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The transformation of ACC to vaterite; an in situ SAXS/WAXS study

The transformation of ACC to vaterite; an in situ SAXS/WAXS study

Mineralogical Magazine 75.3: 559

Vaterite (mu -CaCO3 ) is rarely observed in natural systems, as it is thermodynamically unstable with respect to calcite and aragonite [1]. However, some organisms produce and stabilize vaterite as biominerals [2]. At high supersaturation vaterite forms via a nanoparticulate, poorly-ordered and metastable precursor, amorphous calcium carbonate (ACC). In the pure system, ACC transforms to vaterite within minutes, and subsequently to calcite [3]. The detailed mechanisms of the ACC to vaterite transformation are however still lacking due to the fast kinetics of this first step of crystallization. In this study we demonstrate the use of in situ time-resolved synchrotron-based Small- and Wide-angle X-ray Scattering (SAXS/WAXS) combined with off-line characterization to quantify the kinetics and crystallization mechanisms of vaterite at fast time scales. The SAXS/WAXS data collected at 1 second / frame for up to 32 minutes, revealed that the ACC to vaterite transformation occurs in 3 stages. The initial stage (0-2 min) was governed by ACC precipitation and growth (to O approximately 38 nm). During this stage approximately 70 % of the total vaterite (O approximately 10 nm) formed via spherulitic growth at the expense of the metastable ACC [4]. The end of the spherulitic growth (at 2 min) was concomitant with a drop in supersaturation below the limit for spherulitic growth of vaterite (SI nearly equal 1.4 [4]). During the 2nd stage (2-7 min), remnant ACC lead to a further increase of the amount of vaterite ( approximately 30 %) due to ACC dissolution and vaterite nanocrystal reprecipitation. During this stage the vaterite crystallite size increased from approximately 10 to approximately 35 nm. Finally during the 3rd stage (> 7 min), no de nuovo precipitation of vaterite occurred, however the vaterite crystallite size continued to increase reaching approximately 60 nm at the end of the experiment (32 min). This increase in crystallite size was solely governed by Ostwald Ripening.

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