Y-790981: Ureilite

         About 1/8 fragment bound by three intersecting flat fractured faces and one round face with partly weathered black crust, which shows polygonal fractures characteristic of ureilites. Round dark crystals with relatively coarse-grained size can be seen on the fractured surface with some spaces between the crystals. White needle crystals are found in the cavities of the surface. Y-790981, 42-5 is predominantly composed of anhedral to euhedral olivine (87%) and pigeonite (8%) with dark carbonaceous (C) matrix (5%) at the grain boundaries. The olivine crystals display fracturing and undulatory extinction and subgrain boundaries. Average olivine core composition is Fa₂₁, and in contact with C matrix the composition is enriched in Mg by reduction, and sometimes a Fe-free pigeonite is produced at the rim. The pigeonites have some tiny inclusions. The pigeonite composition Ca₈Mg₇₄Fe₁₈ is similar to those of Y-74123 and is rich in Al₂O₃ (1.1 wt%) and Cr₂O₃ (1.1 wt%). In some crystals there are inclusions of diopside Ca₃₇Mg₅₈Fe₅, more Mg-rich than that expected from the coexisting pair. The most notable texture of the pigeonite is dusty dark appearance.