Transformation Of Amorphous Calcium Carbonate Research Articles

Oxygen isotope fractionation during amorphous to crystalline calcium carbonate transformation at varying relative humidity and temperature

Crystalline calcium carbonate isotope compositions have been widely used to reconstruct past environments. However, if their isotopic compositions are modified because of crystallization from an amorphous precursor, their reliability as paleo-geochemical proxies can be compromised. This study explored the changes in the oxygen isotope compositions during the transformation of amorphous calcium carbonate (ACC) into crystalline carbonate under different conditions of relative humidity (RH of 33–95 %), temperature (T of 5 °C and 20 °C) and in the presence/absence of atmospheric CO2. The data showed that at low RH and T (e.g., RH ≤ 45 % and 5 °C) when a complete ACC-crystalline carbonate transformation did not take place then the original ACC δ18O values (δ18OCaCO3 = −15.9 ± 1.0 ‰, VPDB) were preserved throughout the experimental runtime (up to 144 days). In contrast, in fully crystallized CaCO3 (e.g., at RH ≥ 60 %) the δ18OCaCO3 values increased rapidly over the first few days, followed by a slower and gradual increase. By the end of the experiments (i.e., after 103–144 days) the crystalline δ18OCaCO3 values ranged from −10.4 ‰ to −8.1 ‰ in the presence of atmospheric CO2 and from −12.6 ‰ to −9.5 ‰ in the CO2-free experiments. These changes in oxygen isotope compositions of the CaCO3 reaction products (calcite and/or vaterite) were mainly driven by exchange with H2O from the hydrated ACC i.e. the synthesis fluid. In CO2-present experiments, oxygen isotope fractionation factors between the CaCO3 reaction products and the synthesis fluid (18αc–w) approached or exceeded oxygen isotope equilibrium values. This could be explained by a decrease in the initially high pH of the aqueous fluid released from ACC dissolution during CO2 hydration/hydroxylation, which would have increased the oxygen isotope exchange kinetics between H2O and dissolved inorganic carbon (DIC). In some experiments, the hydration/hydroxylation of 18O-enriched CO2, due to isotopic salt-effects, might have also resulted in 18O-enriched calcium carbonates and calculated fractionation factors that exceeded equilibrium values. In the CO2-free experiments, isotopic equilibrium between the crystalline phase and the synthesis fluid was not reached. This oxygen isotope disequilibrium suggests that without the pH lowering effect of the hydroxylation/hydration of CO2, the CO32− released during ACC/calcite dissolution-reprecipitation may have not isotopically equilibrated with the high pH synthesis fluid due to the long equilibration times required to reach isotope equilibrium at high pH values, leading to the self-buffering of δ18OCaCO3 values. The results suggest that the oxygen isotopic compositions of natural carbonates formed from ACC transformation in air and at low water/solid ratio (e.g., biominerals or carbonates formed in caves) are complex and cannot be used as simple proxies if the reaction kinetics (RH/CO2/T) and H2O sources are not known and quantified.

Enhancing the carbonation of γ-C2S through MgCl2 incorporation

With the growing awareness of CO2 emissions reduction in the cement industry, minerals like γ-C2S with lower calcium content and formation temperatures have shown potential as sustainable alternatives to ordinary Portland cement. Although γ-C2S possesses considerable carbonation reactivity, incorporating additives can further optimize its carbonation hardening performance. This study investigated the effect of different dosages of internally incorporated MgCl2 on the carbonation behavior of γ-C2S. Test results demonstrated that incorporating 1.87 % MgCl2 led to the maximum compressive strength and CO2 uptake. The compressive strength and CO2 uptake following 24 h of carbonation increased from 63.2 MPa and 18.23 % to 84.2 MPa and 23.95 %, respectively. XRD and TG analyses indicated that Mg2+ promoted the transformation of amorphous calcium carbonate into more stable crystalline calcite and the formation of magnesium calcite and nesquehonite. SEM imaging revealed that the generated carbonation products tightly filled the pores, enhancing the compactness and strength. However, excessive MgCl2 coating on particle surfaces hindered further carbonation. These findings reveal opportunities to further optimize the carbonation hardening properties of calcium silicate minerals and precisely tailor the morphology of calcium carbonate products for sustainable construction.

Transformation of amorphous calcium carbonate in the presence of magnesium, phosphate, and mineral surfaces

Amorphous calcium carbonate (ACC) is known as a precursor to crystalline calcium carbonate polymorphs (CaCO3) in various natural and synthetic systems. While ACC has been shown to form by heterogeneous nucleation in biological systems, it is less obvious whether it plays any role in inorganic settings where carbonate minerals grow on pre-existing surfaces. Tight assemblies of calcium carbonate (CaCO3) phases and clay minerals occur in rock-forming quantities in marlstones, are abundant in Earth’s critical zone, in the sediments of aqueous environments, and in the atmosphere as aerosol particles. Previous studies suggested that the co-occurrence of clay and carbonate minerals (especially smectite and calcite) results from the heterogeneous nucleation of CaCO3 on the surface of smectite. In order to understand the role of ACC in the heterogeneous nucleation of carbonate minerals and the effects of common silicate mineral surfaces on the stability of ACC, we synthesized ACC both in the presence and absence of smectite, kaolinite, and quartz, monitored the pH of the solution during the experiments, and studied the precipitated phases with various transmission electron microscopy techniques. Since Mg2+ and (PO43−) ions are known to prolong the lifetime of ACC and are important components in natural aqueous environments, we tested their effects on ACC transformation in both homogeneous and heterogeneous systems.In all studied systems ACC was the first condensed carbonate phase. The presence of smectite in the solution reduced the lifetime of ACC and led to the rapid formation of calcite. We suggest that the main factor driving the transition was the enhanced growth of ACC particles on the surfaces of smectite flakes. On the other hand, the presence of kaolinite and quartz slightly increased ACC stability; ACC typically rimmed the silicate particles and transformed slower than separated, individual ACC particles. When Mg2+ was present in the solution, ACC stability significantly increased and led to the formation of aragonite instead of calcite in both homogeneous and heterogeneous systems. The presence of (PO4)3− ions caused only a minor delay in ACC transformation and the crystalline product was calcite, irrespective of the presence of silicate minerals. These observations suggest that smectite minerals play important roles in the formation of carbonate minerals by providing a suitable surface for the rapid growth and coalescence of ACC particles, leading to larger particle sizes which, in turn, result in the conversion of ACC into calcite.

Acceleration of Wound Healing through Amorphous Calcium Carbonate, Stabilized with High-Energy Polyphosphate

Amorphous calcium carbonate (ACC), precipitated in the presence of inorganic polyphosphate (polyP), has shown promise as a material for bone regeneration due to its morphogenetic and metabolic energy (ATP)-delivering properties. The latter activity of the polyP-stabilized ACC ("ACC∙PP") particles is associated with the enzymatic degradation of polyP, resulting in the transformation of ACC into crystalline polymorphs. In a novel approach, stimulated by these results, it was examined whether "ACC∙PP" also promotes the healing of skin injuries, especially chronic wounds. In in vitro experiments, "ACC∙PP" significantly stimulated the migration of endothelial cells, both in tube formation and scratch assays (by 2- to 3-fold). Support came from ex vivo experiments showing increased cell outgrowth in human skin explants. The transformation of ACC into insoluble calcite was suppressed by protein/serum being present in wound fluid. The results were confirmed in vivo in studies on normal (C57BL/6) and diabetic (db/db) mice. Topical administration of "ACC∙PP" significantly accelerated the rate of re-epithelialization, particularly in delayed healing wounds in diabetic mice (day 7: 1.5-fold; and day 13: 1.9-fold), in parallel with increased formation/maturation of granulation tissue. The results suggest that administration of "ACC∙PP" opens a new strategy to improve ATP-dependent wound healing, particularly in chronic wounds.

The transformation of amorphous calcium carbonate to calcite and classical nucleation theory

The interfaces of calcite with water, dehydrated and hydrated amorphous calcium carbonate are studied with molecular dynamics simulations. The interfacial energies demonstrate that the calcite interface is most stable when in contact with water or low concentration solutions rather than amorphous calcium carbonate. These values are used to test the interplay between supersaturation and the interfacial energy for calcite. They demonstrate that a dissolution-reprecipitation process should always be energetically preferred to a solid state transformation of amorphous calcium carbonate to calcite.

The underlying role of sodium tripolyphosphate on the cementitious mechanism of calcium carbonate binder

Calcium carbonate binder is well-known to reduce the carbon footprint and brings great economic and environmental benefits. However, the cementitious mechanism is unclear due to raw material diversity. We prepare calcium carbonate crystalline polymorphs-vaterite and calcite in the presence of sodium tripolyphosphate (STPP), and the cementitious mechanism is revealed. Herein, the optimal colloidal structure with vaterite as the binding phase is obtained at 5% STPP and the 1d compressive strength reaches 16.99 MPa. The cementitious property can be attributed to the crystallization transformation of amorphous calcium carbonate (ACC) and the electrostatic attraction of particles. In the cementitious process, the effect of STPP on the polymorph selection is determined by STPP contents. At low contents (＜5%), STPP is more effective in inhibiting the nucleation and growth of calcite than vaterite. At high contents (＞5%), calcite is entirely inhibited while the formation of vaterite takes place on the surface of bulk ACC through the coupling mechanism of dissolution-recrystallization and solid-state growth. These results shed light on the function of STPP during calcium carbonate biomineralization and the cementitious mechanism can help for preparing low-carbon calcium carbonate binder.

Effect of coral powder on the carbonation behavior of γ-C2S

γ-dicalcium silicate (γ-C2S) is a potential sustainable binder owing to its high carbonation reactivity and self-pulverization properties. In this study, porous coral powders were used as a partial replacement of γ-C2S, and the effect on the degree of carbonation, the characteristics of carbonated products, and the compressive strength were investigated. Results suggest that the introduction of coral powders is conducive to the ingress of CO2 and thus the enhanced degree of carbonation, which is evidenced by both the increase of carbonation temperature rise and the extension of the sustained carbonation period. The incorporation of coral powders leads to a significant suppression in the content of amorphous calcium carbonate and an enrichment in crystalline phases. The compressive strength of γ-C2S compacts is increased from 79.1 MPa to 88.5 MPa after γ-C2S is partially replaced with 30%wt coral powders, while the same level of partial replacement by quartz powders results in a proportional reduction in compressive strength which is 52.8 MPa. The mechanical enhancement by coral powders is found to be associated with the unique porous structure which introduces more gas transport channels to increase the internal degree of carbonation, and favors the formation of denser interfacial transition zone between coral powders and the carbonated matrix. In addition, the tiny aragonite crystals in the coral powders are believed to induce the transformation of amorphous calcium carbonate into the crystalline phase, which contributes to the strengthening effect.

Microbial EPS‐mediated amorphous calcium carbonate–monohydrocalcite–calcite transformations during early tufa deposition

AbstractThe Holocene Wolfenden tufa deposit in south‐eastern British Columbia, western Canada, preserves a unique record of the earliest stages of calcium carbonate deposition resulting from microbial extracellular polymeric substances‐mediated precipitation of amorphous calcium carbonate (ACC) with partial transformation to monohydrocalcite (MHC) and subsequently to nanocrystalline calcite. This is the first documentation of tufa mineralogy involving ACC transformation to MHC. Progressive dehydration triggered ACC–MHC–nanocrystalline calcite transformations on bryophytes, algae and cyanobacteria sheaths. The adsorption of extracellular polymeric substances matrix molecules into the ACC and ACC–MHC structures preserved polymorph mineralogy of incomplete transformation. Unusual concentrations of biofilm extracellular polymeric substances filaments provided nucleation sites for the ACC precipitation. The ACC nucleation calcified extracellular polymeric substances filaments and resulted in partially coalesced arrays of nanoscale ACC spheroids. Mesocrystalline structures of MHC reconfigured the concentric growth layers of ACC precipitate with bulbous ACC–MHC protuberances. Nanocrystalline rhombic faces of calcite developed within and on the surfaces of the ACC–MHC protuberances. Dehydration of these concentric growth layers of ACC–MHC resulted in the transformation into nanocrystalline calcite with substrates coalesced into micrite fabrics. Recrystallisation obliterated evidence of the calcified extracellular polymeric substances filaments and resulted in microcrystalline calcite spar domains as the widespread encrustation fabric. Localised magnesium adsorption during nucleation of the ACC within the biofilms resulted in needle calcite crystals without the precursor ACC–MHC transformation process. Microbial extracellular polymeric substances‐mediated precipitation of ACC as a necessarily critical step in the earliest phase of the tufa deposition process, leading to the nucleation of calcite has been underappreciated and generally not considered. These earliest stages of calcium carbonate precipitation are proposed as a possible template for other tufa deposits, where the evidence of microbial extracellular polymeric substances‐mediated precipitation of ACC with transformation to MHC and subsequently to nanocrystalline calcite has been obscured by recrystallisation into micrite and spar fabrics.

Magnesium isotope fractionation during carbonate precipitation associated with bacteria and extracellular polymeric substances

Magnesium (Mg) is abundant on Earth's surface aquatic environments and plays an important role during the precipitation of biogenic CaCO3 minerals. Moreover, Mg isotopes are increasingly used to study a number of bio-induced mineralization processes of calcite. In this study, Mg isotope signatures during calcite and aragonite precipitation in the presence of Curvibacter sp. HJ-1 and of their extracellular polymeric substances (EPS) were investigated and the evolution of mineralogical and liquid composition were tracked over time. The results showed that light Mg isotopes were preferentially incorporated into the precipitated solids in all the experiments with and without bacteria. Significant Mg isotope fractionation was associated with the transformation of amorphous calcium carbonate (ACC) to crystal carbonate (Δ26Mgcrystals-ACC = −0.9‰ and −0.7‰ in biotic and EPS experiments, respectively), and obvious Mg isotope fractionation was noted between solids and liquid (Δ26Mgsolid-liquid achieved −2.2‰ and −2.0‰ in biotic and EPS experiments, respectively). Besides, δ26Mg values were significantly correlated with pH, Mg content in liquid and solid, bacterial density and presence of EPS in biotic experiments. These findings indicated that strain HJ-1 had an effect on Mg isotope fractionation during calcite and aragonite precipitation. The difference in Mg isotope fractionation may be a new tool for understanding the biologically mediated effects on Mg-bearing carbonate precipitation, and serves as a useful alternative for aqueous Mg isotope, which is vital for reconstructing past environmental changes.

Mapping intrashell variation in Mg/Ca of brachiopods to external growth lines: Mg enrichment corresponds to seasonal growth slowdown

Determining secular changes in seawater temperature and Mg/Ca ratio is of high importance in paleooceanographic, paleoenvironmental and paleoecological studies. On the one hand, intra-shell variation in Mg/Ca in brachiopod shells is used to reconstruct historical changes in temperature and in seasonality. On the other hand, intra-shell analyses show a weak relationship between Mg/Ca and δ18O, but a positive correlation between Mg/Ca and Sr/Ca, and an increase of Mg/Ca at growth lines, indicating that Mg/Ca does not simply track seawater temperature alone. Internal growth lines are rarely visible in brachiopod shells, and the relation between growth lines that mark growth slowdown or cessation and Mg/Ca fluctuation is thus not properly understood. Here, using four species of extant brachiopods from the NE Pacific and the SW Pacific, from environments with low seasonality in seawater temperature (<5 °C), and two species of Late Triassic brachiopods (Western Carpathians), we measure intra-shell ontogenetic variation in Mg/Ca, S/Ca and Sr/Ca with laser ablation ICP-MS and electron microprobe. In contrast to previous studies, we map variation in Mg, S, and Sr to external growth lines. We find that, first, zones of Mg/Ca enrichment in the secondary layer form 10–50 μm-thick discrete bands or clusters of micrometric bands that consistently terminate at major external, predominantly annual growth lines that are associated with distinct notches that correspond to mantle retractions in most species. These bands generate distinct peaks in Mg/Ca that stand above low, more uniformly distributed background Mg/Ca values along laser-ablation and microprobe transects. The magnitude of Mg/Ca in these bands is comparable to Mg/Ca in the primary layer, exceeding background values of Mg/Ca in the secondary layer by 5–20 mmol/mol (by a factor of two to five). Second, Mg enrichment in discrete bands is closely associated with an increase in S concentrations in the secondary layer. Therefore, even when Mg and S uptake is expected to be highest under fast precipitation rate, these empirical observations show that Mg and S enrichment occur in the zones that were precipitated at times of a significant growth reduction (e.g., in cold-temperate brachiopods studied here during winters when primary productivity is low). The mechanisms for this paradox are unclear but can be related to uptake of Mg by amorphous calcium carbonate (ACC) to delay the transformation of ACC to calcite during the formation of growth lines. Therefore, intra-shell variation in Mg/Ca in brachiopods with major growth lines is primarily determined by seasonal decline or cessation in shell growth and not by seasonal changes in seawater temperature.

Control of MgSO40(aq) on the transformation of amorphous calcium carbonate to high-Mg calcite and long-term reactivity of the crystalline solid

The Mg and SO4 content of naturally occurring calcite are routinely used as paleoenvironmental proxies. Yet little is known about the mechanisms governing the presence of these ions in carbonate minerals when their formation proceeds via an amorphous precursor. To address this, the transformation of Mg-free amorphous calcium carbonate (ACC) into nanocrystalline high-Mg calcite (HMC) was experimentally studied in solutions containing 27 mM of Mg and a range of 10–90 mM of SO4. The obtained results suggest that ACC is stable for several minutes in the experimental solutions and this amorphous phase actively uptakes Mg and SO4 that are incorporated in its structure. Additionally, the obtained results suggest that the stabilization of ACC is not affected by its Mg content and that the transformation to HMC is effectively controlled by the abundance of the free Mg2+(aq) ion. The transformation of ACC to HMC occurs earlier at elevated SO4 concentrations because SO4 limits the availability of Mg2+(aq) due to the formation of the MgSO40(aq) complex. The HMC that is formed from ACC appears as aggregates composed of nanocrystallites and exhibits Mg and SO4 contents up to 8 and 2 mol% depending on the initial SO4 concentration in the reactive solution. The precipitated HMC was kept in contact with the reactive solution in order to assess its reactivity for up to 1 year of reaction time. Over time, a continuous exchange of Mg and SO4 between calcite and reactive solution was observed resulting in enrichment of Mg and depletion of SO4 affecting the total mass of the aggregates with the distribution of these elements to appear homogeneous in the crystalline solid. The high reactivity and the continuous exchange of solutes between the nanocrystalline calcite and the reactive solutions limits the use of Mg and SO4 content of these HMCs as environmental proxies.

Effect of magnesium on monohydrocalcite formation and unit-cell parameters

Abstract Monohydrocalcite (MHC) is hydrated calcium carbonate, which plays an active role in many geological processes, carbonate biomineralization, and can be used for fundamental science (as a paleoenvironmental indicator) and industry (for removal of hazardous anions). Despite a great number of works, the conditions preferable for MHC formation/stabilization and MHC crystal chemical patterns in relation to Mg and H2O are not clarified yet. In the course of current work, we conducted 38 syntheses to obtain information on MHC formation at different Mg/Ca ratios (0–12), pH (~9–12), and temperature (23 and 3 °C). Newly formed carbonate precipitates were studied by means of X-ray powder diffraction, optical, and scanning electron microscopy, energy-dispersive X-ray spectroscopy, Raman, and Fourier-transform infrared (FTIR) spectroscopies. The phase diagram for MHC, calcite, aragonite, and dypingite as a function of pH and Mg concentration in solution at T = 23 °C and Ca/CO3 = 0.5 was obtained. We demonstrated that MHC could be stable in dry conditions for up to two years and that the time of crystallization is important for the transformation of amorphous calcium carbonate to MHC. Our results on synthetic MHC stability show that the widespread idea that MHC is a short-lived intermediate phase is wrong. For the first time, on the basis of a regular changes in the unit-cell parameters the possibility of significant incorporation of magnesium in MHC has been demonstrated. According to FTIR data, it is shown to be accompanied by an increase in the water content, which leads to multidirectional change in a and c MHC parameters.

Effect of Cd on the Nucleation and Transformation of Amorphous Calcium Carbonate

Effect of Cd on the Nucleation and Transformation of Amorphous Calcium Carbonate

Nonclassical Crystallization of Amorphous Calcium Carbonate in the Presence of Phosphate Ions

Phosphate ions are commonly found in amorphous biominerals and play critical roles in controlling the stability and crystallization pathways of amorphous calcium carbonate (ACC). However, the role of phosphate ions during the transformation process of ACC remains elusive. Here, we show that while the onset of ACC transformation in solution could not be effectively delayed by addition of phosphate ions up to 5% (molar fraction in the initial carbonate solution), interestingly, the transformation process can be significantly prolonged. This is ascribed to the formation of a more stable phosphate-enriched amorphous phase during transformation and the inhibition of vaterite dissolution and calcite crystallization. Phosphate ions can also promote the nonclassical crystallization of calcite, where calcite crystals grow via oriented attachment of nanoparticles. In addition, nanoporous calcite single crystals can be obtained through crystallization of ACC after being embedded in the crystals. Our results are of significance to understand the crystallization mechanisms of amorphous materials and biomineralization processes.

Mechanisms of Phase Transformation and Creating Mechanical Strength in a Sustainable Calcium Carbonate Cement.

Calcium carbonate cements have been synthesized by mixing amorphous calcium carbonate and vaterite powders with water to form a cement paste and study how mechanical strength is created during the setting reaction. In-situ X-ray diffraction (XRD) was used to monitor the transformation of amorphous calcium carbonate (ACC) and vaterite phases into calcite and a rotational rheometer was used to monitor the strength evolution. There are two characteristic timescales of the strengthening of the cement paste. The short timescale of the order 1 h is controlled by smoothening of the vaterite grains, allowing closer and therefore adhesive contacts between the grains. The long timescale of the order 10–50 h is controlled by the phase transformation of vaterite into calcite. This transformation is, unlike in previous studies using stirred reactors, found to be mainly controlled by diffusion in the liquid phase. The evolution of shear strength with solid volume fraction is best explained by a fractal model of the paste structure.

Preparation and characterization of phosphate-stabilized amorphous calcium carbonate nanoparticles and their application in curcumin delivery

Nano-sized amorphous calcium carbonate (ACC) have a great potential for drug transport and drug delivery because of their high drug loading capacity, excellent biocompatibility and biodegradability. However, ACC is the thermodynamically least stable phase of calcium carbonate, and such thermal metastability often transforms it into more stable vaterite or calcite phases, so the inhibition of the phase transformation of ACC is of great significance for its application in drug delivery systems. Herein, phosphate as a morphology controlling additive was introduced to stabilize the metastable ACC. The results showed that the composite nanoparticles (named here the ACCP) with narrow size distribution located at approximately 128 nm (measured by dynamic light scattering) were obtained, when Ca2+ concentration and C/P were 10 mM and 7/3 respectively, prepared at 30 °C for 10 min. The powdered X-ray diffraction (XRD) results showed that phosphate has an excellent ability to stabilize ACC, and even after storage for 60 days at room temperature, the structure of ACCP remains amorphous. The in vitro drug release tests showed that ACCP nanoparticles had a high curcumin (Cur) loading capacity and a sustained drug release property. Moreover, the 1, 1-diphenyl-2-picrylhydrazyl radical (DPPH) radical scavenging activity assay showed that ACCP-encapsulating strategy could effectively prevent the decomposition of Cur. In vitro cytotoxicity tests demonstrated that the resultants have excellent biocompatibility. Therefore, the as-prepared ACCP nanoparticles are promising for drug delivery applications.

Gelatinous Siphon Sheath Templates the Starfruit-Shaped Aragonite Aggregate Growth

Biomimetic synthesis of aragonite with various templates in vitro is an important way to understand the biomineralization process and synthesize nacre-like materials. Herein, we used the siphon sheath from the bivalve Lutraria sieboldii as the substrate for the formation of calcium carbonate. We found that the inner layer of the sheath, which is composed of approximately 40% protein and 60% β-chitin, induced the formation of nearly pure aragonite by the transformation of amorphous calcium carbonate (ACC). More surprisingly, unique starfruit-shaped aragonite aggregates were observed on the substrate and were constructed from many adhered, oriented aragonite tablets. We consider that the acid-rich protein from the inner layer of the siphon sheath triggers the formation of ACC, and the swollen β-chitin regulates the transformation of ACC into aragonite by lattice matching and stereochemical recognition. The various surface adhesion energies of the crystal, the change in growth rates on different crystallographic facets, and the hexagonal features of the aragonite tablets led to the formation of starfruit-shaped aragonite aggregates.

Crystallization kinetics of amorphous calcium carbonate in confinement.

Phase transformations of carbonates are relevant to a wide range of biological, environmental, and industrial processes. Over the past decade, it emerged that crystallization pathways in these systems can be quite complex. Metastable intermediates such as amorphous calcium carbonate (ACC) were found to greatly impact composition, structure, and properties of more stable phases. However, it has been challenging to create predictive models. Rapid transformation of ACC in bulk has been one obstacle in the determination of nucleation rates. Herein, it is reported that confinement in microfluidic droplets allows separating in time the precipitation of ACC and subsequent nucleation and growth of crystalline CaCO3. An upper limit of 1.2 cm-3 s-1 was determined for the steady-state crystal nucleation rate in the presence of ACC at ambient conditions. This rate has implications for the formation of calcium carbonate in biomineralization, bio-inspired syntheses, and carbon sequestration.

Secrets of the Sea Urchin Spicule Revealed: Protein Cooperativity Is Responsible for ACC Transformation, Intracrystalline Incorporation, and Guided Mineral Particle Assembly in Biocomposite Material Formation.

The formation of the sea urchin spicule involves the stabilization and transformation of amorphous calcium carbonate (ACC) and assembly of ACC nanoparticle precursors into a mesoscale single crystal of fracture-resistant calcite. This process of particle assembly or attachment is under the control of a family of proteins known as the spicule matrix [Strongylocentrotus purpuratus (SpSM)] proteome. Recently, two members of this proteome, SpSM50 and the glycoprotein SpSM30B/C-G (in recombinant forms), were found to interact together via SpSM30B/C-G oligosaccharide–SpSM50 protein interactions to form hybrid protein hydrogels with unique physical properties. In this study, we investigate the mineralization properties of this hybrid hydrogel alongside the hydrogels formed by SpSM50 and SpSM30B/C-G individually. We find that the SpSM50 + SpSM30B/C-G hybrid hydrogel is synergistic with regard to surface modifications and intracrystalline inclusions of existing calcite crystals, the inhibition of ACC formation, and the kinetic destabilization of ACC to form a crystalline phase. Most importantly, the hybrid hydrogel phase assembles and organizes mineral particles into discrete clusters or domains within in vitro mineralization environments. Thus, the interactions of SpSM50 and SpSM30B/C-G, mediated by carbohydrate–protein binding, reflect the need for protein cooperativity for the ACC-to-crystalline transformation, intracrystalline void formation, and guided mineral particle assembly processes that are instrumental in spicule formation.

Influence of aqueous Mg concentration on the transformation of amorphous calcium carbonate

In natural aquatic environments amorphous calcium carbonate (ACC) shows a remarkable stability, which is assumed to arise from the incorporation of foreign ions as well as organic ligands. Such impurities, like Mg, are considered to be controlling the transformation of ACC and finally occurring CaCO3 polymorphs. Although transformation of ACC with different Mg contents is extensively studied, the role of the aqueous Mg on ACC transformation is still underexplored. Therefore, experiments on the transformation behaviour of additive-free ACC reacting with Mg-bearing solutions were carried out and monitored using highly time-resolved in-situ Raman spectroscopy. The results show that while ACC transformed instantly to calcite and vaterite in pure water, it exhibited a prolonged stability of up to 11.5 h at elevated Mg concentrations in the reactive solutions. During this process ACC incorporated up to 5 mol% of Mg. The subsequent, transformation pathway of the newly formed Mg-ACC changed from forming Mg-calcite and vaterite to precipitating aragonite with nesquehonite and monohydrocalcite as intermediate phases. This indicates that the distinct ACC transformation mechanisms and thus sequences of minerals formed are controlled by the aqueous Mg concentration of the reactive solution. Hence, the finally formed minerals are not determined by the pre-structuring of the original ACC.