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

The Mie-Gruneisen 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 (alpha-FeOOH) to combined conditions of T = 23-250 degrees C and P = 0-29.4 GPa. We find the zero-pressure thermal expansion coefficient of goethite to be alpha(0) = 2.3 (+/- 0.6) x 10(-5)K(-1) over this temperature range. Our data yield zero-pressure compressional parameters: V-0 = 138.75 (+/- 0.02) angstrom(3), bulk modulus K-0 = 140.3 (+/- 3.7) GPa, pressure derivative K-0(') = 4.6 (+/- 0.4), Gruneisen parameter gamma(0) = 0.91 (+/- 0.07), and Debye temperature Theta(0) = 740 (+/- 5) K. We identify decomposition conditions for 2 alpha-FeOOH -> alpha-Fe2O3 + H2O at 1-8 GPa and 100-400 degrees C, and the polymorphic transition from alpha-FeOOH (Pbnm) to epsilon-FeOOH (P2(1)mn). The non-quenchable, high-pressure epsilon-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) angstrom(3).