Pyrite, niccolite, vaesite, rammelsbergite and marcasite. Settlingstones Mine, North Pennines, Britain
Niccolite (pink, bottom) shows faint reflection pleochroism and bireflectance. It is altered along grain edges and along fractures to rammelsbergite (blue-white, centre bottom) accompanied by vaesite (violet-brown with cleavage, centre, top right). A central vein of coarse-grained pyrite (yellow-white) is intergrown with marcasite (green-blue, centre). Black areas are gangue minerals.
Polished block, plane polarized light, x 160, oil