Chondrites

Chondrites are the most primitive asteroids. They are named for their primary constituent, chondrules, spherical grains that directly condensed out of the solar nebula and which underwent one or more heating and cooling cycles. Individual chondrules are thought to have formed in distinctive locations within the solar nebula, but a chondrite typically contains a mix of chondrites. Chondrites will also contain some CAI's, and a cement (matrix) formed from fine-grained dust.  There are generally many inclusions of assorted minerals, including nickel-iron grains, iron sulfides, magnetite, and many other minerals, some unique to asteroids.

Especially interesting are inclusions called presolar grains, sometimes called stardust. These are minerals thought to have formed in supernovae or the outflows of other suns (such as red giants), and certainly formed well before the other components of the chondrites. They pre-date the solar system, hence the name. Presolar grains typically have distinctive isotopic compositions reflecting their unique and alien birth. Presolar grains include nanodiamonds, graphite, moissanite, titanium carbide, silicon nitride, sapphire, spinel, olivine, and pyroxene, as well as a wide variety of other minerals.

Chondrites are bodies which underwent little or no heating and differentiation, but of course some are more pristine than others. The larger parent bodies would have generated significant internal heat from the decay of relatively short-lived isotopes, melting ice and allowing the formation of clays and other hydrated minerals. Still larger bodies would have generated enough heat to have (at least partially) melted, resulting in other asteroid types.

Chondrites include the following types:

• Ordinary Chondrites are common in the inner asteroid belt, and comprise about 80% of all meteorites that fall to Earth, although they are only 10%-15% of all asteroids.
  • 40% of ordinary chondrites are H chondrites which have a High-iron content
  • 50% of them are L chondrites, with a low-iron content.
  • 10% of them are LL chondrites, with a Low iron and overall Low metal content.
• Carbonaceous Chondrites contain a significant percentage of carbohydrates, and are typically dark. They are common in the outer Asteroid Belt, representing about 75% of all asteroids. Their subgroubs are usually identified by a "type meteorite" for which they are named. Each group is thought to consist of meteorites sharing a common parent asteroid.
  • CI group (Ivuna) may be the most pristine, with a chemical composition nearly identical to the Sun, minus gasses including hydrogen and helium. They contain up to 22% water, and show evidence of warming enough to generate hydrated/hydroxide minerals such as phyllosilicates (including clays). CI chondrites also contain significant organic matter including Polycyclic Aromatic Hydrocarbons (PAH's) and amino acids.
  • CM group (Mighei, although the best known sample is the Murchison meteorite) chondrites contain perhaps 30% small chondrules and CAI's and 70% matrix (dust). About 12% of the Murchison meteorite is water (in the form of hydrous minerals).
  • CO group (Omans) have about 70% small chondrules and CAI's in 30% matrix, and show little aqueous processing.
  • CR group (Renazzo) chondrites are about half matrix and half chondrules with few CAI's, while being fairly rich in metallic nickel-iron. They have generally experienced significant aqueous alteration.
  • CH group (high metal) carbonaceous chondrites have very tiny chondrules (averaging 20 microns), with clumps of dust instead of typical matrix, and with a very small fraction of volatile elements. They are rich in metallic nickel-iron compared to most carbonaceous chondrites.
  • CB group (Bencubbin) carbonaceous chondrites have the highest metal content and the lowest volatile element content.
  • CV group (Vigarano) chondrites contain very abundant CAI's in a roughly equal mix of millimeter-sized chondrites and matrix. The CAI's can be quite large (centimeters). The best-known members of this group are the Allende meteroites.
  • CK group (Karoonda) chondrites are similar to CV's, but contain far fewer CAI's, a higher degree of oxidation, and the greatest degree of thermal metamorphism of all carbonaceous chondrites.
• E-type or Enstatite chondrites (named for their high enstatite (MgSiO₃) content) are extremely reduced; their iron is elemental or sulfides. They comprise about 2% of meteorite falls on Earth. The extremely reducing environment of formation for Enstatite chondrites created a variety of unusual minerals, including oldhamite (CaS), niningerite (MgS), and perryite (Fe-Ni silicide).