In order to study physicochemical states of water in a chalcedony, thin sections were heated from room temperature (RT) to 400 degrees C at 50 degrees 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 degrees C (1.8 cm(-1)/50 degrees C). A broad band due to H2O shifts also linearly from 3425 cm(-1) at RT to 3535 cm(-1) at 400 degrees C (15 cm(-1)/50 degrees 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 degrees C showed a decrease of the total water band area. The RT spectra quenched from 550 degrees 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.