Chemical and physical controls on the transformation of amorphous calcium carbonate into crystalline CaCO3 polymorphs

Abstract

Calcite and other crystalline polymorphs of CaCO 3 can form by pathways involving amorphous calcium carbonate (ACC). Previous studies of ACC provide important insights, but apparent inconsistencies in the literature indicate the relationships between ACC composition, local conditions, and the subsequent crystalline polymorphs are not yet established. This experimental study quantifies the control of solution composition on the transformation of ACC into crystalline polymorphs in the presence of magnesium. Using a mixed flow reactor to control solution chemistry, ACC was synthesized with variable Mg contents by tuning input pH, Mg/Ca, and total carbonate concentration. ACC products were allowed to transform within the output suspension under stirred or quiescent conditions while characterizing the evolving solutions and solids. As the ACC transforms into a crystalline phase, the solutions record a polymorph-specific evolution of pH and Mg/Ca. The data provide a quantitative framework for predicting the initial polymorph that forms from ACC based upon the solution aMg 2+ /aCa 2+ and aCO 3 2-/aCa 2+ and stirring versus quiescent conditions. This model reconciles apparent discrepancies among previous studies that report on the nature of the polymorphs produced from ACC and supports the previous claim that monohydrocalcite may be an important, but overlooked, transient phase on the way to forming some aragonite and calcite deposits. By this construct, organic additives and extreme pH are not required to tune the composition and nature of the polymorph that forms. Our measurements show that the Mg content of ACC is recorded in the resulting calcite with a ≈1:1 dependence. By correlating the composition of these calcite products with the Mg tot /Ca tot of the initial solutions, we find a ≈3:1 dependence that is approximately linear and general to whether the calcite is formed via an ACC pathway or by the classical step-propagation process. Comparisons to calcite grown in synthetic seawater show a ≈1:1 dependence. The relationships suggest that the local Mg 2+ /Ca 2+ at the time of precipitation determines the calcite composition, independent of whether growth occurs via an amorphous intermediate or classical pathway for a range of supersaturations and pH conditions. The findings reiterate the need to revisit the traditional picture of chemical and physical controls on CaCO 3 polymorph selection. Mineralization by pathways involving ACC can lead to the formation of crystalline phases whose polymorphs and compositions are out of equilibrium with the local growth media. As such, classical thermodynamic equilibria may not provide a reliable predictor of observed compositions.

Amorphous Phase Mediated Crystallization: Fundamentals of Biomineralization

“…The possible effects of ASW compositions on calcium carbonate crystallization [12,[91][92][93] and shell formation in living species [88,90,94] have been investigated in detail. Blue et al [12] studied the transformation of ACC in calcium carbonate solutions, and found that the crystallization pathway, the polymorph selection, and the mineral composition were sensitive to the physical (stir or not) and chemical (Mg/Ca, carbonate/Ca ratio) conditions, which cannot be explained by the classical thermodynamic equilibria of crystallization. This provides a new understanding of biomineralization where organisms might control the physical-chemical factors (such as local alkalinity, pH, and supersaturation) during biomineralization to enable the precipitation of metastable phases with the unusual calcite compositions and textures that cannot be obtained by the classical crystal growth models.…”

“…Classical crystallization theories have been widely applied for understanding the crystallization behavior of biomimetic mineralization systems [3,8,9]. However, recent studies have failed to adequately explain many biomimetic crystallization systems using classical crystallization theory [10][11][12], which supposes that crystal nucleation or growth is formed directly from ion-by-ion additions [13]. Recently, researchers began hypothesizing that varied "nonclassical" crystallization pathways were more likely to be a product of biomineralization ( Figure 1) [14], which refers to aggregation-based crystallization [15,16], pre-nucleation clusters [17][18][19][20], and amorphous phase mediated crystallization (APMC) [21][22][23][24].…”

High‐Magnesium Calcite Mesocrystals: Formation in Aqueous Solution under Ambient Conditions

“…[3] Them ain stream in the synthesis of high-Mg calcites is to exploit additives which are either biomolecules extracted from mineral forming organisms [2,[4][5][6] or synthetic compounds rich in carboxylate groups. [23][24][25][26][27] While these studies in aqueous solutions have deepened our understanding of the thermodynamic controls on the phase transformation of calcium carbonate,t he biological relevance of these studies awaits further substantiation because biominerals usually have very unique hierarchical levels of structure formation. [15] Knowing that ACCc ould be stabilized by al arge variety of additives [16,17] and that ion binding can modulate significantly the nucleation events, [18] the functions of the aforementioned additives upon the Mg-calcite formation are at least two folded.…”

Precipitation of Calcium Carbonate Mineral Induced by Viral Lysis of Cyanobacteria

“…TEM images revealed no order in the majority of the detected particles, which was confirmed by the diffuse rings in the selected area electron diffraction patterns but the appearance of nanodomains within the ACC particles (RodriguezBlanco et al, 2008). ACC, which received relatively little attention as one of metastable CaCO3 phases, was increasingly recognized as prevalent during calcification (Blue et al, 2017). ACC may precipitate when the conditions promote high local supersaturations for short periods of time (Blue et al, 2017).…”

“…ACC, which received relatively little attention as one of metastable CaCO3 phases, was increasingly recognized as prevalent during calcification (Blue et al, 2017). ACC may precipitate when the conditions promote high local supersaturations for short periods of time (Blue et al, 2017). Although the saturation index (SI) of the ACC <0 (Table 2), 25 implying the nonspontaneous ACC formation within the solution, the growth of cyanobacteria in the present experiment created an ACC favorable microenvironment at the 10 th day reflected by the removing of Ca…”