Nickel-rich assemblages within Mississippi Valley-style mineralization
Niccolite, pyrite, ullmannite and sphalerite. Settlingstones Mine, North Pennines, Britain
Niccolite (pink, right) shows very faint reflection pleochroism and bireflectance and is overgrown by euhedral ullmannite (blue-white, centre) that contains a sphalerite inclusion (light grey, bottom centre). A larger sphalerite crystal/aggregate (top centre) is partially intergrown with pyrite (yellow-white, left), some of which has a tabular habit (centre). Carbonate (dark grey) is the main gangue.
Polished block, plane polarized light. x 80, air
Sphalerite, niccolite and ullmannite. Settlingstones Mine, North Pennines, Britain
Sphalerite (grey, bottom right) is intergrown with, and encloses, radiating niccolite (pink-brown, centre) which shows faint reflection pleochroism, and with euhedral ullmannite (white, centre right), which shows triangular pits (black, centre right). Although the optical properties of ullmannite are similar to those of galena, it has too high a reflectance and is harder. Quartz is dark grey (left centre), as is carbonate (top right).
Polished block. plane polarized light, x 80, air
Niccolite crystals (pink-brown, left) show bireflectance and reflection pleochroism and arc altered along grain edges to rammelsbergite (white, bottom centre). Sphalerite (grey, right) shows faint light-coloured internal reflections (bottom right) and has chalcopyrite (yellow) overgrowing it along fractures (right and bottom centre). Millerite laths (light yellow, centre) appear less yellow than chalcopyrite, but have higher reflectance and are associated with a poorly polished brown alteration mineral (centre). Black areas are the resin (bottom).
Polished block, plane polarized light, x 160. oil
Niccolite (pink-brown, bottom) crystals showing reflection pleochroism and bireflectance are altered along their edges and along fractures to rammelsbergite (white, higher reflectance, bottom left). Euhedral ullmannite crystals (light blue-white, high reflectance, centre top right) surround niccolite and are enclosed within galena (blue-white, lower reflectance, top). Minor amounts of millerite (yellow, bottom left) are associated with sphalerite (light grey, bottom left). A single sphalerite crystal lies beneath ullmannite, within galena (centre right). Black areas are polishing pits.
Polished block, plane polarized light, x 160. oil
Niccolite and sphalerite. Settlingstones Mine, North Pennines, Britain
Niccolite showing the typical radiating habit (centre), known as an 'ice flower' texture. The strong anisotropy of niccolite is well seen. Sphalerite (grey, bottom centre) shows light-colored internal reflections.
Polished block, partially crossed polars, x80, air
Niccolite (pink, bottom) shows faint reflection pleochroism and bireflectance. It is altered along grain edges and along fractures to rammelsbergite (blue-white, centre bottom) accompanied by vaesite (violet-brown with cleavage, centre, top right). A central vein of coarse-grained pyrite (yellow-white) is intergrown with marcasite (green-blue, centre). Black areas are gangue minerals.
Polished block, plane polarized light. x 160, oil