Pyrite, marcasite and sphalerite. Galapagos Mound, Pacific Ocean

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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

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