Igneous achondrites

The achondritic meteorites can be subdivided into the differentiated achondrites igneous rocks from parent bodies that were extensively melted, and the undifferentiated, or primitive, achondrites from parent bodies that underwent little melting. 

Burbine et al. (2002) tested an extreme case of a possible composition for the surface of Mercury. They made spectral observations of enstatite achondrites (igneous meteorites composed almost entirely of pure MgSi03, with some accessory minerals and essentially no FeO). The spectral features of enstatite achondrites (aubrites see Chapter 1.05) are similar to those for Mercury, but lack the spectral reddening observed in spectra of Mercury and have an additional feature at 0.5 p.m caused by troilite (FeS). This reddening (visible to UV ratio) is the result of space weathering, in which FeO is reduced to very small grains of metallic iron. Thus, the reddening indicates that some FeO must be present on Mercury to produce the nanophase iron. Alternatively,... 

Evolved extraterrestrial materials are generally igneous rocks, which according to their thermal history can be discnssed analogonsly to terrestrial samples. To this category belong planetary bodies, differentiated asteroids, and achondritic meteorites. 

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). 

Okada A., Keil K., Taylor G. J., and Newsom H. (1988) Igneous history of the aubrite parent asteroid evidence from the Norton County enstatite achondrite. Meteoritics 23, 59-74. 

The meteorites discussed are aU samples of asteroids, although the exact sources are generally not known more precisely than that. Current practice is to divide the achondrites into two broad categories—differentiated achondrites and primitive achondrites. The former generally have igneous textures and compositions far removed from those of nebular materials, while the latter have metamorphic textures and compositions less... 

Differentiated achondrites. They are achondrites that exhibit igneous textures or igneous textures modified by impact and/or thermal metamorphism, and that have compositions of lithophUe, siderophile, chalcophile, and atmophile elements that are highly fractionated from the ranges of chondritic materials. 

Uncategorized achondrites. They are achondrites with textures and compositions that do not clearly denote mode of origin (igneous versus metamorphic), or that have not been sufficiently well characterized to permit unambiguous categorization. 

Differentiated achondrites represent the products of classical igneous processes acting on the silicate-oxide system of asteroidal bodies— partial to complete melting, and magmatic crystallization. Iron meteorites represent the complementary metal-sulfide system products of this process. 

The ureilite group is the second largest achondrite group, and exhibits some characteristics of both primitive and differentiated achondrites. However, the dominant silicates of ureilites seem to require a high-temperature, igneous origin, and no tme primitive materials related to the ureilites are known (e.g., Goodrich, 1992). 

Chemical changes involving loss of a constituent, like carbon or water in chondrites, require an open system other changes in Table II could occur in open or closed systems. It should be emphasized that thermal metamorphism can only affect secondary (parent body) characteristics - those listed horizontally in Table II - not primary ones. Postaccretionary processes by which H chondrite-like material can form from L or vice versa are unknown. Achondrites, from melted and differentiated parent bodies - including planets (e.g. Martian meteorites like Nakhla) - have petrologies indicative of igneous processing at temperatures 1000 C. 

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