In-situ infrared spectra of OH in rutile up to 1000 °C

Abstract

The infrared spectra of hydrous, oriented single crystals of synthetic Al-doped and pure rutile were measured from room temperature to 1000 °C at 1 bar using a heating stage. At room temperature, the spectra show only one band at ~3278 cm⁻¹ in pure rutile, and two additional bands at ~3303 and 3323 cm⁻¹ in Al-doped rutile. The main band associated with Al at ~3323 cm⁻¹ loses intensity as temperature increases and nearly disappears already at 300 °C. This suggests that the corresponding defect only forms during cooling and, therefore, water speciation in rutile at room temperature is not representative of subduction zone or upper mantle conditions. All OH bands shift to lower wavenumbers at elevated temperatures with dν/dT = −0.0831 cm⁻¹ K⁻¹ for the main band at 3278 cm⁻¹. This strong low-frequency shift is unexpected for an OH bond involved in hydrogen bonding, indicating decreasing O–O distance for the OH band. Together with the extreme broadening of the band at high temperature, it suggests some weakening of the OH bond strength and a rapid exchange of protons, consistent with rapid hydrogen diffusion in rutile.