Pyrite and haematite. Lasail, Oman
Euhedral pyrite (light yellow, centre) is fractured and surrounded by aggregates of fine-grained, often botryoidal, haematite (light green-grey, centre left). The differences in reflectance and surface colour (bottom centre) between the core and margin of the haematite aggregates are due to variations in grain size of individual haematite crystals. The coarser crystals on the margin have a slightly higher reflectance. Quartz (dark grey) is the gangue.
Polished block, plane polarized light, x 160, air