Pyrite, bornite, haematite and covelline. Cornwall, Britain

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Pyrite (pale yellow-white, bottom) is intergrown with bornite (brown-orange, left). This has altered along its grain edges to covelline (blue), which shows reflection pleochroism from light to dark blue (right centre). Rhythmically banded fine-grained haematite (green-grey, centre) surrounds the sulphides and shows slight differences in reflectance due to grain size variations (centre right). Quartz is dark grey.

Polished block, plane polarized light, x 80, air

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