Ikaite (CaCO3·6H2O) Precipitation as a Function of pH and Mg Concentration

The carbonate mineral ikaite (CaCO₃ · 6H₂O) has been found in marine sediments, in sea ice and as tufa columns in Ikka Fjord, SW Greenland, despite it being thermodynamically unstable at all known conditions on Earth. The reasons for these occurrences are unclear. It has been proposed that cold temperature (<6°C), high pH and the inhibition of calcite nucleation by SO₄^2-and/or Mg²⁺ may favor ikaite precipitation.

We simulated the conditions in Ikka Fjord, where sodium carbonate rich spring water mixes with seawater at 5°C. To test the effects of SO₄^2-and Mg²⁺ we used synthetic seawater which allowed us to remove and/or vary the concentrations of each of these components. The pH was controlled by varying the ratios of NaCO₃ and NHCO₃ in the sodium carbonate solution. Ikaite precipitated in experiments with SO₄^2- removed whereas calcite precipitated in experiments with Mg²⁺ removed.

From our results we constructed a kinetic stability diagram for ikaite and calcite as a function of Mg²⁺ concentration and pH in solution, which shows that both high Mg²⁺ concentration and high pH favor ikaite precipitation. Two possible interpretations of these findings are 1) that Mg²⁺ inhibits calcite nucleation, and 2) that high pH increases supersaturation triggering ikaite nucleation. Our findings have implications for the understanding of ikaite occurrence in nature.