Northwest Africa 6960 (NWA 6960)

(Northwest Africa)

Purchased: 2011 Jun

Classification: Ordinary chondrite (type 3)

History: Purchased by GHupé in June 2011 from a Moroccan dealer at the St. Marie-aux-Mines Mineral Show.

Physical characteristics: Very fresh dark gray stone (441 g) with white to beige (and some dark) chondrules, pale angular grains and low amounts of fresh metal.

Petrography: (A. Irving and S. Kuehner, UWS) Well-formed chondrules of varying sizes (some in the size range for LL chondrites, others much smaller). Some chondrules have pyrrhotite-rich rims and others have fine grained dust rims. Large angular olivine grains also are present, and one unusual clast composed of pigeonite+anorthite was found. Olivine (Fa_0.5-49.5; Cr₂O₃ in ferroan olivine ranges from 0.03-0.20 wt.%, mean 0.14, s.d. 0.05, N = 9), orthopyroxene (Fs_1.0-16.9Wo_0.6-3.2), pigeonite (Fs_22.2-35.4Wo_8.0-5.9). Pyrrhotite occurs as an accessory mineral in the matrix, but kamacite and taenite are both minor constituents.

Geochemistry: Olivine (Fa_0.5-49.5; Cr₂O₃ in ferroan olivine ranges from 0.03-0.20 wt.%, mean 0.14, s.d. 0.05, N = 9), orthopyroxene (Fs_1.0-16.9Wo_0.6-3.2), pigeonite (Fs_22.2-35.4Wo_8.0-5.9). Oxygen isotopes (R. Tanaka, OkaU): analyses of acid-washed subsamples by laser fluorination gave δ¹⁸O 5.236, 4.596; δ¹⁷O 3.330, 2.935; Δ¹⁷O 0.572, 0.514 per mil.

Classification: Chondrite (OC3, Type 3.4). Although oxygen isotopic compositions plot near the field for L chondrites, this very fresh specimen has far too little metal (around 2 vol.%), and some of the chondrules are too large, for this to be an L chondrite. Subtype estimated to be 3.4 based on Cr₂O₃ range in ferroan olivine. Estimation of subtype based on histograms (Fig. 4) in Grossman and Brearley (2005).