Carbonaceous chondrites

Carbonaceous chondrites represent only 5.7% of all falls (just about like the irons). They are chemically the most oxidized of all chondrites. They contain virtually no free metal; all the Fe in them is oxidized. That is not to say that carbonaceous chondrites appear rusty. These meteorites typically display a very dark matrix, black to gray in color, containing relatively large amounts of carbon and other organic matter, including amino acids, the building blocks of proteins and thus of life on Earth. Their matrix also contains whitish irregular-shaped specks known as calcium-aluminum-rich inclusions (CAIs). The CAIs consist of minerals uncommon on Earth, with high concentrations of refractory elements such as titanium (Ti). Grains of interstellar material, including microscopic diamonds, have also been found in the matrix and chondrules of carbonaceous chondrites. The chondrules in carbonaceous chondrites are usually well-defined, but they may, in some (rare) cases, be altogether absent. 

Carbonaceous chondrites are further subdivided into four subgroups, in two different ways: 1) with respect to elemental composition; b) by petrologic type. In the first case, the subdivision is based on differences in the abundance of so-called minor and trace elements (for instance calcium, potassium, iridium and zinc). The resulting four subgroups are designated CI, CM, CO and CV, after their typical representatives, the carbonaceous chondrites I, Murchison, Orgueil and V, respectively. In the second subdivision scheme, the carbonaceous chondrites are classified on a petrological (as opposed to compositional) basis more specifically according to their state of alteration. The resulting four subgroups are designated C1, C2, C3 and C4. As described earlier, grade 3 is the least altered of all. It should be emphasized that the alteration processes involved here took place on the parent bodies of the meteorites, not after their arrival on Earth. The fact that aqueous alteration has affected some carbonaceous chondrites is of fundamental importance: it implies that liquid water was available on their parent worlds. There is no simple correspondence between the compositional (elemental) subdivision and the petrologic subgroups. 

Carbonaceous chondrites being relatively fragile, most of the ones known are falls. Allende and Murchison are particularly famous because they fell in relatively large numbers (due to fragmentation during transit through the Earth's atmosphere) and in very recent times. They have been studied extensively, with modern techniques and before the onset of any significant weathering alteration. 

Carbonaceous chondrites might come from the most primitive asteroids known, the C and/or D-type asteroids. Most C and D-type asteroids are located near the outer reaches of the asteroid belt and may, therefore, be the most remote sources of meteorites available. Interestingly, however, Phobos and Deimos, the two small moons of Mars, are also C and D-type objects (respectively) and are much closer to the Earth. They might once have been rogue asteroids which were captured by Mars.
Some carbonaceous chondrites could conceivably have come from the martian moons. Because of the presence of organic matter of extraterrestrial (although likely not biogenic) origin in carbonaceous chondrites, these meteorites are believed to hold fundamental clues to the origin of life on Earth.

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