Northwest Africa 11351 (NWA 11351)

(Northwest Africa)

Purchased: 2013 Apr 26

Classification: Ordinary chondrite (LL3-6)

History: John Shea purchased a single stone from Big Kahuna meteorites on April 26, 2013 .

Physical characteristics: A black shiny fusion crust surrounded the entire stone. Three distinctly different lithologies are apparent on the cut face: Lithology A having packed, well-defined chondrules; lithology B having some well-defined chondrules surrounded by finer-grained material; and lithology C with poorly defined chondrules.

Petrography: Metal abundance is low (1-2 area %) in all three lithologies. Troilite exceeds metal in lithologies A (14 area%) and C (7 area 5), and is roughly the same (3 area%) in lithology B. In all lithologies, metal has been minimally weathered (<10% replacement). Troilite is weathered along fractures and adjacent silicates are discolored. Lithology A is composed of crisp, well defined, closely-spaced chondrules, many of which are rimmed by metal and troilite. Other chondrules are surrounded by a fine-grained matrix made of olivine and albite grains, similar to the matrix described for Lut 005 (LL3). The texture of lithology A is similar to that of a cluster chondrite (as described by Metzler, 2012). Lithology B is composed of chondrules and chondrule fragments, and resembles a fragmental breccia. Both lithology A and lithology B contain numerous glassy or cryptocrystalline chondrules with bleached rims. Chondrule mesostases in both lithologies contain brown isotropic glass as well as devitrified glass. Much of the low-Ca pyroxene in both lithologies is monoclinic. A single large grain of silica polymorph was found in lithology B. Lithology C contains large angular clasts. In the largest clast, chondrules are defined by the presence of trolite within the chondrules. Feldspars in this clast are typically 10-40 µm in diameter. Many smaller clasts in lithology C contain abundant coarse merrillite and diopside, and coarse (>50 μm) plagioclase feldspar grains. Olivine deformation in lithologies A and B consistent with shock stage S4. Olivine grains in the various clasts of lithology C all indicate higher (S5) shock deformation.

Geochemistry: Lithology A: Olivine (Fa=27.9±0.9, n=56), low-Ca pyroxene (Fs=17.6±4.7Wo=1.0±1.3, n=32) and feldspathic glass (Ab=71.5±19.7Or=3.4±3.3, n=18, with Ab_27.3-92.4). Lithology B: Olivine (Fa=27.6±0.6, n=40), low-Ca pyroxene (Fs=21.5±5.4Wo=1.5±1.2, n=29) and feldspathic glass (4 analyses have Ab_13.8-86.0). Lithology C: Olivine (Fa=30.9±1.0, n=40), low-Ca pyroxene (Fs=25.6±1.3Wo=2.1±0.7, n=29) and plagioclase feldspar (Ab=82.6±1.5Or=5.6±2.0, n=12).

Classification: Olivine compositions for all three clasts are consistent with an equilibrated LL chondrite. The sizes of feldspar grains in lithology C indicate that it contains petrographic type 6 material. Although the olivine compositions in lithologies A and B are equilibrated, the pyroxene compositions are not, and along with abundant clinoenstatite, and isotropic calcic glass, suggests that lithologies A and B are samples of a high type 3 (or borderline 3-4) chondrite.

Specimens: 36.5 g, three epoxy butts and three one-inch round doubly polished thin sections are held at Cascadia. The remaining material is held by John Shea.