Molecular H2O-Silicate Interactions

Water plays a fundamental role in natural dissolution, precipitation and sorption processes. However, relatively little is understood about the chemical and physical interactions that take place between H2O and crystalline silicates at the molecular level. An understanding of the interactions between bulk H2O and a mineral surface requires a description of the bonding, especially the role of hydrogen bonding. In order to address this issue, we are studying the behavior of H2O molecules that are incorporated in the inner surfaces of different silicates such as beryl (Al2Be3Si6O18.H2O), cordierite (Mg2Al4Si5O18.H2O) and bikitaite (Li2Al2Si4O12.2H2O) using polarized single-crystal IR and Raman spectroscopy. In beryl and cordierite two general classes of H2O are present in small structural cavities located along infinite channels parallel to [-001]. The molecules are isolated from each other and can be described as the ?zero-dimensional case?. The H2O internal stretching and bending vibrations can be measured for both classes. Class I involves a single, virtually free H2O molecule having very little interaction with the silicate framework. It is dynamically disordered and its internal vibrations are similar in energy to those of H2O vapor, with an O-H bond energy of about 45 kJ/mole. External librational modes are located around 200 cm-1 and translations around 10 cm-1. In contrast, class II H2O bonds to an alkali cation located in the six-membered tetrahedral ring through the lone-pair of the O atom. At about 5 K the H-bonding with the framework is roughly 1 kJ/mole. In the zeolite bikitaite, the H2O molecules occur in infinite channel ways parallel to [010] and they build a hydrogen-bonded H2O chain that has been termed ?one-dimensional ice?. The molecules in the chains are ordered, whereby one H atom per molecule is not bonded and the second is hydrogen-bonded to a neighboring H2O molecule. The hydrogen-bonded O-H stretching bands in the Raman spectra show little line broadening, which is untypical for many hydrogen-bonded systems. With increasing temperature, hydrogen bonding between the H2O molecules weakens and H2O ultimately diffuses out of the channels by 620 K.