Publication Date:
2015-08-27
Description:
Experimental studies reveal complex dissolution behavior of quartz in aqueous NaCl solutions at high temperatures and pressures, involving variation from salting in to salting out that changes with temperature, pressure, and salt concentration. The behavior is not explainable by traditional electrostatic theory. An alternative hypothesis appeals to complexing of SiO 2 with NaCl and can explain the observations. However, the hypothesis of complexing, as previously applied, is inadequate in several respects: it neglects polymerization of solute silica, regards the SiO 2 -NaCl hybrid complex(es) as anhydrous, which seems unlikely, and invokes an incorrect stoichiometry of the hydrated silica monomer, now known to be Si(OH) 4 •2H 2 O. These neglected features can be incorporated into the complexing model in a revised formulation based on a simple thermodynamic analysis using existing quartz solubility data. The analysis leads to a quasi-ideal solution model with silica monomers, dimers and two distinct hydrous SiO 2 -NaCl hybrid complexes with overall NaCl:H 2 O = 1:6, one Na-bearing and one Cl-bearing. Their (equal) molar concentrations ( X hc ) are governed by a pressure- and temperature-dependent equilibrium constant, K hc =X 2 hc/( a N aC l a 6 H 2 O), where a N aC l and a H 2 0 are the respective activities of the solvent components. The stability of the hybrid complexes (i.e., their concentration) is very sensitive to H 2 O activity. The entire set of experimental quartz-solubility data at 700 °C, 1–15 kbar, is reproduced with high fidelity by the expression log k hc =-4.585+0.2691 P -2.023×10 -3 P 2 ( P is pressure in kbar), including the transition from low-pressure salting in to high pressure salting out. The results indicate that hybrid SiO 2 -NaCl complexes are the main hosts for dissolved silica at NaCl concentrations greater than 6 wt%, which are likely common in crustal fluids. At higher temperatures, approaching the critical end point in the system SiO 2 -H 2 O, the model becomes progressively inaccurate, probably because polymers higher than the dimer become significant as SiO 2 concentration increases. This article is protected by copyright. All rights reserved.
Print ISSN:
1468-8115
Electronic ISSN:
1468-8123
Topics:
Geosciences
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