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

January 1, 1985

Significant amounts of SO42−, Na+, and OH are incorporated in marine biogenic calcites. Biogenic high Mg-calcites average about 1 mole percent SO42−. Aragonites and most biogenic low Mg-calcites contain significant amounts of Na+, but very low concentrations of SO42−. The SO42− content of non-biogenic calcites and aragonites investigated was below 100 ppm. The presence of Na+ and SO42− increases the unit cell size of calcites. The solid-solutions show a solubility minimum at about 0.5 mole percent SO42− beyond which the solubility rapidly increases. The solubility product of calcites containing 3 mole percent SO42− 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 SO42− incorporated in calcites vary as a function of the rate of crystal growth. The variation of the distribution coefficient (D">D) of SO42− in calcite at 25.0°C and 0.50 molal NaCl is described by the equation D = k0+ k1R">D = k0+ k1R where k0">k0 and k1">k1 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.




Publication Year 1985
Title Kinetic and thermodynamic factors controlling the distribution of SO32- and Na+ in calcites and selected aragonites
DOI 10.1016/0016-7037(85)90166-8
Authors E. Busenberg, Niel Plummer
Publication Type Article
Publication Subtype Journal Article
Series Title Geochimica et Cosmochimica Acta
Index ID 70012943
Record Source USGS Publications Warehouse
USGS Organization National Research Program - Eastern Branch