CI chondrite
[1][2] Some specimens which are classified as borderline CIs found in Antarctica are sometimes referred to as a separate group, the CY chondrites.
The abbreviation CI is derived from the C for carbonaceous and in the name scheme of Wasson,[3] the I from Ivuna, the type locality in Tanzania.
[5] The meteorite of the type locality Ivuna in Tanzania fell in 1938 splitting into three pieces of altogether 705 grams (24.9 oz).
This was followed in 1965 by a very bright fall in Revelstoke, British Columbia, but only two tiny fragments of 1 gram (0.035 oz) were found.
[11] CI chondrites are very fragile and porous rocks, which easily disintegrate on their descent through the atmosphere; this explains why mainly small fragments have been discovered so far.
Despite a bolide which "gave promise of being big", it yielded only two tiny fragments weighing below one gram- "the dubious distinction of being the smallest recovered meteorite" [at the time].
[12] CI chondrites are characterized by a black fusion crust which sometimes is difficult to distinguish from the very similar matrix.
The element assay of CI meteorite is used as a geochemical standard, as it has "a remarkably close relationship"[13] to the makeup of the Sun and greater Solar System.
[30][17] 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,[31][32] and presolar grains exist (though too small to be relevant here).
In particular, when the iron abundances of CIs and the Sun did not match,[33][34] it was the solar value that was questioned and corrected, not the meteorite number.
[31][35] 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),[36][37] while the Sun continues burning lithium[38] and possibly other elements[30][32][17] and continually creating helium from e. g., deuterium.
Issues with CI abundances include heterogeneity (local variation),[39][40] and bromine and other halogens, which are water-soluble and thus labile.
Despite the Solar agreement, the common elements carbon and nitrogen rarely condense into minerals for inclusion and recovery as meteorites.
[47][48] However, isotope differences in individual samples (excepting radioisotopes) had once been widely held to be local effects, caused by separation processes (plus spallation, captures, etc.
[66][67] Orgueil,[68][43] Alais, Ivuna[69] and Tonk all assay to higher gas levels than typical meteorites-[70] Revelstoke is too small for traditional measurements.
[72][73] The main petrologic characteristic of Type 1 chondrites, such as CIs, is the lack of recognizable chondrules, thus excepting the sample from Tagish Lake.
The original protosolar condensates olivine and pyroxene, with ionic bonds between their components, are susceptible to water, especially with heating.
Strong aqueous alteration at rather low temperatures (at 50 to 150 °C)[105] – a hallmark of CI chondrites – is indicated by the occurrence of minerals like epsomite, but also by carbonates and sulfates.
Fluid inclusions- crystal voids intact enough to enclose liquids- have been identified in other meteorites,[106][107] and the CI chondrites Ivuna, and likely Orgueil.
[78][90] Magnetite originated from continuing oxidation of sulfides: nominally troilite (stoichiometric FeS) but de facto pyrrhotite (Fe(1-x)S) with pentlandite, pyrite, and their nickel substitutions, etc.
CI-chondrites contain the following minerals: All these ferromagnesian silicates are tiny, equidimensional, idiomorphic grains crystallized at high temperatures.
Yet they have flat, featureless spectra in the bands accessible to common telescopes on Earth, rendering them difficult to identify.
[126][127] The amount of material reaching the Earth as micrometeorites/interplanetary dust is over an order of magnitude-nearly two- greater than as macroscopic objects.
Dust particles and to an extent micrometeorites overcome the fragility filter preventing more CI chondrite recoveries.
Particles of a certain size also benefit from the Poynting-Robertson effect, causing them to encounter Earth at slower relative speeds.
Space probes have upended our conception of comets; in particular, Stardust has returned material from Wild 2 that appears more asteroidal than cometary.
Greater yields from the continent's ice fields have resulted in arguably CI or CI-like specimens, starting with Yamato 82042 and 82162 (Y 82042, Y82162).
[148][149] It was discovered in 2023 that asteroid (3200) Phaethon is the parent body of the CYs using mid-infrared spectroscopy,[150] further supporting the idea of a separate classification for these meteorites.
Tagish Lake is enriched in 17O, but deficient in 18O, placing it closer to the CM meteorites and on the CCAM (carbonaceous chondrite anhydrous mineral mixing) line with the CM-CO clan.
Compared to all the meteorites found to date, CI chondrites possess the strongest similarity to the elemental distribution within the original solar nebula.
