A single crystal of a birefringent Cr-bearing majorite, Mg 3 (Mg (sub 0.34) Si (sub 0.34) Al (sub 0.18) Cr (sub 0.14) ) 2 Si 3 O 12 , was synthesized at 20 GPa and 2000 degrees C using "6-8" type uniaxial split-sphere apparatus. This garnet is tetragonal with the unit-cell parameters a nearly equal c and deviates slightly from cubic symmetry. The structure refinements using single-crystal X-ray diffraction intensity data were carried out by assuming three space groups (one cubic and two tetragonal) to determine the most probable symmetry. The most probable space group is I4 1 /a (tetragonal). The Cr ions show a disordered distribution between the two nonequivalent octahedral sites in the I4 1 /a structure.

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