Altered argentopyrite, pyrargyrite, argentopyrite and acanthite. Tynebottom Mine, North Pennines, Britain

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Argentopyrite (brown, centre, top right) has extensively altered to a fine-grained porous aggregate of pyrite and marcasite (light yellow-white, high reflectance, bottom left) cemented by acanthite (blue-grey, top right). Euhedral unaltered argentopyrite crystals within pyrargyrite (blue, centre) are surrounded by light-sensitive rims which have etched to purple-red coloured pits (centre). With time, under strong illumination, these rims are completely destroyed. Quartz and carbonate are black with internal reflections.

Polished block, plane polarized light, x 160, oil

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