A simple method for vaterite precipitation for isotopic studies: implications for bulk and clumped isotope analysis

Calcium carbonate (CaCO3) plays an important role in the natural environment as a major constituent of the skeleton and supporting structure of marine life and has high economic importance as an additive in food, chemicals and medical products. Anhydrous CaCO3 occurs in the three different polymorphs calcite, aragonite and vaterite, whereof calcite is the most abundant and best characterized mineral. In contrast, little is known about the rare polymorph vaterite, in particular with regard to the oxygen isotope fractionation between H2O and the mineral. Synthetic precipitation of vaterite in the laboratory typically involves rapid processes and isotopic non-equilibrium, which excludes isotope studies focused on the characterization of vaterite under equilibrium conditions. Here, we used a new experimental approach that enables vaterite mineral formation from an isotopically equilibrated solution. The solution consists of a ∼0.007molL−1 CaCO3 solution that is saturated with NaCl at room temperature (up to 6.4molL−1). Vaterite precipitated as single phase or major phase (≥94%) in experiments performed between 23 and 91◦C. Only at 80◦Cwasvaterite a minor phase with a relative abundance of 27%.Thehigh mineral yield per experiment of up to 235mg relative to the initially dissolved CaCO3 amount of on average 360mg enables an investigation of the oxygen isotope fractionation between the mineral and water, and the determination of clumped isotope values in vaterite.