Hello all, As usual I goofed...Norbert said NWA 1950 was closer to a Chassignite and not a Nakhlite...because it is has over 50% olivine. And after reviewing the Bulletin it appears it has about 30% pigeonite(a clinopyroxene). So it really is in between a Chassignite and a Nakhlite. Which got it(1950) generalized as a Shergottite!!!
Sheesh...no wonder I'm confused, John > David, Norbert and others: > > Statement of fact: I am a novice and I don't know exactly what I'm talking > about it. > > Comment: With that said I find this whole subject of naming martian rocks to > be somewhat difficult to sort out, or understand completely. Here goes... > > 1. Chassignites - very, very rare "martian" meteorite that is almost entirely > composed of iron rich olivine, with a small amount of chromite, > orthopyroxene, feldpars, and a tiny bit of hornblende. This rock is the > equivalent of Dunite here on earth. On the gabroic triangle figure/chart, > Dunite would be at the bottom center below the Peridotites that are split > into Wehrlite(clinopyroxene based), Lherzolite(a residual melt/mixture of > Wehrlite and Harzburgite), and Harzburgite(orthopyroxene based). > > 2. Nakhlites - a rare, but becoming less rare "martian" meteorite that is > mostly made up of augite clinopyroxene, and has iron rich olivine, and small > amounts of feldspars and other clinopyroxenes. This rock is the equivalent to > earthly clinopyroxenite and olivine clinopyroxenite which both can be > classified as a type of gabbro on earth. > > 3. Shergottites - a more typical "martian" meteorite that is mostly made up > of clinopyroxene, and has several varieties based on the amount of olivine > and feldspars to go along with the clinopyroxene. Now these are usually > called basalts because of the mineral makeup that is similar for extrusive > (volcanic) basalt and intrusive (dike/sill magma) basalt here on earth which > is also know as gabbro. > > 4. Orthopyroxenite - ALH 84001 is not even close to these others from a > mineral standpoint and it stands alone in the "martian" category. On earth it > is also called an orthopyroxenite which is also known as a norite instead of > a gabbro or basalt. > > A martian or earthly Lherzolite is really not a basalt/gabbro (shergottite), > but a mixture or melt of a clinopyroxenite (Nakhlite) and an orthopyroxenite > (ALH 84001). And depending on the level of one pyroxene versus another it is > closer to one or the other. As Norbert suggested it must have more > clinopyroxene than orthopyroxene, so that is why he said it is closer to a > Nakhlite. > > Is anyone confused yet??? Bottom line is that I agree that NWA 1950 maybe > misrepresented by calling it a shergottite...or the boundaries for being a > shergottite are a lot broader than the other categories. Does anyone have a > clear understanding of this criteria? > > Also, is there any known criteria for the shergottites that differentiates > them from being extrusive or intrusive rocks...and if so, would it not be > clearer to call them either a basalt or a gabbro? > > Going to deep I suppose, > > John > > > > > Hello Norbert, > > > > I wonder why it is still the convention to name this Martian group > > "lherzolitic shergottites". Although this geochemical group was > > historically included as a subgroup within the shergottite class, and > > therefore its members were called lherzolitic shergottites (or > > shergottitic peridotites), there is no genetic relationship between the > > basaltic and lherzolitic subgroups. The term "lherzolites" was proposed > > by Eugster and Polnau in 1997 to represent this unique group of Martian > > meteorites. In fact, they showed that the chemical composition of the > > basaltic shergottite group is closer to the nakhlites than to the > > lherzolites. Furthermore, these groups are resolved from each other on > > an O-isotope plot. > > > > I look forward to my next meeting with Bruno and Carine to add a small > > sample of this rare lherzolite to my collection (but not as big as yours > > Norbert!). > > > > David > > > > ______________________________________________ > > Meteorite-list mailing list > > [EMAIL PROTECTED] > > http://www.pairlist.net/mailman/listinfo/meteorite-list > > ______________________________________________ > Meteorite-list mailing list > [EMAIL PROTECTED] > http://www.pairlist.net/mailman/listinfo/meteorite-list ______________________________________________ Meteorite-list mailing list [EMAIL PROTECTED] http://www.pairlist.net/mailman/listinfo/meteorite-list
