Base metal-tin association
Pyrrhotite, pyrite and galena. Nigadoo River, New Brunswick, Canada
A small galena crystal (blue-white, centre) lies at the junction of a number of pyrrhotite (brown, centre) grains, which show faint differences in reflectance and surface colour due to the presence of monoclinic (darker brown) and hexagonal (lighter brown) pyrrhotite (centre). Euhedral pyrite crystals (pale yellow, high reflectance, right) carry pyrrhotite inclusions (top right). Pyrrhotite is extensively altered to fine-grained pyrite and marcasite which are not distinguishable from each other at this magnification. The alteration is to lath-shaped pyrite, marcasite aggregates (bottom centre) oriented along the (0001) plane of the pyrrhotite and to areas of very fine-grained pyrite-marcasite (top left); these appear brown due to the fine grain size of the sulphides. Probably all the pyrite in this section was originally pyrrhotite. The gangue minerals are dark grey.
Polished block, plane polarized light, x80, air
Pyrrhotite, pyrite and marcasite. Nigadoo River, New Brunswick, Canada
Coarse-grained pyrrhotite (browns) comprises higher reflectance monoclinic pyrrhotite along (0001) within lower reflectance, darker brown hexagonal pyrrhotite margins (centre right). It has altered to pyrite (light yellow), much of which is euhedral (top left), and to laths of marcasite (light blue-white, higher reflectance than pyrite, top right). The alteration can be seen to be crystallographically controlled along the basal cleavage of pyrrhotite. Black areas are silicates showing faint internal reflections (bottom left), -accessory minerals are dark grey (top centre).
Polished block, plane polarized light, x 180, oil
Marcasite, sphalerite, pyrite and galena. Nigadoo River, New Brunswick, Canada
Marcasite (blue-white, centre left) lath-shaped aggregates, are intergrown with euhedral pyrite (light yellow-white, centre) and have totally pseudomorphed pyrrhotite. This is a very characteristic replacement texture of pyrrhotite. The orientation of the marcasite laths is along the (0001) basal plane of the original pyrrhotite. Sphalerite (light grey, top right), which carries galena inclusions (bottom centre), is intergrown with galena (blue-white, bottom left).
Polished block, plane polarized light, x 40, air
Sphalerite, pyrite, chalcopyrite and arsenopyrite. Nigadoo River, New Brunswick, Canada
Sphalerite (grey, top right) has chalcopyrite (yellow) inclusions and an incomplete chalcopyrite rim (bottom right). Two euhedral rhombs of arsenopyrite (white, centre, bottom right) have higher reflectance than coarse-grained pyrite (light yellow, bottom left). Fine-grained pyrite and marcasite have totally replaced pyrrhotite and appear brown-yellow due to their fine grain size. Successive 'shells' of pyrite-marcasite with different grain sizes can be seen and form a poorly defined bird's-eye texture (bottom left) growing out from coarse-grained pyrite. Gangue phases are dark grey (centre).
Polished block, plane polarized light. x40, air
Sphalerite, stannite, chalcopyrite, marcasite and galena. Nigadoo River, New Brunswick, Canada
Sphalerite (grey) has very fine chalcopyrite inclusions along its grain boundaries (bottom left) and a discontinuous veinlet of stannite (green-grey, centre) and chalcopyrite (yellow, centre left), some of which is intergrown with galena (white, centre left). Minor amounts of marcasite (white, high reflectance, left centre) are intergrown with gangue along the same veinlet. Areas of sphalerite which have a faint yellow surface colour (bottom right) contain submicroscopic chalcopyrite inclusions and have suffered chalcopyrite disease.
Polished block, plane polarized light, x 80 air
Chalcopyrite, sphalerite, pyrite and mackinawite. Nigadoo River, New Brunswick, Canada
Sphalerite (grey, bottom left) contains submicroscopic chalcopyrite inclusions (yellow-grey areas) and is intergrown with chalcopyrite (yellow, right) which carries sphalerite exsolution stars (top right). Pyrite (light yellow-white, top right) is euhedral or has been replaced by chalcopyrite (centre). Mackinawite (brown, centre) is intergrown with chalcopyrite and shows polysynthetic twins (oriented north-south), some of which have a colour and reflectance that is very similar to the host chalcopyrite. Black areas are polishing pits.
Polished block. plane polarized light, x 180, air