Northwest Africa 4468

Northwest Africa

Find: Summer 2006

Achondrite (Martian, basaltic shergottite)

History: Found in the western part of the Sahara in summer 2006 and purchased in Laâyoune by G. Hupé in July 2006.
Physical characteristics: A single 675 g ellipsoidal stone, mostly coated by shiny black fusion crust, and broken at one end revealing the pale yellow-green interior.

Petrography: (A. Irving and S. Kuehner, UWS) Large ovoid pyroxene oikocrysts (2–10 mm across) enclosing chadacrysts of Ti-poor chromite and olivine are set in an interstitial matrix of 35% olivine, 30% clinopyroxene, 25% maskelynite (all vol%), Ti-chromite, ilmenite, merrillite, Cl-apatite (as prisms up to 0.8 mm long), and pyrrhotite. Inclusions (up to 300 µm across) surrounded by radial shock relaxation fractures occur within matrix olivine grains; some are quenched melt inclusions and others consist mainly of maskelynite. The specimen is cross-cut by sporadic, thin, dark, glassy shock veins (Irving et al. 2007).

Mineral compositions and geochemistry: Pyroxene oikocrysts are zoned from orthopyroxene cores (Fs_24.5Wo_4.4; FeO/MnO = 35.4) with mantles of pigeonite (Fs_26.3Wo_6.3; FeO/MnO = 29.4) to augite rims (Fs_17.9Wo_33.6; FeO/MnO = 24.9). Olivine chadacrysts (as magnesian as Fa_28.8) typically become progressively more ferroan from pyroxene core to rim (up to Fa_39.9). Matrix olivine (Fa_40.7; FeO/MnO = 50.2), matrix clinopyroxene (Fs_31.3Wo_8.1, Fs_19.6Wo_31.3), matrix maskelynite (An_38.7–54Or_3.9–2.3); maskelynite in inclusions within olivine is inhomogeneous and K-Na-rich (Or_52.6Ab_40.7An_6.7).

Classification: Achondrite (Martian, basaltic shergottite).

Type specimen: A total of 20.2 g and one large 3.8 × 2.8 cm thin section are on deposit at UWS. G. Hupé holds the main mass.