An experimental and computational study of sodium-aluminum complexing in crustal fluids

Abstract

The stoichiometry and stability constants of Na-aluminate (NaAl(OH) 4 0) ion pair were determined from both boehmite solubility and potentiometric measurements. Boehmite solubility was measured at temperatures from 125 to 350 °C at pressures corresponding to equilibrium between liquid and vapor ( P SAT) in the system Na-NH 4-Cl-OH. Potentiometric measurements were performed at temperatures from 75 to 200 °C in Na-Al-Cl-OH solutions, using a Na-selective glass electrode. NaAl(OH) 4 0 stability constants derived independently from these two methods are in excellent agreement and increase markedly with increasing temperature from 0.8 at 25 °C to 209 at 350 °C. Experimental results obtained in this study were combined with corresponding data reported by Castet et al. (1993) for boehmite solubility to generate, within the framework of the revised HKF model, the standard partial molal properties and equations of state parameters for Al(OH) 4 −, Al(OH) 3 0, and NaAl(OH) 4 0. The solubilities of gibbsite, boehmite, and corundum calculated in Na-rich solutions using the thermodynamic data for Al aqueous species generated in this study are in close agreement with their experimental counterparts. Thermodynamic calculations carried out at temperatures up to 800 °C and pressures up to 5 kbar indicate that the formation of NaAl(OH) 4 0 ion pairs can markedly increase the solubility of Al-bearing minerals and thus, Al mobility, both in sedimentary basin and metamorphic fluids at pHs > 4.