NWA 5297

Possibly brachinite-related
(Primitive achondrite, ungrouped in MetBull 98)

standby for nwa 5297 photo
Found March 2008
no coordinates recorded Nine small stones having a combined weight of 130 g were found near Alargoug, Morocco, and subsequently sold in June 2008 to meteorite dealer G. Hupé. A sample was submitted for analysis and classification to the University of Washington in Seattle (A. Irving and S. Kuehner), and NWA 5297 was determined to be a completely metamorphosed chondrite (metachondrite) with similarities to the LL chondrite group.

Northwest Africa 5297 is composed of olivine, orthopyroxene, and very sodic plagioclase, along with minor taenite and Ni-bearing troilite. It is a texturally evolved meteorite that exhibits a poikiloblastic structure (finer-grained inclusions of one mineral embedded within larger crystals of another mineral) and lacks any relict chondrules.

The O-isotopic composition of NWA 5297 was analyzed at the Carnegie Institute in Washington D.C. by D. Rumble III, and subsequently by Greenwood et al. (2017). This meteorite was shown to have affinities to the LL-chondrite group (see diagram below). standby for o-isotopic diagram
Diagram credit: Greenwood et al., Chemie der Erde, vol. 77, p. 24 (2017)
‘Melting and differentiation of early-formed asteroids: The perspective from high precision oxygen isotope studies’
(open access: http://dx.doi.org/10.1016/j.chemer.2016.09.005)
However, NWA 5297 has a greater abundance (~10 vol%) of FeNi-metal than would be expected for an LL-chondrite parent body that had experienced a high degree of metamorphism. Furthermore, it was determined that the the olivine composition of NWA 5297 lies outside the range for equilibrated LL chondrites, and the meteorite was recognized by Dunlap et al. (2015) as being similar to the brachinites. They also found that silicates in NWA 5297 plot in the brachinite field on an Fe/Mn vs. Fe/Mg coupled diagram.

Based on Al–Mg systematics, Dunlap et al. (2015) calculated an upper limit of <4.5679 b.y. ago for the timing of Al/Mg fractionation during differentiation on the NWA 5297 parent body. With all of these characteristics taken together, NWA 5297 might be considered as ungrouped, but possibly related to the brachinites. The specimen of NWA 5297 shown above is a 2.78 g partial slice.

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