Galena, sphalerite, chalcopyrite and pyrite. Parys Mountain, Britain

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Galena (blue-white, centre) shows characteristic triangular cleavage pits (black, centre) due to plucking along the (100) cleavage. It encloses euhedral pyrite (light yellow, high reflectance, centre right) and chalcopyrite (yellow, centre bottom), and is intergrown with sphalerite (light grey, left). Sphalerite carries abundant fine-grained chalcopyrite, partly concentrated along grain boundaries but mainly crystallographically oriented within crystals - this is chalcopyrite disease. The gangue is dark grey, the slightly higher reflectance phases are carbonates (top left, bottom right) which have plucked along their cleavage, the darker phase is quartz showing faint internal reflections (top right). Both pyrite and chalcopyrite show relief against galena. The different orientations of the triangular polishing pits within galena show that it comprises a number of separate crystals.

Polished block, plane polarized light, x 80, air

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