Carbonaceous chondrites classification

The largest class of meteorite finds is stony meteorites, made principally of stone. The general stony classification is divided into three subclasses called chondrites, carbonaceous chondrites and achondrites, and it is at this level of distinction at which we will stop. Before looking at their mineral and isotopic structure in more detail, it is useful to hold the composition of the Earth s crust in mind here for comparison. The Earth s crust is 49 per cent oxygen, 26 per cent silicon, 7.5 per cent aluminium, 4.7 per cent iron, 3.4 per cent calcium, 2.6 per cent sodium, 2.4 per cent potassium and 1.9 per cent magnesium, which must have formed from the common origin of the solar system. 

Meteorites General classification into stony, stony-iron and iron, each with an interesting mineralogy, notably the carbonaceous chondrites... 

Figure 8. Figure (a) after Clayton et al. (1976, 1977). The scales are as in Figure 1. The O isotopic compositions of the different meteorite classes are represented ordinary chondrites (H, L, LL), enstatite chondrites (EFl, EL), differentiated meteorites (eucrites, lAB irons, SNCs) and some components of the carbonaceous chondrites. As the different areas do not overlap, a classification of the meteorites can be drawn based on O isotopes. Cr (b) and Mo (c) isotope compositions obtained by stepwise dissolution of the Cl carbonaceous chondrite Orgueil (Rotaru et al. 1992 Dauphas et al. 2002), are plotted as deviations relative to the terrestrial composition in 8 units. Isotopes are labeled according to their primary nucleosynthetic sources. ExpSi is for explosive Si burning and n-eq is for neutron-rich nuclear statistical equilibrium. The open squares represent a HNOj 4 N leachate at room temperature. The filled square correspond to the dissolution of the main silicate phase in a HCl-EIF mix. The M pattern for Mo in the silicates is similar to the s-process component found in micron-size SiC presolar grains as shown in Figure 7.

Kallemeyn, G. W. and Wasson, J. T. (1981) The compositional classification of chondrites. 1. The carbonaceous chondrite groups. Geochimica et Cosmochimica Acta, 45, 1217-1230. 

The classification of chondrites on the basis of their elemental composition is recent with respect to the more traditional classification based on textural and mineralogical differences 6). In the case of carbonaceous chondrites this classification was revised by Wasson in 19747). The petrological type (from 1 to 6 even if some authors also use 7) is intended to indicate the degree of equilibration and metamor-phical recrystallisation. So 1 indicates the least-equilibrated and 6 the most-... 

Kallemeyn G. W., Rubin A. E., and Wasson J. T. (1991) The compositional classification of chondrites V. The Karoonda (CK) group of carbonaceous chondrites. Geochim. Cosmochim. Acta 55, 881-892. 

Chondrites are subdivided into carbonaceous (C), ordinary (O), and enstatite (E) varieties (Fig. 2.8). Carbonaceous chondrites are volatile rich and contain abundant carbon in their matrix. Because they have a high volatile content they are thought to be the most primitive of all chondrites. Within this group there are a number of varieties named after type specimens designated Cl, CM, CV, etc. An earlier classification used Cl to C3. Cl chondrites are the most primitive meteorites within the carbonaceous chondrite groups and the most primitive of all meteorite types. They are the least chemically fractionated and have the highest volatile content. Ordinary chondrites, as their name implies are the most abundant... 

Figure I. The classification of carbonaceous chondrites based on their primary elemental (chemical) composition and levels of secondary (aqueous and thermal) processing. (Adaptedfrom References 4 and 5.)...

Fig. 18.2 The classification of chondrite meteorites is based on the distribution of iron between FeO and Fe + FeS combined with the fact that different classes of chondrites contain about the same amount of iron. This diagram demonstrates that as the concentration of oxidized iron (FeO) increases, the concentration of reduced forms of iron (Fe + FeS) decreases. E = enstatite chondrites, H = bronzite ehondrites, L = hypersthene chondrites, LL = amphoterite (olivine-pigeonite) chondrites, C = carbonaceous chondrites (Adapted from Fig. 29 of Mason 1962, for falls only)...

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