Northwest Africa 15635 (NWA 15635)

(Northwest Africa)

Purchased: 2022 Oct

Classification: Ungrouped achondrite

History: Purchased by Ziyao Wang in October 2022 from a dealer in Laayoune, Morocco.

Petrography: (A. Irving, UWS and P. Carpenter, WUSL) The specimen consists predominantly of anorthite (~60 vol.%) and orthopyroxene (~35 vol.%) together with minor olivine, accessory Ti-free chromite and rare Fe metal, plus minor secondary Fe oxide and barite. There is considerable textural evidence of disequilibrium among the constituent phases. Orthopyroxene grains are large (up to 1 cm long) with irregular, embayed shapes and apparent reaction rims against plagioclase; marginal zones contain indistinct discontinuous exsolution lamellae of more calcic low-Ca pyroxene, as well as peripheral raft-like clusters of finer grained, intergrown pigeonite and plagioclase. Olivine is irregularly distributed as rounded groups of single and multiple grains (accompanied by chromite) and also as chadacrysts within some orthopyroxene, but is too ferroan to be in equilibrium with the orthopyroxene cores. All plagioclase has a spherulitic polycrystalline quenchiform texture (which is not apparent in back-scattered electron imagery, but visible in cross-polarized transmitted light in the studied thin section).

Geochemistry: Orthopyroxene (cores Fs_23.5-25.2Wo_2.5-3.0; rim Fs_29.9Wo_3.9; FeO/MnO = 20-22, N = 3), low-Ca pyroxene exsolution lamellae (Fs_30.2-32.0, Wo_6.8-3.8, FeO/MnO = 21, N = 2), interstitial pigeonite (Fs_29.4Wo_7.8, FeO/MnO = 20), anorthite (An_97.3-98.1Or_0.0, N = 3), olivine (Fa_33.7-35.6, FeO/MnO = 35-37, N = 4).

Classification: Achondrite (ungrouped, olivine-bearing noritic diogenite-like with quenchiform plagioclase). On the basis of identical characteristic textures, mineral mode and mineral compositions this stone is very likely paired with Al Bir Lahlou 001. Although these specimens have some close mineralogical similarities to other noritic diogenites, the reported oxygen isotopes for ABL 001 plot to more negative Δ¹⁷O values than those for typical diogenites.

Specimens: 16.8 g including one polished thin section at UWB; remainder with WangZ.