Platinum-iron alloy, native gold, osmium-iridium-platinum alloy and osmium-iridium alloy. Kalimantan, Indonesia

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An irregular gold grain has high fineness (top) and looks folded. The platinum grain is a complex mixture of platinum-iron alloy (brown-white, centre left) which encloses small areas of an osmium-iridium-platinum alloy (cream, higher reflectance than platinum-iron, centre bottom) and has a rim of the same composition, namely approximately equal proportions of the three metals. The outermost rim (blue, bottom right) is an osmium-iridium alloy, iridosmine, with approximately 70% osmium. The fine scale 'banding' within the main osmium-iridium-platinum alloy (bottom left) is partly due to thin parallel intergrowths of osmium-iridium within the main 'host'. The parallel striations (oriented north-south, centre right) in the resin are damage due to the electron microprobe beam. The round area (top left) showing colours is due to an air bubble in the immersion oil.

Grain mount, plane polarized light, x 180, oil

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