Smithsonite, altered sphalerite, chalcopyrite, covelline and sphalerite. Oxclose Mine, South Pennines, Britain

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Sphalerite is almost totally altered to smithsonite (light greys, centre) which is the main phase and shows strong bireflectance between grains. Very small relict sphalerite (light grey, bottom centre, centre top left) is harder than the enclosing smithsonite. However, most relict sphalerite is lilac-grey to blue (centre left) and resembles blue copper sulphides. Microprobe analyses of this lilac sphalerite show it to have up to 10% by weight of copper. Covelline is deep blue (top centre, right) and comprises much of the very fine-grained (<10śm) sulphide in the smithsonite. Minor amounts of pyrite (white, top right) are present. Fluorite is dark grey (lower reflectance than the resin, bottom left) and a single lath of poorly polished baryte (oriented north-south, bottom right) is present within a larger lath of smithsonite.

Grain mount, plane polarized light, x 80, air

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