Jonathan Hunt (UCLA), Jan. 31, 2007 Geocheminar

Predicting Raman Spectra of Polymerized Aqueous Silica and Alumina Species in Solution From First Principles.

Abstract: Dissolved silica and alumina play an important role in lithospheric fluid chemistry. Silica concentrations in aqueous fluids vary over the range of crustal temperatures and pressures enough to allow for significant mass transport of silica via fluid-rock interaction. The polymerization of silica, and the possible incorporation of alumina into the polymer structure, could afford crystal-like or melt-like sites to otherwise insoluble elements such as titanium, leading to enhanced mobility. Raman spectroscopy in a hydrothermal diamond anvil cell has been used to study silica polymerization at elevated pressure and temperature, but Raman spectra of expected solutes are not fully understood. We calculate Raman spectra of silica dimers and silica-alumina dimers from first principles using hybrid density functional theory (B3LYP). These spectra take into account the variation in bridging angle (Si-O-Si and Si-O-Al angles) that the dimers will have at a given temperature by calculating a potential energy surface of the dimer as the bridging angle varies, and using a Boltzmann distribution at that temperature to determine the relative population of each geometry.