Chondrites are stony meteorites that have not melted since their aggregation early in the history of the Solar System. They have unfractionated elemental compositions that (apart from the most volatile of elements, such as hydrogen and helium) are close to the composition of the Sun, and thus of the original material from which the Solar System formed. Chondrites, therefore are the most significant meteorites for understanding early Solar System chronology, since they are the most primitive of all meteorites, having experienced only mild thermal or hydrothermal metamorphism since accretion into parent-bodies. Almost all chondrites contain chondrules, spherical to sub-spherical assemblages of olivine, pyroxene and feldspar, up to 1 mm in diameter, that have been partially or totally melted prior to parent-body accretion. Many chondrites also contain CAIs (for Calcium, Aluminium-rich Inclusions): irregular-shaped refractory inclusions (up to approx. 1 cm in size) of the oxide and silicate minerals spinel, hibonite, melilite, etc. CAIs frequently exhibit complex mineralogical zoning, both in their rims and in their cores. Chondrites, then, are composed of high temperature components (CAIs, chondrules) set in a matrix of fragmented chondrules mixed with sulphides, metal and minerals formed at lower temperatures (clay minerals, carbonates, sulphates, organic matter). CAIs and chondrules are primary solids, generally presumed to have formed in the nebula prior to aggregation into parent-bodies, although their precise formation processes are not completely understood. The lower-temperature components, phyllosilicates, carbonates, sulphates, etc, are secondary products of fluid activity within parent-bodies. Also preserved within chondritic matrix are unaltered interstellar dust grains: sub-micron-sized diamonds, together with micron-sized silicon carbide, graphite and aluminium oxide grains. These materials were introduced into the pre-solar nebula from neighbouring stars, prior to parent-body aggregation, and thus pre-date the major chondritic components.
Chondrites are sub-divided on the basis of chemistry, matrix, metal and chondrule contents and chondrule properties (size, type, etc.). The differences between the classes are primary, ie were established as the parent bodies accreted in different regions of the solar nebula. The sub-groups are also distinguished in terms of the oxygen isotopic composition of their major silicate minerals, again generally taken to be a reflection of primordial nebula heterogeneity, but possibly modified as a result of widespread fluid-solid exchange processes. The three main chondrite classes are the Carbonaceous (C), Ordinary (O) and Enstatite (E) chondrites. There are also two smaller groupings, the Rumurutiites (R), named after Rumuruti and the Kakangari (K) grouplet. The C, O and E chondrites are all further sub-divided, again on the basis of chemical composition.
Carbonaceous chondrites - into 7 groups, each (apart from CH) named for its type specimen:
Ordinary chondrites - 3 groups:
Enstatite chondrites - 2 groups:
Subsequent to accretion, most meteorites have experienced varying extents of thermal metamorphism or aqueous alteration. These secondary processes occurred on the meteorites' parent bodies, and did not affect the overall composition of the chondrites. Mobilisation and re-distribution of elements such as Fe, Mg and Ca occurred as primary silicates either became homogeneous in composition through heating, or through formation of secondary mineral phases (eg clay minerals, carbonates) during aqueous alteration. Tertiary processes of brecciation and shock melting are also recognised, as are features resulting from weathering during the terrestrial lifetime of a meteorite. The secondary and tertiary processes experienced by chondrites also give rise to sub-classification schemes that reflect these processes. A petrologic type from 3 to 6 indicates increasing thermal metamorphism. A petrologic type from 3 to 1 indicates increasing aqueous alteration. Type 3 chondrites are the least altered, and are further sub-divided into 3.0 to 3.9, on the basis of silicate heterogeneity and thermoluminescence characteristics. Shock classification, from S1 to S6, represents increasing shock metamorphism (Stffler et al. (1991) GCA 55, 3845). Terrestrial weathering is recorded by weathering category, either from W0 to W6 (Wlotzka (1993) Meteoritics 28, 460), or A to C (Antarctic Meteorite Newsletter, all issues), depending on the classification scheme employed.