THE SYMMETRY OF GARNETS ON THE PYROPE (MG3AL2SI3O12) MAJORITE (MGSIO3) JOIN

Garnets with compositions between majorite and pyrope, Mj(38), Mj(48), Mj(75) and Mj(79) were synthesized at high pressures and temperatures in a 2000-ton uniaxial split-sphere apparatus (USSA-2000) and investi gated using high resolution synchrotron X-ray powder diffraction and t ransmission electron microscopy. The results from both techniques are consistent with the tetragonal field for these garnets extending to a majorite composition just below Mj(75) The cubic-tetragonal structural phase transition in garnet along the majorite - pyrope join is sensit ive to both composition and temperature and is expected to result in a nomalous behavior in elastic shear moduli. This phase transition may o ccur in the transition zone of the earth's mantle and will have import ant effects on the elastic and rheological properties of this region w here these garnets are stable phases.