Northwest Africa 13188 (NWA 13188)

(Northwest Africa)

Purchased: 2018 Jun

Classification: Ungrouped achondrite

Physical characteristics: Partially crusted stone

Petrography: (J. Gattacceca, CEREGE) Igneous rock with overall subopthitic texture. Main minerals are pyroxene (300 μm) and plagioclase (to 400 μm). Abundant vesicles 250 μm in diameter. FeTi oxides (~0.5 vol%) as blebs adjacent to vesicles, or acicular and dendritic within the fine-grained intergrowths of plagioclase and pyroxene that are found around plagioclase crystals. No metal was found. Modal abundances from point counting (vol%): plagioclase 49%, pyroxene 26%, vesicles 8%, fine-grained intergrowth of plagioclase and pyroxene 18%.

Geochemistry: Low-Ca pyroxene Fs_25.4±6.6Wo_8.3±0.7, FeO/MnO=42.2±4.3 (n=5). Ca pyroxene Fs_18.4±4.2Wo_38.1±0.8, FeO/MnO=60.3±11.7 (n=3). Plagioclase An_70.1±4.7Ab_29.0±4.4Or_0.9±0.3 (n=5). The bulk sample has a basaltic composition with 52.3%wt SiO₂; 8.0%wt MgO; 10.1 %wt FeO; 10.5%wt CaO; 14.1 %wt Al₂O₃ (ICP-OES, V. Debaille, ULB), and is enriched in incompatible trace elements: (La/Sm)N=2, (La/Lu)N=3.5 (ICP-MS, V. Debaille, ULB). Oxygen isotopic composition (J. Gattacceca, C. Sonzogni, CEREGE) from two analyses of acid-washed 1.5 mg aliquots of a powdered 128 mg bulk sample is δ¹⁷O=3.885, 4.428, δ¹⁸O=7.552, 8.516, Δ¹⁷O=-0.042, -0.000‰ (slope 0.52, analytical uncertainties 0.08‰, 0.12‰, 0.03‰ respectively).

Classification: Achondrite-ung. The oxygen isotopic composition and mineral chemistry (basaltic composition enriched in incompatible elements) set this rock apart from other achondrites.

Specimens: Type specimen at CEREGE. Main mass with A. Jambon.