Effect of temperature on the transformation of amorphous calcium magnesium carbonate with near-dolomite stoichiometry into high Mg-calcite

Abstract

High Mg-calcite (HMC) is thermodynamically unstable under ambient conditions, yet it has been found in many biogenic and abiotic depositional settings with Mg contents up to 50 mol%. The elevated Mg content of HMC was frequently attributed to an amorphous calcium magnesium carbonate (ACMC) precursor, but the effects of transformation conditions on the Mg content of HMC are still debated and far from being quantified. Therefore, the transformation of ACMC with near-dolomite stoichiometry (∼48 mol% Mg) into HMC has been studied in a MgCl₂–NaHCO₃ buffered solution at moderate pH (7.6) and temperatures from 10 to 80 °C. The obtained chemical data show that the apparent solubility of ACMC (K_ACMC) is lower at higher temperature and the relative increase of K_ACMC as a function of the Mg content is similar between 10 and 80 °C which can be assessed by the equation: log(K_ACMC) = 0.01629 * [Mg]_ACMC − 0.0001096 T² + 0.0545 T −12.919, where T is reported in Kelvin and [Mg]_ACMC refers to the amount of Mg in ACMC in mol%. The Mg content of the final HMC increases from 5 to 40 mol% with increasing temperature, thus is significantly lower compared to the Mg content of the ACMC precursor. Our findings argue for a dissolution and re-precipitation process during ACMC transformation, where at higher temperatures the Mg incorporation into HMC is enhanced due to (i) the reduced solvation energy barrier of aqueous Mg²⁺ and (ii) the high prevailing molar Mg²⁺/Ca²⁺ ratio of the solution after its reaction with the ACMC. Notably, the Mg²⁺/Ca²⁺ ratio of the latter solution is controlled by the temperature-dependent solubility of ACMC. Finally, the findings indicate that the nanocrystalline HMCs formed via transformation of ACMC are physically defective from rapid crystal growth and have higher solubilities compared to well-crystallized Mg-bearing calcites.