June 16 – Licentiate Thesis Presentation by Elin Tollefsen

Chemical controls on ikaite formation

Friday, 16 June 2017
10h00, Ahlmannsalen, Geohuset, Stockholm University

Supervisor:
Alasdair Skelton, Stockholm University
Carl-Magnus Mörth, Stockholm University
Erik Sturkell, University of Gothenburg

Examiners:
Professor Valentin Troll, Uppsala University
Professor Victoria Pease, Stockholm University

 

Abstract

The metastable mineral ikaite (CaCO3 · 6H2O) was considered rare when it was first discovered as tufa columns forming above carbonated submarine springs in Ikka Fjord, southwest Greenland. Ikaite has since then been found in marine sediments and in sea ice. It is also the precursor of a common pseudomorph glendonite. Why this mineral forms instead of the stable phase calcite is unclear. However, it has been suggested that cold temperature (<6°C), the presence of Mg and SO4 in the seawater and high pH in the spring water favoured ikaite formation in Ikka Fjord. This thesis presents an experimental study in which the chemical controls on ikaite precipitation are tested by simulating Ikka Fjord conditions. Natural seawater, synthetic seawater or synthetic seawater with SO4 or Mg removed was mixed at a constant rate with a 0.1 M Na2CO3 + 0.1 M NaHCO3 solution with different ratios of Na2CO3/NaHCO3, representing the carbonated spring water. The results showed that ikaite precipitated in experiments with natural seawater, synthetic seawater or synthetic seawater with SO4 removed. However, in experiments with Mg removed from the synthetic seawater, calcite precipitated instead of ikaite. Ikaite did not precipitate at pH < 9.3, instead calcite or aragonite precipitated. From the results of the 47 experiments a kinetic stability diagram was constructed for the transition from metastable ikaite to calcite as a function of Mg concentration and pH at 5°C. One explanation for these results is the inhibition of calcite by Mg.

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