Kinetic and thermodynamic factors controlling the distribution of SO 32− and Na + in calcites and selected aragonites

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) of SO 4 2− in calcite at 25.0°C and 0.50 molal NaCl is described by the equation D = k 0 + k 1R where k 0 and k 1 are constants equal to 6.16 × 10 −6 and 3.941 × 10 −6 , respectively, and 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.