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

Abstract

The challenge to produce dolomite CaMg(CO3)2 at low temperature (20–35 °C) over laboratory time scales so far has remained 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 allow 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 (e.g., nature of transient crystalline phases) and the kinetics. Nanosized crystallites with short-range order, called disordered dolomite CaMg(CO3)2, precipitated following a complex reaction pathway. First, nesquehonite (MgCO3·3H2O: nucleation time 2.5 h) and then disordered dolomite (CaMg(CO3)2: nucleation time 3.2 h) followed by monohydrocalcite (CaCO3·H2O: nucleation time 3.4 h) formed from ACMC transformation. Nesquehonite and monohydrocalcite are transient phases that nourish the slow precipitation of disordered dolomite, which reached a spectral equilibrium after 7 days of reaction. The direct transformation of ACMC into disordered dolomite was also measured. Our experimental results demonstrate that disordered dolomite precipitates at room temperature when an ideal Mg/Ca ratio, high carbonate alkalinity, and high ionic concentration are reached in abiotic systems. This result suggests the possibility of a physicochemical rather than biotic control on the formation of disordered dolomite at low temperature in several geosystems.