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Pressure-temperature stability studies of FeOOH using X-ray diffraction

¹ Department of Earth and Planetary Science, University of California, Berkeley, McCone Hall 4767, Berkeley, California 94720, U.S.A.
² Advanced Light Source, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, U.S.A.

Correspondence: ^* E-mail: aegleason{at}lbl.gov

The Mie-Grüneisen formalism is used to fit a Birch-Murnaghan equation of state to high-temperature (T), high-pressure (P) X-ray diffraction unit-cell volume (V) measurements on synthetic goethite (-FeOOH) to combined conditions of T = 23–250 °C and P = 0–29.4 GPa. We find the zero-pressure thermal expansion coefficient of goethite to be ₀ = 2.3 (±0.6) x 10^–5 K^–1 over this temperature range. Our data yield zero-pressure compressional parameters: V₀ = 138.75 (±0.02) Å3, bulk modulus K₀ = 140.3 (±3.7) GPa, pressure derivative K₀ ' = 4.6 (±0.4), Grüneisen parameter ₀ = 0.91 (±0.07), and De-bye temperature ₀ = 740 (±5) K. We identify decomposition conditions for 2-FeOOH -Fe₂O₃ + H₂O at 1–8 GPa and 100–400 °C, and the polymorphic transition from -FeOOH (Pbnm) to -FeOOH (P2₁mn). The non-quenchable, high-pressure -FeOOH phase P-V data are fitted to a second-order (Birch) equation of state yielding, K₀ = 158 (±5) GPa and V₀ = 66.3 (±0.5) ų.

Key Words: Goethite • XRD data • diamond-anvil cell • compressibility measuresments