Garnets in the majorite–pyrope system: symmetry, lattice microstrain, and order–disorder of cations
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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 (Mg2+ + Si4+ = 2Al3+) 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.
KeywordsGarnet Phase transition Microstrain Cation disorder Cation substitution
The authors thank J. B. Parise and Y. M. Zhou for their valuable suggestions for X-ray diffraction analysis. The authors thank T. Boffa Ballaran for her help in X-ray diffraction measurements. We thank both editor and two reviewers for their constructive comments. The present study is supported by the Grant-in-Aid for Scientific Research (S) by JSPS to T. Irifune (Grant No. 25220712).
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