[S-MP26] Physics and Chemistry of Minerals

[SMP26-03] P-T dependence of hydrogen-bond geometry of β-CrOOD

Chikara Shito¹, *Hiroyuki Kagi¹, Asami Sano-Furukawa^2,3, Sho Kakizawa⁴, Kazuki Komatsu¹, Riko Iizuka-Oku¹, Katsutoshi Aoki¹, Jun Abe⁵, Shinichi Machida⁵, Takashi Saito³, Takashi Kamiyama³, Noboru Furukawa⁶, Akio Suzuki⁷ (1.Graduate School of Science, University of Tokyo, 2.Japan Atomic Energy Agency, 3.High Energy Accelerator Research Organization, 4.Graduate School of Advanced Science and Engineering, Hiroshima University, 5.Comprehensive Research Organization for Science and Society, 6.Graduate School of Science, Chiba University, 7.Graduate School of Science, Tohoku University)

Keywords:neutron diffraction, hydrous mienerals, high pressure, hydrogen bonds

The Earth’s mantle contains a certain amount of high-pressure hydrous minerals. However, behaviors of hydrogen bonding and physical properties of such hydrous minerals at high-PT conditions have not been clarified. We focused on hydrogen-bond geometry and compression behavior of guyanaite (β-CrOOH). β-CrOOH is a distorted rutile-type hydrous mineral and isostructural to δ-AlOOH, one of the most promising hydrous phases for transporting hydrogen to the core–mantle boundary. In this study, X-ray and neutron diffraction experiments on β-CrOOD were carried out at various P-T conditions to clarify the P-T dependence of hydrogen-bond geometry in β-CrOOD. Crystal structure was refined using the Rietveld method.

The space group of β-CrOOD was determined to be P2₁/c, indicating that the hydrogen atoms are partially disordered at ambient condition. Above 450 K, however, the hydrogen atoms were fully disordered, and the space group was Pnnm. Thermal expansion anomaly was observed at temperature range of 300–500 K, where the phase transition (P2₁/c – Pnnm) occurred. In high-pressure experiments, changes in compression behavior caused by phase transition from P2₁/c to Pnnm were observed at about 3 GPa and 4.5 GPa at 300 K and 250 K, respectively. However, no significant change in compression behavior was observed at 450 K. Therefore, dP/dT of the phase boundary would have a negative slope. Assuming that the phase boundary of δ-AlOOH (P2₁nm–Pnnm) has the same tendency as that of β-CrOOD, δ-AlOOH would have a Pnnm structure in the Earth’s mantle and would have higher seismic velocity than the major minerals in the mantle transition zone.