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

Abstract

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.