Meteorites, classification

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. 

Wasson, J. T. Meteorites. Classification and Properties, Minerals and Rocks, Vol. 10, Springer-Verlag, Berlin, Heidelberg 1974... 

Rubin A. E., Wang D., KaUemeyn G. W., and Wasson J. T. (1988b) The Ningqiang meteorite classification and petrology of an anomalous CV chondrite. Meteoritics 23, 13-23. 

FIGURE 2.8 Meteorite classification from the Natural History Museum of London s Catalog of Meteorites, Fifth Edition (Grady, 2000). 

The latest review of meteorite classifications and properties is given by Wasson (1985). A simplified classification scheme is given in table 8. Bulk lanthanide abundances in chondritic meteorites do not indicate any major cosmochemical fractionation during their formation. Lanthanide patterns in chondrites are relatively uniform, and bulk compositions show no dependence on volatility. No Eu or Yb anomalies are apparent. This information indicates that extreme temperatures were not experienced during their formation. Loss of more volatile elements was common among the H, L, LL, C2, and C3 classes, whereas E chondrites, with fully reduced Fe, have their full complement of volatile elements. 

Rousseeuw [4]. Massart and Kaufman [5] and Bratchell [6] wrote specifically for chemometricians. Massart and Kaufman s book contains many examples, relevant to chemometrics, including the meteorite example [7]. More recent examples concern classification, for instance according to structural descriptions for toxicity testing [8] or in connection with combinatorial chemistry [9], according to chemical... 

D. L. Massart, L. Kaufman and K.H. Esbensen, Hierarchical non-hierarchical clustering strategy and application to classification of iron-meteorites according to their trace element patterns. Anal. Chem., 54 (1982) 911-917. 

Broadly speaking the classification of meteorites follows the geological mineral classification and with 275 mineral species reported so far this quickly becomes complex some classes of meteorite have only one member. The mineral structure does convey essential information about the temperature at which the meteorite formed as well as the reduction-oxidation (redox) environment was the environment in which it formed rich in oxygen Meteorites have been classified into three broad classes ... 

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.

Clayton RN, Onuma N, Mayeda TK (1976) A classification of meteorites based on oxygen isotopes. Earth Planet Sci Lett 30 10-18... 

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... 

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). 

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. 

A classification of nonchondritic meteorites is shown in Table 6.2. These meteorites will be described in the following sections, and their chemistry will be considered in Chapter 11. 

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