Magnetite, calcian rhodochrosite, limonite and chalcopyrite. Harlech Dome, Wales, Britain

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Euhedral magnetite (brown, top left) crystals occur within a matrix of calcian rhodochrosite (light grey, faint bireflectance). Microconcretions of calcian rhodochrosite are replaced by fine-grained iron hydroxides/manganese oxides (blue-brown-grey, centre right) in a matrix of unreplaced calcian rhodochrosite. A veinlet of limonite (blue-grey, top) crosscuts the primary banding which is oriented left-right. A single crystal of chalcopyrite (yellow, centre bottom) illustrates the low reflectance of the other phases. Black areas are polishing pits.

Polished block, plane polarized light, x 40, air

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