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, plane polarized light, x 400, oil