This thesis is concerned with the manifold interactions that occur when alkaline-earth metal carbonates are crystallized in the presence of dissolved silica as an additive. The described work subdivides into two main lines of research. On the one hand, an understanding of the potential roles of silica during crystallization was sought on a fundamental level. That is, the mineral - in this case calcium carbonate - was directly precipitated from silica-containing solutions and the effect on growing particles and phase interconversions were characterized at different stages. On the other hand, experiments were performed in which the components were allowed to interact under conditions of low supersaturation, thus leading to gradual mineralization and enabling complex structuring. This resulted in the spontaneous self-assembly of hierarchical polycrystalline architectures, termed "silica biomorphs", which in the case of barium carbonate displayed delicate morphologies with sinuous shapes such as regular helicoids.
Studies in the field of calcium carbonate precipitation showed that addition of silica is an effective means to arrest the crystallization process at virtually any stage simply by adjusting experimental parameters like species concentrations, pH, or temperature. In dilute systems, it was found that initially nucleated, metastable particles of amorphous calcium carbonate (ACC) become enveloped by a skin of silica due to local gradients in pH at growing surfaces and the intimate dependence of carbonate and silicate solubility on the pH. This spontaneous coating was investigated by a number of techniques and it was confirmed to increase the kinetic stability of the amorphous phase, causing decelerated conversion to stable calcite or fully preventing this energetically favored transformation. When stabilized temporarily, the inner ACC fraction of the as-formed core-shell particles served as a depot for CaCO3, slowly releasing growth units to the solution. Under these circumstances, a broad variety of unusual calcite morphologies was obtained, ranging from single crystals with uncommon habit to aggregates of nanocrystals with shapes beyond crystallographic restraints.
In brines at higher supersaturation of CaCO3, a distinct silica-mediated stabilization could be verified also for the metastable crystalline CaCO3 modifications vaterite and aragonite, as evidenced by on-line diffraction analysis using synchrotron radiation. These findings suggest that the presence of silica, along with a sensible adjustment of conditions, does not only permit to study crystallization mechanisms in general by capturing transient intermediates, but also allows for a more or less concerted selection of polymorphs in the course of crystallization. The latter is clearly of interest for the synthesis of CaCO3 powders with tunable properties in view of industrial applications, while the former and in particular the achieved stabilization of ACC may serve as novel approaches to scale inhibition.
Finally, systems at fairly low CaCO3 supersaturation and relatively high silica concentration were investigated. Here, the added silica was found to interfere with the crystallization process already before nucleation. In fact, so-called prenucleation clusters - recently discovered solute-like precursors of solid CaCO3 - were traced in the samples and found to be protected against otherwise rapidly occurring nucleation by binding of silica in their periphery. This concept was elaborated to develop an experimental procedure for the isolation these elusive species and render their analysis by conventional methods possible. Further, the degree of stabilization could be tuned by varying the pH, such that nucleation proceeded gradually to later stages. Eventually, this strategy enabled a direct observation of the processes underlying nucleation, and a non-classical model for homogeneous nucleation of calcium carbonate is proposed.
Work addressing silica biomorphs was primarily focused on unraveling the morphogenetic mechanisms governing the evolution of complex and biomimetic form in simple inorganic environments. To that end, existing ideas on a pH-based chemical coupling of the speciations of carbonate and silicate in aqueous solution and an associated dynamic interplay during precipitation were assessed and efforts were made to advance and substantiate corresponding models.
By a series of experiments conducted under conditions of forced convection, it was ascertained that growth of silica biomorphs is a local phenomenon taking place only at the front of developing aggregates, where the supersaturation and thus the driving force for precipitation is enhanced as compared to the bulk. In particular, the mineralization of one of the components (BaCO3 and silica) is thought to affect locally the chemistry of the other via the pH, ultimately triggering its precipitation. As a consequence, carbonate and silica are alternately deflected from equilibrium in time and therefore mineralized, depicting a synergetic scenario strongly reminiscent of the Belousov-Zhabotinsky reaction. This leads to a continuous production of silica-bearing BaCO3 nanocrystals, which constitute the emerging silica biomorph.
On larger length scales, the assembly of crystallites grows free from constraints, usually forming flat sheets that later curl and give rise to twisted morphologies by mutual winding of different segments. In light of the performed experiments, it appears as if the morphological evolution of the crystal aggregates is widely determined by the beneficial impact of extrinsic and intrinsic surfaces on nucleation barriers. In this paradigm, smooth curvature in silica biomorphs results from the propensity of forming crystal aggregates to fold back on themselves and use their existing surface as a substrate. In turn, flat morphologies are favored when the assembly grows in direct contact with a foreign interface, such as a container wall.
These particular characteristics, together with the observed structural hierarchy, delineate a fascinating analogy between the obtained abiotic precipitates and biologically produced mineral frameworks found in the tissues and exoskeletons of diverse living organisms. In this manner and thanks to the ease of their preparation and handling, silica biomorphs can be considered valuable laboratory model systems for the study of biomineralization concepts.
With respect to these properties, further work was devoted to widen the range of morphologies and structures accessible with silica biomorphs. This was achieved for instance by introducing specific additives or varying distinct synthesis parameters, partially triggering remarkable alterations in the self-assembly behavior. The observed changes are interpreted in the realm of the envisaged morphogenetic mechanism and can mainly be ascribed to subtle shifts in the speciation of silica under the respective conditions.
Eventually, the kinetics of the formation of silica biomorphs were studied. For this purpose, the concentrations of the involved reagents were monitored over prolonged periods of time and compared with growth rates determined for individual crystal aggregates by video microscopy. Results confirm reaction control for the process, thus supporting the autocatalytic character of growing fronts. The gathered data moreover shed novel light on the role of silica during crystallization as well as the mode and degree of its incorporation into forming aggregates.
The drawn conclusions contribute to a deeper understanding of the phenomena leading to complex self-organization in these simple systems and to some extent devise routes to transfer the inherent concepts to other minerals and the design of novel promising materials
