Solid-Solute Phase Equilibria in Aqueous Solutions X. Solubility Constant and Stability of Norsethite

Abstract

The solubility constant of norsethite, BaMg(CO 3 ) 2 , was determined for the ®rst time. Also, a new method was developed which permits the precipitation of pure norsethite from homogeneous solution. The solubility measurements were performed at 25 C in 0.20 mol Á dm À3 sodium perchlorate media of constant ionic strength. It was shown that norsethite dissolves congruently in this solution. The pH variation method was used to obtain a stoichiometric solubility constant which was extrapolated to zero ionic strength by use of the Davies equation.The resulting thermodynamic solubility constant was incorporated in a comprehensive computer model which permits solubility calculations in complex systems. Thus, the conditions for norsethite formation in the environment and in the laboratory can be predicted. Furthermore, the stability of norsethite with respect to witherite (BaCO 3 ) and the various magnesium carbonate phases was calculated.

Solubilities of metal carbonates

“…Although norsethite is clearly metastable with respect to decomposition into the endmembers, again congruent dissolution was observed in solubility studies [25]. When seed crystals of witherite are added to a solution already saturated with norsethite, the three phase equilibrium indicated by the quasi-peritectic line of Fig.…”

Kinetic oxygen isotope fractionation upon acid liberation of CO2 from artificial and natural norsethite (BaMg(CO3)2), a mineral analogue of dolomite

“…The mineralogy and purity of the slowly formed norsethite were confirmed by infrared spectroscopy as well as powder X‐ray diffraction analysis to be pure BaMg(CO 3 ) 2 32 . Sample Ba‐G was made by the addition of a 0.25 M NaHCO 3 solution to a solution containing 0.015 M BaCl 2 and 0.025 M MgCl 2 42 . The batch closed system was kept for a day at room temperature and shaken for 4 weeks at 80°C, and the products were characterized by chemical analysis and X‐ray diffraction 42 (H. Gamsjäger, personal communication, 1999).…”

“…Sample Ba‐G was made by the addition of a 0.25 M NaHCO 3 solution to a solution containing 0.015 M BaCl 2 and 0.025 M MgCl 2 42 . The batch closed system was kept for a day at room temperature and shaken for 4 weeks at 80°C, and the products were characterized by chemical analysis and X‐ray diffraction 42 (H. Gamsjäger, personal communication, 1999). Sample LON‐1 has been synthesized by Longo and Voight 43 by grinding a mixture of Na 2 CO 3 and chloride salts of barium and magnesium in a Step 8000 ball mill.…”

Solid-solute phase equilibria in aqueous solution: Fundamentals and applications

“…To relate these quantities to concentrations (i.e., solubilities), activity coefficients of the aqueous species are generally required. Since solubility measurements on sparingly soluble salts are often performed in a background electrolyte medium of constant ionic strength, e.g., [2][3][4]8,11], activity coefficients are kept constant and activities can be replaced by concentrations. This means that the activity fractions (eq.…”

“…The thermodynamic implications of various types of equilibria between solid and aqueous electro lyte solutions have been developed by Gamsjäger et al [2][3][4][5][6][7][8][9][10][11][12][13] and others, e.g., [14][15][16][17][18].…”