Brachinites are indeed primitive achondrites. This class of meteorites is defined as those "whose bulk compositions are approximately chondritic, but whose textures are igneous or metamorphic. [They] are generally thought to be ultrametamorphosed chondrites or residues of very low degrees of partial melting on small parent bodies. Winonaites, silicates from IAB and IIICD irons, acapulcoites, lodranites, and recently even ureilites ... have been considered to be primitive achondrites." (Mittlefehldt et al., 1998, Rev. Mineral. 36, Ch. 4). The controversy over brachinites is whether they are residues of partial melting, or cumulates derived from primitive melts. Either way, because they are ultramafic rocks that are relatively close to chondritic in composition, and they have equigranular textures, they are "primitive achondrites" by definition. The most recent work, http://www.lpi.usra.edu/meetings/lpsc2002/pdf/1008.pdf, by Mittlefehldt and Berkley, suggests that some brachinits are cumulates.
jeff At 04:38 PM 4/22/2002, Bernd Pauli HD wrote: >[EMAIL PROTECTED] wrote: > > > I noted that McSween has included the Brachinites > > with the primitive Achondrites in his second edition > > of "Meteorites and their parent planets", whilst > > Norton's Encyclopedia exludes the Brachinites from > > the chapter on primitve achondrites, but rather > > lists them with asteroidal (differentiate) achondrites. > > > > So, should we consider the Brachinites as > > primitive meteorites, or not...? Any ideas? > > >Hello Herbert and List, > >O.R. Norton does not really "e x c l u d e" them because he >says (and also explains the different terms) on page 165: > >"Three a d d i t i o n a l small achondrite groups are currently >recognized and should be briefly mentioned to complete our inventory >of the achondrites. They (= acapulcoites, lodranites, and winonaites) >are called primitive achondrites, and as a group are distinguished >from the differentiated achondrites by having been only partially >melted and therefore not completely diffeentiated." > >Reference: > >NORTON O.R. (2002) The Cambridge Encyclopedia of Meteorites. > > >Best wishes, > >Bernd > >______________________________________________ >Meteorite-list mailing list >[EMAIL PROTECTED] >http://www.pairlist.net/mailman/listinfo/meteorite-list Dr. Jeffrey N. Grossman phone: (703) 648-6184 US Geological Survey fax: (703) 648-6383 954 National Center Reston, VA 20192, USA ______________________________________________ Meteorite-list mailing list [EMAIL PROTECTED] http://www.pairlist.net/mailman/listinfo/meteorite-list
