Northwest Africa 16293 (NWA 16293)

(Northwest Africa)

Purchased: 2018

Classification: Ordinary chondrite (LL4-6)

History: A single 153 g stone was purchased online in 2018 from a Moroccan seller. Six slices totaling 26.8 g were given to Cascadia.

Physical characteristics: Physical Characteristics: Cut faces show abundant rounded and ovoid light gray clasts and a few dark-gray to black clasts set in a medium- to dark-gray matrix.

Petrography: (M. Hutson, A. Ruzicka, Cascadia): The ovoid light gray clasts seen in hand specimen are texturally different in thin section and are referred to as white or light clasts. White clasts are very light-colored, lack any evidence of chondrules, and contain plagioclase feldspar grains (up to 150 µm across) easily visible with an optical microscope. Clast 4 (>13 × 4.5 mm across, cut by section edge) is the largest white clast; twins were observed in cross-polarized light in one large plagioclase feldspar grain. Light clasts contain bright fragments and chondrules with poorly defined margins separated by slightly darker fine-grained material. Clast 5 is the largest light clast, is triangular in shape (4 mm base × 6 mm height), and fills in the space between clast 4 and another white clast. A couple of largely opaque clasts (referred to as dark clasts) are present. Clast 1 is the largest dark clast and is ovoid in shape (2 mm × 1.2 mm). The clast is composed of small (most grains much less than a maximum of 200 µm) sub-angular to sub-rounded grains of olivine and pyroxene, with plagioclase feldspar (~40 µm across).

Geochemistry (M. Hutson, A. Ruzicka, Cascadia): Eight of the larger clasts (four white, two light, one dark, and one matrix) were analyzed. Although the compositions of olivine and pyroxene grains for each clast plot within the range for LL group chondrites on a diagram of Fa vs. Fs, olivine and pyroxene compositions plot slightly outside the range for LL group chondrites on diagrams of Fa vs. Fa s.d., and Fs vs. Fs s.d. Average white clast: olivine: Fa_32.0±0.5, N=34; low-Ca pyroxene: Fs_26.1±0.5Wo_3.1±0.8, N=22; plagioclase feldspar: Ab_82.7±4.1An_14.3±4.8Or_2.9±1.2, N=22. Average light clasts: olivine: Fa_32.0±0.8, N=21; low-Ca pyroxene: Fs_23.5±3.4Wo_2.1±1.1, N=22; plagioclase feldspar for 5 out of 6 grains analyzed: Ab_80.0±6.1An_16.9±6.1Or_3.1±1.1, N=5. The sixth grain is Or-rich: Ab_30.3An_3.8Or_65.9 and is similar to that reported by Kovach and Jones (2010) for Tuxtuac (LL5). Dark clast: olivine: Fa_31.7±0.8, N=3; low-Ca pyroxene Fs_25.8±1.2Wo_1.8±0.9, N=3. Matrix: olivine: Fa_32.0±0.8, N=12; low-Ca pyroxene: Fs_24.1±1.4Wo_1.4±0.8, N=20. The largest metal grains in the thin section were analyzed. All of the large grains were Ni-rich, with Ni between 41.4 and 51.8 wt% (N=52). A few small grains (~ 50 µm across) of another metallic phase were visible in BSE images as inclusions or along edges of the high-Ni metal. Analyses (excluding Co) had low totals, with Ni between 2.6 and 3.8 wt% (N=5). The Ni-beta peak was strongly asymmetric, suggesting the presence of a significant amount of Co. These grains appear to be the high-Co phase described by Rubin (1990), which states that this phase is not kamacite. Oxygen isotopic compositions (K. Ziegler, UNM) of three acid treated aliquots: 1.50 mg δ¹⁷O=3.884, 4.100, 4.048; δ¹⁸O=5.412, 5.804, 5.724; Δ¹⁷O = 1.027, 1.035, 1.046 (all ‰, slope of 0.528, sample mass 1.50 mg, 1.40 mg, 1.70 mg respectively).

Classification: Ordinary chondrite (LL4-6), based on texture (including coarseness of plagioclase feldspar) and mineral chemistry, including degree of equilibration of orthopyroxene. White clasts are LL6, light clasts are L4. Dark clast is fragmental and both olivine and pyroxene are equilibrated. This meteorite is an unusually oxidized member of the LL-group as the iron contents in olivine and pyroxene grains in most clasts are outside of the range for equilibrated LL chondrites when standard deviation is considered. Typical LL-chondrite kamacite is missing; almost all of the metal has a high Nickel content. Oxygen isotopes are at the high end of δ¹⁸O for LL chondrites, and are similar to those reported for Northwest Africa 11811 (LL5-an).

Specimens: Cascadia holds 23.7 g in seven pieces, as well as a polished thin section and material in an epoxy butt.