Carbonaceous chondrites or C chondrites represent some of the most pristine matter known, and their chemical compositions match the chemistry of the Sun more closely than any other class of chondrites. Carbonaceous chondrites are primitive and undifferentiated meteorites that formed in oxygen-rich regions of the early solar system so that most of the metal is not found in its free form but as silicates, oxides, or sulfides. Most of them contain water or minerals that have been altered in the presence of water, and some of them contain larger amounts of carbon as well as organic compounds. This is especially true for the carbonaceous chondrites that have been relatively unaltered by heating during their history. The most primitive carbonaceous chondrites have never been heated above 50°C!
However, there are different clans and groups of carbonaceous chondrites that formed on different parent bodies in different regions of the early solar nebula. The most important groups are designated as CI, CM, CV, CO, CR, CK, and CH chondrites. In the following, we'd like to introduce these groups as well as a new group, the CB chondrites, also known as bencubbinites.
Chondrites of the CI group are named for their type specimen Ivuna, which fell in Tanzania in 1938, and there are only a handful of those rare meteorites known. The CI chondrites represent some of the most primitive, friable, and "ugly" meteorites - and yet they are some of the most interesting ones.
They all belong to the petrologic type 1, which means that they suffered a large degree of aqueous alteration. Hence they don't contain any relict chondrules but instead, a large amount of water, up to 20%, in addition to lots of minerals that have been altered in the presence of water such as hydrous phyllosilicates similar to terrestrial clays, oxidized iron in the form of magnetite, and sparsely distributed crystals of olivine scattered throughout the black matrix. In addition, they contain certain amounts of organic matter like PAHs and amino acids, which are the building blocks of life on earth. Because of that peculiar mixture of water and complex organic compounds the chondrites of the CI group are suspected to contain fascinating clues to the origin of life on our planet and maybe elsewhere in the universe too!
Some researchers suggest the origin of the CI chondrites is from comets that are known to be "dirty snowballs" - a mixture of frozen water and pristine matter. Even if that isn't true, the origin of the CI chondrites is certainly in the outer reaches of our solar system since they never have been heated above 50°C during their formation and their subsequent history. Otherwise, the water would have evaporated quite rapidly and the hydrous phyllosilicates would have been metamorphosed into other minerals due to the loss of water.
This group is named for its type specimen, the meteorite of Mighei, which fell in Ukraine in 1889, and it has many more members than the previous group. About 80 chondrites of the CM group are known, and they mostly belong to petrologic type 2, although certain lithologies in some of its members are known to belong to type 1 (e.g. in Cold Bokkeveld). With about 10% water, they contain less than the CI chondrites and show less aqueous alteration so that some chondrules have been well preserved. Those chondrules consist of olivine and are scattered throughout the black matrix. In that mixture of phyllosilicate and magnetite, similar to the matrix of the CI chondrites, one also finds light-coloured inclusions. These high-temperature silicates are lacking in the CI group.
As with the CI chondrites, the CM chondrites are well known to contain a wealth of complex organic compounds. The well-studied meteorite of Murchison, a CM2 that fell in Australia in 1969, was found to contain more than 230 different amino acids, whereas on earth only 20 different amino acids are known and used as fundamental building blocks of life. Some of these extraterrestrial amino acids were found to exhibit strange isotopic signatures that might indicate that they don't have their origin within our solar system. These amino acids are believed to represent actual interstellar matter from other systems and nebulae that were trapped in this meteorite more than 4.5 billion years ago.
Because of this fact, some researchers have promoted the idea that Murchison and other CM chondrites, e.g. the witnessed falls of Murray, and Nogoya, might be of cometary origin, but recent research indicates that certain dark asteroids within the main asteroid belt are the real source of the CM meteorites. There is for example a certain spectral match between the reflectance spectra of the CM chondrites and the largest asteroid of our solar system, 1 Ceres - an irregular dark chunk of matter in the size of Texas. However, recent research has found an even closer match, at least for Murchison - the asteroid 19 Fortuna which is a good candidate to be the lost parent body of this peculiar meteorite and maybe of the other CM chondrites, too.
The chondrites of this group are named for their type specimen, the meteorite of Vigarano, which fell in Italy in 1910. The CV group has about 50 members, but the number of actual CV falls has to be estimated to be somewhat lower since many of them are paired finds from the hot deserts of Africa and the blue-ice fields of Antarctica.
Most CV chondrites belong to petrologic type 3, and only one has been found to belong to type 2 as well as one other that has been classified as type 4. The structure and composition of these carbonaceous chondrites is more close to that of ordinary chondrites. In a dark-grey matrix of mainly iron-rich olivine, the meteorites of the CV group exhibit large, well-defined chondrules that are made of magnesium-rich olivine, often surrounded by iron sulfide. The meteorites of this group also contain white, irregular inclusions of different size that often make up more than 5% of the meteorite. These inclusions are high-temperature minerals called CAIs (calcium-aluminium inclusions) and are composed of silicates and oxides of calcium, aluminium, and titanium.
These large CAIs, characteristic of CV chondrites, have been intensely studied in the famous meteorite of Allende. Allende fell in Mexico in 1969, shortly before Neil Armstrong took his first step on the Moon. The CAIs of Allende contain fine-grained, microscopic diamonds - and those diamonds exhibit strange isotopic signatures that point to an origin outside of our solar system. They are interstellar grains that have proven to be older than the earth and the sun, and probably they are the product of a nearby supernova, of a dying star that made his last breath when our own system formed. Traces of this supernova have been trapped within the CAIs and preserved in the CV group and other carbonaceous chondrites to this day.
The chondrites of the CV group are further divided into three subgroups. The type specimen Vigarano and some other meteorites belong to the reduced subgroup designated as CV3R. These CVs show a higher chondrule abundance as well as more reduced metal and less magnetite than the other two oxidized subgroups. One of these oxidized subgroups is named for the fall of Allende and has been designated as CV3OxA. The meteorites of this subgroup contain minerals like andradite, grossular, kirschsteinite, nepheline and others that aren't found in any other CV subgroup. The other oxidized subgroup is named for the fall of Bali and is designated as CV3OxB. The members of this subgroup represent the most oxidized CVs and show traces of aqueous alteration as well as phyllosilicates that aren't found in the other two subgroups.
The meteorites of this group are named for their type specimen Ornans that fell in France in 1868 - by that, not far from our home in the French department Doubs. There are only about 25 members of this group if we don't count all the probable pairings - especially from the Dar al Gani region, Libya, where many COs have been found. It's more than probable that all Dar al Gani COs arrived in one or two distinct falls.
All members of this group of carbonaceous chondrites belong to the petrologic type 3, and they show a certain relation to the CV group when it comes to chemistry and composition. Therefore, many researchers suppose that the CV and the CO group represent a distinct clan of carbonaceous chondrites that formed in the same region of the early solar system. However, the conditions under which the COs formed must have been different from the conditions under which the CVs formed because there are obvious differences.
First, the chondrites of the CO group are mostly of a more black appearance (although a few are dark-grey; e.g. the type specimen Ornans) and exhibit much smaller chondrules. These tiny chondrules are packed densely within the matrix, representing over 70% of the entire meteorite. In CV chondrites, this ratio is reversed; only about 30% of the meteorite is composed of large chondrules. As in the CV group, the members of the CO group contain CAIs, but these inclusions are commonly much smaller and more sparsely distributed throughout the matrix. Typical for the COs are clearly visible, small inclusions of free metal, mostly nickel-iron, that appear like tiny flakes on the polished surfaces of a fresh, unweathered CO3. From this we can conclude that the chondrites of the CO group formed under even more reducing conditions than the CVs of the reduced subgroup which don't show that much metal in its free form.
The meteorites of this group are named for Karoonda, a meteorite that fell in Australia in 1930. There are only about 20 different CK members known if we exclude all the pairings that have been found so far in the hot deserts of Africa and on the blue-ice fields of Antarctica. Initially, those meteorites were regarded as members of the CV group and were designated as CV4-5. However, more recently, they have been given their own group since they differ in some respect from all the other carbonaceous chondrites.
The chondrites of the CK group belong to the petrologic types 3 - 6, although most of them have been classified as CK4. They are of a dark-grey or black appearance due to a high percentage of magnetite that is dispersed in a matrix of dark silicates, consisting of iron-rich olivine and pyroxene. All of this indicates that they formed under oxidizing conditions, but they don't show any aqueous alteration or phyllosilicates. Elemental abundances as well as the oxygen isotopic signatures suggest that the CK chondrites are closely related to the chondrites of the CO and CV groups and belong to the same clan. They plot somewhere in between those two groups which can be also seen in the size of the chondrules which are intermediate between the typical sizes of CV and CO chondrules. Most CK chondrites sometimes contain large CAIs. In addition, some CK members exhibit shock veins that indicate a violent history for their parent body or an impact history. However, scientists have not yet identified a spectral match for a possible parent body for these rare carbonaceous chondrites yet.
The chondrites of the CR group are named for their type specimen Renazzo, which fell in Italy in 1824. There are only about 15 CR chondrites known, with Renazzo initially classified as a "type II" CM2 chondrite.
However, the CR chondrites are very different from the CM group, although they also mostly belong to petrologic type 2. Like the CM chondrites, they contain hydrosilicates, traces of water, and magnetite. The main difference is that they contain reduced metal in the form of nickel-iron and iron sulfide of up to 10%. This metal is found in the black matrix as well as in the large and clearly visible chondrules that make up about 50% of the meteorites. Sometimes the orange-coloured chondrules are "armored"; i.e. imbedded into small rims of nickel-iron or iron sulfide. All this is typical for the chondrites of the CR group, and it's quite easy to distinguish them from members of other carbonaceous chondrite groups.
Scientists have searched for the origin of the CR chondrites, comparing different reflectance spectra of asteroids with the spectra of the known CR members. There's quite a good match between the spectra of the CR members and one of the most prominent asteroids in our solar system, 2 Pallas, the second largest asteroid known. In addition, maybe we have other samples of this prospective parent body since modern research suggests that two other groups of carbonaceous chondrites are closely related to the CR chondrites - the CH chondrites and CB chondrites or bencubbinites. Together they form the so-called CR clan that either has its origin in one and the same parent body or at least in a common region of the early solar system in which they formed under similar, more reducing conditions.
This group of carbonaceous chondrites is somewhat of an exception since its name is not derived from a type specimen but from one of the typical properties of those meteorites. The "H" stands for "high metal" since the CH chondrites contain up to 15% nickel-iron. There are only about 10 meteorites known which belong to the CH group if one excludes probable pairings. The first of those meteorites has been found in the Antarctic Allan Hills and has been named ALH 85085, so it can be regarded as the type specimen of the CH group. So maybe it would be better to call them "CA group" holding on to the system of nomenclature that has been used for the other groups of carbonaceous chondrites. However, the "CH" is already established, and it might cause too much confusion to change this term just for keeping with the rules of a stringent nomenclature.
All chondrites of the CH group belong to petrologic types 2 or 3, and chemically they are very close to the CR chondrites and the bencubbinites (CB chondrites). Besides their high abundance of nickel-iron, they show many fragmented chondrules, only a few remaining intact. Most of these chondrules as well as the less abundant CAIs are very small which is typical for the chondrites of the CH group. As with the CR chondrites, the members of the CH group contain certain amounts of phyllosilicates and other traces of aqueous alteration that took place during their history of formation.
Some researchers have suggested that the CH chondrites formed in close proximity to the Sun. This is reflected in the abundance of certain trace elements as well as in mineralogy. It is believed that they condensed in a very early stage from the hot primordial nebula inside what is today the orbit of Mercury and have been later transported to outer and cooler regions of the nebula where they have been more or less preserved to this day. An interesting coincidence is that the planet Mercury might have formed from similar, metal-rich material. This would explain its high density and its extraordinary large metal core that makes Mercury unique among all other terrestrial planets in our solar system.
The meteorites of this newly designated group are named for their type specimen Bencubbin, which has been found in Australia in 1930. Only five meteorites constitute this group, plus one new member that has recently been found by our team, currently under analysis.
The bencubbinites are strange meteorites that contain more than 50% nickel-iron. If you consider this, they could be easily regarded as true stony-irons, but their mineralogical and chemical properties clearly put them into the clan of the carbonaceous chondrites, or more strictly speaking into the CR clan. Besides the free metal, they contain highly reduced silicates as well as armed chondrules similar to those found in the members of the CR group. Some members of the CB group also contain CAIs; e.g. the meteorite HaH 237 from Libya, which was previously classified as a metal-rich CH chondrite.
This shows the close relationship between the CH and CB chondrites, which are both members of the CR clan. It is probable that all members of this clan formed under different conditions in the same region of the primordial solar nebula, but it's also possible that they are all part of one and the same parent body. That being so, 2 Pallas, the second largest asteroid of our solar system, would be a prominent candidate to be the possible common source of the meteorites of this clan. At least the reflectance spectra of the CR chondrites seem to match the spectra of 2 Pallas quite closely suggesting that those meteorites might have been derived from this large asteroid through impact events. On the other hand the meteorites of the CR clan don't show too many signs of an impact history such as brecciation, shock-veins etc. so that they might have been derived from much smaller parent bodies which aren't to be identified that easily.
Some carbonaceous chondrites don't fit into established groups, although they can be easily classified as members of the carbonaceous chondrite clan. They are usually designated as C ungrouped or "C UNGR" and they probably represent other parent bodies of carbonaceous chondrites or source regions of the primordial solar nebula. Some of them show a certain relationship to each other and to other groups of carbonaceous chondrites, but the Meteoritical Society has decided that it needs at least five members to constitute a new group.
However, certain new groups and "grouplets" have been proposed in the past; for example, the Coolidge grouplet named for the meteorite from Coolidge that was found in Kansas, USA, in 1937. There are two other carbonaceous chondrites officially designated as C UNGR that show a similar high matrix/chondrule ratio as Coolidge as well as the same enrichment in refractory elements. Maybe this grouplet and others will gain the status of a fully accepted group as soon as new members are found and recognized in the wealth of new meteorites from the hot deserts of Africa and Asia, as well as from the blue-ice fields of Antarctica.