13:45 〜 15:15
[SMP27-P03] The hydrogen-bonding configuration of fluorine-doped brucite under high pressure
キーワード：ブルーサイト、フッ素置換、水素結合の局所構造、高圧粉末中性子回折
Fluorine (F) is supposed to be the most abundant halogen in the mantle [1]. Hydrous minerals commonly contain F, and play a crucial role in transporting F into the mantle via subduction processes (e.g. [2]). F- is incorporated into hydrous minerals by substituting hydroxyls in the crystal structure, which can lead to modifications of physicochemical properties and the formation of the O-H…F hydrogen bond. The high-pressure and high-temperature stability of hydroxyls in the mineral structure is strongly related to large-scale geological processes, such as slab dehydration, rock melting, and subduction earthquakes (e.g. [3]). Therefore, investigating the O-H…F hydrogen-bonding configuration under high pressure and high temperature is among the fundamental mineralogical issues.
The brucite [Mg(OH)2] (P-3m1, Z = 1) structure is an archetype of hydrous minerals and phases, and is also a typical simplified structure for studying the hydrogen bond. Miao et al. (2022) [4] has synthesized the single-crystal F-doped hydrogenated brucite, Mg(OH)1.78F0.22 and Mg(OH)1.16F0.84 (P-3m1, Z = 1), using the multi-anvil apparatus, and has reported the hydrogen-bonding geometries based on the XRD data under the ambient condition. The proton site has been refined in both the single-site model (the 2d site (1/3,2/3,z), the hydroxyl dipole is aligned parallel to the c axis) and the three-fold split-site model (the 6i site (x,2x,z), the proton is disordered in three equivalent splitting sites around the three-fold axis with an equal occupancy of 1/3). Neutron diffraction studies are necessary to locate the proton site more precisely, and pressure effects on the hydrogen-bonding geometries also need to be investigated.
In this study, we have investigated the crystal structure and the hydrogen-bonding configuration of F-doped hydrogenated brucite, Mg(OH)1.81F0.19, and deuterated brucite, Mg(OD)1.79F0.21, under ambient and high-pressure conditions via neutron powder diffraction. The incorporation of fluorine shortened the a and c axes, resulting in smaller unit cell volumes. Under the ambient condition, the proton site of Mg(OH)2 can be refined by either the single-site model or the three-fold split-site model, while the proton of Mg(OH)1.81F0.19 tends to locate in the 2d site. Both Mg(OD)2 and Mg(OD)1.79F0.21 have the proton in the 6i site. The hydrogen-bonding geometries, i.e., d(O-H/D), d(H/D…O), d(O…O), and ∠O-H/D…O, were determined up to 6.9 GPa for Mg(OH)1.81F0.19 and up to 10.1 GPa for Mg(OD)1.79F0.21. In Mg(OD)1.79F0.21, the inclined angle of the O-D dipole increases steadily with increasing pressure, and the hydrogen-bonding interaction strengthens. Compared with Mg(OD)2 [5], the substitution of F alleviates the pressure-induced strengthening of the hydrogen bond under compression. The results of this study have offered the information regarding the O-H…F hydrogen-bonding geometry in hydrous minerals under high pressure, and are promisingly helpful to future studies of the hydrogen-bonding configuration of other F-bearing hydrous minerals (or phases) with more complex structures.
References: [1] McDonough W. F., Sun S.-s. (1995) Chem Geol, 120:223-253. [2] Grützner T., et al. (2017) Geology, 45(5):443-446. [3] Peacock S. M., Hyndman R. D. (1999) Geophys Res Lett, 26(16):2517-2520. [4] Miao Y., et al. (2022) Am Mineral, 107:2065-2074. [5] Parise J. B., et al. (1994) Am Mineral, 79:193-196.
The brucite [Mg(OH)2] (P-3m1, Z = 1) structure is an archetype of hydrous minerals and phases, and is also a typical simplified structure for studying the hydrogen bond. Miao et al. (2022) [4] has synthesized the single-crystal F-doped hydrogenated brucite, Mg(OH)1.78F0.22 and Mg(OH)1.16F0.84 (P-3m1, Z = 1), using the multi-anvil apparatus, and has reported the hydrogen-bonding geometries based on the XRD data under the ambient condition. The proton site has been refined in both the single-site model (the 2d site (1/3,2/3,z), the hydroxyl dipole is aligned parallel to the c axis) and the three-fold split-site model (the 6i site (x,2x,z), the proton is disordered in three equivalent splitting sites around the three-fold axis with an equal occupancy of 1/3). Neutron diffraction studies are necessary to locate the proton site more precisely, and pressure effects on the hydrogen-bonding geometries also need to be investigated.
In this study, we have investigated the crystal structure and the hydrogen-bonding configuration of F-doped hydrogenated brucite, Mg(OH)1.81F0.19, and deuterated brucite, Mg(OD)1.79F0.21, under ambient and high-pressure conditions via neutron powder diffraction. The incorporation of fluorine shortened the a and c axes, resulting in smaller unit cell volumes. Under the ambient condition, the proton site of Mg(OH)2 can be refined by either the single-site model or the three-fold split-site model, while the proton of Mg(OH)1.81F0.19 tends to locate in the 2d site. Both Mg(OD)2 and Mg(OD)1.79F0.21 have the proton in the 6i site. The hydrogen-bonding geometries, i.e., d(O-H/D), d(H/D…O), d(O…O), and ∠O-H/D…O, were determined up to 6.9 GPa for Mg(OH)1.81F0.19 and up to 10.1 GPa for Mg(OD)1.79F0.21. In Mg(OD)1.79F0.21, the inclined angle of the O-D dipole increases steadily with increasing pressure, and the hydrogen-bonding interaction strengthens. Compared with Mg(OD)2 [5], the substitution of F alleviates the pressure-induced strengthening of the hydrogen bond under compression. The results of this study have offered the information regarding the O-H…F hydrogen-bonding geometry in hydrous minerals under high pressure, and are promisingly helpful to future studies of the hydrogen-bonding configuration of other F-bearing hydrous minerals (or phases) with more complex structures.
References: [1] McDonough W. F., Sun S.-s. (1995) Chem Geol, 120:223-253. [2] Grützner T., et al. (2017) Geology, 45(5):443-446. [3] Peacock S. M., Hyndman R. D. (1999) Geophys Res Lett, 26(16):2517-2520. [4] Miao Y., et al. (2022) Am Mineral, 107:2065-2074. [5] Parise J. B., et al. (1994) Am Mineral, 79:193-196.