Abstract

Dolomitization of CaCO3 in the laboratory at ≥175 °C demonstrates that the transformation has two distinct stages: an induction stage during which no detectable products form and a nucleation-growth stage during which dolomite forms at the expense of the reactant CaCO3. Experiments at 218 °C demonstrate that as the Mg/Ca ratio of the solution increases, the length of the induction stage decreases. Addition of CO2 to the solution decreases the induction stage and also changes the overall transformation mechanism. Previously published data show that the induction stage is affected by the temperature of the reaction and the surface area and mineralogy of the reactant. The induction stage is often the slowest part of the overall transformation. Characteristics of natural dolomites that are consistent with the experimental results include the following: (1) Dolomite selectively replaces fine crystalline CaCO3; (2) most carbonate rocks are 100% dolomite or 100% calcite; (3) limestone-dolomite contacts are usually sharp and involve a decrease in the number of dolomite rhombs, but the size of the rhombs remains constant; (4) dolomite in completely dolomitized rocks is more stoichiometric than dolomite in partially dolomitized rocks; (5) aragonite is selectively dolomitized in modern sabkha sediments; and (6) modern dolomite is most abundant in areas of elevated Mg/Ca ratio or HCO3.

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