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

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 α 0 = 2.3 (±0.6) × 10 -5 K -1 over this temperature range. Our data yield zero-pressure compressional parameters: V 0 = 138.75 (±0.02) Å 3 , bulk modulus K0 = 140.3 (±3.7) GPa, pressure derivative K 0 ′ = 4.6 (±0.4), Grüneisen parameter γ 0 = 0.91 (±0.07), and Debye temperature Θ 0 = 740 (±5) K. We identify decomposition conditions for 2α-FeOOH → α-Fe 2 O 3 + H 2 O at 1-8 GPa and 100-400 °C, and the polymorphic transition from α-FeOOH (Pbnm) to ε-FeOOH (P2 1 mn). The non-quenchable, high-pressure ε-FeOOH phase P-V data are fitted to a second-order (Birch) equation of state yielding, K 0 = 158 (±5) GPa and V 0 = 66.3 (±0.5) Å 3 .