Marcasite, pyrite and sphalerite. Ashover, South Pennines, Britain
Radiating laths of marcasite are intergrown with minor amounts of pyrite (pale yellow, top left) and a single crystal of sphalerite (light grey, bottom right). Pyrite is only lightly famished and sphalerite is untarnished, whereas marcasite shows extensive tarnishing (blues, yellows and browns), some of which is related to twin planes (bottom centre). Dark grey areas are carbonate, black areas are polishing pits.
Polished block, plane polarized light, x 40, air