Spinels of the Kontozero Volcano-Plutonic Complex (Kola Peninsula, Russia)

Six spinel supergroup minerals have been found in the Kontozero volcano-plutonic complex rocks: chromite, ulvospinel, magnesioferrite, maghemite, and titanomaghemite, along with magnetite previously known here. Magnetite, chromite, ulvospinel, and magnesioferrite are connected by gradual transitions through a series of the intermediate compositions according to the isomorphic substitution schemes Cr3+ + Al3+ + Mg2+ ↔ 2Fe3+ + Fe2+ and Ti4+ + Fe2+ ↔ 2Fe3+. Maghemite and titanomaghemite were formed as a result of the low-temperature oxidation of magnetite and ulvospinel. These minerals, in turn, are replaced by hematite or an anatase–hematite assemblage.