Northwest Africa 17018 (NWA 17018)

(Northwest Africa)

Purchased: Aug 2022

Classification: Ordinary chondrite (LL5)

History: The stone was purchased by Jasper Spencer from a Mauritanian dealer in August 2022 via Facebook Messenger. Mr. Spencer was told that the stone was found in Mauritania by nomads during 2022. Mr. Spencer sent a 59.9 g end cut to Cascadia for classification in October 2023.

Physical characteristics: Physical Characteristics: The exterior surface of the end cut is dark brown and bumpy. The cut face shows abundant light and dark angular fragments, and a similar-sized (mm-scale) single well-defined dark gray chondrule, set among a light orange-brown background. Two larger (cm-scale) angular black clasts are present.

Petrography: (M. Hutson, A. Ruzicka, Cascadia): The thin section is comprised of clasts and individual mineral and chondrule fragments. With two exceptions (a blackened clast and a granular clast), clast boundaries are indistinct. A large (~12.4 × 12 mm exposed area) blackened clast is truncated along one edge of the section, and bounded by a black straight vein (~0.9 mm wide) along another edge. BSE imaging shows that the blackened clast is composed of large (~0.5-1.0 mm across) irregularly-shaped areas dominated by either olivine, orthopyroxene, or phosphate minerals. Feldspathic material is not concentrated in large patches, but disseminated as small (~20-40 µm) grains surrounding individual olivine and orthopyroxene grains. The vein along the edge of the clast is composed mainly of roughly aligned sub-angular to sub-rounded olivine and orthopyroxene grains and clasts, with only a few clasts >50 µm. No high-pressure minerals were observed. An elongate granular clast (~4.8 × 2.8 mm exposed area) is composed of sub-angular to sub-rounded olivine and orthopyroxene with intergranular spaces filled with a mix of fine-grained (mostly <20 µm long) high-Ca pyroxene and feldspathic material. A single notable chondrule consists of irregularly-shaped olivine grains surrounding lath-shaped regions of inclusion-filled feldspathic material. BSE imaging and element maps indicate that the inclusions are a mix of chromite and ilmenite, and that the feldspathic material forms bands of varying composition. This is referred to as an FCA (feldspathic-chromite assemblage) chondrule. Excluding the clasts and the FCA chondrule, the rest of the thin section (host) contains sparse, but large (up to 2.7 mm across) patches of heavily weathered (~75-80% replaced by terrestrial weathering products) taenite and sulfides (troilite, phyrrhotite, and pentlandite). With the exception of a couple of coarse feldspar grains surrounded by metal and sulfide, all of the feldspar in the host is < 50 µm across. Additional minerals observed in the host are high-Ca pyroxene, chlor-apatite, and merrillite. Ilmenite was observed only inside the FCA chondrule.

Geochemistry: Olivine and orthopyroxene are equilibrated; no difference was seen between host, clasts, or FCA chondrule. Olivine Fa_31.9±0.4, N=53; orthopyroxene Fs_26.3±0.3Wo_1.6±0.3, N=29. Feldspathic material is variable in composition; grains within the host material have compositions typical of an ordinary chondrite (Ab_84.9±2.0Or_4.2±0.5An_10.9±2.0, N=5), as do the grains that were large enough to analyze within the granular clast (Ab_84.7±1.8Or_3.9±0.6An_11.4±2.4, N=2). The grains in the blackened clast are slightly enriched in Potassium (Ab_81.8±3.3Or_8.5±4.6An_9.6±2.4, with a range of Or_5.0-19.9, N=9). Feldspathic material in the FCA chondrule is segregated into bands and patches of highly variable Calcium content with ranges: Ab_16.4-85.2Or_0.4-3.8An_11.4-83.1, N=17.

Classification: Ordinary chondrite (LL5), based on mineral chemistry of olivine and orthopyroxene, distinctness of fragments, and size of feldspar grains.

Specimens: Cascadia holds 45.8 g in two pieces, as well as a polished thin section and material in epoxy butts; Mr. Jasper Spencer holds the main mass.