Water at high temperatures in a microcrystalline silica (chalcedony) by in-situ infrared spectroscopy: physicochemical states and dehydration behavior

Abstract

In order to study physicochemical states of water in a chalcedony, thin sections were heated from room temperature (RT) to 400 °C at 50 °C intervals under an infrared (IR) microscope. A sharp IR absorption band due to hydroxyls (Si-OH) shifts linearly from 3585 cm⁻¹ at RT to 3599 cm⁻¹ at 400 °C (1.8 cm⁻¹/50 °C). A broad band due to H₂O shifts also linearly from 3425 cm⁻¹ at RT to 3535 cm⁻¹ at 400 °C (15 cm⁻¹/50 °C). These reversible band shifts, without dehydration, of hydroxyl and H₂O can be explained by the increasing hydrogen bond distance and the decreasing coordinating numbers of water molecules, respectively.
     The in-situ IR spectra of the chalcedony thin sections during the isothermal heating at 550 °C showed a decrease of the total water band area. The RT spectra quenched from 550 °C indicated the major decrease of 3585 cm⁻¹ absorbance due to Si-OH with the minor decrease of the 3425 cm⁻¹ absorbance due to H₂O. These results suggest the dominant dehydration of Si-OH species from the microcrystalline quartz and the certain stability of liquid-like H₂O against dehydration in pores and grain boundaries.