Résumé
We have performed an extensive characterization by transmission electron microscopy (including precession electron diffraction tomography and ab initio electron diffraction refinement aswell as electron energy loss spectroscopy) of anhydrous phase B (Anh‐B) formed directly from olivine at 14 GPa, 1400 °C. We show that Anh‐B, which can be considered as a superstructure of olivine, exhibits strong topotactic relationships with it. This lowers the interfacial energy between the two phases and the energy barrier for nucleation of Anh‐B, which can form as a metastable phase. We have calculated the elastic and seismic properties of Anh‐B. From the elastic point of view, Anh‐B appears to be more isotropic than olivine. Anh‐B displays only a moderate seismic anisotropy quite similar to the oneof wadsleyite.