Northwest Africa 7357 (NWA 7357)

Morocco

Purchased: 2019

Classification: HED achondrite (Eucrite, cumulate)

Physical characteristics: The stone’s surface is half covered by fusion crust, which manifests streaming of melt. In places the surface shows significant tan-orange weathering and caliche staining. On interior broken surfaces, light mineral clasts fluoresce orange under 365 nm UV light.

Petrography: This highly shocked breccia consists dominantly of pyroxene and plagioclase (now partly maskelynite). Vaguely discernible vestiges of an original texture, and macroscopic observations, suggest grains were typically much coarser than 1 mm, but most of the material has been either granulated to much finer than 1 mm or shock-melted. A distinctive aspect of the texture is that a component of inferred likely shock-melt origin is dominantly neither glassy nor in the form of veins: Many large areas, constituting about 30% of the rock, show a crystalline but incongruously fine-grained (<20 μm) and subophitic texture, of suspected in situ, or near-in situ, impact-melt origin. This mode of origin is inferred from the sharp textural disparity with, and yet close geochemical similarity to, the surrounding coarser groundmass (which is of distinctively magnesian-cumulate composition; see below). Accessory phases include Cr-spinel, Fe-metal, troilite, and a glassy shock-melt vein. Within a few relatively ungranulated pyroxene grains, exsolved augite lamellae are seen to be typically about 4-7 μm wide. Classification as a cumulate eucrite is based on geochemistry (see below), not the severely impact-modified texture.

Geochemistry: Low-Ca pyroxene (32 analyses) clusters near Fs_34.4Wo_2.5. High-Ca pyroxene blebs and lamellae (4 analyses) cluster near Fs_14.8Wo_43.6. As a subset, pyroxenes in the fine-grained/subophitic enclaves (18 analyses) are not compositionally distinctive, apart from showing mostly intermediate Ca (Wo, 3.2-17.4 mol%), as their low-Ca and high-Ca components are not resolved by EPMA. Pyroxene FeO/MnO (wt; 36 analyses) averages 28.3. Plagioclase (8 analyses) is An_92.2-93.7, average An_92.6±0.5; which includes a subset of 3 analyses, averaging An_93.1, from the fine-grained/subophitic domains. The fine-grained/subophitic domains are thus geochemically fully as cumulate-like as the less shocked, coarser balance of the rock. Classification as a cumulate eucrite is based on geochemistry (consistently magnesian mafic silicates, Na-poor plagiolcase, far more Cr-spinel than ilmenite), not texture. Analyses (6) of a long glassy shock-melt vein average 19.3±1.3 wt% Al₂O₃ with FeO/MgO (wt) = 1.05.

Classification: Eucrite, cumulate, brecciated.

Specimens: 24 g at UCLA; main mass (5 kg) with Gessler.