Solubilities of corundum, wollastonite and quartz in H 2O–NaCl solutions at 800 °C and 10 kbar: Interaction of simple minerals with brines at high pressure and temperature

Abstract

Solubilities of corundum (Al 2O 3) and wollastonite (CaSiO 3) were measured in H 2O–NaCl solutions at 800 °C and 10 kbar and NaCl concentrations up to halite saturation by weight-loss methods. Additional data on quartz solubility at a single NaCl concentration were obtained as a supplement to previous work. Single crystals of synthetic corundum, natural wollastonite or natural quartz were equilibrated with H 2O and NaCl at pressure ( P) and temperature ( T) in a piston-cylinder apparatus with NaCl pressure medium and graphite heater sleeves. The three minerals show fundamentally different dissolution behavior. Corundum solubility undergoes large enhancement with NaCl concentration, rising rapidly from Al 2O 3 molality ( m Al 2 O 3 ) of 0.0013(1) (1 σ error) in pure H 2O and then leveling off to a maximum of ∼0.015 at halite saturation ( X NaCl ≈ 0.58, where X is mole fraction). Solubility enhancement relative to that in pure H 2O, X Al 2 O 3 / X Al 2 O 3 ° , passes through a maximum at X NaCl ≈ 0.15 and then declines towards halite saturation. Quenched fluids have neutral pH at 25 °C. Wollastonite has low solubility in pure H 2O at this P and T ( m CaSiO 3 = 0.0167 ( 6 ) ) . It undergoes great enhancement, with a maximum solubility relative to that in H 2O at X NaCl ≈ 0.33, and solubility >0.5 molal at halite saturation. Solute silica is 2.5 times higher than at quartz saturation in the system H 2O–NaCl–SiO 2, and quenched fluids are very basic (pH 11). Quartz shows monotonically decreasing solubility from m SiO 2 = 1.248 in pure H 2O to 0.202 at halite saturation. Quenched fluids are pH neutral. A simple ideal-mixing model for quartz-saturated solutions that requires as input only the solubility and speciation of silica in pure H 2O reproduces the data and indicates that hydrogen bonding of molecular H 2O to dissolved silica species is thermodynamically negligible. The maxima in X Al 2 O 3 / X Al 2 O 3 ° for corundum and wollastonite indicate that the solute products include hydrates and Na + and/or Cl − species produced by molar ratios of reactant H 2O to NaCl of 6:1 and 2:1, respectively. Our results imply that quite simple mechanisms may exist in the dissolution of common rock-forming minerals in saline fluids at high P and T and allow assessment of the interaction of simple, congruently soluble rock-forming minerals with brines associated with deep-crustal metamorphism.