Water molecules in beryl and cordierite: high-temperature vibrational behavior, dehydration, and coordination to cations

Abstract

Natural samples of typical cyclosilicates beryl and cordierite include water and carbon dioxide molecules in channels formed by the open cavities. Water molecules in the channels have two forms that are distinguished by whether they coordinate to extra-framework cations (type II) or not (type I). We measured polarized infrared (IR) spectra for thin sections of the (100) plane of beryl or the (100) and (010) planes (cb and ca planes) of cordierite under various temperature conditions. The spectral features of major bands clearly showed the distinguishable behavior of types I and II water molecules under high temperature as follows. Over the temperature range from room temperature to 800°C where rapid dehydration did not occur, the decrease in band heights for type II water molecules were smaller than those for type I, and band shifts were more predominant for type II water molecules. The decrease in band heights and band shifts of type I/II bands differed also for beryl and cordierite, which arises from the different ways in which water molecules are fixed in the channels. Dehydration was enhanced at 850°C. The IR spectra at room temperature quenched from 850°C both for beryl and cordierite showed that the vibrational bands related to type II water molecules were stable after those related to type I water molecules disappeared. In addition, frequency changes of type II bands were observed, possibly because of changes of coordination states of type II water molecules to extra-framework cations in the channels.