Northwest Africa 5312, revised classification and description

History: Purchased by Fabien Kuntz in 2007 from a dealer in Rich, Morocco.

Physical characteristics: A 354 g dense stone (354 g) partially covered by moderately weathered fusion crust. The uncrusted surface has a dark brown coloration.

Petrography: (A. Irving, UWS and T. Bunch, NAU) Back-scattered electron imaging of a large polished endcut shows this specimen to be an aggregate of predominantly orthopyroxene and anorthite with accessory kamacite, taenite, silica polymorph, Mg-bearing merrillite, apatite and troilite. Some orthopyroxene grains are oikocrysts containing olivine chadacrysts. Sparse angular grains of olivine are present as well as sporadic breccia clasts. One such clast (13 mm by 11 mm) consists of angular olivine and orthopyroxene grains within a finer grained plagioclase-free, chromite and troilite-bearing matrix, and other breccia clasts (as large as 3.5 mm) are composed of orthopyroxene and olivine surrounded by fine-grained, polygonized orthopyroxene+olivine+apatite+chromite+troilite (or symplectic intergrowths of these minerals).

Geochemistry: (S. Kuehner, UWS and J. Wittke, NAU) Orthopyroxene (Fs_20.6-25.1Wo_1.5-2.6, FeO/MnO = 25-31, N = 3), olivine (Fa_22.1-31.1, FeO/MnO = 42-48, N = 3), chromite (cr# = 0.73), taenite (Ni = 42.5 wt.%), merrillite (MgO = 3.6 wt.%). Oxygen isotopes (D. Rumble, CIW): acid-washed subsamples analyzed by laer fluorination gave, respectively, δ¹⁷O 1.545, 1.484; δ¹⁸O 3.364, 3.149, Δ¹⁷O -0.224, -0.173 (all per mil).

Classification: Mesosiderite.

Specimens: The entire 354 g stone including one polished thin section is at PSF.