Minor sulphide mineralization in granite

Chalcopyrite, sphalerite, pyrite and marcasite. Jersey, Channel Islands, Britain

Click hereChalcopyrite (yellow, bottom left) encloses small sphalerite stars (grey), whereas sphalerite (light grey, bottom right) has chalcopyrite inclusions (yellow), many of which are crystallographically oriented; this is chalcopyrite disease. Lath-shaped intergrowths of pyrite (light yellow-white, centre top) and marcasite (blue-green white, slightly higher reflectance) are pseudomorphs after pyrrhotite crystals. It is difficult to distinguish between the disulphides at this magnification. The low reflectance rounded area (centre bottom) within chalcopyrite is a limonite pseudomorph after pyrrhotite. Rhombic carbonate (dark grey, top right) shows faint bireflectance and internal reflections. Black areas are polishing pits.

Polished block, plane polarized light. x 80, air

Chalcopyrite, sphalerite and pyrite. Jersey, Channel Islands, Britain

Click hereChalcopyrite (yellow) contains exsolved sphalerite stars (light grey, centre) and two crystals of pyrite (light yellow, higher reflectance, top right) growing into a void (black). A very thin veinlet that is only just discernible (running north-south, centre) is marked by elongated polishing pits (black) and by stannite (light grey, higher reflectance than sphalerite, centre) cutting across the central sphalerite star. Black areas are polishing pits.

Polished block, plane polarized light, x90. air

Pyrite, sphalerite, chalcopyrite, galena and limonite. Jersey, Channel Islands, Britain

Click herePyrite (light yellow-white) encloses sphalerite stars (grey, centre, centre left) and minor amounts of poorly polished galena (white, top centre below chalcopyrite). Chalcopyrite (yellow, top right above galena) surrounds pyrite and is extensively altered to banded limonite (medium grey, right centre) which is intergrown with very finely disseminated chalcocite (blue-grey). Quartz (left centre) and phyllosilicate (dark grey) are the gangue phases. The presence of sphalerite stars which are typical of sphalerite exsolution in chalcopyrite, but are not common in pyrite, suggests that this pyrite has pseudomorphed chalcopyrite.

Molybdenite. Jersey, Channel Islands, Britain

Click hereCoarse blades and laths of molybdenite show strong bireflectance and reflection pleochroism. The strong basal cleavage of molybdenite (left) parallel to (0001) is clearly seen, as are deformation effects similar to kink banding (centre). The dark grey area (bottom) is quartz. Four trigonal carbonate crystals showing bireflectance are lighter grey (bottom left). Black areas are polishing pits.

Polished block. plane polarized light, x40, air

Molybdenite. Jersey, Channel Islands, Britain

Click hereMolybdenite laths surround a tabular silicate crystal (dark coloured with internal reflections, top centre). Molybdenite shows strong anisotropy effects (blue-white to green-grey), controlled by primary crystal orientation and its subsequent deformation.

Polished block, crossed polars, x40, air

Bismuthinite, chalcopyrite, wolframite and native bismuth. Viloco, Bolivia

Click hereEuhedral to subhedral wolframite (grey-brown, low reflectance, centre right) is fractured and shows alteration about its margins. Chalcopyrite (yellow, left) is intergrown with bismuthinite (top centre), which shows strong reflection pleochroism (grey-blue to blue-white),and bireflectance. Poorly polished native bismuth (cream-white, high reflectance, centre) occurs within bismuthinite. Silicate phases are black.

Polished block, plane polarized light. x 160. oil