Dec 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003trgeo...1..291m&link_type=abstract
Treatise on Geochemistry, Volume 1. Editor: Andrew M. Davis. Executive Editors: Heinrich D. Holland and Karl K. Turekian. pp. 71
Other
6
Scientific paper
This chapter covers the major and minor achondrite groups, and three newly described unique achondrites. The discussion of other unique achondrites by Mittlefehldt et al. (1998) is still current. The silicates of the stony-iron mesosiderites show many similarities with the howardites, and the silicate inclusions in IAB irons are closely related to the stony winonaites. Therefore, these will be included here. Finally, some IIE irons contain nonchondritic silicate inclusions, and they will also be considered.The meteorites discussed are all 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 distinct from those of nebular materials. I will define these categories more precisely as:Differentiated achondrites. They are achondrites that exhibit igneous textures or igneous textures modified by impact and/or thermal metamorphism, and that have compositions of lithophile, siderophile, chalcophile, and atmophile elements that are highly fractionated from the ranges of chondritic materials.Primitive achondrites. They are achondrites that exhibit equilibrated, metamorphic textures, possibly modified by impact. Rare members have relict chondritic textures, but examples with classic, unequilibrated chondritic (i.e., type 3) textures are absent. They have compositions of lithophile, siderophile, chalcophile, and atmophile elements that are at most only moderately fractionated from the range of nebular materials. Some members may be quite fractionated, but the group as a whole is dominated by primitive materials.The majority of silicate inclusions of IIE irons do not easily fit into either of these categories. These I will call uncategorized as: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.
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