Effect of pH on the precipitation of synthetic CaCO3 polymorphs and determination of Mg/Ca ratios in synthetic low-magnesium calcite: An experimental investigation

Jeremy M Weremeichik1, Aleksandra Novak1, Aleksey Sadekov2, Rooban Venkatesh K.G. Thirumalai1 and Rinat I Gabitov3, (1)Mississippi State University, Mississippi State, MS, United States, (2)University of Cambridge, Cambridge, United Kingdom, (3)Mississippi State University, Geosciences, Mississippi State, MS, United States
The intent of the work presented is to determine the effect which the Mg/Ca ratio in fluid has on the Mg partition coefficient (KMg) between synthetically precipitated low-magnesium calcite and solution. In nature, planktonic foraminifera produce low-magnesium calcite with distinct Mg/Ca that is typically found to be within the range of ~1.4 to ~10 mmol/mol (e.g. Regenberg et al., 2009; Sadekov et al., 2009). Our intent was to replicate Mg/Ca found in foraminifera by conducting experiments where low-Mg calcite was grown inorganically at a near-constant salinity and temperature.

Synthetic low-Mg calcite was precipitated using a continuous pumping method whereby NaHCO3 and Mg-doped CaCl2 solutions were pumped into a NaCl solution. Continuous pumping allows for delivery of solutions to a reaction vessel at a constant rate while simultaneously maintaining fluid volume by removing fluid at a constant rate equal to the delivery rate. In order to vary the Mg/Ca ratio while keeping salinity near-constant, magnesium and calcium molar concentrations were varied but the sum of the concentrations was kept equal to the concentration of NaHCO3 or Na2CO3.

Optical microscopy analysis of samples showed that the use of Na2CO3 as the carbonate ion source in experiments precipitated predominately aragonite with needle-like crystals whereas use of NaHCO3 yielded calcite with rhombohedral crystals. Powder XRD analysis of a sample collected from one of the Na2CO3 experiments confirmed that the dominant CaCO3 polymorph precipitated was aragonite. This suggests that aragonite is being produced at higher pH values using Na2CO3 and calcite is being produced at lower values of pH using NaHCO3. We intend use XRD, AAS, and ICP-MS to confirm that rhombohedral crystals are calcite and determine Mg/Ca of fluid and those crystals.


Regenberg et al. (2009) Earth Planet. Sci. Lett., 278, doi: 10.1016/j.epsl.2008.12.019.

Sadekov et al. (2009) Paleoceanography, 24, doi: 10.1029/2008PA001664.