Northwest Africa 4801

Algeria

Find: May 2007

Achondrite (angrite)

History: Greg Hupé purchased the single stone in Erfoud, Morocco. The stone is broken into four pieces (total weight 252 g) and was found in Algeria in May 2007.

Physical characteristics: Friable specimen composed of relatively coarse dark reddish-brown, pale yellow-green, white and black grains, with some large white patches (polycrystalline anorthite aggregates). No fusion crust is evident.

Petrography: (A. Irving and S. Kuehner, UWS) The overall texture is granular with the presence of some polygonal grain aggregates and triple grain junctions. Grain size ranges from 0.1–1.2 mm. It is composed mostly of Al-Ti clinopyroxene and pure anorthite (some as polycrystalline aggregates up to 0.5 cm across), with sporadic large Cr-pleonaste grains (up to 200 mm across), calcic olivine, pleonaste, poikilitic merrillite and minor troilite and altered kamacite. Subhedral olivine grains are enclosed by clinopyroxene, and both these and pleonaste are surrounded by interstitial anorthite and merrillite. Pleonaste occurs as small grains throughout the specimen, but also as mantles around the large Cr-pleonaste grains, and those mantles contain abundant chadacrysts of subhedral olivine and clinopyroxene. Small cavities (2–5 μm across) within clinopyroxene contain olivine and anorthite grains. Kirschsteinite is absent, and merrillite is more abundant than in most other known angrites.

Geochemistry: Clinopyroxene (Fs_11.8Wo_56.9; Al₂O₃ = 10.6 wt%, TiO₂ = 2.4 wt%, FeO/MnO = 133), olivine (Fa_45.5Ln_2.4, FeO/MnO = 88), spinel (Cr-pleonaste: Cr₂O₃ = 6.3 wt%, Cr/(Cr + Al) = 0.070, Mg/(Mg + Fe) = 0.451; pleonaste: Cr₂O₃ = 1.9–2.4 wt%, Cr/(Cr + Al) = 0.021–0.026, Mg/(Mg + Fe) = 0.462–0.467). Oxygen isotopes (D. Rumble, CIW): δ¹⁸O = 3.595, 3.544; δ¹⁷O = 1.809, 1.823; Δ¹⁷O = -0.082, -0.041 ‰.

Classification: Achondrite (angrite).

Specimens: A total of 20.0 g, one polished thin section and one polished mount are on deposit at UWS. GHupé holds the main mass.