The hydrogen-absorbing ability of a mantle mineral in its struc-ture determines the capacity of the water reservoirs hosted by the mineral.Water reservoirs at the base of Earth\'s mantle directly influence the fate of water brought down by slab subduction and the seismic heterogeneity such as ultralow-velocity zones (ULVZs)at the core-mantle boundary.Pyrite-FeO2Hx (0 ≤ x ≤ 1) presents a possibility of such reservoirs in the deep mantle [1-3].Ever since the discovery of this mineral phase,however,its chemistry at the lower mantle conditions has been debated [1,2].We conducted kinetics experiments of pyrite-FeO2Hx dehydrogenation at 110 GPa/2100 K,110 GPa/2300 K,and 120 GPa/2300 K and P-V-T equa-tion of state analysis using in situ synchrotron X-ray diffraction.We found that x approaches 0.80,0.75,and 0.79,respectively,at the above conditions.The collective P-V-T data yield K0 =241(13)GPa,K\'=4.2(4),dK/dT =-0.028(1) GPa/K,α0 =4.32(13) × 10-5 K-1,and α1 =0.31(10) × 10-8 K-2 for the composition of x =0.75± 0.04.Our first-principles calculations indicate that FeO2H0.75 with a slightly distorted pyrite structure is stable at 100 GPa.These results indicate that this mineral is likely present in the deep man-tle with rather a partially dehydrogenated composition than FeO2 or FeOOH.The results also clarify the difference between the ULVZs originated from pyrite-FeO2Hx and those from partial melting in terms of shear and compressional wave seismic velocity reduction ratio δlnVs/δlnVp.