Pyrrhotite, chalcopyrite, native gold and galena. Cabaçal 1, Mato Grosso, Brazil




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125µm


The final polishing conditions are not known. Native gold shows relief, compare gold (golden-yellow, high reflectance, strong relief, centre top) within the silicate gangue with gold (centre bottom) intergrown with chalcopyrite (yellow-green, bottom centre) and pyrrhotite (brown, right) where it shows less relief Minor amounts of galena (blue-grey, top centre) are intergrown with pyrrhotite and gold, but are difficult to see. The high reflectance and colour of gold give adjacent crystals of chalcopyrite and pyrrhotite anomalously low reflectance and colours. These effects are more marked to the eye than the photomicrograph suggests. Gangue phases (greys) are quartz (left) and phyllosilicates (centre). Black areas are polishing pits.


Polished block, plane polarized light, x 160, air



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