H-D interdiffusion in brucite at pressures up to 15 GPa

Abstract

Proton diffusion in brucite was investigated by conducting hydrogen-deuterium (H-D) exchange experiments using multi-anvil high-pressure apparatuses at pressures from 3 to 15 GPa and temperatures in the range of 750-1050 K. The diffusion couple was composed of a natural proton-dominated brucite single crystal surrounded by synthesized D-doped brucite polycrystals. Micro-Raman spectroscopy was used to determine the diffusion profiles of the samples. The D/H diffusion profile across the boundary between single crystal and polycrystalline D-doped brucite showed an asymmetric pattern characterized by faster diffusion in aggregates. The D/H interdiffusion rate determined from the analysis of the single crystal side indicates that the interdiffusion rate increases with increasing H/D ratio. The H-D interdiffusion rate in the direction perpendicular to the c-axis is about 0.5 orders of magnitude higher than that in the direction parallel to the c-axis. At 3 GPa, the H-D interdiffusion coefficients [D(m 2 /s)] along and perpendicular to the c-axis of brucite at compositions of C O no D rm = 0.2 in the single-crystal region were determined to be 3.30(±1.77) ×10 −11 exp[-48.2(±5.8) (kJ/mol)/ RT] and 1.43(±1.33) ×10 −9 exp[-67.5(±23.2) (kJ/mol)/RT], respectively. The H-D interdiffusion rate perpendicular to and along the c-axis increased about one order of magnitude by compression from 3 to 10 GPa, but the pressure enhancement became weaker above 10 GPa. From 10 to 15 GPa, there is almost no pressure dependence of proton diffusion for both directions. As pressure increases up to 10 GPa, enhancement of the proton migration is strongly correlated with the activation of the atomic interaction and decrease of O … O′ distance induced by compression. The positive pressure effect on the proton diffusion in brucite suggests that proton diffusion in higher-pressure hydrous phase becomes faster because of the shorter O … O′ distance.

High‐Temperature Vibrational Spectra Between Mg (OH)2 and Mg (OD)2: Anharmonic Contribution to Thermodynamics and D/H Fractionation for Brucite

“…The frequency ratios of the OD/OH stretching modes versus temperature, which are measured by both Raman and Fourier transform infrared (FTIR) spectra for these brucite samples. Data points from the literature (at ambient temperature) are plotted in open symbols for Mg (OH) 2 (five‐pointed star: Guo et al, ; diamond: Dawson et al, ; pentagon: Duffy, Hemley, et al, ), Ca (OH) 2 (up triangle: Dawson et al, ), as well as β ‐Co (OH) 2 (left and right triangles: Mockenhaupt et al, ).…”

Pressure‐induced enhancement of proton conduction in brucite

“…The activation energies parallel and perpendicular to the c axis of the single‐crystal brucite were determined to be 0.72 (±0.03) eV and 0.62 (±0.03) eV, respectively. These activation energies of a single crystal are comparable to those (0.50–0.70 eV) obtained in H‐D interdiffusion experiments [ Guo et al ., ]. This consistency indicates that electrical conduction and diffusion in brucite are due to the same type of defect species.…”

“…The potential barrier between proton sites in adjacent OH − groups is proportional to the O · · · O' distance [ Martens and Freund , ]. Since the O · · · O' distance is shortened by compression under high pressure, more protons can easily be transferred from donors (OH) to acceptors (O') through an H‐bond‐type proton exchange [ Freund and Wengeler , ; Guo et al ., ].…”

“…The starting material was a natural single‐crystal Mg(OH) 2 , which was the same material as used in the previous study on proton diffusivity [ Guo et al ., ]. The crystallographic orientation of each crystal was determined by a precession X‐ray camera, with a characteristic K α X‐ray of Mo.…”

“…Therefore, brucite can be regarded as an ideal subject to investigate the effect of pressure on the electrical conductivity of hydrous minerals. Proton diffusivity in brucite is strongly enhanced at higher pressures, which has been inferred from hydrogen‐deuterium (H‐D) interdiffusion experiments [ Guo et al ., ]. Proton conduction likely dominates the conductivity of hydrous minerals because of the high mobility of the protons [ Yoshino , ].…”