Sample No.: ALHA77219

Location: Allan Hills

Field No.: 78012502

Weight (gms): 637.1

Meteorite Type: Mesosiderite

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Physical Description:

This is not a complete specimen. It is rounded to subangular and a dark brown weathering rind covers the entire stone. Many inclusions of various sizes are present on the highly weathered surface; these objects are glassy, dark greenish-black, and range from angular to subrounded. The inclusions are up to 1 cm in diameter. Approximate diameter: 12 x 8 x 7 cm.

Petrographic Description: Brian Mason

The polished thin section is dominated by large orthopyroxene clasts (up to 6 mm across in a groundmass consisting largely of crushed and comminuted orthopyroxene (grains up to 0.6 mm across). The groundmass also contains about 20% of nickel-iron (kamacite and taenite) in grains averaging about 0.3 mm, rare grains of plagioclase (An90), troilite, chromite, merrillite, and a silica polymorph (probably tridymite). The groundmass encloses a rounded aggregate (3 mm across) of fine-grained (up to 0.15 mm) olivine, composition Fa26. The composition of the orthopyroxene clasts and groundmass is somewhat variable, Fs24-28, with CaO ranging from 0.5-2.3%; one grain of diopsidic pyroxene was analyzed, Wo43En45Fs12. The major material of this section is characteristic of a diogenite; however, the presence of a considerable amount of nickel-iron and the olivine enclave is unique for this class of achondrites, and suggests a possible affinity to the mesosiderites. The identification as a mesosiderite has been confirmed by examination of a cut surface of the whole specimen.