The Classification of Meteorites

Undifferentiated meteorites these are derived from asteroids which never underwent the heating which leads to fusion. They consist of millimetre-sized spherules (chondrules) embedded in a matrix. 

Differentiated meteorites they come from asteroids which have been through a fusion process which led to a more or less clear separation into nucleus, mantle and crust. 

According to the Catalogue of Meteorites (1985), there are four main groups of meteorites  

Chondrites Achondrites Stony iron meteorites Iron meteorites 

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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 ... 

There are a number of excellent (but rather technical) chapters in recent books that describe the classification of meteorites in greater detail than presented here. The following are highly recommended. All have excellent photographs, but some of these resources probably offer more information than most readers can use. 

Over the years, scientists have developed a very detailed and somewhat complex system for the classification of meteorites. It is based on the chemical composition and (to a somewhat lesser degree) the physical characteristics of meteorites. The chart below summarizes the main elements of the system. 

Meteorites have been collected for several centuries and about 2,000 specimens had accumulated in museums of the world prior to 1970 when meteorites were discovered in Antarctica. The meteorites in these museums include three different types stones, irons, and stony irons. Each of these types has been subdivided into different groups depending on the minerals of which they are composed, their chemical compositions, the inclusions they contain, their textures, and the abundance of volatile matter. An abbreviated version of the classification of meteorites is contained in Table 18.1 (see also Davis 2005 Krot et al. 2005 Hartmann 2005 Lipschutz and Schultz 1999 McSween 1999 Taylor 1992). 

Figure 1 Element/Si mass ratios of characteristic elements in the major groups of chondritic (undifferentiated) meteorites. Meteorite groups are arranged according to decreasing oxygen content. The best match between solar abundances and meteoritic abundances is with Cl-meteorites. For classification of meteorites,...

A rather different approach to understanding the condensation of the solar nebula came from the work of the geochemist V.M. Goldschmidt carried out in the 1920s. Goldschmidt proposed, what has now become, a widely used geochemical classification of the elements. This work was in part based upon the study of meteorites, and so his classification is very relevant to the understanding of planetary processes. 

Yttrium has been found in samples of lunar crystalline rocks (namely in relatively yttrium-rich mineral and grain-si/e fractions, light and dark clasts) collected during the Apollo 11-15 and Luna 16 missions. A relatively high amount of yttrium was found in these samples (Gmelin et al. 1980). Indeed, the proportion of yttrium to rare earth elements (REE including ratios and correlations of Y and REE to other elements) was used as a criterion for the classification of lunar rocks. Yttrium and Rare Earths were also a subject of study in the space, for example, in stellar spectra in the solar atmosphere, and in different types of meteorites and mesosiderites. 

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)...

Table 18.5 Classification of meteorite specimens collected on the ice fields adjacent to the Allan Hills that carry the code ALH. Based on data from Score and Lindstrom (1990). Note that the abundances of the ALH meteorite specimens are similar to those of falls and differ from the abundances of finds ...

Sears, D.W.G., Dodd, R.T. (1988) Overview and classification of meteorites. In Meteorites and the Early Solar System, edited by Kerridge, J.F., Matthews, M.S. Tucson, AZ University of Arizona Press, pp. 956-983. 

Analytical Instrumentation. Before the Apollo Moon landings, meteorites were the only extraterrestrial materials available for astrogeologists to study. Usually the classification of a meteorite requires a certain amount of destructive analysis. In many cases, the most interesting and rare meteorites are available only in very small quantities, thereby hmiting tbe amount of material available for analysis. Similarly, only small amounts of the rocks recovered from the Moon were available for analysis. The National Aeronautics and Space Administration (NASA) deliberately preserved a large quantity of lunar material for future scientists to study with instruments not yet invented. They realized that another trip to the Moon might not occur for many years. 

Some recent examples of the uses for clustering in chemical problems include the following chemical information systems, selection of compounds for biological testing, classification of meteorites, composition of atmospheric particles, characterization of gas chromatographic stationary phases, and studies of IR speara. ... 

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. 

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.

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). 

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. 

Alkalis versus silica diagram used for geochemical classification of volcanic rocks. Martian meteorites, Gusev crater rocks and soils analyzed by the Spirit rover, and Bounce Rock analyzed by the Opportunity rover generally plot in the field of basalts, as does the average Mars Odyssey GRS analysis. Compositions derived from TES spectra (Surface Types 1 and 2) and the Mars Pathfinder dust-free rock plot in the basaltic andesite and andesite fields. After McSween et at. (2009). 

Wasson, J.T., Choi, B.-G., Jerde, E.A. and Ulff-M0ller, F. (1998) Chemical classification of iron meteorites XII. New members of the magmatic groups. Geochimica et Cosmochimica Acta, 62(4), 715-24. 

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