Crystallization of Mixed Alkaline-Earth Carbonates
in Silica Solutions at High pH
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Abstract
The ability of silica to influence
the mineralization of alkaline-earth
carbonates is an outstanding example for the formation of biomimetic
structures in the absence of any organic matter. Under suitable conditions,
silica-stabilized carbonate nanocrystals can spontaneously self-assemble
into hierarchical materials with complex morphologies, commonly referred
to as “silica biomorphs”. However, growth of these crystal
aggregates has largely been restricted to the higher homologues in
the alkaline-earth series, i.e., SrCO<sub>3</sub> and BaCO<sub>3</sub>, while corresponding architectures of the much more relevant calcium
carbonate are quite difficult to realize. To systematically address
this problem, we have crystallized metal carbonates in the presence
of silica at high pH, using barium and strontium chloride solutions
that contained increasing molar fractions of Ca<sup>2+</sup>. The
resulting materials were analyzed with respect to their composition,
structure, and crystallography. The obtained data demonstrate that
the growth process is already strongly affected by small amounts of
calcium. Indeed, morphologies typically observed for SrCO<sub>3</sub> and BaCO<sub>3</sub> remained absent above certain thresholds of
added Ca<sup>2+</sup>. Instead, globular and hemispherical structures
were generated, owing to fractal branching of carbonate crystals as
a consequence of poisoning by silica. These alterations in the growth
behavior are ascribed to relatively strong interactions of hard calcium
ions with silicate species in solution, shifting their speciation
toward higher oligomers and even inducing partial coagulation. This
notion is confirmed by additional experiments at increased ionic strength.
Our results further demonstrate that the observed hemispherical particles
exhibit distinct polymorphism, with orthorhombic solid solutions (aragonite-type
(Sr,Ca)CO<sub>3</sub> and (Ba,Ca)CO<sub>3</sub>) being formed at lower
Ca<sup>2+</sup> contents, whereas Sr<sup>2+</sup>/Ba<sup>2+</sup>-substituted
calcite prevails at higher Ca<sup>2+</sup> fractions. In the case
of Ba<sup>2+</sup>/Ca<sup>2+</sup> mixtures, there is moreover an
intermediate range where virtually identical morphologies were confirmed
to be Ba<sup>2+</sup>-doped vaterite. These findings extend the variety
of structures and compositions accessible in these simple systems,
and may explain difficulties previously encountered in attempts to
prepare CaCO<sub>3</sub> biomorphs at standard conditions