<p>The iron-bearing Fe_xTi_yO_z minerals are important terrestrial materials for plate tectonics studies and paleo-magnetism in geophysics. The bulk modulus of the Fe_xTi_yO_z minerals, such as Fe₃O₄ (magnetite), Fe₂TiO₄. (ulvöspinel), FeTiO₃ (ilmenite), Fe₂TiO₅ (pseudobrookite) are related to the compressions of cation sites and vacant site. Cation site partly occupied by Ti atom shows a smaller <i>K</i>_<i>o</i> than that of only Fe atom. The compressibility increases with increasing Ti content in Fe_xTi_yO_z. The vacant sites in the unit cell are much larger volumes than cation sites in these four structures. The bulk moduli of these samples are very similar to those of their vacant sites. The compressibility of the cation site is much larger than those of the vacant sites. The cation distribution in the solid solutions have been carried out by neutron diffraction and X-ray diffraction studies. The Fe/Ti distribution has been examined as a function of pressure. The cation sites and vacant sites show different compressibility with increasing pressure. The cation sites of these minerals at high-pressure conditions were investigated to understand their strong electronic correlations. The crystal internal potential change under high pressure is in an equilibrium state of the external pressure by virial theorem. The structure changes such as high-low electron spin transition, Jahn–Teller effect and <i>d-p-π </i>hybridization in the Fe–O bonds are elucidated by present high-pressure experiments. The <i>d-p</i>-<i>π</i> hybridization in the octahedral cation site was discussed by molecular orbital calculation and it brings the deformation of the octahedral cation site. The deformation triggers the structure changes in the high-pressure polymorphs.</p>