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Akimotoite, ilmenite-type MgSiO₃ high-pressure polymorph can be stable in the lower-mantle transition zone along average mantle and subducting slab geotherms. Significant amounts of Al₂O₃ can be incorporated into the structure, having the pyrope (Mg₃Al₂Si₃O₁₂) composition. Previous studies have investigated the effect of Al₂O₃ on its crystal structure at nearly endmember compositions. In this study, we synthesized high-quality ilmenite-type Mg₃Al₂Si₃O₁₂ phase at 27 GPa and 1073 K by means of a Kawai-type multi-anvil press and refined the crystal structure at ambient conditions using a synchrotron X-ray diffraction data via the Rietveld method to examine the effect of Al₂O₃. The unit-cell lattice parameters were determined to be <i>a</i> = 4.7553(7) Å, <i>c</i> = 13.310(2) Å, and <i>V</i> = 260.66(6) ų, with <i>Z</i> = 6 (hexagonal, <i>R</i><inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mover accent="true"><mn>3</mn><mo>¯</mo></mover></semantics></math></inline-formula>). The volume of the present phase was placed on the akimotoite-corundum endmember join. However, the refined structure showed a strong nonlinear behavior of the <i>a</i>- and <i>c</i>-axes, which can be explained by Al incorporation into the MgO₆ and SiO₆ octahedral sites, which are distinctly different each other. Ilmenite-type Mg₃Al₂Si₃O₁₂ phase may be found in shocked meteorites and can be a good indicator for shock conditions at relatively low temperatures of 1027–1127 K.