• journal_title：American Mineralogist
• Contributor：Bjorn O. Mysen
• Publisher：Mineralogical Society of America
• Date：2010-
• Format：text/html
• Language：en
• Identifier：10.2138/am.2010.3539
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：95
• issue：11-12
• firstpage：1807
• section：Articles

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 Q^o and Q¹ species, whereas at lower temperature and pressure only Q^o species were detected. The Q¹/Q^o abundance ratio is positively correlated with temperature (and silica content). An excess volume of mixing, V_H2O^excess, derived from a comparison of EOS of pure H₂O and those of silicate-saturated H₂O, is related to the Q¹/Q^o abundance ratio in silicate-saturated fluid, X_Q¹/X_Q^o. The V_H2O^excess = M_H2O(1/d^m−1_H2O/0.88), V_H2O^excess ~ 0 when X_Q¹/X_Q^o approaches 0.