Chromite, ferritchromit, sperrylite and pentlandite. Unst, Shetland Islands, Britain

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Chromite (medium grey-brown) is altered to higher reflectance ferritchromit (light grey, centre left) about crystal boundaries and along fractures. More extensive alteration has produced spongy ferritchromit with minor relict chromite (bottom right). A single crystal of a platinum group mineral (white, high reflectance, centre right) with the optical properties of sperrylite is seen in a silicate-infilled fracture between altered chromite. Two grains of pentlandite (yellow-brown, centre left) are seen in ' a ferritchromit rim. Dark grey areas are silicates, black areas are polishing pits.

Polished block, plane polarized light, x 80, air

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