Sphalerite, pyrite and chalcopyrite. Bayda, Oman
Pyrite (light yellow-white) forms euhedral crystals (top centre) or poorly polished spongy aggregates (right centre). Sphalerite (left, centre) has suffered extensive chalcopyrite disease along twin planes. The variation in surface colour of the diseased sphalerite (yellow-brown to purple-brown) is controlled by the density and crystallographic orientation of the fine chalcopyrite inclusions. Other areas of the sphalerite are inclusion-free and show characteristic grey surface colours and low reflectance (left). The crystal on the right, which appears to be chalcopyrite, enclosing a low-reflectance phase; is an example of extreme chalcopyrite disease with little relict sphalerite. The degree of chalcopyrite replacement is clearly related to crystallographic domains in the original sphalerite.
Polished thin section, plane polarized light, x 160, oil