Abstract

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 (α-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 α0 = 2.3 (±0.6) × 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 γ0 = 0.91 (±0.07), and De-bye temperature Θ0 = 740 (±5) K. We identify decomposition conditions for 2α-FeOOH → α-Fe2O3 + H2O at 1–8 GPa and 100–400 °C, and the polymorphic transition from α-FeOOH (Pbnm) to ε-FeOOH (P21mn). The non-quenchable, high-pressure ε-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) Å3.

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