Quartz solubilities in NaCl solutions with and without wollastonite at elevated temperatures and pressures

Abstract

The effect of NaCl on the solubility of quartz with and without wollastonite in aqueous solutions was investigated at temperatures between 200 and 600°C and pressures of 0.5, 1.0, 1.75, and 2.0 kbars. At 0.5 kbar and temperatures of 300°C and greater, quartz solubility increases with increasing NaCl concentration up to 1.86 molal; that is, it salts in. The extent of salting-in increases with increasing temperature. At 500°C, quartz solubility in 0.83 m NaCl solution is about 0.2 log units higher than in pure water. At 1 kbar, quartz does not start to salt-in until the temperature is above 400°C. Below 400°C at 1 kbar, quartz tends to salt-out in NaCl solutions up to 4.0 molal and the extent of salting-out increases with decreasing temperature. Above 400°C at 1 kbar, the degree of salting-in also increases with temperature with quartz solubility in 0.83 m NaCl solution being about 0.15 log units greater than in pure H 2O. Within experimental error, at 1.75 and 2 kbars and between 300 and 600°C, there is no significant saltingin or salting-out effect on quartz solubility in NaCl solutions up to 0.83 molal. Our solubility data combined with experimental measurements of previous investigators were interpreted with a Setchenow-type equation which accounts for both solvent effects and short-range interactions between charged solute species and aqueous silica. The calculation indicates that Born-type dipole-dipole interactions between NaCl solutes and the aqueous silica monomer are not strong enough to cause the quartz salting-in in NaCl solutions that is observed at supercritical conditions.