Abstract: Effect of pH on the precipitation of synthetic CaCO3 polymorphs and determination of Mg/Ca ratios in synthetic low-magnesium calcite: An experimental investigation (2016 Ocean Sciences Meeting)

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

Abstract:

The intent of the work presented is to determine the effect which the Mg/Ca ratio in fluid has on the Mg partition coefficient (K^Mg) 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.

References

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.

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