Sphalerite, pyrite, galena and marcasite. Shullsburg, Wisconsin, USA

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Sphalerite (light grey) shows faint internal reflections (centre left) and encloses lath-shaped pyrite (yellow-white, centre left) and a partially replaced rhomb of marcasite (white, high reflectance, Centre). Pyrite crystals are replaced by galena (blue-white, top). Dark grey area is carbonate, black areas are polishing pits. In crossed polars the relict marcasite shows optical continuity, showing it to have been one crystal.

Polished block, plane polarized light, x 80, air

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