Mechanistic Insights into the Crystallization of Amorphous Calcium Carbonate (ACC)

Many organisms use amorphous calcium carbonate (ACC) during crystalline calcium carbonate biomineralization, as a means to control particle shape/size and phase stability. Here, we present an in situ smalland wide-angle X-ray scattering (SAXS/WAXS) study of the mechanisms and kinetics of ACC crystallization at rapid time scales (seconds). Combined with offline solid and solution characterization, we show that ACC crystallizes to vaterite via a threestage process. First, hydrated and disordered ACC forms, then rapidly transforms to more ordered and dehydrated ACC; in conjunction with this, vaterite forms via a spherulitic growth mechanism. Second, when the supersaturation of the solution with respect to vaterite decreases sufficiently, the mechanism changes to ACC dissolution and vaterite crystal growth. The third stage is controlled by Ostwald ripening of the vaterite particles. Combining this information with previous studies, allowed us to develop a mechanistic understanding of the abiotic crystallization process from ACC to vaterite and all the way to calcite. We propose this is the underlying abiotic mechanism for calcium carbonate biomineralization from ACC. This process is then augmented or altered by organisms (e.g., using organic compounds) to form intricate biominerals. This study also highlights the applicability of in situ time-resolved SAXS/WAXS to study rapid crystallization reactions. ■ INTRODUCTION The anhydrous crystalline calcium carbonate (CaCO3) polymorphs, which form under ambient conditions, are calcite, aragonite, and vaterite. Their crystallization in both abiotic and biotic systems is often preceded by the formation and subsequent transformation of amorphous calcium carbonate (ACC). Synthetic calcium carbonates with specific particle sizes, shapes, and structures can be produced using the ACC formation pathway. Large quantities are manufactured for many industrial applications (e.g., paper manufacture and pharmaceuticals); however, in some cases, their precipitation is highly undesirable (e.g., scale formation in oil pipes), which leads to high repair/replacement costs. Calcium carbonates also form in a wide variety of natural environments (e.g., soils and sediments) and are ubiquitous at the Earth’s surface. For example, calcium carbonate is the major component of ancient limestone deposits or modern marine reefs. Most modern, natural calcium carbonate phases are formed by organisms as biominerals, which fulfill a wide variety of functions (e.g., stability and protection). The most common and stable calcareous biominerals are calcite and aragonite. However, in some cases, less stable vaterite is biomineralized (e.g., in spicules of an ascidian and various fish otoliths). Despite their importance, a clear understanding of the fundamental processes controlling the crystallization of calcium carbonate is still lacking. Many biomineralizing organisms utilize the ACC pathway to precisely control the particle shape and crystalline polymorph during the formation of their shells or spines etc. For example, sea urchin larvae produce highly elongated single crystals of calcite by the controlled deposition and transformation of ACC within a biological membrane. Such natural biological processes have informed biomimetic studies of crystal growth and design and are now used to manipulate the shape and size of synthetic calcium carbonate particles. The structure and chemistry of ACC is complex with several forms of ACC classified according to their water content, local order, and mode of formation (e.g., abiotic vs biogenic). A key variable is the amount of structural water. Hydrated-ACC can contain up to ∼1.6 mol of water per mole of CaCO3, yet several less hydrated and even anhydrous forms of ACC have been described. For example, Radha et al. produced both disordered and less disordered ACC with different degrees of hydration. Observations by Politi et al. during sea urchin spicule formation showed that initially hydrated ACC forms, which transformed to anhydrous ACC before crystallizing to calcite via a secondary nucleation process. The enthalpies of the ACC phases in relation to the crystalline calcium carbonates show that energetically the sequence of increasing stability and possible crystallization pathway is as follows: disordered, hydrated ACC → less disordered, less hydrated ACC → anhydrous ACC → vaterite → aragonite → calcite. This sequence highlights that all ACC phases have higher formation Received: May 17, 2012 Published: May 21, 2012 Article pubs.acs.org/crystal © 2012 American Chemical Society 3806 dx.doi.org/10.1021/cg300676b | Cryst. Growth Des. 2012, 12, 3806−3814 enthalpies than the crystalline polymorphs; thus, ACC can act as a precursor to any of the anhydrous crystalline phases. However, in inorganic systems, the transformation of ACC to its crystalline counterparts is often extremely rapid (seconds to minutes), and because of their inherent instability, the ACC precursors are difficult to characterize using ex situ techniques (e.g., TEM and FTIR). Therefore the proposed sequence of polymorph formation has not yet been observed in full or quantified in detail. Abiotically synthesized ACC rapidly transforms to vaterite, calcite, or aragonite, with the polymorph formed dependent on a number of factors including time, fluid composition, presence of organic molecules, and temperature. In most cases, pure ACC will transform to calcite via a vaterite intermediate at low temperatures (<30 °C) and to aragonite via vaterite at higher temperatures (>60 °C). At low temperatures, the addition of magnesium tends to favor the direct formation of calcite from ACC, without a vaterite intermediate. In contrast, sulfate has been shown to increase the stability and persistence of vaterite significantly at low temperatures and may be key to understanding the stabilization of vaterite in biological and environmental systems. The mechanisms and kinetics of the later stages of the crystallization pathways have been shown to be controlled by the dissolution of vaterite and precipitation of calcite, with the rate controlled by the surface area of calcite. However, the mechanism of the ACC to vaterite transformation is still not clear, with several possible mechanisms proposed. Many studies have suggested that ACC dissolves and vaterite spheres formed via homogeneous nucleation of nanocrystalline vaterite particles, followed by fast aggregation to form micrometer sized polycrystalline spheres. A solid state mechanism for the ACC to vaterite crystallization has also been proposed, with the ACC particles dehydrating and recrystallizing to form vaterite. Finally, a recent study, based on imaging of inorganically precipitated vaterite, suggested that vaterite forms via ACC dissolution coupled to spherulitic growth. Resolving the mechanisms and kinetics of ACC crystallization in abiotic systems is key to developing a detailed understanding of how calcium carbonate phases form in both natural and synthetic processes. In particular, the mechanism of transformation between the individual phases forming as the system moves toward thermodynamic equilibrium from disordered hydrated ACC to fully crystalline calcite needs to be fully quantified. However, it should be noted that many biomineralization processes occur entirely within a biological membrane with little or no free water and in the presence of organic macromolecules. Therefore, any abiotic mechanism of ACC crystallization in solution may be altered or manipulated by the organism during biomineral formation. In this study, we performed in situ smalland wide-angle Xray scattering (SAXS/WAXS) experiments with a time resolution of 1 s to study the direct transformation of ACC to vaterite in solution and the effect of sulfate on this reaction. Combined with offline characterization of the solid phases and solution composition, we show that the crystallization of ACC to vaterite occurs in three distinct stages. First, the initial ACC phase dehydrates and vaterite forms via rapid spherulitic growth. This is followed by an intermediate stage where vaterite continues to form from the dissolving ACC, and finally, the vaterite particle grows via surface-controlled Oswald ripening. In addition, the presence of sulfate decreased the overall crystallization rate and reduced the particle growth rate during