Northwest Africa 11991 (NWA 11991)

(Northwest Africa)

Purchased: 2018 Jan

Classification: Ordinary chondrite (LL3)

History: Found in northwest Africa.

Physical characteristics: In cut slices, interior is light gray to light brown with slight weathering staining, has well defined chondrules and metal contents typical of LL chondrites.

Petrography: (A. Ruzicka, S. Goudy, Cascadia) Thin section shows well-defined chondrules, many with glassy mesostases, and good examples of cluster chondrite texture with little intervening matrix. Multiple bleached chondrules. Analysis in reflected light shows thin section contains roughly 2.3% metal with ~10% metal weathered, and roughly 4.2% troilite; largest slice contains 2.8±0.3% metal and 5.4±0.6% troilite (area% values). Based on olivine, shock stage is S5. Matrix is composed chiefly of <10 μm grains of ferroan olivine and albitic feldspar, reminiscent of that found in Lut 005 and Lut 006.

Geochemistry: (M. Hutson, Cascadia) Olivine (Fa_21.6±9.9, N = 156; Cr₂O₃ 0.07±0.05 wt%, N = 126).

Classification: LL3, estimated subtype 3.5. LL indicated by low metal content and minor weathering. Type 3 indicated by texture, presence of chondrule glass, and variable olivine composition. Subtype estimated as 3.5 based on olivine Fa variability (subtype 3.4-3.6) and chrome content in olivine.

Specimens: A total of 48.4 g in two slices, a polished thin section, and potted butt are available at Cascadia. Mr. James Tobin holds the main mass.