Covelline, haematite, bornite, digenite and wittichenite. English Lake District, Britain
Digenite (light blue) is extensively altered about its margins to covelline (dark blue), some of which shows reflection pleochroism from dark to light blue (bottom left), and only the cores of digenite crystals remain unaltered (centre). Digenite is also replaced by bornite (brown, top left), by wittichenite (cream-white, top left) and by fine-grained haematite (green-grey, right), which also replaces all the other sulphides. Dark grey areas are quartz. Black areas are polishing pits.
Polished block, plane polarized light, x 80, air