Abstract
ABSTRACT During high temperature (150-300°C) dolomitization experiments the rate of dolomitization increases with temperature, reactant surface area, reactant solubility, ionic strength and Mg2+/Ca2+ of the solution. One would predict these results from simple kinetic theory. The discoveries that SO42-slows the rate, Li+ increases the rate and the induction period is long for dolomitization could not have been predicted. Dolomitization is a three-step reaction: (1) Nucleation: Nucleation of very high-Mg calcite (35-40 mole % MgCO3, VHMC) or nonstoichiometric dolomite is followed by nucleation of more stoichiometric dolomite on CaCO3. The composition of the VHMC or nonstoichiometric dolomite is a function of Mg2+/Ca2+ of the solution. Other variables such as dissolved Fe can affect the composition of the nuclei. (2) Induction period: Nucleation occurs during the induction period but the most of this period is post-nucleation growth of VHMC, nonstoichiometric and stoichiometric dolomite nuclei. Changes in the solution and substrate surface area affect the duration of the induction period. (3) Replacement period: (a) CaCO3 is replaced by VHMC or nonstoichiometric dolomite. VHMC nucleates faster than stoichiometric dolomite and therefore begins to replace the reactant first. (b) CaCO3 and VHMC and/or nonstoichiometric dolomite are rapidly replaced by stoichiometric dolomite. This phase of the reaction is sensitive to solution variables such as Mg2+/Ca2+ and ionic strength. The three-phase reaction model is consistent with eight characteristics of natural dolomites. (1) Very Ca-rich dolomite is common only in modern dolomites. (2) There is a direct relationship between Mg2+/Ca2+00 solution and Mg/Ca ratio in the dolomite. (3) The stoichiometry of dolomites in some ancient rocks is directly related to the percentage of dolomite in the rock. (4) Suppression of stoichiometric dolomite nucleation allows the persistence of metastable Ca-Mg-CO3 phases. (5) Dolomite-limestone contacts are often sharp. (6) Dolomite selectively replaces fine-grained CaCO3. (7) Dolomite crystals often have cloudy centers and clear rims. (8) Dolomite textures are mainly determined by the crystal size of the reactant.
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