Left to right: pieces of the Allende, Yukon, and Murchison meteorites.
A carbonaceous chondrite is a rare type of stony meteorite which contains large amounts of the magnesium-rich minerals olivine and serpentine and a variety of organic compounds, including amino acids. Although fewer than 100 carbonaceous chondrites are known, accounting for only about 5% of chondrite falls, they provide a great deal of information about the origin of the Sun and planets, and even of life itself (see organic matter, in meteorites).
Carbonaceous chondrites are the most primitive and unaltered type of meteorite known, with an elemental composition probably similar to that of the nebula from which the Solar System formed. In addition to silicates, oxides, and sulfides, they contain, most distinctively, water or minerals that have been altered in the presence of water, together with large amounts of carbon, including organic compounds. The most pristine carbonaceous chondrites have never been heated above 50°C. Different groups of carbonaceous chondrites have been identified that came from parent bodies in different parts of the solar nebula.
|Some well-known carbonaceous chondrites|
|where found||date of fall||mass recovered|
|Alais, France||Mar 15, 1806||6 kg|
|Cold Bokkeveld, South Africa||Oct 13, 1838||3 kg|
|Kaba, Hungary||Apr 15, 1857||3 kg|
|Orgueil, France||May 14, 1864||12 kg|
|Nogoya, Argentina||Jun 30, 1879||4 kg|
|Mighei, Russia||Jun 18, 1889||8 kg|
|Indarch, Russia||Apr 7, 1891||27 kg|
|Mokoia, New Zealand||Nov 26, 1908||4 kg|
|Haripura, India||Jan 17, 1921||0.5 kg|
|Ivuna, Tanzania||Dec 16, 1938||0.7 kg|
|Murray, Kentucky, USA||Sep 20, 1950||12.6 kg|
|Allende, Mexico||Feb 8, 1969|
All groups of carbonaceous chondrites except the CH group are named for a characteristic type specimen.
CI chondrites, only a handful of which are known, are named for the Ivuna meteorite. They have very few chondrules and are composed mostly of crumbly, fine-grained material that has been changed a lot by exposure to water on the parent asteroid. As a result of this aqueous alteration, CI chondrites contain up to 20% water in addition to various minerals altered in the presence of water, such as clay-like hydrous phyllosilicates and iron oxide in the form of magnetite. They also harbor organic matter, including polycyclic aromatic hydrocarbons (PAHs) and amino acids, which makes them important in the search for clues to the origin of life in the universe. It remains uncertain whether they once had chondrules and refractory inclusions that were later destroyed during the formation of hydrous minerals, or they lacked chondrules from the outset.
CIs have never been heated above 50°C, indicating that they came from the outer part of the solar nebula. They are especially interesting because their chemical compositions, with the exception of hydrogen and helium, closely resemble that of the Sun's photosphere. They thus have the most primitive compositions of any meteorites and are often used as a standard for gauging how much chemical fractionation has been experienced by materials formed throughout the solar system.
CM chondrites are named for the Mighei meteorite that fell in Mykolaiv province, Ukraine, in 1889. They contain small chondrules (typically 0.1–0.3 millimeters in diameter) and similar-sized refractory inclusions. They also show less aqueous alteration than, and about half the water content of, CI chondrites. Like CIs, however, they contain a wealth of organic material – more than 230 different amino acids in the case of the famous Murchison meteorite. Comparisons of reflectance spectra point to the asteroid 19 Fortuna or, possibly, the largest asteroid, 1 Ceres, as candidate parent bodies.
CV chondrites are named for the Vigarano meteorite that fell in Italy in 1910. They resemble ordinary chondrites and have large, well-defined chondrules of magnesium-rich olivine, often surrounded by iron sulfide, in a dark-gray matrix of mainly iron-rich olivine. They also contain calcium-aluminum inclusions (CAIs) – the most ancient minerals known in the solar system – that typically make up more than 5% of the meteorite.
CO chondrites are named for the Ornans meteorite that fell in France in 1868. They some similarities in composition and chemistry to the CV chondrites and may have formed with them in the same region of the early solar system. As in the CV group, CAIs are present but are commonly much smaller and spread more sparsely in the matrix. Also typical of COs are small inclusions of free metal, mostly nickel-iron, that appear as tiny flakes on the polished surfaces of fresh, unweathered samples.
CK chondrites are named for the Karoonda meteorite that fell in Australia in 1930. They were initially thought to be members of the CV group but are now grouped separately since they differ in some respect from all other carbonaceous chondrites. Their dark gray or black coloration is due to a high percentage of magnetite dispersed in a matrix of dark silicates consisting of iron-rich olivine and pyroxene. This shows they formed under oxidizing conditions, yet there is no sign of aqueous alteration. Elemental abundances and oxygen isotopic signatures suggest that CKs are closely related to CO and CV types. Most CK chondrites contain large CAIs and some show shock veins that point to a violent impact history.
CR chondrites are named for the Renazzo meteorite that fell in Italy in 1824. They are similar to CMs in that they contain hydrosilicates, traces of water, and magnetite. The main difference is that CRs contain reduced metal in the form of nickel-iron and iron sulfide that occurs in the black matrix as well as in the large chondrules that make up about 50% of the meteorites. A possible parent body is Pallas, the second largest asteroid. The CH and CB chondrites are so closely related to the CRs that all three groups may have come from the same parent or at least from the same region of the solar nebula.
CH chondrites are named for their high metal content. They contain up to 15% nickel-iron by weight and are closely related in chemical composition to the CRs and CBs. They also show many fragmented chondrules, most of which, along with the less abundant CAIs, are very small. As with the CRs, the CHs contain some phyllosilicates and other traces of alteration by water. One theory suggests that the CHs formed very early in the solar system's history from the hot primordial nebula inside what is today the orbit of Mercury, later to be transported to outer, cooler regions of the nebula where they have been preserved to this day. Mercury may have formed from similar, metal-rich material, which would explain its high density and extraordinary large metal core.
CB chondrites, also known as bencubbites, are named for the prototype found near Bencubbin, Australia, in 1930. Only a handful of these unusual meteorites are known. All are composed of more than 50% nickel-iron, together with highly reduced silicates and chondrules similar to those found in members of the CR group.
C ungrouped chondrites (C UNGRs)
C ungrouped chondrites fall outside the other groups and probably represent other parent bodies of carbonaceous chondrites or source regions of the primordial solar nebula.
And finally ...
One of the more outlandish and amusing proposals concerning these objects came from Leslie C. Edie of Bellmore, Long Island. In the Apil. 13, 1962 issue of Science,1 he suggested that the long-chain molecules within carbonaceous chondrites might represent encoded information put there by an extraterrestrial civilization (see genome, interstellar transmission). The meteorites were then hurled out into space in the hope that they would eventually be found by another race!
1. Edie, L. C. "Messages from Other Worlds," letter in Science, 136, 184 (1962).