Abstract
The high-pressure behavior of the mineral brucite, Mg(OH)2, is of great geochemical and geophysical interest, because brucite-type minerals serve as a simple analog for more complex, hydrogen-bearing oxide and silicate minerals in the deep-earth. A combined neutron and synchrotron x-ray powder diffraction study of hydrogenated and deuterated brucite was conducted at ambient temperature and at pressures
to 9 and 20 GPa, using a Paris-Edinburgh (neutron diffraction) and a diamond anvil cell (synchrotron x-ray radiation), respectively. The two materials were synthesized by the same method and companion diffraction measurements of the two materials were conducted under the same conditions.
Our experimental results show that the lattice-parameters of the a axis, parallel to the sheets of Mg-O octahedra, decrease only slightly with pressure with no effect of H-D substitution. However, the c axis of Mg (OD)2 is shorter and may exhibit greater compressibility with pressure than that of Mg (OH)2. Consequently, the unit-cell volume of deuterated brucite is slightly, but systematically smaller than that of
hydrogenated brucite. Fitting to a third-order Birch-Murnaghan equation shows that values of the bulk modulus for hydrogenated and deuterated brucite are indistinguishable from each other within the experimental errors. The measured effect of H-D substitution on the unit-cell volume 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.
