The effects of tarnishing and carbon coating

Marcasite, pyrite and sphalerite. Ashover, South Pennines, Britain

Radiating laths of marcasite are intergrown with minor amounts of pyrite (pale yellow, top left) and a single crystal of sphalerite (light grey, bottom right). Pyrite is only lightly famished and sphalerite is untarnished, whereas marcasite shows extensive tarnishing (blues, yellows and browns), some of which is related to twin planes (bottom centre). Dark grey areas are carbonate, black areas are polishing pits.

Chalcocite. arsenopyrite, chalcopyrite and bornite. Wheal Jane, Cornwall. Britain

The section is tarnished. Euhedral rhombic arsenopyrite (white) is fractured and replaced by chalcocite. Chalcopyrite (tarnished to yellow-red) is replaced by Bornite (tarnished to purple-brown) and chalcocite (pale blue) which is untarnished. Quartz and tourmaline (dark grey) are the gangue minerals. Black areas are polishing pits.

Chalcocite, arsenopyrite, chalcopyrite and bornite. Wheal Jane, Cornwall, Britain

This is the same field of view as previous section but after repolishing. Chalcopyrite (yellow) and bornite (brown) now show their characteristic colours. The alteration of chalcopyrite to bornite and chalcocite has been controlled by grain boundaries, fractures and cleavage directions within chalcopyrite.

Pyrite, sphalerite and galena. Ireland

This section is tarnished. Sphalerite (light grey, left and right) and galena (white, bottom left) are only slightly tarnished or untarnished. Pyrite is extensively tarnished and shows delicate growth zones as banding. On repolishing only the outermost growth bands are seen. The tarnishing shows the direction of growth and that pyrite has been fractured and re-cemented and/or replaced by galena and sphalerite. These textures would be more difficult to demonstrate after repolishing.

Native gold and platinum-iron alloy. Kalimantan, Indonesia

This section is a carbon-coated gold concentrate from a modern placer deposit. Irregular to rounded native gold grains with gold enrichment rims are red-purple to blue-purple respectively. A rounded grain of platinum-iron alloy is brick red (centre). Low-reflectance minerals, mainly zircon and quartz, appear very similar to the mounting material which comprises the bulk of the surface. Black areas arc polishing pits, the rounded ones are erupted air bubbles in the resin. The surface colours depend upon the thickness of the carbon-coat, but the light blue-white colour of the coated resin is characteristic.

Native gold and platinum-iron alloy. Kalimantan, Indonesia

This is the same field of view as previous section but after removal of the carbon-coating (it is rotated by 90°). Many gold grains (yellow) have porous gold enrichment rims (top left) and some show white-yellow higher reflectance cores and yellow lower reflectance rims (centre left). The cores have a lower fineness than their margins which are 980 fine. The platinum-iron alloy is white with high reflectance. A single grain of quartz (dark grey, centre) is coarser grained than euhedral to subhedral zircon (grey) and has a lower reflectance. Black areas are polishing pits.