Single-crystal X-ray structure refinement of Al-bearing superhydrous phase B

Superhydrous phase B is an important dense hydrous magnesium silicate (DHMS) phase in a hydrous peridotite system for understanding the deep water cycle in the Earth’s interior. A large amount of Al can be substituted in the crystal structure of superhydrous phase B, but its position in the crystal structure has yet to be determined. Here, we investigated the crystal structure of Al-bearing superhydrous phase B (Mg_8.1Si_2.0Al_1.9H_5.9O₁₈) by single-crystal X-ray diffraction (SC-XRD), together with Raman and Fourier-transform infrared (FT–IR) spectroscopies to determine the positions of Al and predict the positions of H. From crystal structure refinement, we clarified that Al was located in the octahedral Mg1, Mg2, and Si1 sites with a Pnnm space group. Furthermore, the result is consistent with a previously proposed substitution reaction: 2Mg²⁺ + Si⁴⁺ ⇄ 2Al³⁺ + 2H⁺ + □_Mg. The obtained FT–IR spectra, difference Fourier maps, and bond-valence revealed new H positions at (1) O₆–H⋯O₄, which is a part of the AlO₆ octahedron; (2) O₅–H⋯O₃, which is the same as the Mg-endmember superhydrous phase B, and (3) O4-H⋯O1, which is the shared edge of the Mg2O₆ and Mg4O₆ octahedra. By summarizing the present study together with the previous studies, it is predicted that the cause of the phase transition between Pnnm and Pnn2 occurs at ~ 1300 ℃ due to the synthesis temperature, which is not due to the substitution of metal elements.