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). Abroad 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 absorbaance 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.