QUARTZ SOLUBILITIES IN NACL SOLUTIONS WITH AND WITHOUT WOLLASTONITE AT ELEVATED-TEMPERATURES AND PRESSURES

The effect of NaCl on the solubility of quartz with and without wollastonite in aqueous solutions was investigated at temperatures between 200 and 600-degrees-C and pressures of 0.5, 1.0, 1.75, and 2.0 kbars. At 0.5 kbar and temperatures of 300-degrees-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-degrees-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-degrees-C. Below 400-degrees-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-degrees-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 H2O. Within experimental error, at 1.75 and 2 kbars and between 300 and 600-degrees-C, there is no significant salting-in 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.