|— Group —|
Carbonaceous chondrites with a CI chondrite in the center (Tagish Lake - CI 2)
|Total known specimens||Six|
|TKW||17 kilograms (37 lb)|
|Alternative names||CI chondrites, C1 chondrites, CI chondrite meteorites, C1 chondrite meteorites|
CI chondrites, sometimes C1 chondrites, are a group of rare stony meteorites belonging to the carbonaceous chondrites. Samples have been discovered in France, Canada, India, and Tanzania. Compared to all the meteorites found so far, their chemical composition most closely resembles the elemental distribution in the sun's photosphere.
The abbreviation CI is derived from the C for carbonaceous and from the I for Ivuna, the type locality in Tanzania. The 1 in C1 stands for the type 1 meteorites in the classification scheme of Van Schmus-Wood. Type 1 meteorites normally have no recognizable chondrules.
There are very few finds of CI chondrites, six so far altogether. The oldest find dates back to the year 1806: two meteorites were seen near Alès (or Alais) in France. Consequently, two pieces weighing 6 kilograms were discovered at Saint-Étienne-de-l'Olm and Castelnau-Valence, small villages S-E of Alès. In 1864 another fall happened in France at Orgueil near Montauban. The meteorite had disintegrated into 20 pieces weighing a total 10 kilograms. In 1911 a meteorite was seen near Tonk (Rajasthan) in India. Only a few fragments were recuperated that weighed a mere 7.7 grams (0.27 oz). 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. All in all roughly 17 kilograms of CI-chondrites exist so far.
During the Apollo 12 mission a meteorite was found 1969 on the moon, which was first thought to be a CI chondrite, but later turned out to be a closely related CM chondrite. In 2000 a fall occurred at Tagish Lake in the Yukon Territory. This meteorite is meanwhile included within the CI chondrites, although it contains chondrules. It has been designated CI 2.
CI chondrites are very fragile and porous rocks, which easily disintegrate on their descent through the atmosphere, which explain why mainly small fragments have been discovered so far. A good example is the very bright Revelstoke fall which yielded only two tiny fragments weighing below one gram. CI chondrites are characterized by a black fusion crust which sometimes is difficult to distinguish from the very similar matrix. The opaque matrix is rich in carbonaceous material and contains black minerals like magnetite and pyrrhotite. At some places white, water-bearing carbonates and sulfates are incorporated.
The main characteristic of CI chondrites is the lack of recognizable chondrules (an exception being the sample from Tagish Lake). Yet small chondrule fragments and calcium-aluminium-rich inclusions (CAI's) do occur, but are quite rare.
CI chondrite mineralogy is dominated by a fine-grained phyllosilicate matrix, hosting carbonates, sulfates, sulfides, and magnetite. CI-chondrites contain the following minerals:
All these ferromagnesian silicates are tiny, equidimensional, idiomorphic grains crystallized at high temperatures.
- magnetite. Occurs as framboids, spherulites and platelets.
- cubanite. Intergrown with pyrrhotite.
Carbonaceous minerals include:
The ferromagnesian minerals are isolated and show no signs of alteration. As concerns the genesis of the montmorillonite and the serpentine-like minerals it is assumed that they were produced from magnesium-rich olivines and pyroxenes by aqueous alteration.
CI chondrites contain between 17and 22 weight % water. Their high porosity (of up to 30%) seems to be correlated to that fact. The water is not occurring freely, but is rather tied up in water-bearing silicates. Strong aqueous alteration at rather low temperatures (at 50 to 150 °C)  – a hallmark of CI chondrites – is indicated by the occurrence of minerals like epsomite, but also by carbonates and sulfates. Liquid water must have penetrated the parent body through cracks and fissures and then deposited the water-bearing phases.
Iron is present with 25 weight %, but mainly in oxidised form (magnetite). Iron sulfides like pyrrhotite, pentlandite, troilite and cubanite do occur, but elemental iron is absent. The Mg/Si ratio of 1.07 is rather high. Only CV chondrites are more strongly enriched in magnesium. The Ca/Si ratio of 0.057 is the lowest of all carbonaceous chondrites. As regards the oxygen isotopes CI chondrites have the highest values in δ 17O and δ 18O among the carbonaceous chondrites. The ratio 17/18 compares with terrestrial values.
Because of their high porosity CI chondrites have only a density of 2.2 g/cm3.
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 (in the sun lithium 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.
CI chondrites and the closely related CM chondrites are very rich in volatile substances, especially in water. It is assumed that they originally formed in the outer asteroid belt, at a distance surpassing 4 AU - the reason for this being the so-called snow line situated at this distance and representing a temperature of 160° K. At these conditions any water present condensed to ice and was therefore preserved. 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).
- Dodd, R. T.: Meteorites: A Petrologic-Chemical Synthesis. Cambridge University Press, New York 1981
- Zolensky, M. E. & McSween, H.Y.: University of Arizona Press, Tucson 1988
- Zolensky, M. E. & Thomas, K. L. (1995). GCA, 59, p. 4707 – 4712
- Mason, B.: Meteorites. John Wiley and Son Inc., New York 1962
- 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
- Van Schmus, W. R. & Hayes, J. M.: Chemical and petrographic correlations among carbonaceous chondrites. In: Geochimica Cosmochimica Acta. 38, 1974