High temperature infrared spectra of hydrous microcrystalline quartz

A series of in-situ high temperature infrared (IR) measurements of water in an agate sample and in a milky quartz has been conducted in order to understand the nature of water in silica at high temperatures (50–700?°C) and the dehydration behavior. IR absorption bands of water molecules trapped in the milky quartz showed a systematic decrease in intensities and a shift from 3425?cm^?1 at 50?°C toward 3590?cm^?1 at 700?°C without any loss of water. This indicates a change in IR absorption coefficients corresponding to different polymeric states of water at different temperatures. The broad 3430?cm^?1 band in the agate sample also showed a systematic decrease in IR intensity and a band shift toward higher frequency with increasing temperature (～700?°C). This indicates that the agate sample also contains fluid inclusion-like water. For this agate sample, a dehydration of loosely hydrogen-bonded molecular water occurred at lower temperatures (<200?°C). At higher temperatures (>400?°C), sharp bands around 3660 and 3725?cm^?1 (3740?cm^?1 at 50?°C) due to surface silanols, appeared. This indicates dehydration of H₂O molecules that are hydrogen bonded to surface silanols. SiOH species in the agate are divided into three groups, namely SiOH group located at structural defects, surface silanols hydrogen bonded to each other and free surface silanols. Former two dehydrate below 700?°C and the dehydration rate of the SiOH at structural defects is faster than the other. IR spectra show that SiOH species decrease continuously even after the dehydration of most of H₂O molecules. All these results provide realistic bases for the change in physicochemical states of different OH species in silica at high temperatures.