HR: 1340h
AN: V23A-1224    [Abstracts]
TI: Speciation of Aqueous Silica Using Raman Spectroscopy and First Principles Calculations
AU: * Hunt, J D
EM: jhunt@ess.ucla.edu
AF: UCLA, Department of Earth and Space Sciences, Los Angeles, CA 90095-1567, United States
AU: Kavner, A
EM: akavner@igpp.ucla.edu
AF: UCLA, Department of Earth and Space Sciences, Los Angeles, CA 90095-1567, United States
AU: Schauble, E A
EM: schauble@ucla.edu
AF: UCLA, Department of Earth and Space Sciences, Los Angeles, CA 90095-1567, United States
AU: Manning, C E
EM: manning@ess.ucla.edu
AF: UCLA, Department of Earth and Space Sciences, Los Angeles, CA 90095-1567, United States
AB: This study presents Raman spectra of high-pH silica solutions taken at ambient conditions with varying silica concentrations. Dissolved silica plays an important role in lithospheric fluid chemistry. Over the range of crustal temperatures and pressures, silica concentrations in quartz-saturated aqueous fluids vary sufficiently to allow for significant mass transport of silica via fluid-rock interaction. Polymerization of aqueous silica plays an important role in elevating dissolved SiO₂ concentrations, and could afford silicate-melt-like or crystal-like sites into which otherwise insoluble elements such as titanium could substitute, leading to enhanced mobility for those elements. It would therefore be useful to understand what the independent effects of concentration, composition (pH and incorporation of other elements), pressure, and temperature are on silica polymerization. Raman spectra of silica solutions have previously been obtained [Ref. 1, 2], but those studies did not vary aqueous silica concentration at a fixed P and T. As a foundation for future studies of polymerization of aqueous silica at high P and T, we collected Raman spectra of high-pH silica solutions at ambient conditions with varying silica concentrations. Total silica concentration was varied while keeping pH, P, and T constant. First principles calculations of explicitly solvated silica monomers and dimers are used to interpret the experimental spectra. The spectra show that as total silica concentration increases, the ratio of silica in dimers to silica in monomers increases as well, shown by the ratio of the dimer peak height at 600 cm^-1 to the monomer peak height at 780 cm^-1. This has been thermodynamically predicted [Ref. 3], and has been indirectly observed in high P-T solubility measurements [Ref. 4], but ours is the first experiment to directly observe that this is the case while keeping temperature and pressure constant. These results are a promising first step towards hydrothermal diamond anvil cell experiments aimed at exploring the effects of temperature, pressure, and concentration on silica polymerization. 1. Zotov and Keppler (2002). Chem. Geol., 184: 71-82. 2. Dutta and Shieh (1985). Appl. Spectrosc., 39: 343-346. 3. Gerya, et al. (2005). Eur. J. Mineral., 17: 269-283. 4. Newton and Manning (2003). Contrib. Min. Pet., 146: 135-143.
DE: 1000 GEOCHEMISTRY
DE: 3610 Geochemical modeling (1009, 8410)
DE: 3616 Hydrothermal systems (0450, 1034, 3017, 4832, 8135, 8424)
DE: 3630 Experimental mineralogy and petrology
DE: 3934 Optical, infrared, and Raman spectroscopy
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting