Structure and twinning of single-crystal MgSiO3 garnet synthesized at 17 GPa and 1800 °C

Single crystals of MgSiO₃ garnet have been synthesized at 17 GPa and 1800 °C in a uniaxial split-sphere anvil device, and the structure has been determined by refinement of single-crystal X-ray diffraction data. This garnet is tetragonal with unit-cell parameters a = 11.501(1) Å, c = 11.480(2) Å, V_cell = 1518.6(4) ų. The space group is I4₁/a, and all crystals display pervasive twinning by reflection on (110), making the apparent Laue symmetry close to 4/mmm. Two Mg positions are coordinated by eight anions with average Mg-O bond lengths of 2.284 and 2.279 Å. Mg and Si are at least partially ordered over the octahedral sites with mean Si-O = 1.807 Å and mean Mg-O = 1.998 Å. The three tetrahedral sites are all occupied by Si with mean bond lengths of 1.622,1.652, and 1.641 Å.