Digenite and covelline. Mount Gabriel, County Cork, Ireland

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The main sulphide (blue-green) is banded on a very fine scale but has the optical properties of digenite. However, X-ray diffraction suggests that it is an intimate mixture of digenite and covelline. It is replaced along its edges by radiating sheaves of covelline which show extreme anisotropy from deep red (centre left) to golden-yellow (bottom centre). Dark grey phases with internal reflections are silicate gangue minerals, mainly quartz, which show euhedral crystal forms (left centre).

Polished block, plane polarized light, x 180, oil

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