Morphology of oriented ilmenite inclusions in olivine from garnet peridotites (Central Alps, Switzerland)

Olivine with oriented ilmenite inclusions occurs in garnet peridotites and in basic layered intrusions. In all cases, the ilmenite inclusions are elongated parallel to [010](ol) but exhibit different morphology (rods versus platelets). These morphological differences observed for ilmenite inclusions in olivine can be correlated with variations in the chemical composition of the precipitating phase. Determination of the topotactic misfits between host and inclusion have been calculated for three chemical compositions along the ilmenite-geikielite join. The elongation of ilmenite parallel to [010](ol) corresponds to the direction along which the mismatch between the two phases is the smallest, i.e., less than or equal to 1.1%. In section perpendicular to [010], the lattice misfit is compensated by dislocation coronas along the interface between ilmenite and olivine. The rod morphology of the ilmenite inclusions concurs with minimum topotactic misfits between the lattices of olivine (Fo90) and Mg-bearing ilmenite and does not require a perovskite precursor. The formation of ilmenite inclusions in olivine requires only cation diffusion within a relatively undisturbed oxygen framework.