Polishing characteristics and problems

Pyrrhotite, chalcopyrite, magnetite and pentlandite. Kambalda, Western Australia

Click hereThe section has been partially polished with 6µm diamond paste. Individual grains of pyrrhotite (brown) are clearly seen due to differential polishing. Crystals in some orientations still retain many polishing pits (black) whereas others are well polished. Chalcopyrite (yellow, centre bottom) and magnetite (light grey, bottom right) are present. Pentlandite (pale yellow-brown, centre right and close to a diagonal fracture) has higher reflectance than pyrrhotite and is just visible within the largest pyrrhotite crystal as small pentlandite flame-like exsolution bodies along a fracture- Dark grey area (bottom right) is silicate, black areas are polishing pits. Coarse features, especially grain boundaries, are well shown after initial polishing, but fine features are still difficult to see.

llmenite and haematite. Kimberlite pipe. Unknown Provenance

Click hereThis section has been polished with 1µm diamond paste, but shows extensive plucking. Host magnesium-rich ilmenite crystals (brown) carry lath-shaped haematite exsolution bodies (blue-grey). Plucked areas (black) have the same size, shape and orientation as the haematite, suggesting that it has been preferentially removed. If more than one phase is plucked in a section then it becomes difficult to estimate modal percentages.

Bornite, stromeyerite, chalcocite, pyrite and tetrahedrite group mineral. Unknown Provenance

Click hereThis section has been polished with ¼ µm diamond paste. Although the section is scratch-free has enormous relief and hence shows strong shadows about the harder phases. Euhedral to subhedral pyrite (pale yellow, high reflectance), a tetrahedrite group mineral (light grey, centre and top centre) and quartz (dark grey, bottom centre) show high relief against the softer copper and silver sulphides. Bornite (brown-red, left) has an symplectite-like intergrowth with, and inclusions of, chalcocite (light blue). Stromeyerite (light lilac-grey, centre right) also is complexly intergrown with chalcocite. Although relief accentuates the symplectite texture of the softer phases, they remain poorly polished in areas close to high relief minerals. Minerals forming thin rims around the harder phases would be 'lost' within the shadows. Bornite (top and bottom right) is purple coloured rather than brown, this is tarnish which has been protected by the adjacent quartz crystals from removal.

Chloanthite, rammelsbergite, pyrargyrite, argentopyrite, acanthite and altered argentopyrite. Tynebottom Mine, North Pennines, Britain

Click hereThis section has been polished with ¼µm diamond paste but shows the effects of overpolishing. Chloanthite (cream-white) is overgrown by rhombic rammelsbergite (pale blue-white, slightly lower reflectance, centre) on the edge next to pyrargyrite. Pyrargyrite (blue) and argentopyrite (brown) overgrow the arsenides. In argentopyrite the slightly higher reflectance patches with a lighter surface colour in the centre of the crystals are more overpolished than the margins. The pyrargyrite crystal (bottom centre left) also shows the same effect. Acanthite (light blue-grey, bottom left) is less strongly coloured than pyrargyrite and more poorly polished. Fine-grained porous aggregates of pyrite, marcasite and an uncharacterized mineral, AgFe11S8, have pseudomorphed argentopyrite (bottom right) but individual crystals cannot be identified at this magnification. Calcite (dark grey) is the main gangue.

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

Click hereThe 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.

Sphalerite, galena, haematite and magnetite. Jersey, Channel Islands, Britain

Click hereFinal polishing with ¼ µm diamond paste. Despite a wide variation in hardness between the phases, there is little relief. Sphalerite (light grey, left) is intergrown with galena (White, centre) and euhedral magnetite (brown, bottom centre). Haematite (pale blue, bottom centre) laths are being replaced by galena (centre). The gangue is quartz (dark grey, bottom centre) and abundant coarse-grained carbonate with a slightly higher reflectance than quartz.