Amorphous Calcium–Magnesium Carbonate (ACMC) Accelerates Dolomitization at Room Temperature under Abiotic Conditions

The challenge to produce dolomite CaMg(CO3)2 at low temperature (20-35°C) over laboratory time scales remains so far unsuccessful, which has led to long-lasting scientific debates in the last two centuries. This mineral exerts a major control on the natural carbon dioxide sequestration into various sedimentary, basaltic and mantellic rocks. The present study reports on specific abiotic conditions that allows the precipitation of disordered dolomite, high Mg-calcite and high Ca-magnesite at room temperature over time scales of hours to days. Here we show that an amorphous calcium magnesium carbonate (ACMC) phase accelerates dolomitization at room temperature. ACMC is initially precipitated by mixing a carbonate (HCO3-/CO32-=1; pH~10.3~pKa2) alkaline solution with a Mg-Ca ionic solution (Mg molar fraction between 0 and 1). Then, time-resolved in situ Raman spectroscopy monitored the transformation of ACMC into Mg-rich carbonate minerals. The initial Mg molar fraction controlled both the reaction mechanism (...