Northwest Africa 12937 (NWA 12937)

(Northwest Africa)

Purchased: 3 Feb 2019

Classification: HED achondrite (Eucrite)

History: Cascadia purchased an NWA sample from Sidi Mohamed Ismaily at the Tucson GEM show on February 3, 2019

Physical characteristics: No remnant fusion crust is visible on the exterior. Most of the exterior is covered by a dark brown weathering rind; one surface has a lighter rusty brown matte surface. A few dark ridges stick up from the background, and one waxy grain of feldspar (5 mm × 7 mm) is visible on the weathered surface. Most of the cut faces appear dark gray, fine-grained, and featureless in hand specimen, with the exception of a thin (1-5 mm) rim of coarser-grained material (white and brown grains) near the surface.

Petrography: (M. Hutson, A. Ruzicka, Cascadia): In thin section, sample is a breccia with clast textures ranging from hypidiomorphic to subophitic. Most clasts are fine-grained, with a coarse-grained clast visible on the edge of the section. Clasts are separated by a dark fine-grained matrix. Both pyroxene and plagioclase grains appear partially recrystallized in cross-polarized light, with patches of remnant twinning in many pyroxene grains and only a few of the largest feldspar grains. Feldspar is crystalline. The sample is moderately weathered with small calcite veins cutting across the section. In BSE imaging, feldspar grains contain numerous inclusions, which are concentrated on the outer margins of the larger grains. Pyroxene grains are heavily fractured. Remnant areas of twinned augite and low-Ca pyroxene, are offset by microfaults, and grade into mottled regions defined by a subtle core-rim structure. In some areas pigeonite rims surround low-Ca pyroxene cores, in other areas rims of low-Ca pyroxene surround pigeonite cores. The mottled regions are roughly 5-25 microns across, the same size as the fine-scale recrystallization seen in optical microscopy. Areas between clasts contain melt veins that cut across silicates. Phases present include pyroxene, feldspar, chromite, ilmenite, and silica polymorph

Geochemistry: Mineral compositions and geochemistry: (M. Hutson, A. Ruzicka, Cascadia; K. Ziegler, UNM). Despite textural variation, the low-Ca pyroxene (Fs_63.3±1.0Wo_2.5±0.5, N=14), augite (Fs_27.8±0.7Wo_43.6±0.7, N=14 ), and feldspar (An_89.9±0.9Or_0.5±0.2, N=13) grains are equilibrated. Intermediate composition pyroxenes form a mixing line between augite and low-Ca pyroxene, with a well-defined cluster in the pigeonite field (Fs_59.6±1.4Wo_6.1±1.8, N=12). Oxygen data (mean and standard deviation, all in per mil and linearized): δ¹⁷O=1.770±0.077, δ¹⁸O=3.855±0.140, and Δ¹⁷O=-0.266±0.012 (N=3).

Classification: Achondrite (eucrite) based on chemistry, texture, and oxygen isotope composition.

Specimens: Cascadia holds the main mass in one large piece (463.2 g) and several fragments, with a total mass of 471.9 g, in addition to a polished thin section and a mounted butt.