Bornite, chalcopyrite and chalcocite. Jersey, Channel Islands. Britain

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A dried oil droplet (multicoloured, bottom right) is present within altered bornite. Although its colours are similar to those of tarnished bornite the circular shape is characteristic. Unaltered bornite is present in trace amounts and is extremely difficult to see (centre, top right). It is pink-brown and has a slightly lower reflectance than altered bornite (yellow-brown) which is the main phase. Altered bornite carries thin chalcopyrite (yellow, higher reflectance) and chalcocite (blue, lower reflectance) lamellae oriented along (111). Extensive alteration of this type has been called 'idaite' or interpreted as exsolution between chalcopyrite and bornite.

Polished block, plane polarized light, x 160, oil

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