Northwest Africa 13968 (NWA 13968)

(Northwest Africa)

Purchased: 2019

Classification: Rumuruti chondrite (R3)

History: Acquired by Mark Lyon at 2019 Tucson Gem and Mineral Show in Tucson, AZ

Physical characteristics: (L. Garvie, ASU) Dark, shiny, sandblasted stone lacking fusion crust. Chondrules visible over the surface of the stone and on the cut surface. A 2 × 3 cm cut surface is dark showing an abundance of light-colored chondrules, largest to 1.5 mm, and abundant Fe-Ni sulfides. Cut surface shows two prominent 1-cm breccia clasts. Clasts are noticeable by their darker and more abundant matrix compared with the bulk of the stone. Very little weathering visible in the stone.

Petrography: (A. Herbst and W. Cortez, ASU) Approximately ~30% chondrules, 55 vol% matrix, 10 vol% subhedral to euhedral olivine grains, 5% sulfides, and <1 vol% Al-rich chondrules and CAIs. Sulfides occur as micron and submicron sized grains dispersed in the matrix, nodules to 700 µm, and as mantles around chondrules. Two prominent breccia clasts were observed, ~8 mm and ~5 mm in size. Chondrules as 0.4 ± 0.2 mm (n=74), though one to 1.5 mm. Chondrules are dominantly porphyritic olivine-pyroxene, but also show barred olivine and radial pyroxene textures. EPMA results show a rare diopside-rimmed CAI (~0.1 mm in size) composed of spinel and ilmenite inclusions identified within the larger breccia clast. The matrix within the two clasts is fine-grained while the matrix outside of the clasts is coarser-grained. The bulk matrix hosts lithic fragments of pyroxene and subhedral to anhedral olivine grains as well as compositionally zoned olivine. Phosphates (merrillite and Cl-apatite-rich) aggregates along with one euhedral spinel are also identified within the matrix of the bulk meteorite. Platinum-group bearing metal-rich inclusions are located within metal sulfide hosts, including a ~5 µm gold and platinum grain embedded in a pentlandite host, a ~600 nm osmium-rich grain embedded in a troilite host, and a ~3 µm iridium-arsenide grain in a pentlandite host.

Geochemistry: (A. Herbst and W. Cortez, A. Wittmann, ASU) Olivine Fa_26.9±14.0, range 0.6-48.3 (n=57); low-Ca pyroxene En_83.5±9.0Fs_15.1±8.6Wo_1.4±2.1 (n=30); high-Ca pyroxene En_45.5±5.5Fs_11.8±2.8Wo_42.7±8.2 (n=4).

Classification: Petrography (texture and average chondrule size) and geochemistry consistent with an R3 chondrite.