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.