Sphalerite, chalcopyrite and pyrrhotite. East Pacific Rise, Pacific Ocean

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The outer margin of a zinc-rich chimney. Sphalerite (grey) is coarse-grained and has inclusion-free cores but minor chalcopyrite (yellow) on its margin. Smaller sphalerite crystals have a hexagonal morphology (centre right) suggesting wurtzite rather than sphalerite was originally precipitated. Fine-grained pyrrhotite laths (brown, left), which have largely altered to pyrite and marcasite, are precipitated 'black smoke'. Black areas are polishing pits.

Polished block, plane polarized light, x 80, air

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