Hydrogen Dissolution Mechanisms in Bridgmanite by First-Principles Calculations and Infrared Spectroscopy

Understanding hydrogen dissolution mechanisms in bridgmanite (Bgm), the most abundant mineral in the lower mantle, is essential for understanding water storage and rheological and transport properties in the region. However, interpretations of O-H bands in Fourier transform infrared spectroscopy (FTIR) spectra of Bgm crystals remain uncertain. We conducted density functional theory (DFT) calculations on vibrational characteristics of O-H dipoles and performed polarized FTIR measurements to address this issue. DFT calculations for four substitution models—Mg vacancies, Si vacancies, Al³⁺ + H⁺ substitution for Si⁴⁺, and Al substitution with Mg vacancies—reveal distinct O-H bands with different polarizations. Deconvolution of polarized FTIR spectra on Mg_0.88Fe²⁺_0.035Fe³⁺_0.065Al_0.14Si_0.90O₃ and Mg_0.95Fe²⁺_0.033Fe³⁺_0.027Al_0.04Si_0.96O₃ crystals shows five major O-H bands with distinct polarizations along principal crystallographic axes. These experimental and calculated results attribute O-H bands centered at 3,463–3,480, 2,913–2,924, and 2,452–2,470 cm⁻¹ to Mg vacancies, Si vacancies, and Al³⁺ + H⁺ substitution for Si⁴⁺, respectively. The total absorbance coefficient of bridgmanite was calculated to be 82,702(6,217) L/mol/cm². Mg and Si vacancies account for 43%–74% of the total water content, making them dominant hydrogen dissolution mechanisms in Bgm. The band frequencies for the Mg and Si vacancies in Bgm are drastically different from those in olivine and ringwoodite, corresponding to the significant changes in O-H bond strengths and in the Si and Mg coordination environments from upper-mantle to lower-mantle minerals. These results highlight the need to incorporate hydrogen dissolution mechanisms in Bgm for understanding electrical conductivity and rheology of the lower mantle.