METAL AND SCHREIBERSITE IN MAYO BELWA,AN ENSTATITE ACHONDRITE

The Mayo Belwa meteorite (an aubrite) contains discrete metal grains ranging in size from less than 1 μm to 300 μm across, and schreibersites up to 25 μm across. Intergrowths of metal and schreibersite also occur. These phases are distributed heterogeneously throughout the meteorite and are present both in the fine-grained matrix and within silicate crystals. The concentrations of the elements Ni, Fe, Si, Co, P, were determined in grains larger than 4 μm across. Most metal grains have 1–8% Ni, the total range being 0.8–23.5% Ni. The 1–8% Ni metal may be sub-divided into two compositional groups, one relatively compact, having 1–3.8% Ni, 0.1-0.3% Si; the second having a much greater range in both Ni and Si (～ 4–12%, 0.1-1.2% respectively). There is no zonation in the Si contents of individual grains. Neither the size of the grains nor their environment correlates with their Ni or Si contents, though there is a tendency for low Ni, low Si metal to be within enstatite crystals. Schreibersite (8–14.4% Ni) occurs as isolated grains or associated with low-Ni metal; it generally contains less than 500 ppm Si. The wide range in the Ni contents of the metal distinguishes it from the metal of the E-chondrites, and argues against an E-chondrite source for this metal.