Niccolite, sphalerite, chalcopyrite, millerite and rammelsbergite. Settlingstones Mine, North Pennines, Britain

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Niccolite crystals (pink-brown, left) show bireflectance and reflection pleochroism and arc altered along grain edges to rammelsbergite (white, bottom centre). Sphalerite (grey, right) shows faint light-coloured internal reflections (bottom right) and has chalcopyrite (yellow) overgrowing it along fractures (right and bottom centre). Millerite laths (light yellow, centre) appear less yellow than chalcopyrite, but have higher reflectance and are associated with a poorly polished brown alteration mineral (centre). Black areas are the resin (bottom).

Polished block, plane polarized light, x 160, oil

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