Modern seafloor sulphides

Pyrite and marcasite. Galapagos Mound, Pacific Ocean

Radiating aggregates of poorly crystalline pyrite (browner, lower reflectance, centre) form cores to euhedral or subhedral pyrite (yellow-white, centre right) which is intergrown with minor marcasite (blue-white, higher reflectance, top centre) in voids.

Polished block, plane polarized light, x 80, air

Pyrite and marcasite. Galapagos Mound, Pacific Ocean

Radiating, botryoidal, poorly crystalline pyrite (brown, lower reflectance, bottom centre) is overgrown by euhedral to subhedral pyrite (yellow-white, higher reflectance, centre bottom) and minor marcasite (white, centre bottom left). The east-west linear feature is an organic structure-a worm tube? that is partially infilled by pyrite (centre). Dark grey areas are matrix and resin (bottom left).

Polished block, plane polarized light, x 40, air

Pyrite. Galapagos Mound, Pacific Ocean

Euhedral to subhedral pyrite (yellow-white) is banded, as seen by slight variations in reflectance (top left). A large organic structure is partially infilled by very fine-grained pyrite (left and right centre) which has picked out some of the internal structure. Black areas are polishing pits.

Polished block. plane polarized light. x 40, air

Pyrite, marcasite and sphalerite. Galapagos Mound, Pacific Ocean

Pyrite (light yellow) forms euhedral crystals, many with poorly crystalline cores (top right), or irregular banded poorly crystalline aggregates (brown-white, lower reflectance, top left). A thin rim of silica (grey, top left) encloses pyrite. Subhedral to euhedral sphalerite (light grey, bottom right) inclusions occur in pyrite. Radiating crystals of marcasite (white, centre) surround pyrite and show incipient alteration to limonite along growth zones. Black areas are polishing pits.

Polished block. plane polarized light, x80, air

Pyrite, marcasite, chalcopyrite and sphalerite. Galapagos Mound, Pacific Ocean

Pyrite (light yellow-white) forms euhedral crystals (centre) which contain small sphalerite (light grey, centre right) inclusions or is interbanded with marcasite (blue-white, higher reflectance than pyrite, centre top, top right). The lower reflectance growth bands within the pyrite-marcasite intergrowths are either due to very fine-grained pyrite or the presence of small non-sulphide inclusions. Anhedral chalcopyrite (yellow, centre) is present within the silica matrix. Black areas are polishing pits.

Polished block, plane polarized light, x80, air

Pyrite, marcasite and sphalerite. Galapagos Mound, Pacific Ocean

Pyrite (light yellow) forms euhedral crystals enclosing non sulphides (black, top right) and minor sphalerite (light grey, centre right). Marcasite (blue-green white, bottom centre) occurs as inclusions within the large pyrite (centre right) but also surrounds it. The main marcasite (bottom centre) shows incipient alteration to limonite; this alteration is crystallographically controlled along growth zones. Smaller euhedral pyrite crystals (left) contain abundant non-sulphide inclusions (black) and minor sphalerite (centre left).

Polished block, plane polarized light, x 160, oil