Control of MgSO₄⁰(aq) on the transformation of amorphous calcium carbonate to high-Mg calcite and long-term reactivity of the crystalline solid

The Mg and SO₄ content of naturally occurring calcite are routinely used as paleoenvironmental proxies. Yet little is known about the mechanisms governing the presence of these ions in carbonate minerals when their formation proceeds via an amorphous precursor. To address this, the transformation of Mg-free amorphous calcium carbonate (ACC) into nanocrystalline high-Mg calcite (HMC) was experimentally studied in solutions containing 27 mM of Mg and a range of 10–90 mM of SO₄. The obtained results suggest that ACC is stable for several minutes in the experimental solutions and this amorphous phase actively uptakes Mg and SO₄ that are incorporated in its structure. Additionally, the obtained results suggest that the stabilization of ACC is not affected by its Mg content and that the transformation to HMC is effectively controlled by the abundance of the free Mg²⁺(aq) ion. The transformation of ACC to HMC occurs earlier at elevated SO₄ concentrations because SO₄ limits the availability of Mg²⁺(aq) due to the formation of the MgSO₄⁰(aq) complex. The HMC that is formed from ACC appears as aggregates composed of nanocrystallites and exhibits Mg and SO₄ contents up to 8 and 2 mol% depending on the initial SO₄ concentration in the reactive solution. The precipitated HMC was kept in contact with the reactive solution in order to assess its reactivity for up to 1 year of reaction time. Over time, a continuous exchange of Mg and SO₄ between calcite and reactive solution was observed resulting in enrichment of Mg and depletion of SO₄ affecting the total mass of the aggregates with the distribution of these elements to appear homogeneous in the crystalline solid. The high reactivity and the continuous exchange of solutes between the nanocrystalline calcite and the reactive solutions limits the use of Mg and SO₄ content of these HMCs as environmental proxies.