Sphalerite, pyrite and chalcopyrite. Bayda, Oman

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Euhedral to poorly crystalline pyrite (light yellow-white, left) surrounds sphalerite (centre) which has suffered extensive replacement by chalcopyrite; this is an extreme example of chalcopyrite disease. The replacement of sphalerite by very fine-grained chalcopyrite is crystallographically controlled, following growth zones and poorly defined twinning. The differences in surface colour (orange to purple) are due to orientation effects of the fine chalcopyrite inclusions. Under crossed polars the anisotropy effects are enormous. The outer margin of the sphalerite (grey, top left) is chalcopyrite-free and belongs to a later generation of sphalerite. Quartz (dark grey with internal reflections) is the gangue.

Polished block, plane polarized light, x 160, oil

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