Solubility investigations in the amorphous calcium magnesium carbonate system

Amorphous precursors are known to occur in the early stages of carbonate mineral formation in both biotic and abiotic environments. Although the Mg content of amorphous calcium magnesium carbonate (ACMC) is a crucial factor for its temporal stabilization, to date little is known about its control on ACMC solubility. Therefore, amorphous Ca _x Mg _1-x CO ₃ ·nH ₂ O solids with 0 ≤ x ≤ 1 and 0.4 ≤ n ≤ 0.8 were synthesized and dispersed in MgCl ₂ -NaHCO ₃ buffered solutions at 24.5 ± 0.5 °C. The chemical evolution of the solution and the precipitate clearly shows an instantaneous exchange of ions between ACMC and aqueous solution. The obtained ion activity product for ACMC (IAP _ACMC = "solubility product") increases as a function of its Mg content ([Mg] _ACMC = (1 - x) × 100 in mol%) according to the expression: log(IAP _ACMC ) = 0.0174 (±0.0013) × [Mg] _ACMC - 6.278 (±0.046) (R ² = 0.98), where the log(IAP _ACMC ) shift from Ca (-6.28 ± 0.05) to Mg (-4.54 ± 0.16) ACMC endmember, can be explained by the increasing water content and changes in short-range order, as Ca is substituted by Mg in the ACMC structure. The results of this study shed light on the factors controlling ACMC solubility and its temporal stability in aqueous solutions.