Speciation and mixing behavior of silica-saturated aqueous fluid at high temperature and pressure
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Bjorn O. Mysen
Abstract
The effect of dissolved silica on the PVT properties of H2O and structure of silica-saturated aqueous fluids in equilibrium with quartz in the SiO2-H2O system has been determined in situ with the materials at temperature (up to 800 °C) and pressure (up to 1350 MPa) in a constant-volume hydrothermal diamond anvil cell. Pressure was measured with the Raman shift of 13C synthetic diamond and was also calculated from the PVT properties of pure H2O. The two sets of pressures thus obtained differ by ≤50 MPa at T < 500 °C. At higher temperatures (and pressures), the pressure difference increases and reaches about 350 MPa at 800 °C.
The structure of the silica-saturated aqueous fluid was probed with microRaman and microFTIR methods. Coexisting molecular H2O and OH-groups, bonded to Si4+, exist above ~400 °C and ~0.4 GPa with their abundance-ratio, OH/H2O°, positively correlated with temperature. Hydrogen bonding diminishes with temperature and cannot be detected in the silica-saturated aqueous fluid above ~400 °C and ~0.4 GPa. This behavior resembles that of pure H₂O under similar temperature and pressure conditions.
Speciation of dissolved silica in the fluid at 400 °C/760 MPa, and above, comprises Q0 and Q1 species, whereas at lower temperature and pressure only Q0 species were detected. The Q1/Q0 abundance ratio is positively correlated with temperature (and silica content). An excess volume of mixing, VexcessH₂O, derived from a comparison of EOS of pure H2O and those of silicate-saturated H2O, is related to the Q¹/Q⁰ abundance ratio in silicate-saturated fluid, XQ1/XQ⁰. The VexcessH₂O = MH₂O(1/dm-1H₂O/0.88), VexcessH₂O ~ 0 when XQ¹/XQ⁰ approaches 0.
© 2015 by Walter de Gruyter Berlin/Boston
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Articles in the same Issue
- Characterization of cation environments in polycrystalline forsterite by 25Mg MAS, MQMAS, and QCPMG NMR
- Structure of nanocrystalline phyllomanganates produced by freshwater fungi
- First-principles study on variation of lattice parameters of mullite Al4+2xSi2–2xO10–x (x = 0.125, 0.250, 0.375)
- Natrolite may not be a “soda-stone” anymore: Structural study of fully K-, Rb-, and Cs-exchanged natrolite
- Metal retention, mineralogy, and design considerations of a mature permeable reactive barrier (PRB) for acidic mine water drainage in Northumberland, U.K.
- A relationship between d104 value and composition in the calcite-disordered dolomite solid-solution series
- XRD, micro-XANES, EMPA, and SIMS investigation on phlogopite single crystals from Mt. Vulture (Italy)
- Volume behavior of the 10 Å phase at high pressures and temperatures, with implications for H2O content
- Interfacial tension between immiscible liquids in the system K2O-FeO-Fe2O3-Al2O3-SiO2 and implications for the kinetics of silicate melt unmixing
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