Thermodynamics of aragonite-strontianite solid solutions: Results from stoichiometric solubility at 25 and 76°C
Geochimica et Cosmochimica Acta
- Niel Plummer and E. Busenberg
Dissolution of synthetic strontianite-aragonite solid solutions was followed analytically to stoichiometric saturation using large solid to solution ratios in CO2-H2O solution at 25 and 76°C. The compositional dependence of the equilibrium constant was calculated from the composition of saturated (stoichiometric) solutions and used to calculate the activities and activity coefficients of CaCO3 and SrCO3 in the solid Ca(1−x)SrxCO3 at 25 and 76°C. The results show that the solid-solution is not regular but unsymmetrical. The excess free energy of mixing is closely modeled for all compositions by the relation
where A0 is 8.49 ± 0.30 and 7.71 ± 0.20 KJ/mole and A1 is −4.51 ± 0.20 and −3.36 ± 0.40 KJ/mole at 25 and 76°C, respectively. The equilibrium constant is denned as a function of the SrCO3 mole fraction, x, by the relation
where R is the gas constant, T is in Kelvins and KA and KS are the aragonite and strontianite equilibrium constants.
The experimental results indicate the Henry's law coefficients of SrCO3 in aragonites containing 0 to 6 mole percent SrCO3 are approximately 91± 8 and 23 ± 1 at 25 and 76°C, respectively and for strontianites the Henry's law coefficients and applicable compositional ranges are approximately 7.3 ± 0.3 (0.84 ≤ x ≤ 1.00) and 3.3 ± 0.5 (0.50 ≤ x ≤ 1.00) at 25 and 76°C, respectively. Substitution of small amounts of Sr in aragonite and Ca in strontianite initially increases the stability of the solid. The most stable aragonites and strontianites contain 0.58 ± 0.03 and 12.5 ± 1.1 mole percent SrCO3 and CaCO3 at 25°C and 3.1 ± 0.3 and 17.2 ± 1.1 mole percent SrCO3 and CaCO3 at 76°C, respectively. The spinode occurs over the regions 0.065 ± 0.001 ≤ x ≤ 0.620 ± 0.014 at 25°C and 0.103 ± 0.007 ≤ x ≤ 0.585 ± 0.019 at 76°C where all compositions are unstable. A miscibility gap occurs over the compositional ranges 0.0058 ± 0.0003 ≤ x ≤ 0.875 ± 0.011 at 25°C and 0.031 ± 0.003 ≤ x ≤ 0.828 ± 0.011 at 76°C and is in reasonable agreement with reported compositions of natural aragonites and strontianites. Marine aragonites are neither at equilibrium nor stoichiometric saturation with surface seawater. The experimentally observed distribution coefficient of Sr in aragonite is 12 times larger than the calculated equilibrium value (0.095) at 25°C. Naturally occurring strontianites contain large amounts of calcium primarily because Ca/Sr ratios in natural waters are typically large.
Neither equilibrium nor stoichiometric saturation is observed at 76°C during laboratory recrystallization of strontianite-aragonite solid solutions even after apparent 100 percent conversion to a narrow secondary composition and demonstration of a nearly constant composition system for periods of 300 hours.
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- Thermodynamics of aragonite-strontianite solid solutions: Results from stoichiometric solubility at 25 and 76°C
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- Geochimica et Cosmochimica Acta
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- 19 p.
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