Classification of meteorites is largely based on their mineralogical and petrographic characteristics and their whole-rock chemical and O-isotopic compositions According to the currently used classification scheme, meteorites are divided into chondrites, primitive achondrites, and achondrites There are 15 chondrite groups, including 8 carbonaceous (CI, CM, CO, CV, CK, CR, CH, CB), 3 ordinary (H, L, LL), 2 enstatite (EH, EL), and R and K chondrites Several chondrites cannot be assigned to the existing groups and may represent the first members of new groups Some groups are subdivided into subgroups, which may have resulted from asteroidal processing of a single group of meteorites Each chondrite group is considered to have sampled a separate parent body Some chondrite groups and ungrouped chondrites show chemical and mineralogical similarities and are grouped together into clans The significance of this higher order of classification remains poorly understood The primitive achondrites include ureilites, acapulcoites, lodranites, winonaites, and silicate inclusions in IAB and IIICD irons and probably represent recrystallization or residues from a low-degree partial melting of chondritic materials The genetic relationship between primitive achondrites and the existing groups of chondritic meteorites remains controversial Achondrites resulted from a high degree of melting of chondrites and include asteroidal (angrites, aubrites, howardites-diogenites-eucrites, mesosiderites, 3 groups of pallasites, 15 groups of irons plus many ungrouped irons) and planetary (martian, lunar) meteorites

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Systematics and Evaluation of Meteorite Classification

Mineralogy and composition of matrix and chondrule rims in carbonaceous chondrites

In this chapter, we review current classification of meteorites, which is based on several primary classification parameters the whole-rock chemical compositions, oxygen isotopic compositions, carbon and nitrogen abundances and isotopic compositions, stable-isotope anomalies, mineralogy and petrography. Secondary classification parameters – petrologic type and shock metamorphism stages – provide clues to the thermal and shock history of meteorites, and degree of terrestrial weathering. We summarize the major mineralogical and geochemical characteristics of chondrite groups (H, L, LL, EH, EL, R, K, CI, CM, CR, CO, CK, CV, CH, and CB), and nonchondritic meteorites, both asteroidal (winonaites, acapulcoites-lodranites, brachinites, ureilites, angrites, howardites–eucrites–diogenites, mesosiderites, pallasites, and irons) and planetary (lunar and Martian – shergottites, nakhlites, chassignites, and orthopyroxenites). Finally, we discuss possible genetic relationships among meteorite groups.

Classification of Meteorites and Their Genetic Relationships

The presence of abundant phyllosilicates in many carbonaceous chondrites indicates a prevailing activity of low-temperature aqueous alteration in primitive asteroids. However, among the hydrous carbonaceous chondrites known, more than 20 samples show evidence of having been heated at elevated temperatures with corresponding phyllosilicate dehydration. The mineralogical features of dehydration suggest that the heating occurred in situ in meteorites, which demonstrates that there are some hydrated asteroids that have been heated at a certain period after aqueous alteration. Recent studies have uncovered details of heating and dehydration processes in hydrous carbonaceous chondrites: step-by-step changes in mineralogy, trace element chemistry, carbonaceous materials, and reflectance spectra have been clarified. Based on data from synchrotron X-ray diffraction analysis of the matrix, heated hydrous carbonaceous chondrites have been classified as Stages I-IV, with the temperature of heating increasing from I to IV. In spite of recent progress, heat sources are poorly defined, mainly due to a lack of chronological information on the timing of the heating, and therefore more data are needed to fully clarify the thermal metamorphism of hydrous carbonaceous chondrites.

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Post-hydration thermal metamorphism of carbonaceous chondrites

Modal mineralogy of CI and CI-like chondrites by X-ray diffraction

As a part of the consortium study on antarctic meteorites with af­ finities to CI-chondrites we studied the samples of Yamato (Y)-86720, Y-82162, and Belgica (B)-7904. These carbonaceous chondrites are unique samples and do not perfectly fit in the traditional classification schemes. Therefore, they have to be considered as very important samples to carry distinct information about processes in the early solar system. Y-82162 is a very fine-grained carbonaceous chondrite. Based on the occur­ rence of abundant clasts (up to several mm in size) we suggest that this sample is a chondritic breccia. The dominating phases are phyllosilicates; abundant sulfide grains are scattered throughout the entire sample. However, the abundances of sulfides vary from clast to clast. Y-86720 contains about 13 vol% of light objects embedded in a fine-grained, phyllosilicate-rich groundmass. Some of these objects appear to be relict chondrules; however, they essentially consist of phyllosilicates. Most light, round to irregularly-shaped components exhibit well-preserved ac­ cretionary dust mantles (\"dark rims\") similar to those found in CM-chondrites. Y-86720 is mineralogically more closely related to the CI-chondrites than to any other chondrite group; texturally, however, it appears to be an intermediate chondrite between CI and CM as also suggested by bulk chemical criteria (G. W. KALLEMEYN; Papers Presented to the 13th symposium on Antarctic Meteorites, June 7-9, 1988, Tokyo, NIPR, 132, 1988). B-7904 contains 18 vol% of objects larger than about 70 μm in size. 42 vol% of these components are chondrules or chondrule fragments. The most abundant constituents are, however, olivine-bearing, fragment-like objects (45.9 vol%) unknown from other chondrites. The olivines within these components are embedded in a fine-grained brownish-grey matrix. Other constituents include fine-grained CAis, olivine aggregates, and mineral fragments. B-7904 is a new kind of carbonaceous chondrite and we do not like to classify this meteorite as a CM-type chondrite because of the following reasons: a) A great number of chondrules in B-7904 is much larger (0.5-3 mm) than measured for the mean size of chondrules in CM­ chondrites (0.3 mm). b) Many components (olivine-bearing, fragment-like objects, Cr, Al-rich fine-grained particles) are unknown from CM-chondrites. c) The oxygen isotope composition and the low H20-contents are untypical for CM-chondrites.

A. Bischoff

Papers

Mineralogy and petrography of the anomalous carbonaceous chondrites Yamato-86720, Yamato-82162, and Belgica-7904