Chromite associations

Chromite, chalcopyrite and rutile. Bushveld, Republic of South Africa

Euhedral, fractured cubic crystals of chromite (medium grey) are intergrown with silicate (dark grey). A single lath of rutile (light grey, centre) is partially enclosed in chromite and partly in silicate. Minor amounts of interstitial chalcopyrite (yellow, left centre) are present. Black areas are polishing pits.

Polished block, plane polarized light, x40, air

Chromite. The Great Dyke, Zimbabwe

Euhedral chromite crystals (grey) are fractured, some fractures are along a poorly defined cleavage (bottom centre) and are accompanied by incipient alteration (higher reflectance areas, centre right). Silicate (dark grey) forms the matrix to the chromite and replaces it (bottom right).

Polished block, plane polarized light, x 160, air

Chromite, pentlandite, violarite and rutile. Bushveld, Republic of South Africa

Euhedral chromite crystals (green-grey) enclose exsolved fine rutile needles (light grey, right) which are crystallographically oriented along (111) planes of the chromite. Coarse-grained rutile crystals (light grey, top centre and bottom left) overgrow chromite. Interstitial pentlandite (light-brown, centre) has partially altered along its cleavage to less well polished violarite (brown, lower reflectance than pentlandite). The silicate matrix is black.

Polished block, plane polarized light, x 160, oil

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

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

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

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, x80, air

Chromite and magnetite. Unst, Shetland Islands, Britain

A euhedral chromite (green-grey, centre) is fractured (black), the fractures are less well-developed in the magnetite (light brown) overgrowth. The host dunite retains relict olivine (dark grey, bottom right) within secondary silicates (dark grey, featureless). Black areas are polishing pits. The differences in surface colour between the chromites in this and earlier plates are due to compositional variations. The chromite in this section is closer to the ideal composition.

Polished thin section, plane polarized light, x 160. oil