In situ X-ray observation of decomposition of hydrous aluminum silicate AlSiO₃OH and aluminum oxide hydroxide d-AlOOH at high pressure and temperature

We clarified the stability limit of phase Egg, AlSiO₃OH, a candidate for water reservoir in the siliceous sediment of slabs in the transition zone conditions by in situ X-ray observation using high energy X-ray from synchrotron radiation source of SPring-8. Phase Egg is stable at least up to 1625 °C at 17 GPa. We observed decomposition of phase Egg into δ-AlOOH and stishovite at pressures greater than 23 GPa at temperatures below 1200 °C. No water release occurs associated with the decomposition. At temperatures above 1200 °C at 23 GPa, we observed decomposition of phase Egg into corundum+stishovite+fluid. We also determined the phase boundary of the decomposition reaction of δ-AlOOH to corumdum+fluid based on the in situ X-ray diffraction at high pressure and temperature.