<p>An alumina-rich silicate phase of calcium ferrite-type (CF) phase is a major mineral in crustal rocks subducted into the lower mantle. However, its chemical composition with increasing pressure and temperature remains unclear yet. In this study, phase relations in the CaAl₂O₄-MgAl₂O₄ system were determined at 15–25 GPa and 1100–2000&#xa0;°C by using a multi-anvil apparatus. In the pressure-temperature range, CaAl₂O₄-rich CF phase coexisted with another alumina-rich silicate of hexagonal (NAL) phase in the (1-x)CaAl₂O₄-xMgAl₂O₄ system (x = 5–70&#xa0;mol%). The solubility of the MgAl₂O₄ component in the CF phase increases with pressure and temperature, from 3.8&#xa0;mol% to 23.9&#xa0;mol%, while the composition of the NAL phase was almost constant. In the MgAl₂O₄-rich system, we found that almost no CaO was incorporated into MgAl₂O₄ phases in the present pressure-temperature range. MgAl₂O₄ spinel (Sp) transformed to decomposition phases of periclase (Per) or Mg₂Al₂O₅ (mLd) phase and corundum (Crn), and then the CF phase formed at higher pressures, showing phase assemblages of NAL + Sp, NAL + Crn + Per or mLd, and NAL + CF with increasing pressure. An additional experiment in the NaAlSiO₄-CaAl₂O₄ system at 20 GPa and 1400&#xa0;°C showed both Na- and Ca-rich CF phases incorporated Ca²⁺ and Na⁺, respectively. These results together with previously determined phase relations in NaAlSiO₄-MgAl₂O₄ can explain limited CaO contents of CF phases in crustal materials subducted into the lower mantle. In addition, pressure-temperature changes in the MgAl₂O₄ solubility of CF-type CaAl₂O₄ can be used as a new pressure calibrant in the multi-anvil experiments.</p>