Northwest Africa 8781 (NWA 8781)

(Northwest Africa)

Purchased: 2014 Jul

Classification: Carbonaceous chondrite (Ungrouped)

History: Several black stones found together near Foum Zguid, Morocco, in July 2014 were purchased by Rachid Chaoui and were subsequently sold to Stefan Ralew and Darryl Pitt.

Petrography: (A. Irving and S. Kuehner, UWS, L. Garvie, ASU) Composed of very small chondrules (210±60 µm, N=20) and small mineral fragments in a black, very fine-grained matrix (~50 vol.%). No phyllosilicates were detected by powder-XRD in the matrix. No CAI were observed. Minerals are olivine, orthopyroxene, diopside, stained kamacite and troilite.

Geochemistry: Olivine (Fa_1.1-70.7, Cr₂O₃ in ferroan examples = 0.21-0.40 wt.%, N = 8), orthopyroxene (Fs_1.0-7.0Wo_1.2-0.4, N = 3), diopside (Fs_2.1Wo_40.8; Fs_0.4Wo_48.2). Oxygen isotopes (K. Ziegler, UNM): analyses of five 1-2 mg acid-washed non homogenized subsamples by laser fluorination gave, respectively, δ¹⁷O -10.020, -9.832, -6.298, -5.897, -4.490; δ¹⁸O -6.085, -5.472, -1.822, -0.895, 1.221; Δ¹⁷O -6.807, -6.943, -5.336, -5.424, -5.135 (all per mil). These compositions plot on the CCAM trend, but distinctly below the trend for CO chondrites

Classification: Carbonaceous chondrite (ungrouped). This specimen is not a CV3 chondrite (chondrule size much too small, no CAI), not a CK chondrite (no magnetite), not a CO3 chondrite (oxygen isotopes are different, no CAI), and not a CM chondrite (matrix apparently anhydrous, oxygen isotopes displaced to more ¹⁶O-rich compositions). Therefore it seems to be an ungrouped carbonaceous chondrite.

Specimens: 20.1 g including one polished thin section at UWB; 170 g with Kuntz; other samples with Mr. S. Ralew and DPitt.