We report stability field of a new hydrous phase Δ-AlOOH, which is a high pressure polymorph of diaspore. This phase is stable at pressures from 17 GPa to at least up 25 GPa at around 1000 to 1200 ¿C, which are likely conditions expected in the slabs descending into the lower mantle. This phase contains significant amounts of octahedrally coordinated Mg and Si in solid solution. The oxygen arrangement of Δ-AlOOH is similar to that of the post-stishovite phase, CaCl2 type SiO2, which exists in the deep lower mantle. Δ-AlOOH may play an important role as a water reservoir in subducting slabs. ¿ 2001 American Geophysical Union