D'Orbigny

Buenos Aires, Argentina

Found 1979 July

Angrite

A 16.55 kg stone, mostly covered with dark gray fusion crust, was found in a corn field after a farmer hit it with a plow.  Classification and mineralogy (F. Brandstätter and G. Kurat, NHMV): has a sub­ophitic texture, exhibiting laths of plagioclase (An_>99, up to 2 mm) with subhedral to euhedral augite and anhedral to subhedral olivine; augite shows marked optical zoning, with cores containing 1.6 wt% TiO₂, 7.8 wt% Al₂O₃, X_Mg [Mg/(Mg + Fe + Ca)] = 26.8 mol%, X_Fe = 21.9 mol%, and X_Ca = 51.2 mol%, and reddish-brown rims with 4.0 wt% TiO₂, 8.0 wt% Al₂O₃, X_Mg = 0.6 mol%, X_Fe = 46.7 mol%, and X_Ca = 52.6 mol%; olivine is zoned, with cores having Fa_39.7La_1.3 (La = larnite), 0.9 wt% CaO, 0.13 wt% Cr₂O₃, 0.45 wt% MnO, and rims having Fa_75.9La_11.5, 6.6 wt% CaO, 0.12 wt% Cr₂O₃, 0.93 wt% MnO; in places, anhedral kirschsteinite (Fa_62.0La_34.0, 20.0 wt% CaO, 0.11 wt% Cr₂O₃, 0.75 wt% MnO) is intergrown with olivine; in places, abundant ultrabasic glass fills pore spaces (SiO₂ ≈40 wt%); minor to accessory phases include transparent brownish spinel (up to 500 µm), ulvöspinel, troilite, and a silicophosphate; contains abundant round vugs, up to 2.3 cm in diameter, and druses containing augite and rare centimeter-sized green olivine crystals (Fa_9–11); rock is fresh, but pores contain variable amounts of caliche. Specimens: main mass with finder; type specimen, 184 g, NHMV.