Magnetite–chromite ore occurs as small lenses or dikes at the peripheral parts of serpentinites of northern Greece. Cores of chromite are surrounded by small zones of ferrit-chromit and rims of magnetite. Electron microprobe study across zoned chromite grains shows compositional variation ranges unknown to date in spinel minerals. Cr, Al and Mg decrease as Fe and Ni increase outward from the core; Mn increases gradually outwards and attains its greatest values (up to 14.4 wt.%, MnO) in the periphery of chromite cores and in the ferrit-chromit, and drops off to zero in the magnetite rim.
There is an asymmetry of the Mn peaks across the zoned chromite grains with an asymmetrical magnetite rim. The increase of Mn content proportional to the magnetite rim and a manganese balance in the zoned chromite suggest that the trend of the increasing Mn content is related to the progressive metasomatism of the chromite. The high Mn content in the enrichment zone is probably due to substitution of Mn for Mg and in part for Fe2+, the Mn deriving from chromite already transformed to magnetite.