Northeast Africa 057 (NEA 057)

Chad

Purchased: 2023 Dec

Classification: Ordinary chondrite (H5, melt breccia)

History: A small stone was found by hunters at an unspecified location within the Boutel Fil strewnfield and purchased in Chari-Baguirmi, Chad from the finder by Ziyao Wang in December 2023.

Physical characteristics: A single dark stone (23.4 g) partially coated by black fusion crust.

Petrography: (A. Irving, UWS, and P. Carpenter, WUSL) This specimen consists predominantly of a relatively coarse-grained melt rock portion enclosing rare type 5 chondrite clasts. The melt rock portion is composed of elongate skeletal, strongly-zoned olivine phenocrysts (up to ~1 mm in length) within an ultra fine-grained mesostasis containing crystallites of relatively ferroan olivine, low-Ca pyroxene and subcalcic augite plus glass. Sparse spherical grains composed of high-Ni kamacite and troilite (up to 0.9 mm in size, stained by secondary Fe oxides) are also present in the mesostasis. A small type 5 chondrite clast (~3 mm across) within the studied thin section contains sparse relatively small chondrules within a recrystallized matrix containing stained kamacite and troilite.

Geochemistry: Melt rock portion: olivine (cores Fa_8.7-8.9, N = 2; rims Fa_25.9-38.0, N = 2; mesostasis crystallites Fa_24.5-43.4, N = 4), low-Ca pyroxene crystallite in mesostasis (Fs_42.8Wo_4.9), subcalcic augite crystallite in mesostasis (Fs_32.8Wo_25.5). Chondrite clast: olivine (Fa_19.1±0.0, N = 2), low-Ca pyroxene (Fs_16.6±0.1Wo_1.6±0.1, range Fs_16.5-16.6Wo_1.5-1.7, N = 2). Oxygen isotopes (K. Ziegler, UNM): analyses of acid-washed subsamples by laser fluorination gave, respectively, δ¹⁷O 2.913, 2.940; δ¹⁸O 4.238, 4.267; Δ¹⁷O 0.675, 0.687 per mil (all data linearized, TFL slope = 0.528). Magnetic susceptibility log χ (× 10^-9 m³/kg) = 4.89.

Classification: Ordinary chondrite (relatively coarse-grained melt breccia with isotopic affinities to H chondrites, metal-deficient). This specimen is evidently paired with Boutel Fil (b). Although it has an oxygen isotopic composition consistent with H chondrites and contains an H5 chondrite clast, the overall metal content and magnetic susceptibility are significantly lower than for typical H chondrites.

Specimens: 9.05 g including one polished thin section at UWB; remainder with WangZ.