. 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, and the pyrimidine uracil, alongside non-biological compounds like triazines.
Amino acids are present in CI chondrites at concentrations of approximately 70-75 nmol/g, with a relatively simple distribution dominated by beta-alanine. 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 including dolomite, calcite, and breunnerite, as well as various sulfur compounds such as alkyl and aromatic disulfides, though some sulfur content may result from terrestrial weathering oxidative processes.

Comparison with other chondrite groups

CI chondrites stand apart from all other meteorite groups due to their extensive aqueous alteration, with minimal visible chondrules and calcium-aluminum-rich inclusions, and no reported amoeboid olivine aggregates. 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, exceeded only by CV chondrites, alongside the lowest Ca/Si ratio among all carbonaceous chondrites. 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 minerals, along with significant amounts of magnetite, carbonates, and sulfates. These mineralogical differences reflect varying water-to-rock ratios and alteration temperatures during parent body processing.

Formation and Alteration

Solar Nebula Conditions Required for CI Formation

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 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.
Although classified as Type 1 chondrites, CIs do contain rare chondrule fragments, anhydrous minerals, and CAIs. 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.