Northwest Africa 12680 (NWA 12680)

(Northwest Africa)

Purchased: 2018 Oct 5

Classification: Ordinary chondrite (L5, melt breccia)

History: Colvin purchased from Michael Gilmer (Galactic Stone and Ironworks) on October 5, 2018.

Physical characteristics: The exterior of the meteorite surrounds an 8 mm thick slice received by Cascadia. This ribbon of exterior consists of a weathered surface ranging from almost black to dark brown with patches of reddish oxide and pink/tan caliche. Cut faces show medium to dark gray, angular clasts, up to 2.3 × 1.8 cm in area, set in a dark gray to black matrix. Most clasts lack clearly defined chondrules in hand specimen. A small clast, which was not sectioned contains numerous readily distinguished chondrules.

Petrography: (M. Hutson, A. Ruzicka, Cascadia) A single blackened clast (approximately 2 × 0.6 cm area) fills most of the thin section, and a small triangular portion of another blackened clast (0.2 × 0.5cm; base × height) is visible in one corner of the section, with a melt-rich region along one edge of the section and between the clasts. Chondrules in both clasts are readily delineated. Silicates in both clasts are crosscut by small sulfide veins. Metal and sulfide grains appear fluidized with edges defined by the shapes of the surrounding silicate grains, and with some metal grains containing angular fragments of silicate grains. The melt region between clasts consists of weakly zoned olivine and pyroxene grains between about 20-40 μm across set in a much finer grained (< 5 μm across) matrix composed of grains of metal/sulfide, sulfide, and zoned pyroxene. This melt region contains larger metal-troilite grains, in which the troilite contains smaller blebs of metal. Coarser veins of mixed metal and sulfide cut across the melt regions. Areas containing networks of feldspathic glass are present.

Geochemistry: Mineral compositions and geochemistry: Olivine and pyroxene grains in the two clasts are essentially identical, and can be considered together: Fa_24.7±0.3, N=24; Fs_21.4±0.5Wo_1.4±0.5, N=22. Single zoned olivine and pyroxene grains in the melt region have slightly lower iron contents than those in the clasts: Fa_{21.2± 2.2}, N=15; Fs_20.1±1.4, N=9.

Classification: Ordinary chondrite (L5-melt breccia) based on chemistry and texture. The two clasts are L5 chondrite fragments based on chemistry and texture. The presence in hand specimen of a small clast with more readily defined chondrules suggests that there may be clasts of a different petrographic type in the meteorite. The extensive melt matrix surrounding the clasts indicates this is a melt breccia.

Specimens: Cascadia holds 45.5 g in three pieces, in addition to a polished thin section and a mounted butt. The main mass is held by Colvin: Christopher Colvin, 20036 Fern Glen Ln. N., Forest Lake MN 55025).