E. Busenberg — Page 2

USGS1987

Thermodynamics of aragonite-strontianite solid solutions: Results from stoichiometric solubility at 25 and 76°C

Dissolution of synthetic strontianite-aragonite solid solutions was followed analytically to stoichiometric saturation using large solid to solution ratios in CO 2 -H 2 O 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 CaCO 3 and SrCO 3 in the solid Ca (1− x ) Sr x CO 3 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 A 0 is 8.49 ± 0.30 and 7.71 ± 0.20 KJ/mole and A 1 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 SrCO 3 mole fraction, x , by the relation where R is the gas constant, T is in Kelvins and K A and K S are the aragonite and strontianite equilibrium constants. The experimental results indicate the Henry's law coefficients of SrCO 3 in aragonites containing 0 to 6 mole percent SrCO 3 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 SrCO 3 and CaCO 3 at 25°C and 3.1 ± 0.3 and 17.2 ± 1.1 mole percent SrCO 3 and CaCO 3 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.

Geochimica et Cosmochimica Acta↗

USGS1985

Kinetic and thermodynamic factors controlling the distribution of SO32- and Na+ in calcites and selected aragonites

Significant amounts of SO 4 2− , Na + , and OH − are incorporated in marine biogenic calcites. Biogenic high Mg-calcites average about 1 mole percent SO 4 2− . Aragonites and most biogenic low Mg-calcites contain significant amounts of Na + , but very low concentrations of SO 4 2− . The SO 4 2− content of non-biogenic calcites and aragonites investigated was below 100 ppm. The presence of Na + and SO 4 2− increases the unit cell size of calcites. The solid-solutions show a solubility minimum at about 0.5 mole percent SO 4 2− beyond which the solubility rapidly increases. The solubility product of calcites containing 3 mole percent SO 4 2− is the same as that of aragonite. Na + appears to have very little effect on the solubility product of calcites. The amounts of Na + and SO 4 2− incorporated in calcites vary as a function of the rate of crystal growth. The variation of the distribution coefficient ( D "> D ) of SO 4 2− in calcite at 25.0°C and 0.50 molal NaCl is described by the equation D = k 0 + k 1 R "> D = k 0 + k 1 R where k 0 "> k 0 and k 1 "> k 1 are constants equal to 6.16 × 10 −6 "> 6.16 × 10 −6 and 3.941 × 10 −6 "> 3.941 × 10 −6 , respectively, and R "> R is the rate of crystal growth of calcite in mg·min −1 ·g −1 of seed. The data on Na + are consistent with the hypothesis that a significant amount of Na + occupies interstitial positions in the calcite structure. The distribution of Na + follows a Freundlich isotherm and not the Berthelot-Nernst distribution law. The numerical value of the Na + distribution coefficient in calcite is probably dependent on the number of defects in the calcite structure. The Na + contents of calcites are not very accurate indicators of environmental salinities.

Geochimica et Cosmochimica Acta↗

USGS1981

Crystal growth of calcite from calcium bicarbonate solutions at constant P CO 2 and 25°C: a test of a calcite dissolution model

A highly reproducible seeded growth technique was used to study calcite crystallization from calcium bicarbonate solutions at 25°C and fixed carbon dioxide partial pressures between 0.03 and 0.3 atm. The results are not consistent with empirical crystallization models that have successfully described calcite growth at low P CO 2 (< 10 −3 atm). Good agreement was found between observed crystallization rates and those calculated from the calcite dissolution rate law and mechanism proposed by Plummer et al . (1978).

Geochimica et Cosmochimica Acta↗