The symmetry of garnets on the pyrope (Mg₃Al₂Si₃O₁₂) -majorite (MgSiO₃)

Garnets with compositions between majorite and pyrope, Mj₃₈, Mj₄₈, Mj₇₅ and Mj₇₉ were synthesized at high pressures and temperatures in a 2000-ton uniaxial split-sphere apparatus (USSA-2000) and investigated using high resolution synchrotron X-ray powder diffraction and transmission electron microscopy. The results from both techniques are consistent with the tetragonal field for these garnets extending to a majorite composition just below Mj₇₅. The cubic-tetragonal structural phase transition in garnet along the majorite - pyrope join is sensitive to both composition and temperature and is expected to result in anomalous behavior in elastic shear moduli. This phase transition may occur in the transition zone of the earth's mantle and will have important effects on the elastic and rheological properties of this region where these garnets are stable phases.