Northwest Africa 12938 (NWA 12938)

(Northwest Africa)

Purchased: 12 Mar 2019

Classification: HED achondrite (Eucrite)

History: Dr. Richard Palin of the Colorado School of Mines purchased a single stone in March 2019 from a dealer on ebay (identified as matt992011). On the ebay listing, the seller says that he purchased a NWA, unclassified, probable HED from a Moroccan dealer, three years before the 2019 listing. Dr. Palin sent images of the hand specimen, a type specimen, and preliminary EMP data to Cascadia asking for confirmation that the sample was an HED, and for aid in classification.

Physical characteristics: Cascadia received an end piece covered with a dark brown desert varnish. The cut face shows intermingled lighter and darker gray patches with small crystals visible in the lighter patches. Photographs of the entire specimen shows patches of remnant fusion crust surrounding uncoated euhedral to subhedral feldspar grains of varying size and abundance.

Petrography: (M. Hutson, A. Ruzicka, Cascadia; R. Palin, CSM): The thin section shows two lithologies, with a sharp (breccia) contact between. One is coarser-grained with a cumulate texture, and the other finer-grained with a subophitic to intergranular texture. Phases present include pyroxene, feldspar, chromite, ilmenite, zircon, a calcium phosphate, and silica polymorph, and terrestrial calcite fills fractures. The cumulate clast contains inclusion-filled feldspars up to 1 mm in length. Feldspar grains in the finer-grained clast are always less than 400 microns long, with most less than 200 microns in length. In both clasts, in cross-polarized light, many of the smaller feldspar grains are isotropic; the larger grains contain remnant twins, with patches that are isotropic. Pyroxene grains are heavily fractured and stained brown, with darker staining near the edge of the sample. In BSE, pyroxene grains show remnant twinning grading into untwinned regions. Twins are often offset by one to several microfaults. A small melt region occurs at the contact between the two clasts and is rich in a silica polymorph. A vein dominated by silica polymorph originates in this melt region and cuts across the finer-grained clast; small areas of whole-rock melt occur along this vein.

Geochemistry: Mineral compositions and geochemistry: (M. Hutson, A. Ruzicka, Cascadia; K. Ziegler, UNM). Despite textural variation, the low-Ca pyroxene (Fs_60.3±0.8Wo_2.3±0.4, N=27), augite (Fs_25.7±0.5Wo_43.7±0.3, N=13 ) and feldspar (An_84.9±1.Or<1, N=49) grains are equilibrated. Chromite Cr# = 84.1±1.7 (N=5). Oxygen data (mean and standard deviation, all in per mil and linearized): δ¹⁷O=1.745±0.128, δ¹⁸O=3.795±0.209, and Δ¹⁷O=-0.259±0.019 (N=3).

Classification: Achondrite (eucrite) based on chemistry, texture, and oxygen isotope composition. The sample is brecciated, but the minerals have the same chemical composition, so this appears to be a genomict breccia.

Specimens: Cascadia holds 14.4 g in two pieces, in addition to a polished thin section and a mounted butt. Dr. Richard Palin of the Colorado School of Mines holds the main mass.