Mineralogy and petrography of Almahata Sitta

Michael Zolensky, NASA Johnson Space Center, Houston, USA
Almahata Sitta is an anomalous, polymict ureilite (verified by oxygen isotope and bulk compositional data. Some lithologies are typical ureilite types, but others are anomalous. Anomalous features include lack of zoning of olivine, large compositional range of silicates, high abundance and large size of pores, crystalline pore wall linings, and overall fine-grained texture. Tomography reveals that the pores define thin, discontinuous ''sheets'' connected in three dimensions, suggesting that they outline grains that have been incompletely welded together. The crystals lining the pore walls are probably vapor phase deposits. Therefore Almahata Sitta may represent an agglomeration of fine-grained, incompletely reduced pellets formed during impact, and subsequently welded together at high temperature.