Mississippi Valley-style lead-zinc deposits
Bravoite, nickeliferous pyrite, marcasite, smithsonite and limonite. Oxclose Mine, South Pennines, Britain
Bravoite (pink-brown) is euhedral (top right) to anhedral (centre bottom) and has lower reflectance than nickeliferous pyrite (light brown-white, centre bottom) which overgrows it. Both phases are earlier than marcasite (light green-white, bottom right). Smithsonite (light greys, bottom centre) shows bireflectance between adjacent crystals. Bravoite has altered to limonite (light grey) along growth zones (centre top) and twin planes (top right, in euhedral bravoite). Dark grey areas are fluorite (centre) crystals. Black areas are polishing pits.
Polished block, plane polarized light. x80, air
Bravoite, nickeliferous pyrite, pyrite, chalcopyrite and covelline. Oxclose Mine, South Pennines, Britain
Subhedral bravoite (pink-brown, left) is enclosed within nickeliferous pyrite (light brown-white, top centre). Coarse grained, faintly zoned, euhedral nickeliferous pyrite (centre right) has a partial pyrite rim (slightly higher reflectance, centre), but this is difficult to see. Chalcopyrite (yellow, centre) is partially altered to covelline (blue).
Coarse-grained grain mount, plane polarized light, x 160, oil
Galena, chalcopyrite, tetrahedrite and covelline. Clevedon, Somerset, Britain
Galena (white, bottom left) is rimmed by chalcopyrite (yellow) and also encloses it (top right). Tetrahedrite (light grey, centre) is intergrown with chalcopyrite which is partially replaced by covelline (deep blue, top left, bottom right). Baryte (dark grey) is the gangue.
Polished thin section. plane polarized light, x55, air
Marcasite. Ashover, South Pennines, Britain
An intergrowth of marcasite crystals shows their extreme anisotropy, variation in grain size, and twinning along (101) as coarse single twins (top left) and as polysynthetic twinning (centre left).
Polished block, crossed polars, x 80, air
Galena, chalcopyrite, enargite, covelline and sphalerite. Clevedon, Somerset, Britain
Galena (blue-white, top) has a rim of chalcopyrite (yellow, centre right) which is partly altered to covelline laths (red, centre right) growing into a vug. Euhedral enargite (centre) is zoned from a lower reflectance (blue-grey) core to brown margin. Minor sphalerite (grey, centre top) occurs between chalcopyrite and enargite but also overgrows enargite into the vug (centre); this is difficult to see (grey with faint internal reflections). Gangue minerals below the surface of the section in the vug show strong internal reflections.
Polished block, plane polarized light, x450, oil
Nickeliferous marcasite and bravoite. Oxclose Mine, South Pennines, Britain
Euhedral, zoned bravoites have lower reflectance than the enclosing zoned marcasite. Bravoite shows higher reflectance cores and lower reflectance outer zones and has a pentagonal dodecahedral habit (centre left). The enclosing marcasite is coarse-grained and also shows faint nickel-rich zoning (centre). Nickel-poor marcasite is unzoned, has a slightly higher reflectance, and occurs on the margin of the nickeliferous marcasite (bottom right). Different crystals show reflection pleochroism (green-blue to yellow, bottom centre).
Polished block. partially crossed polars, x400, oil