Northwest Africa 16789 (NWA 16789)

(Northwest Africa)

Purchased: 2024 Jan

Classification: Ungrouped achondrite

History: Acquired in January 2024 by Shun-Chung Yang and Marcin Cimala from Ali Mftah in Algeria.

Physical characteristics: Two slightly weathered stones with observable glassy fusion crust.

Petrography: (D. Sheikh, Cascadia) Sample is a cumulate rock displaying a poikilitic texture primarily comprised of smaller (~0.1-1.5 mm) anhedral to subhedral grains of pigeonite and augite chadacrysts surrounded by larger (>1 mm) oikocrysts of plagioclase feldspar. Pyroxene compositions are relatively equilibrated whereas plagioclase is zoned and varies in compositon from albite to labradorite. Accessory phases identified include troilite, ilmenite, merrillite, and chromite; an interstitial microlitic glass is present throughout the sample that surrounds and cuts across mineral grains.

Geochemistry: Pigeonite (Fs_39.2±1.0Wo_8.3±0.4, range Fs_36.3-40.4Wo_7.4-9.3, Fe/Mn = 18±1, n=20), Augite (Fs_20.8±3.3Wo_37.6±1.0, range Fs_10.8-23.0Wo_36.8-39.8, Cr₂O₃ (in wt. %) = 1.6±0.3, Fe/Mn = 20±1, n=22), Zoned Plagioclase (An_22.5±16.5Or_0.2±0.4, range An_5.9-54.7Or_0.1-1.3, n=29), Chromite (Cr# = 90.9±0.6, n=6). Interstitial microlitic glass bulk composition estimated using multiple spots (in wt. %): Na₂O = 6.5±0.5, MgO = 0.6±0.1, Al₂O₃ = 12.6±0.1, SiO₂ = 70.0±0.6, P₂O₅ = 0.9±0.1, SO3 = 0.6±0.1, K₂O = 1.8±0.7, CaO = 0.8±0.3, TiO₂ = 1.5±0.1, MnO = 0.1±0.1, FeO = 4.6±0.2, n=11).

Classification: Achondrite (ungrouped). Sample texture, mineral chemisty, and interstitial microlitic glass bulk composition are essentially identical to what has been observed in NWA 6698 (Vaci, 2021; Vaci et al., 2021), indicating a likely pairing between both samples.

Specimens: 20.4 g at Cascadia, 150 g with Shun-Chung Yang, 11 grams with Marcin Cimala.

Vaci (2021) not found