Pyrite, arsenopyrite, TiO2 minerals. Vaal Reef, Witwatersrand, Republic of South Africa

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Pyrite (light yellow, bottom right) has a lower reflectance than arsenopyrite (white, top left, centre right), one grain of which shows reflection pleochroism from pink-white to white (centre right). Both pyrite and arsenopyrite are rounded grains. Two aggregates of fine-grained TiO2 (blue-grey, bottom left and top right) are pseudomorphs after iron-titanium oxide minerals. Quartz is dark grey, softer gangue phases have plucked out.

Polished block, plane polarized light, x 80, air

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