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CI chondrite
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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.
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.
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 is the most abundant element in CI chondrites (46 wt%.), 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. 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%. 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.
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. 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). 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 (10s μm in size) 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 (raspberry-like clusters), 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, 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 (forsterite with fayalite Fa10–20), 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 (found in microscopic veins), vaterite, carbonates, and sulfates are present.
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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.
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.
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 is the most abundant element in CI chondrites (46 wt%.), 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. 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%. 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.
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. 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). 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 (10s μm in size) 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 (raspberry-like clusters), 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, 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 (forsterite with fayalite Fa10–20), 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 (found in microscopic veins), vaterite, carbonates, and sulfates are present.