Magnetite and haematite. France

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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

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