Northwest Africa 4537

Morocco

Find: 2005

Achondrite (aubrite)

History: Purchased in 2006, by Matteo Chinellato, from a Moroccan dealer in Erfoud.

Physical characteristics: A single green complete stone lacking fusion crust with a total weight of 261 g and displaying minute spots of yellow-orange staining and larger dark brown on the exterior.

Petrography: (V. Moggi Cecchi and G. Pratesi, MSP) The thin section displays a uniform very fine-grained equigranular texture consisting of bladed grains of enstatite with sporadic clasts of enstatite (up to 0.7 mm across). The grain size is variable from 80 to 210 μm in width and up to 270 μm in length. Relatively few enstatite grains exhibit lamellar twinning. Other silicate phases include plagioclase, blades of daubreelite up to 50 μm in width, and rare troilite, Fe,Ni metal and schreibersite grains rounded. Minor barite and calcite are present, probably due to terrestrial weathering.

Geochemistry: Pyroxene (En_98.5Wo_1.5; Al2O3 = 0.19 wt%), plagioclase (An_14.3Or_4.1). Oxygen isotopes (R. Greenwood, I. Franchi, OU) δ¹⁷O = 2.650‰; δ¹⁸O = 5.021‰; Δ¹⁷O = +0.039‰.

Classification: Achondrite (aubrite). Minor weathering effects on troilite, and minor yellow-orange staining on exterior surface.

Type specimen: A total of 136 g, one polished thin section and one block are on deposit at MSP; the Museo di Storia Naturale dell’Università di Firenze holds the remaining 125 g mass.