Arsenopyrite, covelline, altered sphalerite, stannite and chalcocite. Cligga Head, Cornwall, Britain

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Coarse-grained subhedral to euhedral arsenopyrite (white, high reflectance, right) has been replaced by banded covelline (deep blue, bottom left). Altered sphalerite (bottom centre) shows an anomalous, but characteristic, brown-grey surface colour associated with its replacement by copper-rich minerals. Chalcocite (light blue, centre) has replaced sphalerite about its grain edges and along veinlets. Chalcocite has altered to covelline (deep blue, centre). Minor amounts of stannite (brown-yellow, centre right) surround the altered sphalerite. Quartz (light grey, centre) is the main gangue material.

Polished block, plane polarized light, x 80, air

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