Northwest Africa 16503 (NWA 16503)

(Northwest Africa)

Purchased: 2022

Classification: HED achondrite (Eucrite, polymict)

History: Craig Zlimen purchased a single unclassified northwest African stone from an Algerian dealer in 2022 and gifted the stone to Zachary Zlimen. Cascadia received a 46.8 g piece in June 2023.

Physical characteristics: Physical Characteristics: The exterior of the end cut received by Cascadia is irregular, light tan colored in low areas and dark brown in higher areas; there is no evidence of a fusion crust. The cut face displays speckled angular clasts set amidst a fine-grained, dark gray groundmass.

Petrography: (M. Hutson, A. Ruzicka, Cascadia): In thin section, the sample consists of lithic clasts set in a host comprised of angular mineral fragments surrounded by fine-grained clastic matrix, and heavily crosscut by carbonate veins. Lithic clasts include basaltic (largest ~7.8 × 3.3 mm exposed area), gabbroic (largest ~3 × 4 mm exposed area), and opaque (largest ~1.2 × 0.9 mm exposed area) rock fragments. Gabbroic clasts and opaque clasts make up < 10% of the lithic clasts. BSE imaging of the largest opaque fragment at low magnification shows a basaltic texture with elongate crystals (~100 μm long) of pyroxene and plagioclase feldspar; higher magnification reveals that these elongate crystals are composed of much smaller (~5 μm) blocky grains meeting at triple junctions. Mineral chemistries, EDS maps, and BSE images were obtained for this opaque clast, the three largest basaltic clasts, and the largest gabbroic clast (clast 4). The three basaltic clasts are partially surrounded by thin feldspathic veins containing small (~10 μm) pyroxene fragments. Additional phases observed in thin section include a silica phase, merrillite, ilmenite, chromite, sparse troilite in one small unanalyzed opaque-rich clast, and (in gabbroic clast 4 only) isolated small grains of fayalitic olivine, as well as thin veins of fayalitic olivine and plagioclase feldspar filling fractures in pyroxene.

Geochemistry: On a pyroxene quadrilateral, all but two of the 43 pyroxene grains analyzed have compositions which define three distinct clumps: pigeonite (host grains and all clasts with basaltic textures) Fs_61.8±1.0Wo_7.2±1.1, range Fs_59.8-63.5Wo_5.8-9.4, molar Fe/Mn=30.8±0.9, N=26; augite (host grains and clasts with basaltic texture) Fs_34.1±0.5Wo_39.3±0.5, range Fs_33.5-34.7Wo_38.8-39.9, molar Fe/Mn=32.2±1.0, N=7; and orthopyroxene (clast 4): Fs_54.9±1.1Wo_4.3±1.1, range Fs_53.0-56.1Wo_3.1-5.6, molar Fe/Mn=32.6±1.9, N=9. Two grains lie along a line between the pigeonite and augite clumps. Plagioclase feldspar compositions are variable, with feldspar compositions of all clasts falling within the range defined by host feldspar grains: Ab_14.0±6.0Or_1.2±0.7An_84.9±6.5, range Ab_5.9-26.2O0.4-2.6rAn_71.2-93.6, N=18 (host only). Olivine (clast 4): olivine Fa_71.9±1.1. N=7.

Classification: HED achondrite (Eucrite, polymict) based on mineral chemistry and texture, with clast 4 having distinctly difference pyroxene compositions from the remaining analyzed material.

Specimens: Cascadia holds 43.7 g in two pieces, as well as a polished thin section; Zachary Zlimen (Minnesota Meteorites) holds the main mass.