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Chemical controls on ikaite formation

Journal article
Authors E. Tollefsen
G. Stockmann
A. Skelton
C. M. Morth
C. Dupraz
Erik Sturkell
Published in Mineralogical Magazine
Volume 82
Issue 5
Pages 1119-1129
ISSN 0026-461X
Publication year 2018
Published at Department of Earth Sciences
Pages 1119-1129
Language en
Links dx.doi.org/10.1180/mgm.2018.110
Keywords ikaite, magnesium, pH, precipitation, Ikka Fjord, calcite inhibitors, calcium-carbonate hexahydrate, ikka-fjord, southwest greenland, crystal-growth, mono lake, precipitation, temperature, aragonite, mg2+, crystallization
Subject categories Mineralogy

Abstract

The hydrated carbonate mineral ikaite (CaCO3 center dot 6H(2)O) is thermodynamically unstable at all known conditions on Earth. Regardless, ikaite has been found in marine sediments, as tufa columns and in sea ice. The reason for these occurrences remains unknown. However, cold temperatures (<6 degrees C), high pH and the presence of Mg2+ and SO42 in these settings have been suggested as factors that promote ikaite formation. Here we show that Mg concentration and pH are primary controls of ikaite precipitation at 5 degrees C. In our experiments a sodium carbonate solution was mixed with seawater at a temperature of 5 degrees C and at a constant rate. To test the effect of Mg2+ and SO42 we used synthetic seawater which allowed us to remove these elements from the seawater. The pH was controlled by different ratios of Na2CO3 and NaHCO3 in the carbonate solution. We found that ikaite precipitated when both seawater and synthetic seawater from which SO4 had been removed were used in the experiments. However, ikaite did not precipitate in experiments conducted with synthetic seawater from which Mg had been removed. In these experiments, calcite precipitated instead of ikaite. By varying the Mg concentration of the synthetic seawater and the pH of the sodium carbonate solution, we constructed a kinetic stability diagram for ikaite and calcite as a function of Mg concentration and pH. One possible explanation of our finding is that Mg2+ inhibits calcite nucleation and thereby allows metastable ikaite to form instead.

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