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

E. Busenberg; Niel Plummer

doi:10.1016/0016-7037(85)90166-8

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

E. Busenberg·Niel Plummer

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Abstract

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.

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E. Busenberg, Niel Plummer. 1985. Kinetic and thermodynamic factors controlling the distribution of SO32- and Na+ in calcites and selected aragonites. https://doi.org/10.1016/0016-7037(85)90166-8

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