Northwest Africa 10074 (NWA 10074)

(Northwest Africa)

Purchased: 2014

Classification: Primitive achondrite (Acapulcoite)

History: Purchased by Michael Farmer at the Tucson Gem and Mineral Show, 2014

Physical characteristics: Stone is ~70% covered in brownish fusion crust, with well-developed roll-over lip. Sawn surface is light greenish brown with a smattering of small (<0.5 mm) metal and troilite grains.

Petrography: Recrystallized into polygonal and subhedral grains, typically <0.3 mm, with occasional larger (to 1 mm) grains of orthopyroxene. Silicates have homogenous compositions. Many of the low-Ca pyroxene grains contain a core of small metal/troilite and rare chromite grains. These metal/troilite grains are typically <50 μm long and rounded to euhedral. Troilite covers ~7 areal % and Fe-Ni metal ~3 areal %. Most metal grains have high Ni rim and plessitic core. Kamacite is uncommon. Troilite unshocked and single crystal. Accessory chromite. Shock and weathering low.

Geochemistry: (L. Garvie, ASU) Olivine Fa_11.4±0.1, FeO/MnO=21.2±1/0, n=10; low Ca pyroxene Fs_10.8±0.1Wo_1.4±0.3, FeO/MnO=13.3±0.8, Cr₂O₃=0.2±0.0 wt%, n=9; high Ca pyroxene Fs_4.8±0.1Wo_43.8±0.4, FeO/MnO=11.1±0.7, Cr₂O₃=1.3±0.0 wt%, n=7; Feldspar An_14.3±0.2Or_4.4±0.2, n=7. Large metal grains have Ni=13.3±0.5 wt% and Co=0.5±0.1 wt% (N = 13). Small metal grains in silicates have more variable Ni from 5.9 to 9.5 wt%. (K. Ziegler, UNM): Analyses of three acid-washed subsamples by laser fluorination dual inlet mass spectrometry gave, respectively, δ¹⁷O 1.176, 1.021, 0.991; δ¹⁸O 3.949, 3.724, 3.691; Δ¹⁷O -0.909, -0.945, -0.958 per mil (reference TFL slope = 0.528, values are linearized)

Classification: Acapulcoite