Phase transformations in natural serpentine and chlorite have been studied at 25-50 GPa and 900-1280 degrees C by in-situ X-ray diffraction measurements using the multianvil apparatus with sintered diamond anvils. Antigorite was found to transform to a phase assemblage including phase H, at pressures above about 35-40 GPa corresponding to those in the upper part of the lower mantle. The zero pressure bulk modulus K-0 of phase H was determined to be 160 GPa, assuming its pressure derivative K' = 4. The thermal stability of phase H is significantly enhanced by a solid solution with a delta-AlOOH component, suggesting that aluminous phase H in cold slabs would deliver a certain amount of water into the deepest part of the lower mantle. (C) 2015 Elsevier B.V. All rights reserved.