Theoretical Raman spectrum and anharmonicity of tetrahedral OH defects in hydrous forsterite

Substitutional mechanisms involving hydrogen incorporation at vacant tetrahedral sites play a major role in water incorporation in olivine. Infrared (IR) absorption spectra of hydrous forsterite samples usually display a cluster of narrow and weakly anharmonic OH stretching bands at wavenumbers above 3500 cm⁻¹. A broader absorption band displaying pronounced temperature-dependent shift and broadening is commonly superimposed to this diagnostic spectrum and was tentatively assigned to interstitial OH groups. A less frequently observed band with similar temperature-dependent characteristics is related to a coupled incorporation of hydrogen and boron at the tetrahedral site. Here, we re-examine these interpretations by computing the theoretical Raman spectrum and investigating the local vibrational properties of OH groups at the tetrahedral site of forsterite. The present results show that the two anharmonic bands are both to be ascribed to the protonated O2 site in the clumped $(B,H)Six$ and the $(4H)Six$ defects. The peculiar orientation of the corresponding OH groups does not allow H-bond sharing and leads to efficient vibrational phase relaxation of the stretching mode through a hindered rotational mode coupled to the vibrational density of states of the host. The occurrence of interstitial OH groups previously proposed to interpret specific anharmonic bands of the forsterite IR spectrum is highly challenged by this new explanation. These results confirm that, at high pressure and high temperature, hydrogen incorporation in forsterite is essentially dominated by $(4H)Six$ defects.