Chondritic meteorites texture

Chondrules display a wide variety of compositions and textural types (see Fig. 8.5). The volume fraction occupied by chondrules in chondritic meteorites ranges from 85% (ordinary chondrites) down to 0% (Cl chondrites). Without question the most abundant type of chondrule is dominated by ferromagnesian (Fe, Mg-rich) silicates (Figs. 8.4 and 8.5 Lauretta et al. 2006). Ferromagnesian chondrules are primarily composed of olivine, pyroxene (minor Ca-rich pyroxene), glass, spinels, Fe, Ni-rich metal, FeS, and other minor phases (< 1 vol%). These are subdivided into FeO-poor and FeO-rich, or Type-I and Type-II chondrules (Brearley Jones 1998 Jones et al. 2000, and references therein Lauretta et al. 2006, and references therein). The division is delineated by the Mg-number [or Mg defined as 100 x Mg/(Mg + Fe)] of the olivine and low-Ca pyroxene > 90 is FeO-poor, and < 90 is FeO-rich, which roughly translates in a bulk difference of 10 wt% FeO < 10 wt% = FeO-poor and >10 wt% = FeO-rich. 

Primitive achondrites exhibit metamorphic textures, as appropriate for the solid residues from which melts were extracted. In effect, these meteorites represent an extension of the highly metamorphosed type 6 chondrites, which show no eutectic melting of metal and sulfide. In a few cases, some primitive achondrites have recognizable chondritic textures, but often they are so thoroughly recrystallized that chondrules are not identifiable. 

In a limestone deposit at Kinnekulle, Sweden, a remarkable collection of more than 40 highly altered meteorites (Fig. 9.18) totaling 7.7 kg have been collected during routine guarrying operations (Schmitz etal., 1997,2001,2003). The 3.2-m-thick limestone layer in which they are found was deposited over -1.75 Myr in the mid-Ordovician. Despite being almost completely replaced by calcite, barite, and phyllosilicates, the meteorites are easily identified by their chondritic texture. Their identification as meteorites is confirmed by measurements of platinum-group elements. The chemical characteristics of relict spinels indicate that they are either L or LL chondrites. 

Based on their bulk compositions and textures, meteorites can be divided into two major categories, chondrites and nonchondritic meteorites, the latter include the primitive achondrites and igneously differentiated meteorites (Figure 1). 

The recently described metal-rich Tafassasset meteorite has an oxygen isotopic composition similar to CR chondrites and an equigranular texture (Bourot-Denise et al., 2002). It is, however, characterized by uniformly ferrous ohvine (Fa3o) and orthopyroxene (FS25), and the presence of albitic plagioclase (An24 4s), chromite, and merrilite it may be the first metamorphosed CR chondrite. 

Olivine-rich, accretionary rims around CAIs, which have grain sizes >5-20 pm, were first described in the altered CV chondrite Allende by MacPherson et al (1985). In this meteorite, the accretionary rims have four layers that differ in texture and mineralogy. The innermost layer... 

Fagan T. J., Krot A. N., and Keil K. (2000) Calcium, aluminum-rich inclusions in enstatite chondrites (I) Mineralogy and textures. Meteorit. Planet. Sci. 35, 771-783. 

The available data suggest that heat sources for melting primitive bodies (presumably chondritic in chemical composition and texture) that yielded differentiated meteorites were within rather than external to parent bodies. Important sources no doubt include impact shock-heating and radioactive heating from radionuclides - both extant and and extinct... 

Each of the meteorites varieties in Table 18.1 consists of different groups of objects based on their bulk chemical and mineral compositions, textures, and other criteria. For example, the ordinary chondrites, which are the most abundant stony meteorites, are subdivided in Fig. 18.2 into groups identified by letter E, H, L, and LL. The average chenucal compositions of meteorites... 

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