HR: 0800h
AN: DI41A-0341    [Abstracts]
TI: Pressure-Volume-Temperature Equation of State for alpha-FeOOH Using X-ray Diffraction
AU: * Gleason, A E
EM: arianna@eps.berkeley.edu
AF: University of California, Berkeley, Dept. of Earth and Planetary Science 307 McCone Hall #4767, Berkeley, CA 94767, United States
AU: Kunz, M
EM: mkunz@lbl.gov
AF: Lawrence Berkeley National Laboratory, Advanced Light Source 1 Cyclotron Rd. MS 2-400, Berkeley, CA 94767, United States
AU: Jeanloz, R
EM: jeanloz@berkeley.edu
AF: University of California, Berkeley, Dept. of Earth and Planetary Science 307 McCone Hall #4767, Berkeley, CA 94767, United States
AB: Hydrogen strongly influences the physical properties of mantle materials. Understanding the stability and properties of simple hydroxides at high pressures and temperatures offers an important first step toward quantifying more complex hydrogen-bearing compounds relevant to the Earth's interior. We focus on iron-oxy- hydroxide because Fe is a major chemical component of the deep Earth, with valence (hence chemical-bonding properties) dependent on pressure. Powder x-ray diffraction measurements on goethite (α-FeOOH), collected at combined pressures and temperatures of 0-35 GPa and 23°-400°C, and fit to a third- order Birch-Murnaghan equation of state, yield a zero-pressure isothermal bulk modulus and pressure derivative of K_0T = 124 (±3) GPa and K'_0T = 4 (±1). We measure the zero-pressure thermal expansivity of goethite, finding α = 2.3 (±0.6) x 10^-5K^-1 between 23 and 400°C. These thermoelastic parameters are compatible with the values for diaspore (AlOOH), an isostructural analogue that forms a continuous solid solution with goethite (its high-pressure phase, δ-AlOOH, is thought to be a dehydration product of lawsonite in the transition zone). And at even higher pressures, ~50 GPa or more, where the mechanical and coulombic energy densities are comparable to major electronic transitions, we expect a change in Fe-(OH) interaction. Finally, we propose that a high- to low-spin transition in trivalent iron could result in a significant molar-volume reduction, manifesting itself as a first-order phase transition at high pressures.
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3919 Equations of state
DE: 3924 High-pressure behavior
DE: 3954 X-ray, neutron, and electron spectroscopy and diffraction
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting