Northwest Africa 13993 (NWA 13993)

(Northwest Africa)

Find: 2020

Classification: HED achondrite (Eucrite, brecciated)

History: The specimen was collected in northwest Africa, purchased by Terry Boudreaux in 2020 and subsequently acquired by the Field Museum in the same year.

Physical characteristics: The specimen is a single, 807 g stone partly covered with a glassy fusion crust that has preserved flow lines. The brecciated interior is light gray-to-tan in color and hosts at least two distinct, fine-grained matrix lithologies. Within the matrix exist mineral and lithic fragments as well as darker (dark gray to black) rounded clasts, approximately 0.5-1 cm in diameter.

Petrography: The specimen has a brecciated, ophitic to sub-ophitic texture. Larger (up to 1.5 mm) euhedral pyroxene and plagioclase grains. BSE imaging reveals that the modal abundances of pyroxene and plagioclase are nearly equal and are set in fine-grained subhedral matrix. Both low-Ca pyroxene (pigeonite) and high-Ca pyroxene (augite) are present, but pigeonite is more abundant by far (representing ~85% of the pyroxene modal abundance). Pigeonite exhibits very narrow (~0.1 µm) augite exsolution lamellae, and the original igneous Fe/Mg/Ca zoning is preserved. Within the matrix exist large (up to 1 cm) rounded clasts with variably fine-grained (up to 1 mm) textures. Intergranular silica polymorph, chromite, troilite, and Fe metal are also present. A secondary brecciated matrix lithology, which makes up approximately 20% of the area of the cross-section studied, is larger-grained and more pyroxene-rich than the primary matrix lithology. Dendritic-textured chrome-spinel is a common minor phase throughout the specimen. There are also light-colored veins which contain long plagioclase/silica polymorph needles. Oxidized Fe is visible in the hand sample as groupings of red grains, up to 1 cm in diameter, primarily localized to the secondary matrix lithology.

Geochemistry: Low-Ca pyroxenes exhibit a compositional range of Fs_48.5-55.9Wo_5.7-13.3, with an average of Fs_53.3±2.09Wo_6.9±1.86. These grains have an average FeO/MnO of 38.9±1.1 (n=30). High-Ca pyroxenes exhibit a compositional range of Fs_32.0-49.5Wo_21.2-49.3, with an average of Fs_38.7±7.89Wo_33.9±14.4 and an FeO/MnO of 41.4±0.72 (n=4). Feldspars have the limited range of An_90.49±0.78 (n=12). Plagioclase K₂O contents are very low, ?0.2 wt%.