STRUCTURAL RELATION OF HYDROUS RINGWOODITE TO HYDROUS WADSLEYITE

Abstract

The sequential displacement mechanism based on the oxygen-lattice cubic closest packing (c.c.p.) in the <2 0 1> direction was proposed in this study. All displacements of cations are within the OT and the O layers with the length of displacement vector being around 1.7 or 2.9 A, contrary to displacement of around 5.0 A for models proposed previously. The difference in atomic arrangement between hydrous wadsleyite and hydrous ringwoodite is small. The atomic arrangement of the O layer of hydrous wadsleyite is essentially the same as that of hydrous ringwoodite when Mg vacancies preferentially exist in the O layer. The partial occupancies of normally vacant tetrahedral sites reported in the hydrous-β and hydrous-γ structures may possibly be caused by the existence of Mg vacancies at the octahedral sites through phase transition from hydrous-β to hydrous-γ or from hydrous-γ to hydrous-β phases.