Figure 2. Classification of meteorites hierarchy of sub-divisions into groups and classes. There is not always a clear cut distinction between types: e.g., there are many iron meteorites that contain silicate inclusions related to chondritic meteorites. Both melted and unmelted stony meteorites are made from the same elements as terrestrial rocks: Si, O, Fe, Mg, Ca and Al. Like terrestrial rocks, these meteorites are assemblages of silicate minerals: pyroxene, olivine and plagioclase, but unlike terrestrial rocks, they also contain iron-nickel metal and sulphides.
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. Chondrites 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). The CAIs (for Calcium, Aluminium-rich Inclusions) are irregular-shaped refractory inclusions (up to ~ 1 cm in size) of the oxide and silicate minerals spinel, hibonite, melilite, etc. (Figure 3). CAIs frequently exhibit complex mineralogical zoning, both in their rims and in their cores.
Chondrules are 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 (Figure 4). Chondrules are a study in themselves; the most comprehensive recent reference is the workshop proceedings edited by Hewins et al. . 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 predate the major chondritic components.
Chondrites sub-groups are distinguished on the basis of chemistry, matrix, metal and chondrule contents and chondrule properties (size, type, etc.). The differences between the classes are primary, i.e. 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 (Figure 5), again generally taken to be a reflection of primordial nebula heterogeneity, but possibly modified as a result of widespread fluid-solid exchange processes.
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