In addition to the silicaceous chondrules, chondrites contain variable amounts of free metal (Fe, Ni). Chondrites are subdivided into 3 main groups. In order of decreasing degree of oxidation of the iron (Fe) that they contain, these are: carbonaceous chondrites, ordinary chondrites, and enstatite chondrites.
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Within each of these groups, chondrites are also classified according to their petrologic type; that is, on the basis of the degree of alteration (or alteration grade) to which they have been subjected on their parent body prior to arriving on Earth. This degree of alteration ranges from 1 to 6, grade 3 corresponding to the least altered state. The grade decreases from 3 to 1 as aqueous alteration (alteration by liquid water) intensifies, while the grade increases from 3 to 6 as thermal metamorphism (alteration by heating) increases. At grade 6, chondrites have undergone such intense heating and attendant recrystallization that chondrules may be almost completely obliterated. Carbonaceous chondrites have mostly undergone aqueous alteration (none are of grade higher than 4), while most ordinary and enstatite chondrites have undergone thermal metamorphism (none are of grade lower than 3).
- 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.
- Ordinary chondrites
- Ordinary chondrites represent 79% of all falls. They are subdivided into 3 subgroups on the basis of content in free metal (Fe, Ni): H (high), L (low), LL (low low, i.e. very low). All ordinary chondrites are rich in the mineral olivine.
- H chondrites
- H chondrites are characterized by relatively high iron, nickel and sulfide contents (16-22% by weight). The sulfide is essentially iron sulfide (FeS), a mineral known as troilite. Because bronzite is the dominant form of pyroxene in these meteorites, H chondrites are also known as olivine-bronzite chondrites. H chondrites may be identified by the fact that their matrix displays abundant iron flakes and facets, which are often oxidized. Alteration grades for H chondrites range from 3 to 6. (There are no H1 or H2 chondrites). As described earlier, H6 chondrites have undergone significant thermal metamorphism, whereas H3 chondrites have not. Most H chondrites are of the H5 petrologic type.
- L chondrites
- L chondrites are characterized by a low free metal and sulfide content (7-16% by weight). Because hypersthene is the dominant form of pyroxene in these meteorites, L chondrites are also known as olivine-hypersthene chondrites. Alternation grades for L chondrites range from 3 to 6. Most L chondrites are of the L6 petrologic type.
- LL chondrites
- LL chondrites have the lowest free metal and sulfide content (less than 7% by weight). As for the L chondrites, hypersthene is their dominant form of pyroxene. To distinguish the LLs from the L chondrites, LL chondrites are sometimes referred to as amphoterites. The matrix of LL chondrites displays very little visible iron. Alteration grades for amphoterites range from 3 to 6. Most Ll chondrites are of the LL6 petrologic type.
- Enstatite chondrites
- Enstatite chondrites (or E chondrites) represent only 1.0% of all falls. They have the highest free metal and sulfide content (23-35% by weight) and the lowest oxidation state of all chondrites. Their silicate phase is almost purely enstatite (MgSiO3), an iron-free form of pyroxene. A distinction is sometimes made between EH and EL chondrites on the basis of their free metal and sulfide contents (H for high, L for low). Quite distinctively, all the iron in E chondrites is visible as free metal. Alteration grades for E chondrites range from 3 to 6. Most E chondrites are of the E6 petrologic type.