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How short-lived ikaite affects calcite crystallisation

DOI: 10.1021/acs.cgd.7b00743 DOI Help

Authors: R. Besselink (German Research Center for Geosciences) , J. D. Rodriguez Blanco (University of Copenhagen; Trinity College Dublin) , T. M. Stawski (German Research Center for Geosciences) , L. G. Benning (German Research Center for Geosciences; Free University of Berlin; University of Leeds) , D. J. Tobler (University of Copenhagen)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Crystal Growth & Design

State: Published (Approved)
Published: September 2017
Diamond Proposal Number(s): 9904

Abstract: The pathways of CaCO3 crystallisation are manifold, often involving one or several metastable amorphous or nanocrystalline intermediate phases. The presence of such intermediates is often overlooked or not observed, because they are short-lived and/or occur at small molar fractions. However, their occurrence does not just impact the mechanisms and pathways of formation of the final stable CaCO3 phase, but also affects their crystal size, shape and structure. Here we document the presence of such a short-lived intermediate through in situ and time-resolved simultaneous small and wide angle X-ray scattering (SAXS/WAXS) combined with high resolution electron microscope observations. The presence of short-lived nanocrystalline ikaite changes the crystallisation mechanism of calcite and consequently affects its size and morphology: When ikaite forms concomitant with the dissolution of amorphous calcium carbonate (ACC) but prior to calcite formation, fairly large glendonite-type calcite crystals grow despite the presence of citrate ligands that usually reduce growth rate. These were ideal seeding crystals for further crystallization from supersaturated ions in solution. In contrast, in the absence of ikaite the crystallisation of calcite proceeds through the already documented transformation from ACC, resulting in fine-grained spherulitic calcite with sizes ≈ 8 times smaller than when ikaite was present. Noteworthy is, that the formation of the intermediate ikaite, although it consumes less than 3 mol % of the total precipitated CaCO3, still clearly affected the calcite formation mechanism. This highlights the need for more time-resolved and nanoscale studies to unravel the mechanism of CaCO3 crystallisation, for improved understanding of biomineralisation processes and for the design of enhanced industrial CaCO3 products

Subject Areas: Earth Science, Chemistry

Instruments: I22-Small angle scattering & Diffraction