Kinetic and thermodynamic factors controlling the distribution of SO 32- and Na + in calcites and selected aragonites
Abstract
Significant amounts of SO 42-, Na +, and OH - are incorporated in marine biogenic calcites. Biogenic high Mg-calcites average about 1 mole percent SO 42-. Aragonites and most biogenic low Mg-calcites contain significant amounts of Na +, but very low concentrations of SO 42-. The SO 42- content of non-biogenic calcites and aragonites investigated was below 100 ppm. The presence of Na + and SO 42- increases the unit cell size of calcites. The solid-solutions show a solubility minimum at about 0.5 mole percent SO 42- beyond which the solubility rapidly increases. The solubility product of calcites containing 3 mole percent SO 42- 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 42- incorporated in calcites vary as a function of the rate of crystal growth. The variation of the distribution coefficient ( D) of SO 42- 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.
- Publication:
-
Geochimica et Cosmochimica Acta
- Pub Date:
- March 1985
- DOI:
- 10.1016/0016-7037(85)90166-8
- Bibcode:
- 1985GeCoA..49..713B