Dolomite stoichiometry and Ostwald's Step Rule

Abstract

Nucleation kinetics responsible for Ostwald's Step Rule in the transition of calcite to dolomite at 193°C in 1 M 0.66 Mg to Ca ratio solutions were investigated. Nucleation and crystal growth kinetics were determined from differences in the induction period between isothermal experiments and experiments cycled between 193°C and room temperature. The cycled reactions used 12 h and 48 h heating periods interchanged with 12 h at room temperature. All three experimental designs produced high magnesium calcite (HMC), nonstoichiometric dolomite, and near ideal dolomite. No differences were seen in product size or morphology among designs. The length of the induction period of HMC was the same in all three experimental designs. This indicates that stable nucleation of HMC requires less than 12 h of heating and that the remaining induction period reflects HMC crystal growth. Isothermal and 48 h experiments produced a more stoichiometric dolomite (approx. 46% Mg) after 107 to 109 h of heating. The 12 h series did not detect this phase until 181 h of heating. This indicates that nucleation of the more stoichiometric phase was suppressed. Nucleation theory, these experiments, and the structural and thermodynamic properties of HMC and more stoichiometric dolomite lead us to conclude that the surface free energy of HMC is less than that of more stoichiometric dolomite. Ostwald's Step Rule in these calcite to dolomite transformations is consistent with surface free energy controlled nucleation kinetics.