Achondrite, ungrouped

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Purchased 2016
no coordinates recorded

A relatively fresh meteorite weighing 36.67 g was found in Northwest Africa and subsequently purchased by J. Higgins in October 2016 from a dealer in Nouakchott, Mauritania. A sample was analyzed at the University of Washington in Seattle (A. Irving and S. Kuehner) and NWA 11187 was classified as an ungrouped achondrite that has mineralogical and geochemical affinities to the ureilite group.

Northwest Africa 11187 is a coarse-grained achondrite consisting of large poikilitic grains of pigeonite enclosing small chadacrysts of Cr-poor forsteritic olivine (Irving and Kuehner, MetBull 106). Neither the Cr-poor olivine nor the poikilitic texture are typical for ureilites, although poikilitic clasts have previously been observed (e.g., polymict ureilite NWA 1926; T.E. Bunch, 2005). Graphite and kamacite are present in NWA 11187 as minor constituents.

Oxygen-isotope values for NWA 11187 were ascertained at the University of New Mexico (K. Ziegler), and the values plot near the CCAM line near the resolved field for ureilites (see the oxygen three-isotope plot). An in-depth study of multiple samples from the acapulcoite–lodranite clan was conducted by Li et al. (2018), with the ungrouped achondrite NWA 11187 also included in the study. Utilizing a coupled Δ17O vs. ε54Cr (ε54Cr = –0.85 [±0.11]) diagram, they demonstrated that while NWA 11187 does plot among the broad field of the ureilites, it is also close to the compositionally diverse members composing the acapulcoite–lodranite clan (see diagram below).

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Diagram credit: Li et al., GCA, vol. 242, p. 92 (2018)
'Evidence for a Multilayered Internal Structure of the Chondritic Acapulcoite–Lodranite Parent Asteroid'

Northwest Africa 11187 shows evidence for low shock and a low degree of terrestrial weathering. The main mass of this meteorite resides in the Royal Ontario Museum. The specimen of NWA 11187 shown above is a 0.25 g partial slice that was taken from the ~1 g slice shown below.

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Photos courtesy of John Higgins—Outer Space Rocks