Northwest Africa 15588 (NWA 15588)

(Northwest Africa)

Purchased: 2019

Classification: CL3

History: Purchased by David Huffman in 2019 from a Moroccan dealer.

Physical characteristics: A single, irregularly shaped, black stone (109 g) with red-brown weathered exterior surfaces. Cut interior surfaces are mostly black with sparse shiny metal.

Petrography: (A. Irving, UWS and P. Carpenter, WUSL) The specimen contains a significant amount of altered kamacite, which caused difficulties with proper petrographic assessment both by optical thin section microscopy and back-scattered electron imagery. The extensive staining by secondary terrestrial Fe hydroxides obscures the primary assemblage to some extent. Well-delineated unequilibrated chondrules (apparent diameter 790±260 µm, N = 20) are evident in thin section, but the proportion of matrix cannot be accurately assessed. Accessory phases include taenite, troilite, calcic plagioclase, pleonaste and Al-Ti-diopside.

Geochemistry: Olivine (Fa_10.0±5.4, PMD = 44%, range Fa_1.3-16.3, N = 10; Cr₂O₃ in ferroan olivine 0.02-0.11 wt.%, mean 0.07±0.03 wt.%, N = 8), low-Ca pyroxene (Fs_6.7±4.6Wo_2.0±1.0, range (Fs_1.0-12.4Wo_0.9-3.0, N = 4), subcalcic diopside (Fs_4.5Wo_29.5), diopside (Fs_2.5Wo_39.5), Al-Ti-diopside (Fs_1.8Wo_57.1, Al₂O₃ 13.1 wt.%, TiO₂ 4.9 wt.%). Oxygen isotopes (D. Ibarra, BrownU): analyses of acid-washed subsamples by laser fluorination gave, respectively δ¹⁷O -1.370, -2.140; δ¹⁸O 2.754, 2.154; Δ¹⁷O -2.819, -3.271 per mil (TFL slope value = 0.526). All data were normalized to San Carlos olivine analyzed concurrently. Magnetic susceptibility log χ (× 10^-9 m³/kg) = 4.89.

Classification: Carbonaceous chondrite (CL3). Oxygen isotopes plot close to the CCAM line but consistently to slightly higher δ¹⁷O values, and lie on an extension of the trend for five specimens established as CL chondrites (Metzler et al., 2021). The presence of relatively calcic plagioclase and mineral phases found in CAI are also consistent with a carbonaceous chondrite affinity, but the significant metal content and elevated magnetic susceptibility are not attributes of carbonaceous chondrites other than CL chondrites. The range in Fa contents of olivine in this specimen (PMD = 44%) is much larger than that found in either of the other two known CL chondrites of subtype 3.9, and the ferroan olivines have Cr₂O₃ contents of up to 0.1 wt.%. Therefore, this specimen must be of a lower subtype than previous examples, and is estimated to be of subtype 3.5 or 3.6.

Specimens: 25.3 g including one polished thin section and one polished mount at PSF; remainder with Mr. D. Huffman.