Thermal equation of state of phase egg (AlSiO3OH): implications for hydrous phases in the deep earth

Phase egg (AlSiO3OH), an important hydrous phase in the Al2O3–SiO2–H2O ternary system, was found in the superdeep diamond from the mantle transition zone. Here, we have investigated the compressibility and the crystal structural evolution of phase egg using the synchrotron-based single-crystal X-ray diffraction and infrared spectroscopy combined with diamond anvil cells up to 32 GPa and 900 K. Present results show that the hydrogen atom flips its position and forms a new hydrogen bond configuration at ~ 14 GPa and room temperature, leading to a first-order phase transition of phase egg, but this behavior is smeared out because of thermal disordering of hydrogen atom at high temperatures. These findings provide important implications for revealing the complex state of hydrogen in the deep earth. Along the cold and warm subducting slab geotherms, the density and bulk velocity of phase egg are greater than those of preliminary reference Earth model in the mantle transition zone, presenting negative buoyancy force for deep-water transportation. Based on the obtained thermal elastic parameters, we further demonstrate that topaz-OH transforms to phase egg with a ~ 11.7% increase in bulk velocity and a ~ 4.2% increase in density, and phase egg decomposes into δ-AlOOH and stishovite with a ~ 6.2% increase in bulk velocity and a ~ 4.0% increase in density under subduction zone conditions.