Northwest Africa 16812 (NWA 16812)

Algeria

Find: 2024

Classification: Ureilite

History: Found in Algeria in 2024; later purchased by Matthew Stream, who sent 20.2 grams to Cascadia for classification.

Physical characteristics: Single stone with weathered fusion crust.

Petrography: (D. Sheikh, Cascadia) Sample displays an overall recrystallized texture comprised of mm-sized olivine grains (most of which are polycrystalline and comprised of smaller recrystallized grains ~10 microns in size meeting at ~120° triple junctions) displaying Mg-rich reduced rims, and polycrystalline low-Ca pyroxene (interstitial to olivine, original grain boundaries ~1 mm, comprised of smaller compositionally zoned recrystallized grains ~15 μm in size meeting at ~120° triple junctions). Accessory phases identified include graphite, Fe-Ni metal (primarily along olivine grain boundaries and/or alongside low-Ca pyroxene), troilite, and chromite; subcalcic augite inclusions (<15 microns in size) occur interstitial to recrystallized olivine and low-Ca pyroxene grains. Vesicles are present throughout the sample (primarily concentrated within polycrystalline low-Ca pyroxene).

Geochemistry: Olivine (Fa_12.6±4.8, range Fa_0.9-19.8 (Fa_18.4-19.8 in olivine cores, Fa_0.9-9.5 along olivine rims depending on specific grain), CaO (wt. %) = 0.3±0.1, Cr₂O₃ (wt. %) = 0.6±0.1, Fe/Mn = 24±9, n=39), Low-Ca Pyroxene (Fs_9.2±4.1Wo_3.8±1.6, range Fs_2.3-14.6Wo_2.0-6.9, Cr₂O₃ (wt. %) = 0.9±0.1, Fe/Mn = 17±8, n=28), Subcalcic Augite (Fs_8.5±4.6Wo_31.1±2.4, range Fs_1.1-12.5Wo_27.1-33.8, Al₂O₃ (wt. %) = 1.0±0.7, TiO₂ (wt. %) = 0.2±0.1, Cr₂O₃ (wt. %) = 1.2±0.4, Fe/Mn = 14±8, n=8).

Classification: Ureilite (recrystallized).

Specimens: Cascadia has 17.9 g in one slice, along with a polished thin section and material in an epoxy brick; main mass with Matthew Stream.