A kinetic model for distribution coefficients and application to Mg-calcites

A review of experimental and natural Mg-calcite occurrences indicates that no simple relation exists between $\text{mMg}\text{mCa}$ in solution and $\text{X}{}^{\mathrm{Mg}}\text{X}{}^{\mathrm{ca}}$ in growing calcite crystals. The great variability of the data suggests a strong influence of precipitation kinetics on the distribution of Mg⁺² between solution and crystal. We have derived a kinetic formulation for the distribution coefficient (λ^Mg = X^Mg/X_Ca/ mMg/mCa) based upon the existence, at steady state conditions, of a constant mass surface phase. The resulting formulation is consistent with both experimental and natural Mg-calcite compositions. The primary factors which influence the value of λ^Mg are temperature and the ratio of K_sp^MCO3 to the activity product (M⁺²]CO₃^?2]/MCO₃]) for magnesite and calcite. The results suggest that Mg-calcite composition (X^Mg/X^Ca) is at best a crude measure of the mMg/mCa ratio in paleosolutions.