Abstract
A neutron powder diffraction study of hydrogenated and deuterated brucite was conducted at ambient temperature and at pressures to 9 GPa, using a Paris-Edinburgh high-pressure cell at the WAND instrument of ORNL High Flux Isotope Reactor. The two materials were synthesized by the same method and companion measurements of neutron diffraction were conducted under the same conditions. Our refinement results show that the lattice-parameters of the a axis, parallel to the sheets of Mg-O octahedra, decreases only slightly with pressure with no effect of H-D substitution. However, the c axis of Mg(OD)2 is shorter and decreases more rapidly than that of Mg(OH)2 with increasing pressure. Consequently, the unit-cell volume of deuterated brucite is slightly, but systematically smaller than that of normal brucite, which were fitted to third-order Birch-Murnaghan equation: K0=41.46�0.52 and 39.04�0.30 GPa for Mg(OH)2 and Mg(OD)2, respectively with a fixed K0� value of 10. Our results suggest that H-D substitution of brucite and other hydrous minerals with H-bonding likely influences their structural and dynamic properties under pressure, including the position of hydrogen atoms. The measured effect of H-D substitution on the unit-cell volume also demonstrates that brucite (and other hydrous minerals) preferentially incorporate deuterium over hydrogen under pressure, suggesting that the distribution of hydrogen isotopes in deep-earth conditions may differ significantly from that in near-surface environments.
