Magnetite and haematite. France
An andesite contains subhedral to euhedral magnetite crystals (light brown, centre) that are partially oxidized to haematite (blue-white, centre left). The alteration proceeds along crystal edges and along curved fractures. A large pyroxene crystal (bottom right) has higher reflectance than the fine-grained, poorly polished, groundmass silicates.
Polished block, plane polarized light, x 40, air