Unstable to Stable Transformations during Dolomitization

Dolomitization of calcite at 218°C in hydrothermal bombs resulted in a series of unstable precursors to the final product, stoichiometric, ordered dolomite. The first phase formed was high-Mg calcite with approximately 35 mole % MgCO₃. This was replaced by Ca-rich, poorly ordered dolomite that was, in turn, replaced by ordered, stoichiometric dolomite. The rate of dolomitization and the rate at which unstable phases transformed to stoichiometric dolomite was related to the Mg²⁺/Ca²⁺ ratio of the solution. There was no simple relationship between the Mg²⁺/Ca²⁺ ratio solution and that of the products. SEM analyses of the products are interpreted to indicate that the unstable to stable transformation occurs by three, possibly simultaneous, mechanisms: (1) The small rhombs in the final products indicate that new crystals continue to nucleate during the reaction. (2) The large rhombs indicate that overgrowths of the more stable phase grew on the less stable phase. (3) The very irregular interiors indicate that the initial unstable products underwent a intercrystalline-scale dissolution and reprecipitation replacement.