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. All carbonaceous chondrites are primitive and undifferentiated meteorites that formed in oxygen-rich regions of the primordial solar nebula so that most of the metal is not found in its free form but in the form of silicates, oxides, or sulfides. Most of them contain water or minerals that have been altered in the presence of water, and some of them do even contain amounts of carbon as well as different organic compounds, such as amino acids. This is especially true for the carbonaceous chondrites that have been more or less unaltered by heating during their history. The most primitive carbonaceous chondrites have never been heated above 50°C!
Representing less than 6% of all witnessed falls, carbonaceous chondrites are extremely rare. They are also of highest scientific interest as they also often contains calcium-aluminum-rich inclusions (so-called CAIs). These CAIs consist of minerals uncommon on Earth, with high concentrations of refractory elements such as titanium, as well as grains of interstellar or pre-solar materials, including nano-diamonds. Other carbonaceous chondrites do contain amino acids, the building blocks of life on Earth, as well as substantial amounts of water, raising the question of the origin of life itself!
However, the carbonaceous chondrites are a rather heterogenous class, and there are different clans and groups of carbonaceous chondrites that formed on their respective parent bodies in different regions of the early solar nebula. The most important groups are designated as CI chondrites (carbonaceous chondrites Ivuna type), CM chondrites (carbonaceous chondrites Mighei type), CV chondrites (carbonaceous chondrites Vigarano type), CK chondrites (carbonaceous chondrites Karonda type), CO chondrites (carbonaceous chondrites Ornans type), CR chondrites (carbonaceous chondrites Renazzo type), and CH chondrites (carbonaceous chondrites metal rich). Each of these groups will be described on its own page, as well as a relatively new group, the CB chondrites, also known as the bencubbinites. Besides that, there are also several ungrouped carbonaceous chondrites that don't fit neatly into the existing classification schemes, and some of those will also be introduced on a separate page.