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Chemical controls on ikaite formation
Stockholm University, Faculty of Science, Department of Geological Sciences.
Stockholm University, Faculty of Science, Department of Geological Sciences.
Stockholm University, Faculty of Science, Department of Geological Sciences.ORCID iD: 0000-0002-3732-7993
Stockholm University, Faculty of Science, Department of Geological Sciences.
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Number of Authors: 62018 (English)In: Mineralogical magazine, ISSN 0026-461X, E-ISSN 1471-8022, Vol. 82, no 5, p. 1119-1129Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
2018. Vol. 82, no 5, p. 1119-1129
Keywords [en]
ikaite, magnesium, pH, precipitation, Ikka Fjord, calcite inhibitors
National Category
Earth and Related Environmental Sciences
Research subject
Geology
Identifiers
URN: urn:nbn:se:su:diva-163729DOI: 10.1180/mgm.2018.110ISI: 000452016700008OAI: oai:DiVA.org:su-163729DiVA, id: diva2:1277182
Available from: 2019-01-09 Created: 2019-01-09 Last updated: 2020-04-03Bibliographically approved
In thesis
1. Experimental, petrological and geochemical investigations of ikaite (CaCO3·6H2O) formation in marine environments
Open this publication in new window or tab >>Experimental, petrological and geochemical investigations of ikaite (CaCO3·6H2O) formation in marine environments
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Carbonates are a group of minerals that play an essential role in several processes on planet Earth, for example in the global carbon cycle and as a product of biomineralisation. Calcite (CaCO3) is by far the most common mineral in the carbonate group, and the stable form of carbonate at Earth surface conditions. However, calcite growth is often kinetically limited and polymorphs of calcite or hydrous calcium carbonates will form instead under certain circumstances. In this thesis, I investigate a hydrous form of calcium carbonate, ikaite (CaCO3 · 6H2O), which occasionally forms under conditions where normally calcite formation would be expected.

Ikaite is metastable at surface conditions and has only been observed in nature at temperatures below 7°C. In Ikka Fjord, southwest Greenland, several hundred ikaite columns occur at the bottom of the fjord. Previous studies in Ikka Fjord have shown that ikaite columns are forming above submarine springs that are extremely sodium carbonate rich (pH ~10.5). An association with the surrounding igneous rocks, which comprise nepheline syenite and carbonatite, has been suggested. In the first part of this thesis, I investigate this association. A petrographic study of rocks samples from the igneous complex showed that the combined alteration of the minerals siderite and nepheline could explain the composition of the submarine spring water, and thereby the unique formation of ikaite columns at this site.

It is from the mixture of sodium carbonate spring water and seawater that ikaite precipitates in Ikka Fjord, despite the fact that all other calcium carbonates are supersaturated in this mixture. Why ikaite precipitates and not the other forms of calcium carbonate was investigated by a series of experiments in the second and third parts of this thesis. Previous studies have suggested that ikaite was favoured by the low temperature in the fjord (<7°C) and the presence of phosphate (95- 263 μmol/kg) in the submarine spring water, which is known to inhibit calcite growth even at only trace concentrations. In the second part of this thesis, we simulated Ikka Fjord conditions in laboratory and showed that ikaite precipitation is not controlled by the presence of phosphate in the mixture. Instead, after a second series of experiments I found that it is the presence Mg in seawater that inhibits calcite growth and therefore favour ikaite precipitation.

Ikaite is metastable and at temperatures above 7°C the mineral will transform or decompose to calcite and water. The transformation can occur pseudomorphically and pseudomorphs after ikaite have been found worldwide in the sediment record. Pseudomorphs after authigenic ikaite in sediments are named glendonite, and because of the narrow temperature range of ikaite observations in nature, glendonite has been used as a paleotemperature indicator. In the fourth part of this thesis, I explore the temperature range of ikaite nucleation by a series of experiments and found that ikaite nucleation can occur up to at least 35°C. This challenges the use of glendonite as a paleotemperature indicator.

Place, publisher, year, edition, pages
Stockholm: Department of Geological Sciences, Stockholm University, 2020. p. 36
Series
Meddelanden från Stockholms universitets institution för geologiska vetenskaper ; 380
Keywords
ikaite, petrology, experiment, geochemistry, marine environments, pseudomorphs, calcium carbonate, Ikka Fjord, glendonite, nepheline, siderite, paleotemperature, carbonatite
National Category
Geology
Research subject
Geology
Identifiers
urn:nbn:se:su:diva-180194 (URN)978-91-7911-022-2 (ISBN)978-91-7911-023-9 (ISBN)
Public defence
2020-05-13, De Geersalen, Geovetenskapens hus, Svante Arrhenius väg 14, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 4: Accepted.

Available from: 2020-04-20 Created: 2020-03-23 Last updated: 2020-05-25Bibliographically approved

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