High temperature stability limit of phase egg, AlSiO3(OH)

The stability relations of phase egg, A1-SiO3(OH), have been investigated at pressures from 7 to 20 GPa, and temperatures from 900 to 1700 degrees C ina multi-anvil apparatus. At the lower pressures phase egg breaks down according to the univariant reaction, phase egg = stishovite + topaz-OH, which extends from 1100 degrees C at 11 GPa to 1400 degrees C at 13 GPa where it terminates at an invariant point involving corundum. At pressures above the invariant point, the stability of phase egg is limited by the breakdown reaction, phase egg = stishovite + corundum + fluid, which extends from the invariant point to 1700 degrees C at 20 CPa, Stishovite crystallized in the A1(2)O(3)-SiO2-H2O system contains A1(2)O(3) and the amount of A1(2)O(3) increases with increasing temperature. It is inferred that the A1(2)O(3) content is controlled by the charge-balanced substitution of Si4+ by A1(3+) and H+. Aluminum-bearing stishovite coexisting with an H2O-rich fluid may contain a certain amount of water. Therefore, phase egg and stishovite in a subducting slab could transport some H2O into the deep Earth.