Acanthite, argentopyrite, pyrargyrite and altered argentopyrite. Tynebottom Mine, North Pennines, Britain
125µm
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
