Northwest Africa 8418 (NWA 8418)

(Northwest Africa)

Purchased: 2014 Apr

Classification: Carbonaceous chondrite (C3)

History: Purchased by Darryl Pitt in April 2014 from a dealer in Erfoud, Morocco.

Petrography: (A. Irving and S. Kuehner, UWS) Relatively fresh specimen composed of relatively large, separated granular chondrules in a finer grained metal-bearing matrix. Chlorapatite occurs throughout the matrix, along with Ni-poor kamacite, Ni-bearing lawrencite, Si-bearing Fe metal and rare merrillite. Some portions of this specimen contain large (up to 15.5 mm by 12 mm), ellipsoidal CAI consisting of porous aggregates (grainsize ~0.5 mm) of mainly Al-Ti-diopside, olivine and anorthite, with accessory spinel, troilite and taenite, surrounded by complete circumferential rims (~ 1 mm thick) of chlorapatite (each one rimmed in turn by a thin outer band of calcite). Phosphate and carbonate mineralogy was established by a combination of electron microprobe analysis and cathodoluminescence microscopy.

Geochemistry: Olivine (Fa_0.3-36.2; FeO/MnO = 130-131, N = 4), orthopyroxene (Fs_1.9-2.3Wo_1.0-0.9, N = 3), pigeonite (Fs_1.7Wo_9.5; Fs_24.9Wo_5.5), augite (Fs_5.7Wo_48.2), Al-Ti-diopside in CAI (Fs_0.1Wo_59.7; Al₂O₃ 15.4 wt.%, TiO₂ 3.5 wt.%). Oxygen isotopes (K. Ziegler, UNM): analyses of three acid-washed subsamples by laser fluorination gave δ¹⁷O -3.994, -5.750, -5.299; δ¹⁸O 0.444, -1.773, -0.823; Δ¹⁷O -4.228, -4.814, -4.864. These values fall on the trend for CV and CK chondrites.

Classification: Carbonaceous chondrite (C3) with affinities to CV chondrites. The abundance of apatite in this specimen is an anomalous feature for carbonaceous chondrites.

Specimens: 22.2 g including one polished thin section at UWB. The remaining material is held by DPitt.