Northwest Africa 16786 (NWA 16786)

(Northwest Africa)

Purchased: Jun 2022

Classification: HED achondrite (Eucrite, monomict)

History: A single 23 g stone was purchased by Jasper Spencer from Maxime Denoncin at the Ensisheim show in June 2023. Maxime Denoncin had purchased this stone from Mostafa Hnini during 2022 and was told that this was a northwest African stone found in 2022. Mr. Spencer sent a 6.5 g end cut to Cascadia for classification.

Physical characteristics: Physical Characteristics: The end cut shows white grains and light-colored clasts set amidst a dark gray matrix. The exterior of the end cut is covered by a medium gray weathering patina.

Petrography: (M. Hutson, A. Ruzicka, Cascadia): Clasts contain coarse-grained (mm-scale) pyroxene and plagioclase feldspar grains. Plagioclase grains display mosaic extinction on a small scale (10s of micrometers) in cross-polarized light, with a few µm-scale isotropic regions. BSE images show that pyroxene grains are heavily fractured, with twins that are offset and deformed by numerous microfaults. Some pyroxene grains show evidence for incipient melting, with exsolution lamellae grading into irregular shapes within grains, and irregular grain edges intergrown with inclusion-filled plagioclase feldspar grains. One area studied in detail shows the edges of a pyroxene grain grading into a crystallite-filled glassy vein that contains schlieren. The vein crosses smooth plagioclase feldspar which contains vugs. Additional phases observed are Si-polymorph, chromite, ilmenite, troilite, one zircon grain, one zirconium oxide grain.

Geochemistry: (M. Hutson, B. Agyemang, and A. Ruzicka, Cascadia): Pyroxene and feldspar grains are equilibrated. Low-Calcium pyroxene Fs_64.0±0.8Wo_2.3±0.3, range=Fs_62.3-65.1Wo_1.8-3.1, Fe/Mn (at.%)=28.9±0.7, N=15; high-Calcium pyroxene Fs_28.6±0.5Wo_43.5±0.4, range=Fs_27.9-29.7Wo_42.5-44.3, Fe/Mn (at.%)=28.4±0.7, N=18; plagioclase feldspar An_90.4±0.6Or_0.6±0.2Ab_9.0±0.5, range=An_89.6-91.4Or_0.4-1.0Ab_7.9-9.6, N=13; Ti-Al-bearing chromite: Fe_1.3Cr_1.2Ti_0.3Al_0.2O₄.

Classification: HED Achondrite (Eucrite, monmict), based on texture and mineral chemistry.

Specimens: Cascadia holds 3.3 g in one piece, as well as a polished thin section and material in an epoxy butt; Mr. Jasper Spence holds the main mass.