Northwest Africa 11992 (NWA 11992)

(Northwest Africa)

Purchased: 2007, Sept

Classification: Ordinary chondrite (LL3)

History: Found in northwest Africa, donated to Cascadia July 2012.

Physical characteristics: In cut slices, exterior is dark gray and shiny, interior is gray with well-defined chondrules and has less metal than typically observed in fresh ordinary chondrites. Minimal weathering staining.

Petrography: (A. Ruzicka, S. Goudy, Cascadia) Thin section shows well-defined chondrules with cluster chondrite texture and little intervening matrix. Glassy chondrule mesostases present but not abundant. Roughly half of low-Ca pyroxene grains show twinning suggestive of clinoenstatite polytype. Shock stage is S4. Shock effects are heterogeneous with S1 and S4 olivine grains predominant. Some bleached chondrules. Minimal weathering effects.

Geochemistry: (M. Hutson, Cascadia) Olivine (Fa_30.0±4.0, N = 140; Cr₂O₃ 0.05±0.02 wt%, N = 56), low-Ca pyroxene (Fs_20.2±6.8Wo_1.4±1.1, N = 143).

Classification: LL3 chondrite, estimated subtype 3.7. Iron contents in olivine and low-Ca pyroxene unusually high for LL3. LL indicated by low metal content and low weathering grade and overlap with LL for olivine Fa and low-Ca pyroxene Fs. Type 3 indicated by texture, presence of chondrule glass, abundance of clinoenstatite, and variable olivine and pyroxene compositions. Subtype indicators can be best reconciled with subtype of 3.7 to 3.8: Fa contents suggest 3.7-3.9, Fs contents suggest 3.4-3.9, olivine chrome contents suggest 3.5.

Specimens: A total of 22.7 g in one slice, a polished thin section, and potted butt are available at Cascadia. Mr. McKenzie holds the main mass.