Geocheminar fall-2021

Pyrite-structured FeOOH_x, also known as p-FeOOH_x, has been found to be a phase at lower mantle pressure and temperature conditions. From X-ray diffraction studies, p-FeOOH_x falls into the Pa-3 space group, consisting of a corner-sharing FeO6 octahedra cross-linked with hydrogen bonds, which become symmetric at high pressure. The partially filled 3d electron orbitals give rise to two electronic configurations where Fe2+ can either be in a high-spin or low-spin state. Many iron-bearing oxides and silicates are known to undergo a pressure-induced transition from the high-spin to the low-spin state at conditions of the Earth’s lower mantle. Such spin transitions are important because they shrink the volume and increase the density, affecting buoyancy relationships and seismically observable elastic wave velocities. In FeOOH_x measuring the volume is important as a means for determining hydrogen content (x), which is still unknown and may be variable, and can be determined only by comparison with equations of state of known hydrogen content. However, the spin transition in p-FeOOH_x is poorly understood with only a few experimental measurements constraining the pressure at which it occurs. Here, we investigate the high-spin to low-spin transition in p-FeOOH_x with x=1 using density functional theory (DFT) as implemented in VASP. We find the high-spin to low-spin transition to be a gradual change at both high and low temperatures with a stable mixed-spin state during the transition, calling for an alteration of the thermodynamic ideal mixing model previously proposed by other studies where the transition is sharp at low temperatures. We find the addition of a favorable interaction term between high-spin and low-spin states to the enthalpy of mixing accounts for the first principles results. I will discuss implications for determination of the hydrogen content of this phase and its significance for the deep Earth.