Northwest Africa 11119 (NWA 11119)

Mauritania

Purchased: 2017

Classification: Ungrouped achondrite

History: Purchased by Abdelhadi Aithiba in Mauritania 2017, reportedly found December 2016.

Physical characteristics: Single stone, partially covered in light green fusion crust, broken surface reveals scattered bright green pyroxene crystals up to 3 mm set in a finer grained matrix with some light-green and gray crystals. Spherical vugs and irregular shaped cavities (lined with crystal faces) were observed, some up to 4 mm. The sample is highly friable.

Petrography: (P. Srinivasan, JSC/UNM) Microprobe examination of a polished mount gives a calculated volume mineralogy of 38% plagioclase, 22% silica phase, 21% high-Ca pyroxene, 18% low-Ca pyroxene, 0.7% oxides (ulvospinel, ilmenite), and 0.3% sulfides. The silica phase, which makes up about 22% by volume of this meteorite, is a mix of tridymite and cristobalite with minor quartz, in proportions of approximately 60% tridymite, 37% cristobalite, 3% quartz. No metal was detected. The major phases occur as subhedral, porphyritic grains surrounded by a fine-grained matrix. The matrix has many acicular and compositionally heterogeneous grains resembling quench melt crystals or exsolution intergrowths. Rare fayalitic olivine was detected in one area of the matrix. The high reported alumina content from the micropropbe data is probably due to tiny oxide inclusions which seem to be abundant. Numerous inclusions with exsolution textures were observed.

Geochemistry: (P. Srinivasan, JSC/UNM) Plagioclase An_84.9±9.9Ab_14.8±9.5Or_0.3±0.5, n=60; silica phase SiO₂=96.2±0.6, Al₂O₃=1.2±0.4 (wt%), n=41; clinoenstatite Fs_10±1.3Wo_4.6±0.1, Fe/Mn=6.8±0.2, Fe/Mg=0.1±0.02, n=8; Pigeonite Fs_52.7±24.6Wo_15.8±3.3, Fe/Mn=7.7±1.1, Fe/Mg=8.1±9.5, n=20; augite Fs_22.9±20.4Wo_33.3±5.7, Fe/Mn=6.8±1.8, Fe/Mg=1.6±3.6, n=167. Porphyritic plagioclase grains contain Na-rich rims. Matrix pyroxene grains are highly zoned, with ~5-35 wt% FeO in a single grain. Major element bulk composition based on modal mineralogy: (all in wt%) SiO₂ 57.0, TiO₂ 1.6, Al₂O₃ 12.9, Cr₂O₃ 0.3, FeO 8.6, MnO 1.0, MgO 7.0, CaO 11.0, Na₂O 0.7, K₂O 0.03, S 0.2. Oxygen isotopes (K. Ziegler, UNM): 4 acid-washed fragments analyzed by laser fluorination gave δ¹⁸O= 8.226, 7.781, 8.121, 7.671; δ¹⁷O= 3.364, 3.133, 3.398, 3.046; Δ¹⁷O= -0.979, -0.975, -0.890, -1.004 (linearized, all per mil, TFL slope=0.528).

Classification: Ungrouped achondrite. This appears to be a unique ungrouped achondrite, It shares similar oxygen isotope values with NWA 7325 and ureilites, however it has a distinct mineralogy with abundant silica phase and an extremely wide range of pyroxene compositions.

Specimens: 23 g, plus a probe mount, on deposit at UNM, Abdelhadi Aithiba holds the main mass.