CRYSTAL-CHEMISTRY OF CA-BEARING MAJORITE

Single crystals of Ca-bearing majorite, a garnet with composition (Ca0.49Mg2.51)(MgSi)Si3O12, have been synthesized at 18.2 GPa and 2050-degrees-C. This sample is the first silicate garnet to display ordering on both octahedral and dodecahedral sites-behavior that may increase the compositional flexibility of garnet, affect clement partitioning at high pressure, and stabilize the garnet structure in the transition zone and upper portion of the lower mantle. The garnet is tetragonal [space group I4(1)/a, Z = 8, a = 11.5816(9), c = 11.5288(13) angstrom, V = 1546.39(29) angstrom3] and, like MgSiO3 majorite, displays twinning by twofold rotation about [110].