Symmetry change of majorite solid-solution in the system Mg₃Al₂Si₃O₁₂-MgSiO₃

Abstract

Six single crystals of Mg₃(Mg_xSi_xAl_2-2x)Si₃O₁₂ 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 °C with a “6-8” type uniaxial splitsphere apparatus. Single-crystal X-ray diffraction studies revealed discontinuities in compositional dependence of the molar volume, equivalent isotropic temperature factors (B_eq), and mean bond lengths between x = 0.24 and 0.38. Single crystals in the compositional range 0 ≤ x ≤ 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 B_eq obtained by cubic refinement, the most probable space group in the range 0.38 ≤ x ≤ 0.64 is I4₁/acd (tetragonal), which is the maximal subgroup of the space group Ia3̅d (cubic). Given that the previous reports that crystals with 0.8 ≤ x ≤ 1.0 have the tetragonal space group I4₁/a, the majorite solid-solution in this system undergoes the series of symmetry changes, Ia3̅d →I4₁/acd→I4₁/a, with increasing MgSiO₃ component. The symmetry changes from Ia3̅d to I4₁/acd cannot be explained by the cation ordering on the octahedral site. Strong electrostatic interaction between the dodecahedral (Mg²⁺) and tetrahedral (Si⁴⁺) cations was observed from atomic thermal motion and electron density distribution. Because one of the site symmetries of the two nonequivalent tetrahedral sites in I4₁/acd structure loses the center of symmetry with the symmetry reduction from Ia3̅d to I4₁/acd, the symmetry reduction may be caused by the electronic polarization of the cations due to the neighboring cation-cation interaction.