Northwest Africa 10085 (NWA 10085)

(Northwest Africa)

Purchased: Feb 2013

Classification: K4

Physical characteristics: A desert-varnished and mostly fusion-crusted stone was purchased at the Tucson Gem and Mineral Show in 2013.

Petrography: (J. Utas, UCLA) The meteorite is comprised of approximately ~60% matrix, 15-20% chondrules, and ~20% limonite. The average chondrule diameter is ~450 μm. The thin section is dominated by two large chondrules: a 2 mm PP chondrule and a 1.5 mm POP chondrule. Enstatite laths are dispersed throughout the matrix and limonite, with most laths measuring less than 200 by 30 μm. A few are as large as 400 by 50 μm. Sulfide grains ~2-10 μm in diameter are common, but few larger ones are preserved; one 300 μm sulfide grain remains. Three iron phosphide inclusions up to ~150 μm were observed; one was associated with euhedral chromite crystals ~5 to 50 μm in diameter. Many 5-10 μm grains of unoxidized kamacite and a few grains of taenite are still present in the section.

Geochemistry: Mineral compositions and geochemistry: J. Utas (UCLA) Kamacite (N=6, total 99.0%): 93.1% Fe, 6.7% Ni (5.38% to 8.73%), 0.40% Co, 0.19% Mg, 0.13% P, 0.07% Si (0.02% to 0.18%), and 0.05% Mn (just above detection limit). Ti and Cr were not observed above detection limits. Presence of trace Mg in kamacite may indicate contamination from silicate minerals; the above Si value may be slightly higher than the actual value. One taenite grain was observed, with 75.0% Fe, 24.7% Ni, 0.19% Co, 0.09% P. Ti, Cr, Si, and Mg were not detected (total 99.3%). Schreibersite (N=5, total 100.4%): 46.2% Fe, 29.3% Ni, 24.4% P, other elements not observed. Sulfide (N=7, total 98.0%): 50.5% Fe, 49.05% S, 0.31% Cr, 0.05% P (just above detection limit). Plagioclase (N=2, total 100.5%): An_13.3Ab_83.2Or_3.6. Oxygen isotopes: (K. Ziegler, UNM) δ¹⁷O = 1.886‰, 2.674‰, 1.411‰ δ¹⁸O = 5.629‰, 6.873‰, 5.017‰, Δ¹⁷O = -1.086‰, -0.955‰, -1.238‰. Samples: 1.4, 1.5, and 1.7 mg, respectively, matrix and chondrule.

Classification: (J. Utas and A.E. Rubin, UCLA) (K4) Fa, Fs, and Wo averages plot between values obtained for Kakangari, Lea County 002, and LEW 87232. Kamacite and taenite Fe, Ni, and trace elemental abundances are also intermediate. The only differences observed between this stone and other K (type 2-3) meteorites are this stone’s petrographic grade and the apparent average grain size of matrix mineral assemblages. These differences are consistent with the stone having experienced a higher degree of thermal metamorphism than other, unequilibated, K chondrites.