Kinetic and thermodynamic factors controlling the distribution of SO32? and Na+ in calcites and selected aragonites |
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Authors: | Eurybiades Busenberg L Niel Plummer |
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Institution: | U.S. Geological Survey, Mail Stop 432, Reston, VA 22092 U.S.A. |
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Abstract: | 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 () of SO42? in calcite at 25.0°C and 0.50 molal NaCl is described by the equation where and are constants equal to and , respectively, and 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. |
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