Recent systematic studies of mineral solubilities in water to high pressures up to 50 kbar call for a suitable thermodynamic formalism to allow realistic fitting of the experimental data and the establishment of an internally consistent data base. The very extensive low-pressure (< 5 kbar) experimental data set on the solubility of SiO2 in H2O has in the last few years been extended to 20 kbar and 1300°C, providing an excellent experimental basis for testing new approaches. In addition, solubility experiments with different SiO2-buffering phase assemblages and in situ determinations of Raman spectra for H2O-SiO2 fluids have provided both qualitative and quantitative constraints on the stoichiometry and quantities of dissolved silica species. We propose a thermodynamic formalism for modeling both absolute silica solubility and speciation of dissolved silica using a combination of the chain reaction approach and a new Gibbs free energy equation of water based on a homogeneous reaction formalism. For a given SiO2-buffer (e.g., quartz) and the coexisting H2O-SiO2 fluid both solubility and speciation of silica can be described by the following two reactions:

  • - monomer-forming standard reaction:  
    \[\mathrm{SiO_{2(s)}\ {+}\ 2(H_{2}O)L\ {=}\ (SiO_{2}){\bullet}(H_{2}O)_{2}}\]
  • - polymer-forming chain reaction:  
    \[\mathrm{(SiO_{2})_{n{-}1}{\bullet}(H_{2}O)_{n}\ {+}\ (SiO_{2}){\bullet}(H_{2}O)_{2}\ {=}\ (SiO_{2})_{n}{\bullet}(H_{2}O)_{n{+}1}\ {+}\ (H_{2}O)_{L},}\]
where 2 ≤ n ≤ ∞, and (H2O)L stands for “liquid-like” (associated, clustered) water molecules in the aqueous fluid. We show that reactions (A) and (B) lead to the simplified relationships ΔG°(mono)r,P,T = ΔH°(mono),rTΔS°(mono),r + ΔCp°(mono),r [T − 298.15 - Tln(T/298.15)] + ΔV°(mono),r(P − 1), and ΔG°(poly),r,P,T = ΔH°(poly),rTΔS(poly),r + ΔV°(poly),r (P − 1) (where the ΔG°r,P,T, are the standard molar Gibbs free energy changes in reactions (A) and (B) as a function of pressure P and temperature T; the ΔH°r, ΔS°r, ΔCp°r, and ΔV°r are standard molar enthalpy, entropy, isobaric heat capacity and volume changes, respectively, in reactions (A) and (B) at reference temperature To = 298.15 K and pressure To = 1 bar) that provide excellent descriptions of the available H2O-SiO2 data set in terms of both SiO2 solubility and silica speciation. Discrepancies between directly determined solubility data and data obtained from in situ Raman spectra are ascribed to (i) possible experimental problems of equilibration and (ii) inherent difficulties of interpreting Raman spectra of dilute H2O-SiO2 solutions. In agreement with recent findings, our model indicates that dissolved silica in quartz-buffered aqueous solutions is considerably polymerized, exceeding 20–25 % at all temperatures above 400°C.

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