Northwest Africa 6712 (NWA 6712)

(Northwest Africa)

Purchased: Sept 2009

Classification: HED achondrite (Eucrite)

History: Samples were donated to Cascadia by Mr. Fred Olsen.

Physical characteristics: Two pieces fit together forming an overall faceted shape. Red-brown patina covers much of the surface, although a dark-grey fusion crust is visible in places, and one side shows a light-grey interior. Cut face shows a dominantly fine-grained interior, with clasts, including one large and relatively coarse basalt clast (~2-3 cm). A discolored weathering zone extends 3-4 mm inward from one surface.

Petrography: (A.Ruzicka, Cascadia) Prevalent lithology consists of basalt clasts (0.7-2.4 mm) that grade into a finer grained matrix of pigeonite, plagioclase, and augite clasts (<0.3 mm). Fractured and slightly porous plagioclase largely fills interclast areas. Large, coarse basalt clast has a subophitic texture dominated by twinned plagioclase with undulose-to-mosaic extinction (up to 0.8-1.6 mm long × 0.1-0.2 mm wide) and heavily fractured pigeonite with undulose-to-mosaic extinction (up to 0.6-1.5 mm); this clast is internally brecciated. Clouding by precipitates is ubiquitous in plagioclase and pyroxene but spatially variable. Throughout the section, augite occurs as lamellae in pigeonite or as discrete grains and clasts. Small calcite veins locally fill fractures.

Geochemistry: (A. Ruzicka, M. Hutson, and T.J. Schepker, Cascadia) Major minerals have similar compositions in the largest and smaller basalt clasts and in the clastic matrix: plagioclase (Ab_12±1.7An_86.7±1.7Or_1.3±0.8, n = 41), pigeonite (Wo_8.0±2.4En_32.2±1.1Fs_59.8±2.1, Fe/Mn = 32.3 wt, n = 36), augite (Wo_40.±1.6En_28.0±0.9Fs_31.3±1.2, Fe/Mn = 31.6, n = 20).

Classification: Achondrite (eucrite). Equilibrated, heavily brecciated and moderately deformed, apparently monomict breccia.

Specimens: A total of 20.8 g, 1 thin section, and potted butt are on deposit at Cascadia. Mr. McKenzie holds the main mass.