Pyrite. Galapagos Mound, Pacific Ocean
Euhedral to subhedral pyrite (yellow-white) is banded, as seen by slight variations in reflectance (top left). A large organic structure is partially infilled by very fine-grained pyrite (left and right centre) which has picked out some of the internal structure. Black areas are polishing pits.
Polished block, plane polarized light, x 40, air