Rammelsbergite, bismuthinite, skutterudite and chalcopyrite. Great Bear Lake, Canada

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The cobalt-nickel arsenides are rammelsbergite and skutterudite which are zoned and overgrow each other. The main arsenide is rammelsbergite, which shows faint reflection pleochroism (white and brown-white, bottom centre) and zoning from brown-white cores to white rims (top right). Rammelsbergite is overgrown by euhedral skutterudite (white, centre), which is euhedral and harder (bottom right) and which, in turn, has zoned rammelsbergite overgrowths (centre, bottom right). Thin dark zones between skutterudite and overgrowing rammelsbergite (centre) are due to replacement and alteration of skutterudite. Extensive replacement of a large skutterudite crystal by fluorite (black) is seen on the extreme bottom right where only a relict outer zone of skutterudite remains. Bismuthinite (blue-white) shows reflection pleochroism and bireflectance (top right) and has replaced cobalt-nickel arsenides. A single crystal of chalcopyrite (yellow, centre) is present.

Polished block, plane polarized light, x 160, oil

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