Quartz solubility in sodium carbonate solutions at high pressure and temperature

Abstract

We present new experimental data on the synthetic quartz (Qtz) solubility in the Na2CO3 aqueous solutions ranging in salt concentration from 0.3–3.5 m (mole/kg H2O). The data have been collected in the temperature (T) range 500–700 °C at pressure (P) 4 kbar and 600 °C/5 kbar in an internally heated pressure vessel, with phase assemblage bracketing as a solubility monitor in quench experiments: first disappearance of crystalline Qtz in a succession of runs with constant Na2CO3 concentration but decreasing the Qtz/solution weight ratio was taken for the solubility value. It was possible to “bracket” the solubility within about 1 wt% by carefully observing the presence or absence of crystalline Qtz in the run products. Qtz solubility varies systematically with P, T, and particularly, with the Na2CO3 molality, increasing from 0.7 m for the 0.3 m Na2CO3 to 4 m for the 3.5 m Na2CO3 (at 600 °C/4 kbar). In all cases the SiO2 molality in the solutions exceeds that of Na2CO3, which suggests the presence of polymerized silica species in the solutions. Quantum chemical computations showed the dimer H5Si2O7− as the most stable species, which corresponds to the dehydrated species Si2O4OH− in the framework of the revised Helgeson–Kirkham–Flowers equation of state (HKF). A complete set of thermodynamic quantities for Si2O4OH−, consistent with the revised HKF and the DEW model (Sverjensky et al., 2014) has been retrieved from the experiments. Calculations with the obtained set of thermodynamic parameters reproduce very well the Qtz solubility experiments performed in a simpler H2O-NaOH fluid at 600 °C/4 kbar. Model calculations of silica and Ca solubility in the 15 wt% (3 m) NaCl aqueous solution interacting with a siliceous carbonate rock (Qtz + Calcite saturated) along a 13°/km geotherm, which take into account Si2O4OH−, predict much higher bulk Si and Ca contents in the fluid phase than in equilibrium with the corresponding individual minerals for P above 4–5 kbar.