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

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Chromite crystals (grey-brown) are altered to ferritchromit (light grey) about their crystal edges and along microfractures. The alteration is progressive, first to a light-grey, well polished phase and then to a poorly polished white-grey phase (bottom right). Very fine-grained sulphides (< 1-2 lam) are associated with this alteration. The sulphides include pentlandite grains (yellow-brown, top left) lying along a microfracture within chromite. The secondary silicate gangue shows abundant internal reflections.

Polished block, plane polarized light, x 160, oil

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