Niccolite, pyrite, ullmannite and sphalerite. Settlingstones Mine, North Pennines, Britain
Niccolite (pink, right) shows very faint reflection pleochroism and bireflectance and is overgrown by euhedral ullmannite (blue-white, centre) that contains a sphalerite inclusion (light grey, bottom centre). A larger sphalerite crystal/aggregate (top centre) is partially intergrown with pyrite (yellow-white, left), some of which has a tabular habit (centre). Carbonate (dark grey) is the main gangue.
Polished block, plane polarized light, x 80, air