Summary: Chemically analysed samples of magnetites carrying exsolved ulvöspinel and ilmenite have been heated in vacuo at various temperatures without change in overall chemical composition and the changes occurring studied microscopically, magnetically, and by X-rays.
Magnetite with exsolution bodies of ulvöncl can be homogenized probably below 600°C.; thermomagnetic curves and microscopic textures suggest that the solvus curve is very steep at the Fe3O4 end of the Fe3O4-Fe2TiO4 series.
On heating magnetite containing ilmenite lamellae up to 950°C., some interchange of ions between the phases occurs (Fe3O4+FeTiO3 → Fe2TiO4+Fe2O3, the ilmenite lamellae taking Fe2O3 into solid solution. The accompanying host magnetite not only gains Fe2TiO4, but at the same time dissolves some FeTiO3. Above 950°C. extensive decomposition takes place, with the production of pseudobrookite, and homogenization has not been achieved. It is suggested that under magmatic conditions extensive solid solution exists in which the FeTiO3 component may be regarded as having a spinel (γ) structure; after exsolution, monotropic inversion may occur to yield a rhombohedral ferri-ilmenite (Chevallier et al., 1955), which on slow cooling changes in composition towards FeTiO3.
Magnetite with exsolution bodies of both ulvöspinel and il menite shows on heating the features of both the above binary types.