Ci ChondritesEdit

CI chondrites

CI chondrites, also known as Ivuna-type carbonaceous chondrites, are among the most primitive and chemically informative meteorites recovered on Earth. They are distinguished by their near-solar elemental abundances, high water content, and abundant hydrated minerals, all of which point to extensive aqueous alteration on their parent body in the early solar system. The name derives from the Ivuna meteorite discovered in Tanzania in 1938, a type specimen for this class; other famous members include Orgueil, a meteorite recovered in 1864 in France. As a group, CI chondrites are rare and highly weathered, in part because they are chemically fragile and easily altered by water and terrestrial exposure after delivery to Earth. They are a key resource for understanding the composition of the solar nebula and the early processes that operated on carbon-rich bodies in the asteroid belt.

Classification and Nomenclature

• Origin of the name: The designation CI comes from Ivuna, the type specimen. The “I” and “C” reflect their classification as carbonaceous chondrites with distinctive chemical signatures.
• Relationship to other meteorite groups: CI chondrites sit within the broader family of carbonaceous chondrites, but they are set apart by their lack of chondrules, extreme aqueous alteration, and compositions that closely resemble solar photospheric abundances for many elements.
• Subtypes and petrologic characteristics: CI chondrites are typically described as having undergone substantial aqueous alteration and are commonly classified by a low petrologic type, indicating early, pervasive aqueous processing on their parent body. This alteration has consequences for mineralogy, chemistry, and the preservation of organic material.
• Notable members: Orgueil and Ivuna are among the best-known CI chondrites; other specimens from various locations have reinforced the definition of this group and its terran alteration history.

Mineralogy and Petrography

• Mineral suite: The CI chondrite matrix is rich in hydrated minerals, especially phyllosilicates such as serpentine- and clay-group minerals. The presence of these minerals attests to long-lived aqueous activity on the meteorite’s parent body.
• Absence of chondrules: Unlike many carbonaceous chondrites, CI chondrites typically lack chondrules, the rounded silicate inclusions common in other groups, reinforcing their classification as highly altered, primitive material.
• Metal and sulfides: CI chondrites are relatively deficient in metallic iron and sulfides compared with many other meteorite classes, consistent with extensive alteration and loss or redistribution of metal during aqueous processing.
• Organic-rich matrix: The matrix hosts a diverse assemblage of organic compounds, including both simple and more complex molecules, which have implications for prebiotic chemistry in the early solar system.

Geochemistry and Isotopic Signatures

• Elemental abundances: CI chondrites are often described as having nearly solar abundances for many elements, making them a benchmark for primordial solar system composition. They serve as a reference point for comparing other meteorites and planetary materials.
• Volatiles and hydration: The high content of water-bearing minerals reflects extensive hydration, which has altered the original mineralogy and redistributed volatiles during alteration on the parent body.
• Oxygen isotopes and other isotopes: The isotopic compositions of oxygen and other elements in CI chondrites reveal the presence of distinct solar system reservoirs and contribute to debates about the mixing and processing of materials in the early solar nebula.
• Organic matter and amino acids: CI chondrites harbor a suite of organic compounds, including amino acids and hydrocarbons, whose distributions and enantiomeric patterns are studied for clues about prebiotic chemistry and the delivery of organics to early Earth.

Hydration, Alteration, and the Early Solar System

• Aqueous alteration: The defining feature of CI chondrites is extensive aqueous alteration, which converted primary silicates into hydrous minerals and mobilized elements within the matrix. This process records the interaction of liquid water with primitive solids in the early solar system.
• Water-rich worldviews: The hydration state of CI chondrites provides evidence for the presence of liquid water on their parent body, a factor that has implications for the thermal and chemical evolution of carbon-rich asteroids.
• Terrestrial weathering and preservation: Because CI chondrites are physically fragile and highly reactive, they are particularly susceptible to terrestrial alteration after exposure on Earth. Careful handling and analysis are required to distinguish inherited features from Earthly contamination.
• Implications for planet formation: The combination of near-solar elemental abundances with strong aqueous processing makes CI chondrites valuable for testing models of the distribution of water and volatiles in the early solar system and for constraining the materials that contributed to the formation of terrestrial planets.

Cosmochemistry and Prebiotic Significance

• Solar-system baseline: CI chondrites have long been used as a standard against which other meteorites and planetary materials are compared when reconstructing the composition of the solar nebula.
• Organic chemistry: The presence of a wide array of organic molecules, including amino acids, in CI chondrites has made them central to discussions about the delivery of prebiotic ingredients to early Earth and the broader question of how life’s building blocks may have originated elsewhere in the solar system.
• Isotopic reservoirs and presolar grains: The isotopic compositions and the distribution of presolar grains in CI chondrites contribute to our understanding of stellar contributions to the solar system’s initial material and the extent of processing in the solar protoplanetary disk.

Parent Body and Origin

• Likely source region: CI chondrites are associated with carbon-rich material in the asteroid belt, where low-velocity accretion and prolonged aqueous alteration could occur on exposed surfaces of primitive bodies.
• Evolutionary history: Their histories reflect chemical processing in the outer regions of the early solar system, with water-driven alteration playing a central role in transforming their original mineralogy.
• Relationship to other meteorites: Comparing CI chondrites with other carbonaceous chondrites, as well as with non-carbonaceous meteorites, helps researchers to map the diversity of early solar system materials and the processes that distributed volatiles and organics.

Controversies and Debates

• Representativeness of solar composition: While CI chondrites are often treated as near-solar baselines for bulk compositions, some researchers argue that their heavy alteration and volatile loss on the parent body mean they do not perfectly reflect the Sun’s primordial makeup. Ongoing work seeks to disentangle nebular inheritance from parent-body processing.
• Depth of aqueous alteration on their parent body: There is debate about the extent, duration, and conditions of aqueous activity on the CI parent body, including what drove hydrothermal systems and how uniformly alteration affected the entire body.
• Origins of organic material: The provenance and synthesis pathways of CI chondrite organics are debated, with discussions about interstellar vs. solar nebula chemistry, in-situ aqueous synthesis, and the degree to which terrestrial contamination may affect detected amino acids and other organics.
• Preservation biases: Given their fragility and susceptibility to terrestrial weathering, interpretations based on CI chondrites may be biased toward the pieces that survived delivery and recovery, complicating attempts to reconstruct the full diversity of their original materials.

See also

• Ivuna
  
• Orgueil
  
• carbonaceous chondrite
  
• chondrite
  
• phyllosilicate
  
• amino acids
  
• organic compounds
  
• isotopes
  
• cosmochemistry
  
• solar system