Six single crystals of Mg 3 (Mg x Si x Al (sub 2-2x) )Si 3 O 12 with x = 0.05, 0.13, 0.24, 0.38, 0.52, and 0.64 (the majorite solid-solution) were synthesized at 20 GPa and 2000 degrees C with a "6-8" type uniaxial split-sphere apparatus. Single-crystal X-ray diffraction studies revealed discontinuities in compositional dependence of the molar volume, equivalent isotropic temperature factors (Be q ), and mean bond lengths between x = 0.24 and 0.38. Single crystals in the compositional range 0< or =x< or =0.24 show no birefringence, whereas those of x = 0.64 have a slight optical anisotropy. Moreover, the cell symmetry for x = 0.64 obtained using synchrotron X-ray radiation is tetragonal with a slight deviation from cubic. On the basis of site splitting expected from compositional dependence of Be q obtained by cubic refinement, the most probable space group in the range 0.38< or =x< or =0.64 is I4 1 /acd (tetragonal), which is the maximal subgroup of the space group Ia3d (cubic). Given that the previous reports that crystals with 0.8< or =x< or =1.0 have the tetragonal space group I4 1 /a, the majorite solid-solution in this system undergoes the series of symmetry changes, Ia3d --> I4 1 /acd --> I4 1 /a, with increasing MgSiO 3 component. The symmetry changes from Ia3d to I4 1 /acd cannot be explained by the cation ordering on the octahedral site. Strong electrostatic interaction between the dodecahedral (Mg (super 2+) ) and tetrahedral (Si (super 4+) ) cations was observed from atomic thermal motion and electron density distribution. Because one of the site symmetries of the two nonequivalent tetrahedral sites in I4 1 /acd structure loses the center of symmetry with the symmetry reduction from Ia3d to I4 1 /acd, the symmetry reduction may be caused by the electronic polarization of the cations due to the neighboring cation-cation interaction.