Xenotime, monazite and pyrite. Groverake, North Pennines, Britain
Xenotime (darker grey, centre left) forms the core to later monazite (lighter grey, centre right) and both occur within quartz (grey), the main mineral. Very fine-grained pyrite (white, centre) infills later fractures. Black areas are polishing pits.
Polished block, plane polarized light, x 160, air