HR: 1340h
AN: V43A-1422    [Abstracts]
TI: Pressure-Induced Valence Change and Hydrogen Disorder in Fe(OH)$_{2}$
AU: * Speziale, S
EM: speziale@uclink.berkeley.edu
AF: University of California, Department of Earth and Planetary Science, Univ. of California, Berkeley, CA 94720-4767 United States
AU: Milner, A
EM: ela@post.tau.ac.il
AF: Tel Aviv University, School of Physics and Astronomy, Tel Aviv University, Tel Aviv, 69978 Israel
AU: Pasternak, M P
EM: moshepa@post.tau.ac.il
AF: Tel Aviv University, School of Physics and Astronomy, Tel Aviv University, Tel Aviv, 69978 Israel
AU: Zaug, J M
EM: zaug1@llnl.gov
AF: Lawrence Livermore National Laboratory, Lawrence Livermore National Laboratory, Livermore, CA 94551 United States
AU: Jeanloz, R
EM: jeanloz@berkeley.edu
AF: University of California, Department of Earth and Planetary Science, Univ. of California, Berkeley, CA 94720-4767 United States
AB: Vibrational and M\"{o}ssbauer spectroscopy reveal that pressure causes the hydrogen lattice of brucite-structured Fe(OH)$_{2}$ to become disordered above 10 GPa, leading to self-oxidation of Fe$^{2+}$----$^{P}$----$>$Fe$^{3+}$$+{\it e}$^{-}$. This result documents one mechanism by which water (hydrogen) dissolved in the Earth's mantle can induce changes in valence and chemical bonding at depth, regardless of buffering by other mineral phases. Infrared absorption (IR) and Raman (R) spectroscopy at 7-21 GPa and 293 K show that the A$_{2u}$ (IR) and A$_{1g}$ (R) OH-stretching modes decrease with pressure (-1.3 $\pm$ 0.1 cm$^{-1}$/GPa and -4.9 $\pm$ 0.2 cm$^{-1}$/GPa, respectively) in a manner expected for hydrogen-bonded systems. Both modes show non-linear broadening, with a $\sim$ 4-fold increase in width over the experimental pressure range. This is interpreted as a signature of the hydrogen-ion positions becoming disordered, as previously proposed for the isostructural Mg(OH)$_{2}$, Ca(OH)$_{2}$ and Co(OH)$_{2}$, and attributed to the effects of H - H Coulombic repulsion caused by compression along the {\it c}- crystallographic axis. In the case of Fe(OH)$_{2}$, M\"{o}ssbauer spectroscopy reveals the internal (self-) oxidation of the Fe site at starting at similar pressures as the hydrogen disordering. Displacement (and disordering) of H alters the Coulomb potential at the iron site, thus furnishing the driving force for delocalization of the weakly bonded 3d electron of Fe. Once the electron is lost from Fe, the site remains oxidized on decompression apparently because of the affinity of iron for the 3+ oxidation state. Thus, the hydrogen disordering is observed to be reversible upon decompression, whereas the self-oxidation is not. Such changes in valence can strongly affect the phase stability, minor- and trace-element partitioning, and transport properties of ''water''-bearing minerals in the Earth's mantle.
DE: 3630 Experimental mineralogy and petrology
DE: 3924 High-pressure behavior
DE: 3934 Optical, infrared, and Raman spectroscopy
DE: 1025 Composition of the mantle
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting