| Summary: | Phase transitions of Mg<sub>2</sub>TiO<sub>4</sub> and Fe<sub>2</sub>TiO<sub>4</sub> were examined up to 28 GPa and 1600 °C using a multianvil apparatus. The quenched samples were examined by powder X-ray diffraction. With increasing pressure at high temperature, spinel-type Mg<sub>2</sub>TiO<sub>4</sub> decomposes into MgO and ilmenite-type MgTiO<sub>3</sub> which further transforms to perovskite-type MgTiO<sub>3</sub>. At ~21 GPa, the assemblage of MgTiO<sub>3</sub> perovskite + MgO changes to 2MgO + TiO<sub>2</sub> with baddeleyite (or orthorhombic I)-type structure. Fe<sub>2</sub>TiO<sub>4</sub> undergoes transitions similar to Mg<sub>2</sub>TiO<sub>4</sub> with pressure: spinel-type Fe<sub>2</sub>TiO<sub>4</sub> dissociates into FeO and ilmenite-type FeTiO<sub>3</sub> which transforms to perovskite-type FeTiO<sub>3</sub>. Both of MgTiO<sub>3</sub> and FeTiO<sub>3</sub> perovskites change to LiNbO<sub>3</sub>-type phases on release of pressure. In Fe<sub>2</sub>TiO<sub>4</sub>, however, perovskite-type FeTiO<sub>3</sub> and FeO combine into calcium titanate-type Fe<sub>2</sub>TiO<sub>4</sub> at ~15 GPa. The formation of calcium titanate-type Fe<sub>2</sub>TiO<sub>4</sub> at high pressure may be explained by effects of crystal field stabilization and high spin−low spin transition in Fe<sup>2+</sup> in the octahedral sites of calcium titanate-type Fe<sub>2</sub>TiO<sub>4</sub>. It is inferred from the determined phase relations that some of Fe<sub>2</sub>TiO<sub>4</sub>-rich titanomagnetite inclusions in diamonds recently found in São Luiz, Juina, Brazil, may be originally calcium titanate-type Fe<sub>2</sub>TiO<sub>4</sub> at pressure above ~15 GPa in the transition zone or lower mantle and transformed to spinel-type in the upper mantle conditions.
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