Meteoritical Bulletin: Entry for Northwest Africa 4799

Writeup from MB 94:

Northwest Africa 4799

Algeria

Find: May 2007

Achondrite (aubrite)

History: Purchased by Greg Hupé in May and October 2007 from a Moroccan dealer in Tagounite.

Physical characteristics: Eighty-six light brown stones and fragments (total 365 g); most are complete, some are broken, and some have remnant fusion crust.

Petrography: (A. Irving and S. Kuehner, UWS) The overall texture is that of a brecciated igneous rock, with larger angular to rounded clasts (some showing cumulus texture) in a matrix of smaller grains. The dominant mineral is pure enstatite with lesser amounts of interstitial albite (some in graphic intergrowths with a silica polymorph) and accessory Si-bearing kamacite, Ti-Cr-bearing troilite, oldhamite, niningerite, Ti-bearing daubreelite and schreibersite. The enstatite grains exhibit polysynthetic twinning indicative of inversion from former clinoenstatite. Grain boundary coatings of goethite or limonite and orange staining along enstatite cleavage planes, as well as veinlets of calcite, are evidently products of terrestrial desert weathering.

Geochemistry: Enstatite (En_99.5Fs_0.1Wo_0.4), kamacite (Fe 92.1 wt%, Ni 4.5 wt%, Si 3.4 wt%).

Classification: Achondrite (aubrite).

Specimens: A total of 20.1 g and one polished thin section are on deposit at UWS. GHupé holds the main mass.

NWA 4799: reclassification.

NWA 4799, NWA 7214, NWA 7809, and NWA 11071 contain unfractionated modal abundances of plagioclase, troilite, and metal, which is similar to other enstatite chondrite meteorites. These NWA samples contain higher abundances of metal (7-14 vol.%) compared to aubrites. The only silicate phases in these samples are enstatite and plagioclase, whereas aubrites present olivine and diopside. Meteorites of enstatite chondrite parentage typically do not contain diopside or olivine. Other distinguishing features are metal/troilite ratios and Ti concentrations in troilite. Aubrites contain lower metal/troilite ratios and high Ti in troilite, whereas enstatite chondrite meteorites show the opposite characteristics (as these NWA meteorites do). The metal in these samples presents a higher concentration of Si compared to the aubrites (4 wt.% Si in the NWA samples versus 0.9 wt. % Si in aubrites. Graphite was also observed in these samples, which is not typically found in aubrites. The NWA meteorites show an affinity for an EH chondritic parent body origin, as seen by low Ti in troilite, high Si in metals, the presence of niningerite and graphite, and the lack of diopside and forsterite. As metal and troilite are observed as inclusions within enstatite grains, it suggests the rocks were formed by complete melting. For the complete petrographic, mineralogical and geochemical analyses of these four meteorites, and references to data listed above, see Udry et al. (2019)