Crystallization of Calcium Carbonate Observed In-situ by Combined Small- and Wide-angle X-ray Scattering

Abstract

The transformation of amorphous colloidal calcium carbonate into single microcrystals was observed in supersaturated solutions in situ. The observations are done by simultaneous time-resolved small-angle and wide-angle X-ray scattering experiments (TR-SAXS/WAXS) at a third generation synchrotron source. TR-SAXS/WAXS demonstrates that the particles generated by reaction of Ca 2+ and CO 3 2-ions are amorphous. The transformation of these amorphous CaCO 3 particles proceeds via dissolution and subsequent heterogeneous nucleation of the crystalline modification on the walls of the quartz capillary containing the reacting mixture. The crystalline modifications thus generated could be identified. No solid-solid transition is observed. TR-SAXS/WAXS is therefore well-suited to follow the mineralization from aqueous solution in great detail.The formation of calcium carbonate from supersaturated solutions has been studied for more than a century and it has received renewed interest, especially in the field of biomineralization. 1 The metastable forms of calcium carbonate can be investigated more precisely nowadays thanks to the improvement of the experimental techniques. 2 In particular amorphous calcium carbonate (ACC) has emerged as a precursor to the formation of more stable crystalline forms. 3 A topic of intense research activity is presently the detailed mechanism of phase transformation leading from the initial amorphous material to the final, thermodynamically stable, crystalline modification (calcite). Different mechanisms are being discussed, as, e.g., a solid/solid phase transition and a dissolution/crystallization process. This process also depends strongly on reagent concentrations and reaction conditions. 4 The transition from the amorphous state into a stable, crystalline modification is difficult to be studied in a timeresolved experiment. A number of experimental techniques have been applied to the study of the calcium carbonate formation, including optical, electron, and X-ray microscopies, dynamic light scattering, turbidimetry, and conductivity methods. 2,5 These methods, however, do not give the structural information as a function of time. On the other hand, the application of X-ray scattering methods is particularly well suited because it allows in-situ measurements of the size, morphology, and interactions of colloidal particles growing in solution. 6,7 Hence, scattering experiments probing directly the crystal structure of a given population of particles would be helpful to investigate the mineralization leading to a stable crystalline form.Recently, we studied the nucleation and growth of colloidal spheres made of calcium carbonate (CaCO 3 ) by using timeresolved small-angle X-ray scattering (TR-SAXS). 6 This method allows us to follow the growth of the particles from ∼20 ms after mixing the reagent solutions to their final size with a time resolution down to ∼0.1 s. In particular, we showed that the evolving particles consist of amorphous calcium carbonate. This could be deduced from the low m...

Mesocrystals—Ordered Nanoparticle Superstructures

“…[137] Even during the aggregation of clusters to amorphous but solid species, there appear to be liquid intermediates, which can transform to mesocrystals or single crystal-liquid hybrid particles exhibiting a soap film like texture or the typical minimal surfaces of liquids like the P-surface. [139] The corresponding time scales related to mesocrystal formation are even broader spanning from the sub-ms range for primary particle formation [140,141] to many hours of structural ripening, as observed for the (NH 4 ) 3 PW 12 O 40 example. [42] This spans at least about 8 decades in time (10 À3 -10 5 s).…”

The role of sulfate groups in controlling CaCO3 polymorphism

“…Under such conditions, the rapid formation of a most soluble and disordered phase such as vaterite is kinetically favored (Sawada, 1998), in agreement with the Ostwald step rule (Sö hnel and Garside, 1992). It is noteworthy that other more soluble and disordered phases, like ACC, might form prior to vaterite, as reported by numerous authors Loste et al, 2003;Pontoni et al, 2003). Although neither the XRD nor the IR spectroscopic analyses showed any evidence of the presence of this phase in the precipitates, its presence in the very initial stages of the process cannot be dismissed.…”

“…The nucleation and growth of metastable CaCO 3 polymorphs is commonly related to the predominance of kinetic factors over thermodynamic properties (Ogino et al, 1987;Sawada, 1998). High supersaturation levels at nucleation and the initial stages of growth result in the formation of ACC, vaterite and aragonite (Sawada, 1998;Bolze et al, 2002;Pontoni et al, 2003). In agreement with Ostwald's step rule (Sö hnel and Garside, 1992), these three crystalline forms ultimately transform into the more stable calcite via dissolution-crystallization reactions of various degrees of complexity (Fernández-Díaz et al, 2009).…”

The Effect of Additives on Amorphous Calcium Carbonate (ACC): Janus Behavior in Solution and the Solid State

“…Thus, zones depleted of ACC particles have often been observed adjacent to calcite crystals growing on solid substrates from suspensions of ACC. [ 31 , 32 ] Simultaneous SAXS/ WAXS studies of the transformation of ACC have demonstrated that the heterogeneous nucleation and growth of calcite on the walls of the reaction vessel is accompanied by dissolution of the ACC, [ 33 ] while measurement of the changes in the solution composition which occur on crystallization of ACC to vaterite and calcite also show that the solution composition (as expressed in the ion activity product) remains at the level of the least stable polymorph present. [ 16 ] The solution is therefore supersaturated with respect to calcite and vaterite during crystallization of the ACC to these phases, which demonstrates that the rate-determining step is the growth of the crystalline phases rather than dissolution of the ACC.…”