Northwest Africa 4520

Northwest Africa

Find: 2000

Achondrite (ureilite)

History: Purchased in Alnif in 2000.

Physical characteristics: A total mass of 23.2 g comprises a single stone that is a flat ellipsoid with the longest axis of 3.5 cm. The external color is dark brown, covered with moderately weathered fusion crust and some partly broken edges.

Petrography: (H. Takeda, Chiba; A. Yamaguchi, NIPR) The sample contains olivine only, observed in two polished thin sections made 2 cm apart. One section shows a typical ureilite texture with carbonaceous veins, in which Fe-metal, silica, and rare Na-K-rich aluminous silicate are present. Olivine ranges in size from ~1.5 × 0.9 mm to 0.6 × 0.3 mm. A large part of the other thin section consists of large olivine crystals up to 3.2 × 2.7 mm in size, with thin grain boundaries essentially without opaque materials. Small olivine grains (0.2 × 0.1 mm) with carbonaceous veins are present around the large olivine part. Olivine crystals have minor cracks, but shock effects are minor. Some grain boundaries are filled with Ca carbonate from interaction with the terrestrial environment.

Mineral compositions and geochemistry: Olivine (cores = Fo_79–82, rims = Fo₉₀). Oxygen isotopes: (M. Kusakabe, OkaU) δ¹⁷O = 3.32, δ¹⁸O = 8.07 (both ‰).

Classification: Achondrite (olivine-rich ureilite), minor weathering.

Type specimen: A total of 6 g and two thin sections are on deposit at NIPR. Fectay hold the main mass.