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Purchased September 2017
no coordinates recorded

A single stone meteorite weighing 840 g was found in Northwest Africa and subsequently purchased by Azelmat Nor Eddine in Guelmim, Morocco. Petrographic and chemical analyses were conducted at the Institut für Planetologie in Münster, Germany (K. Metzler), and it was determined that NWA 11756 is a brachinite.

Northwest Africa 11756 consists primarily of olivine (76 vol%; Fa26.8 [±0.2]; mean Fe/Mn = 58) with grain boundaries forming 120° triple junctions, low-Ca pyroxene (12 vol%), high-Ca pyroxene (5 vol%), and chromite (~1 vol%), along with accessory Cr-Al-spinel (K. Metzler), troilite, plagioclase, and chlorapatite (Crossley et al., 2020). The meteorite has experienced a high degree of terrestrial weathering and much of the primary FeNi-metal (kamacite) has been transformed into secondary Ni-bearing hydroxides along grain boundaries.

Similar to some other brachinites, very fine-grained assemblages of orthopyroxene and opaques (metal and sulfide) line some olivine grain boundaries, a feature that has been attributed to fluctuations in redox conditions (e.g., Rumble III et al, 2008). Several scenarios for the reduction of primary olivine were reviewed by Goodrich et al., 2017), including reaction with methane to form orthopyroxene + metal (Irving et al., 2013), and by sulfidization by a S-rich fluid or gas to form orthopyroxene + sulfide (e.g., Singerling et al., 2013). A petrographic and mineralogical investigation of several brachinites including NWA 11756 led Zhang et al. (2022) to conclude that these pyroxene-troilite intergrowths, as well as spatially associated chlorapatite (showing chromite exsolution and replacement by merrillite), are a byproduct of the sulfidization of olivine by a S-rich vapor during impact heating.

Crossley et al. (2020) distinguished two subgroups among the brachinite clan: oxidized and reduced (see diagram below). The reduced subgroup has oxygen fugacities (related to the partial pressure of available oxygen) ≤IW–1.45, an olivine Fa content <30 with Fe/Mn ratio <60, chondritic Ir/Os and Pt/Os ratios, HSE that are hosted primarily in FeNi-metal, and these meteorites may contain minor phases including orthopyroxene, plagioclase, and FeNi metal. In addition to the meteorites shown in the diagram below, the reduced subgroup includes such brachinites and brachinite-like meteorites as NWA 12733, NWA 5400 (ACHO-ung), and MIL 090405/6 (ACHO-ung); see the NWA 3151 page for details about the oxidized subgroup as distinguished by Crossley et al. (2020).

Fe–Mg–Mn Redox Trend for Brachinites and Brachinite-like Achondrites
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Diagram credit: Crossley et al., MAPS, vol. 55, #9, fig. S9 (2020)
'Sulfide-dominated partial melting pathways in brachinites'

Further details about this group can be found on the other brachinite pages on this website. The specimen of NWA 11756 shown above is a 4.92 g slice, while that below shows the reverse side, with excellent photography by Azelmat Nor Eddine.

standby for nwa 11756 photo
click on photo for a magnified view

Photo courtesy of Azelmat Nor Eddine—Saharock Meteorites