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We present a systematic experimental study on the phase transition, lattice microstrain, and order-disorder of cations for garnets in the majorite-pyrope system. Polycrystalline gem-quality garnets were synthesized at high pressure and high temperature using a Kawai-type multi-anvil apparatus. A phase transition from a cubic to tetragonal structure is clearly observed for garnets with the majorite content of more than 74 mol % through X-ray diffraction (XRD) and Raman scattering studies. Microstrain of garnets, evaluated with the Williamson-Hall plot on XRD profiles, shows a nonlinear dependence of the garnet compositions. The variation of the XRD peak broadening suggests the lattice microstrain of these garnets may be associated with the local structural heterogeneities due to the substitution of different cations via the coupled substitution (Mg + Si = 2Al) in the garnet structure. The width variation of Raman scattering peaks indicates that cation disorder occurs in the garnet structure for intermediate compositions. It is found that intermediate garnets and end-members have a minimum of microstrain, while those between end-members and intermediate compositions possess a larger microstrain. [ABSTRACT FROM AUTHOR]

The elastic wave velocities of polycrystalline MjPy garnet along the majorite-pyrope system have been measured at pressures up to 21 GPa and temperatures up to 2,000 K using ultrasonic interferometry in conjunction with in situ X-ray diffraction techniques in a Kawai-type multi-anvil apparatus. The elastic moduli of MjPy garnet and their pressure and temperature derivatives are determined by a two-dimensional linear fitting of the present experimental data, yielding: K = 161.5 (7) GPa , ∂K/ ∂P = 4.42 (4) , ∂K/ ∂T = −0.0154 (2) GPa/K , G = 86.2 (2) GPa , ∂G/ ∂P = 1.28 (1) , ∂G/ ∂T = −0.0096 (5) GPa/K. The present results together with those of the studies on the majorite-pyrope solid solutions suggest the pressure and temperature derivatives of elastic moduli are insensitive to the majorite content in the majorite-pyrope system. The velocity gradients of the majoritic garnets in the majorite-pyrope system are 3 ~ 6 times lower than those required to account for the high seismic velocity gradients observed in the mantle transition zone. [ABSTRACT FROM AUTHOR]