A single fusion-crusted stone weighing 1,500 g was found in Morocco or Algeria. The meteorite was subsequently purchased by G. Hupé while visiting in Morocco. A type specimen was submitted for analysis to the University of Washington in Seattle (A. Irving and S. Kuehner), and NWA 3151 was classified as a brachinite, the largest member of the group found to date. A separate 26.5 g stone, NWA 5191, has been determined to be a possible pairing to NWA 3151 (A. Ibhi, LPMM, and M. vanGinneken and L. Folco, MNA-SI).

Northwest Africa 3151 is a coarse-grained (0.7–1.6 mm), olivine-rich (~95 vol%) dunite (Irving et al., 2005). It contains minor clinopyroxene, FeNi-metal, chromite, and troilite, and rare sodic plagioclase and orthopyroxene. It is most similar to the paired brachinites EET 99402/407. Terrestrial oxidation of the metal phase has formed hydroxides among the grains, but the silicates remain unaltered. Initial O-isotopic data for NWA 3151 plot within the brachinite field (D. Rumble, III, Carnegie Institution, Washington D.C.), but additional measurements have established more positive values than those of Brachina. Notably, two other Saharan stones, the brachinite NWA 595 and the primitive achondrite NWA 4042, share very similar O-isotopic values with NWA 3151 and the other brachinites (with the exception of Brachina).

In MAPS Vol. 41 (2006), #5288, A. Irving and D. Rumble III have argued that "if all these specimens (including Brachina) derive from the same parent body, then it must be isotopically quite heterogeneous." While this is plausible—and a wide isotopic dispersion is also observed among the winonaites and the acapulcoite/lodranite samples (Rumble III et al., 2008)—the disparity in O-isotopes that exists between Brachina and the Saharan brachinites which is revealed in this study is consistent with an origin for the Saharan brachinites on a parent body separate from that of Brachina. Other evidence supports such a multiple parent body scenario; some brachinite members exhibit characteristics of primitive achondrites, i.e., have near-chondritic compositions, while others appear to have experienced igneous fractionation with element depletions and to contain melt inclusions in olivine. Evidence is also ambiguous among brachinites as to whether they represent cumulates or metamorphic processes. It could be inferred that this varied group represents a diversity of petrogenetic models representing more than a single parent body.

However, if the isotopically and petrographically diverse suite of brachinites did originate on a common, ancient, heterogeneous parent body, then its size should be commensurate with the low degree of heating exhibited by some of the known samples. Given this concept that the brachinites all derive from a common parent body, the range of the group may be too narrowly defined, and perhaps some of the brachinite-like primitive achondrites like Zag (b), Divnoe, and Tafassasset also share a genetic relationship.

Besides NWA 3151, few brachinites are currently known. Despite the fact that the brachinite Reid 027 was found in close proximity to Reid 013, which establishes a definite likelihood for their pairing, the two brachinites exhibit differences in both grain size and plagioclase composition; for some scientists, these variations support the decision to maintain their independent status until more thorough comparisons can be made. LEW 88763 is classified as a brachinite but is probably an acapulcoite.

Further details on this rare group of meteorites can be found on the other brachinite member pages of this website. The specimen of NWA 3151 shown above is a 0.498 g thin partial slice. The photo below shows the complete uncut mass.

Photo courtesy of Greg Hupé