Quartz solubility in salt-bearing solutions at pressures to 1 GPa and temperatures to 900°C

Katy Evans*

*Corresponding author for this work

    Research output: Contribution to journalArticlepeer-review

    17 Citations (Scopus)

    Abstract

    An expression that describes the solubility of quartz in alkali-chloride-bearing solutions is derived and applied to literature data on quartz solubility. The model is based on a physically realistic conceptual model that involves salt dissociation as a function of pressure, temperature and salt concentration, and the presence of hydrated monomer, dimers and hydrated alkali-silica species. A simplified version of the model that neglects dimerization and water in the alkali-silica species provides excellent fits to the experimental data with two calibration parameters at each pressure and temperature. The calibration parameters represent the relative stabilities of the alkali-silica and hydrated-silica solute complexes, and the degree of salt dissociation. The success of the model provides support for the existence of an alkali-silica species in salt solutions at low fluid densities. The model can be interpolated successfully for NaCl-bearing solutions between 0.1 and 1 GPa and from 400 to 900°C, with standard deviations of the predicted from the experimental data of less than 10% relative for most conditions. Quartz solubility within this range is described by X(SiO2/X(SiO2,ref) = (X(H2 Oapp)/1 + αX(NaClapp))3 +c (αX(NaClapp)/1 + αX(NaClapp)) where Xi,app refers to the apparent mole fraction of i, which is the mole fraction calculated if NaCl is assumed to be totally associated, and X(SiO2,ref) is the mole fraction of SiO2 in the salt-free solution c = 37.17 - 0.00141T - 24.45ρ α = 1 Erf[dXNacl,app], where d is a calibration parameter that depends on pressure and density as shown below. d = 2.75 - 2.733Ρ + 2.38ρ. T is temperature in Kelvin, Ρ is pressure in GPa and ρ is the density in g cm-3. Extrapolation of the expression beyond the calibration range is not recommended. Quartz solubility is additive for mixtures, so silica concentrations in solutions more complex than those covered by the literature data can also be predicted. Fits to the data also enabled regression of thermodynamic data for the proposed SiO2.(NaCl)0.5 species. H298 is -1040.2 ± 7.6 kJ mol-1, S298 is 0.169 ± 0.01 kJ mol-1 K-1 and V298 is 31 ± 4 kJ GPa-1 mol-1. The model is semi-empirical because the fit parameters are correlated with one of the set parameters, and the use of a simplified mathematical description of salt dissociation. Nevertheless, the model provides an improved method for the calculation of quartz solubility in geological solutions.

    Original languageEnglish
    Pages (from-to)451-467
    Number of pages17
    JournalGeofluids
    Volume7
    Issue number4
    DOIs
    Publication statusPublished - Nov 2007

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