Acanthite, argentopyrite, pyrargyrite and altered argentopyrite. Tynebottom Mine, North Pennines, Britain
Euhedral argentopyrite (brown) shows faint sector twinning (top centre) and reflection pleochroism (light to darker brown). Much argentopyrite has altered to a porous aggregate of pyrite and marcasite (light yellow-white, high reflectance, bottom left) which cannot be distinguished at this magnification. The iron sulphides are cemented by acanthite (blue grey, bottom left). Coarse-grained acanthite (blue-grey, soft, right) surrounds argentopyrite and encloses a euhedral crystal of pyrite (high reflectance, top right). Pyrargyrite (blue, centre left) is better polished and has a stronger surface colour than acanthite. Quartz (dark grey, bottom right) and a trigonal carbonate (dark grey, slightly higher reflectance than quartz, bottom centre) are the main gangue minerals. Black areas are polishing pits.
Polished block, plane polarized light, x 160, air