Meteoritical Bulletin: Entry for Northwest Africa 7214

Writeup from MB 102:

Northwest Africa 7214 (NWA 7214)

Western Sahara

Found: 2006

Classification: Enstatite achondrite (Aubrite)

History: A 2200 g crusted stone was found in Western Sahara and purchased by A. Aaronson in 2010.

Physical characteristics: Desert ablation has mostly polished the fusion crust into a translucent veneer with very limited chemical weathering.

Petrography: Composed mostly of fine-to medium-grained (0.3 to 1.2 mm), euhedral to subhedral equilgranular enstatite together with interstitial metal, schreibersite, graphite, a silica phase present as round inclusions in FeS, daubreelite, FeS, brezinaite and oldhamite are typically found together in complex clusters. Plagioclase is present as small intercumulus lath-shaped grains. With the exception of the underside, the stone is very fresh (W0/1) with no oxidation of metal, shock level is low.

Geochemistry: Enstatite is essentially Fe-free (En_99.6Fs_0.1Wo_0.3). Kamacite contains 5.3 wt % Ni and is Si-enriched (Si = 4.0 to 5.8 wt %). FeS contains 2.3 - 3.6wt % Ti and 1.8 wt % Cr. The rare mineral brezinaite consists of S, 45.4 wt %; Cr, 46.3 - 52.1wt %; Fe, 9.1 to 3.0 wt % and 1.1 wt % Mn. Plagioclase is An_4.2.

Classification: Achondrite (aubrite).

Specimens: A total of 20.6 g is on deposit at PSF. DPitt holds the main mass of 2.2 kg.

NWA 7214: 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)