Magnetite, pyrite, haematite, pyrrhotite, ilmenite and spinel. St Peter Port gabbro, Guernsey, Channel Islands, Britain

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Coarse-grained magnetite (brown-grey, left and right) crystals carry crystallographically oriented, exsolved, thin spinel crystals (grey, bottom right) and rare ilmenite laths (brown-grey, lower reflectance than magnetite, bottom left, top centre left). Pyrite (yellow, high reflectance) encloses relict pyrrhotite (brown-yellow, centre right) and has poorly crystalline rims (centre left). It is intergrown with subhedral to euhedral haematite (blue) and a second generation of magnetite (brown-grey, centre bottom). This generation of magnetite is inclusion-free and has a slightly higher reflectance than the main magnetite. Elsewhere in the section (not shown), pyrrhotite is the main sulphide surrounding the first generation of magnetite and shows alteration to pyrite, marcasite and magnetite. Therefore, it is possible that the pyrite and second generation magnetite have replaced pyrrhotite here, and that subsequently magnetite has oxidized to haematite.

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

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