The degradation mechanism of iron oxides caused by alumina during reduction from magnetite has been studied. The experiments were carried out by reducing briquettes containing various mixtures of commercial magnetite fines and Al 2 O 3 in a CO/CO 2 atmosphere at 950°C in thermo-gravimetric equipment. Following reduction, samples were examined using optical microscopy, SEM-EDS and XRD.The diffusion of Al cations into the magnetite during reduction induces swelling and cracking of the briquettes. The composition of the magnetite phase approaches the composition of hercynite during reduction. Assuming the spinel structure is that of hercynite, reduction to non-stoichiometric wüstite (Fe 0.88 O) causes an expansion of over 16 %. This transformation causes high tension forces in the magnetite/hercynite phase in the vicinity of the interface with the wüstite which presumably account for the breakdown of the structure during the reduction.

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Occurrence, ilmenite exsolutions and composition of magnetite in the carbonatite massif of Sokli and in the olivinite of Tulppio have been studied. In the carbonatite massif, magnetite of magmatic carbonatites is hypidio-idiomorphic, contains only very fine-grained ilmenite exsolutions (ulvöspinel), and the minor components TiO-2, MgO, MnO and AI2O3 of magnetite show a decreasing tendency in the successive I II-*• IV magmatic stages. Magnetite of metacarbonatite is xenomorphic, containing a great number of ilmenite exsolutions indicating a complicated thermal history of these rocks. The magnetite of the magmatic carbonatites has lower TiOa and higher MnO, AI2O3, MgO and ZnO contents than that of the metacarbonatites, on the average. Magnetite of the Tulppio olivinite is fine-grained, and the minor element contents, excepting Cr and Si, are lower than those in the carbonatite massif.