Chondrites classification

As seen in Figure 6.3, the taxonomy used for chondrites is a combination of chemical group and petrologic type, as in H4 or 2. This system avoids the problems of previous chondrite classifications, by cleanly separating primary and secondary characteristics. 

Although most chondrites contain the same minerals, the proportions of these and their compositions differ among the 6 or 7 principal chondritic chemical groups. The primary bases for chondrite classification involve proportions of iron as metal and silicate (in which oxidized iron - expressed as FeO - may be present), and total iron (from Fe, FeO, and FeS) content (Figure... 

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.

Urey HC (1947) The thermodynamic properties of isotopic substances. J Chem Soc 562-581 Urey HC, Greiff LJ (1935) Isotopic exchange equilibria. J Am Chem Soc 57 321-327 Van Schmus WR, Wook JA( 1967) A chemical petrologic classification for the chondritic meteorites. Geochim Cosmochim Acta 31 747-765... 

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. 

Classification system for chondrites, adapted from Van Schmus and Wood (1967). A meteorite is classified by identifying its chemical group and petrologic type. Approximate temperatures for metamorphism or alteration are shown at the bottom. The relative abundances of meteorites assigned to various petrologic types are indicated by the shaded proportion of each box (data from Scott and Krot, 2004). 

Because of the isotopic variability and the high cosmic abundance of oxygen, oxygen isotopes are very useful for meteorite classification. Below the condensation temperature of silicates and above the condensation temperature of ices, approximately 25% of the oxygen in the solar nebula is predicted to have occurred in condensed solids, with the remainder in gaseous molecules. Chondrites provide samples of the condensed oxygen in the early solar system. 

The systematic variations in oxygen isotopes provide an independent means of classifying chondrites that generates the same groups as the chemical compositions. The oxygen isotopes also work for classifying non-chondritic meteorites. Oxygen isotopic compositions are somewhat easier to obtain than detailed chemical data and so are often used to nail down a classification. 

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-... 

Refractory inclusions are a class of chondritic components that derive their classification in part from the fact that they are composed of some of the most refractory materials found in primitive planetary samples (Grossman 1972 MacPherson et al. 1988 Connolly 2005 Beckett et al. 2006). In meteoritics, refractory refers to the temperature at which materials condense or evaporate within the gaseous environment in the protoplanetary disk. Thus, refractory inclusions are composed of minerals that are among the first predicted to condense from a gas of solar or enhanced solar composition (Ebel 2006). Their abundance in chondrites can potentially range up to 15 vol%, although most appear to be only a few vol% (Grossman et al. 1988 Russell et al. 1998 Ebel et al. 2008 Hezel et al. 2008). 

Undifferentiated and differentiated meteorites. 03.1.3.2 Cosmochemical classification of elements Cl Chondrites as Standard for Solar Abundances... 

---