Articles | Volume 12, issue 11
Technical note
03 Jun 2015
Technical note |  | 03 Jun 2015

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

T. Kluge and C. M. John

Abstract. 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 ~0.007 mol L−1 CaCO3 solution that is saturated with NaCl at room temperature (up to 6.4 mol L−1). Vaterite precipitated as single phase or major phase (≥94%) in experiments performed between 23 and 91 °C. Only at 80 °C was vaterite a minor phase with a relative abundance of 27%. The high mineral yield per experiment of up to 235 mg relative to the initially dissolved CaCO3 amount of on average 360 mg enables an investigation of the oxygen isotope fractionation between the mineral and water, and the determination of clumped isotope values in vaterite.

Short summary
•Vaterite was synthesized from a NaCl-saturated CaCO3 solution at 23-91°C •Vaterite occurred as pure or dominating phase and amounted up to 235 mg per experiment •The precipitation technique allows thermal and isotopic equilibration and enables oxygen and clumped isotope analyses •18α(vaterite-H2O) has the same temperature dependence as calcite •Vaterite δ18O values can hardly be distinguished from calcite (offset +0.0±0.4‰) •Clumped isotope values are indistinguishable from calibration data
Final-revised paper