High-temperature evolution of OH bands in synthetic hydrous forsterite

In situ high-temperature infrared spectra of hydrous forsterite were conducted from room temperature up to 1253K. The crystal was divided into oriented and unoriented samples. The main peaks have different behavior on temperature: (1)3612 and 3477 cm^-1 is insensitive to the temperature. (2) 3578 and 3566 cm^-1 show similar negative shift. (3) 3551 cm^-1 shift largely to lower wavenumber. Combined with the crystal structure, we explore the possible mechanism of incorporated hydrogen. A comparison of the calculated bond distances shows that the high frequency(>3450 cm^-1) is due to possibly both in the M and Si sites. The band 3612 cm^-1 is related to the O3-O3 (2.99Å) in the M2 octahedron. And the 3578 cm^-1 is attributed to the O1-O2 in the M1(2.85Å). The peak of 3551 cm^-1 is corresponding with one of the protons on O2 pointing away from the tetrahedral center and located in an adjacent interstitial site. In additon, 3477 cm^-1 might be connected with the proton on O1 pointing to the O2 in the Si tetrahedron. Besides, 3566 cm^-1 might be accounted by the protonation of O3-O3 in the Si tetrahedron. The high OH-stretching frequencies in forsterite under upper mantle temperature imply a lower H/D fractionation compared to the wadsleyite.