Controls of temperature, alkalinity and calcium carbonate reactant on the evolution of dolomite and magnesite stoichiometry and dolomite cation ordering degree: An experimental approach

Natural and synthetic dolomites exhibit large scatter of structural and compositional characteristics with respect to variations in their stoichiometry and cation ordering degree (COD), but the physicochemical factors controlling dolomitization reactions are still poorly understood and by far not quantified. In the present study, dolomite was synthesized in batch reactors at temperatures of 150 °C, 180 °C and 220 °C by reacting calcite or aragonite with an artificial solution containing MgCl₂ and NaHCO₃ over a 360 day period. The obtained results on the evolution of the stoichiometry of dolomite and magnesite and on the COD of dolomite indicate that the formation of stoichiometric and well ordered dolomite and of stoichiometric magnesite follow a ripening process and proceed through dissolution and re-precipitation of a sequence of intermediate phases, such as Ca-magnesite, huntite, very-high-Mg-calcite (VHMC) and disordered dolomite. The initial occurrence and the metastability of the precursor phases were dependent particularly on reaction temperature and to a lesser extent on the CaCO₃ source used. Temperature is the major rate-limiting parameter of the evolution of dolomite and magnesite stoichiometry and dolomite COD, corroborating results from prior experiments and natural deposits. From our data, we determined kinetic rates for the stoichiometric ripening of magnesite and dolomite and for the dolomite COD for the first time using a first-order reaction kinetic approach. The kinetics increase with temperature following the linear Arrhenius equation, in turn allowing for activation energies of the respective processes to be determined (44.0, 57.4 and 9.3 kJ·mol⁻¹, respectively). Interestingly we found the reaction rate for magnesite stoichiometric ripening is approximately one third of that for dolomite regardless of the temperature, carbonate alkalinity or CaCO₃ phase used for reaction. Furthermore, we see that the activation energies required for the formation of ideal dolomite decreases in sequence from the precipitation of an initial VHMC phase (∼133.5 kJ·mol⁻¹; Arvidson and Mackenzie, 1999), followed by stoichiometric ripening (57.4 kJ·mol⁻¹) to cation ordering (9.3 kJ·mol⁻¹; for perfect ordering). Extrapolation of the obtained dolomitization rates indicates that about 1.4 and 6.8 Myr are required to approach ideal dolomite at 50 and 25 °C, respectively, explaining the large absence of ordered dolomite in modern sedimentary successions. The present dataset provides new insights into dolomitization pathways and rates and contributes to a better understanding of the wide range of stoichiometries and COD values of dolomite and its evolution versus magnesite observed in the geological record and in laboratory studies.