Carbonaceous chondrites CI
Synonyms: Ivuna-like carbonaceous chondrites
General: The chondrites of the CI group are named for their type specimen Ivuna, a meteorite that fell in Tanzania in 1938, and there is only a mere handful of these rare meteorites known. The CI chondrites represent some of the most primitive matter having been recovered so far - and therefore they are some of the most interesting meteorites.
Description: Mostly small, black, and very friable rocks that remind more of a piece of tar or charcoal than of a stone. CI chondrites exhibit a very low density, and high porosity, and they show a mat black fusion crust that is hardly to distinguish from their black mottled interiors. All of them belong to the petrologic type 1, which means that they suffered a large degree of aqueous alteration.
Mineralogy: Due to the aqueous alteration, CI chondrites 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 such as 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 hold fascinating clues to the origin of life.
Origin and Formation: 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 primordial matter. Even if that's not the case, 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.
Members: This rare group comprises just five members, all of them being witnessed falls: Ivuna, Alais, Orgueil, Tonk Revelstoke