Rodriguez-Blanco, JD, Shaw, S, Bots, P et al. (2 more authors) (2014) The role of Mg in the crystallization of monohydrocalcite. Geochimica et Cosmochimica Acta, 127. 204 - 220. ISSN 0016-7037
Abstract
Monohydrocalcite is a member of the carbonate family which forms in Mg-rich environments at a wide range of Mg/Ca ratios Mg 2+ aq/Ca 2+ aq ≥ 0.17 < 65. Although found in modern sedimentary deposits and as a product of biomineralization, there is a lack of information about its formation mechanisms and about the role of Mg during its crystallization. In this work we have quantitatively assessed the mechanism of crystallization of monohydrocalcite through in situ synchrotron-based small and wide angle X-ray scattering (SAXS/WAXS) and off-line spectroscopic, microscopic and wet chemical analyses. Monohydrocalcite crystallizes via a 4-stage process beginning with highly supersaturated solutions from which a Mg-bearing, amorphous calcium carbonate (ACC) precursor precipitates. This precursor crystallizes to monohydrocalcite via a nucleation-controlled reaction in stage two, while in stage three it is further aged through Ostwald-ripening at a rate of 1.8±0.1nm/h. In stage four, a secondary Ostwald ripening process (66.3±4.3nm/h) coincides with the release of Mg from the monohydrocalcite structure and the concomitant formation of minor hydromagnesite. Our data reveal that monohydrocalcite can accommodate significant amounts of Mg in its structure (χ=0.26) and that its Mg content and dehydration temperature are directly proportional to the saturation index for monohydrocalcite (SI) immediately after mixing the stock solutions. However, its crystallite and particle size are inversely proportional to these parameters. At high supersaturations (SI=3.89) nanometer-sized single crystals of monohydrocalcite form, while at low values (SI=2.43) the process leads to low-angle branching spherulites. Many carbonates produced during biomineralization form at similar conditions to most synthetic monohydrocalcites, and thus we hypothesize that some calcite or aragonite deposits found in the geologic record that have formed at high Mg/Ca ratios could be secondary in origin and may have originally formed via a metastable monohydrocalcite intermediate.
Metadata
Item Type: | Article |
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Authors/Creators: |
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Copyright, Publisher and Additional Information: | Crown copyright (c) 2013 Published by Elsevier Ltd. All rights reserved. NOTICE: this is the author’s version of a work that was accepted for publication in Geochimica et Cosmochimica Acta. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Geochimica et Cosmochimica Acta, 127, (2014) DOI 10.1016/j.gca.2013.11.034 |
Dates: |
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Institution: | The University of Leeds |
Academic Units: | The University of Leeds > Faculty of Environment (Leeds) > School of Earth and Environment (Leeds) |
Depositing User: | Symplectic Publications |
Date Deposited: | 18 Sep 2014 08:56 |
Last Modified: | 16 Jan 2018 17:34 |
Published Version: | http://dx.doi.org/10.1016/j.gca.2013.11.034 |
Status: | Published |
Publisher: | Elsevier |
Identification Number: | 10.1016/j.gca.2013.11.034 |
Related URLs: | |
Open Archives Initiative ID (OAI ID): | oai:eprints.whiterose.ac.uk:80229 |