Polymerization of aqueous silica in H2o-K2O solutions at 25-200 °C and 1 bar to 20 kbar

Understanding the polymerization of aqueous silica is important for modeling fluid-rock interactions at high pressure and temperature. However, it remains unclear how changes in SiO2 concentration, alkalinity, pressure, and temperature affect silica polymerization and depolymerization. We studied silica speciation in alkaline K2O-SiO2-H2O solutions at ambient conditions (0.05, 0.5 and 5 molal Si, 0.5-0.8 K2O/SiO2), as well as at elevated pressure and temperature in a diamond anvil cell (5 molal Si, K2O/SiO2=0.7). The controls on polymerization were assessed by comparing Raman spectroscopic results to ab initio calculations that focused on four neutral species: the monomer, the dimer, and the linear and ring trimers. In addition to a similar to 785 cm(-1) band associated with Si-OH symmetric stretching in the monomer, the calculations show that Si-O-Si stretching in the dimer and trimers yields peaks at similar to 600 cm(-1). A broad 600 cm(-1) feature has been observed in previous studies at high pressure and temperature, and the broadness has been attributed to a soft Si-O-Si angle in the dimer and temperature; however, our calculations show that this is not alone sufficient to explain this feature, and that other species likely contribute. The breathing mode of the ring trimer yields an additional feature at similar to 500 cm(-1), which may be representative of contributions from highly polymerized species to the low-frequency limb of the broad 600 cm(-1) feature. Individual Si-OH stretching and bending in all species gave features at similar to 825-875 cm(-1) and similar to 950-1050 cm(-1) These bands overlap significantly with Si-O- stretching modes of deprotonated species, which are more abundant in the alkaline solutions studied experimentally. Measured Raman spectra at ambient conditions display broad peaks associated with monomers (735-800 cm(-1)), oligomers (475-650 cm(-1)) and deprotonated species (850-1125 cm(-1)). Peak areas attributed to oligomers and deprotonated species increase linearly with total Si concentration, with a small dependence on the K2O/SiO2 ratio, indicating that Raman intensities are proportional to species concentrations. The Raman peak area of the monomer feature does not scale linearly with total SiO2 concentration, however, and depends strongly on the K2O/SiO2 ratio. In broad agreement with NMR results, the Raman spectra show that increasing silica concentration promotes silica polymerization, and that increasing the K2O/SiO2 ratio or temperature depolymerizes aqueous silica. The Raman spectra show that increasing pressure also causes aqueous silica to depolymerize. The results provide a foundation for characterizing aqueous silica polymerization at crustal and upper mantle conditions. (C) 2011 Elsevier B.V. All rights reserved.