Amorphous Calcium Carbonate Particles Research Articles

Earthworm granules: A model of non-classical biogenic calcium carbonate phase transformations.

Different non-classical crystallization mechanisms have been invoked to explain structural and compositional properties of biocrystals. The identification of precursor amorphous nanoparticle aggregation as an onset process in the formation of numerous biominerals (crystallization via particle attachment) constituted a most important breakthrough for understanding biologically mediated mineralization. A comprehensive understanding about how the attached amorphous particles transform into more stable, crystalline grains has yet to be elucidated. Here, we document structural, biogeochemical, and crystallographic aspects of the formation as well as the further phase transformations of the amorphous calcium carbonate particles formed by cultured specimens of earthworm Lumbricus terrestris. In-situ observations evidence the formation of proto-vaterite after dehydration of earthworm-produced ACC, which is subsequently followed by proto-vaterite transformation into calcite through nanoparticle attachment within the organic framework. In culture medium spiked with trace amounts of Mn2+, the cauliflower-like proto-vaterite structures become longer-lived than in the absence of Mn2+. We propose that the formation of calcite crystals takes place through a non-classical recrystallization path that involves migration of proto-vaterite nanoparticles to the crystallization site, and then, their transformation into calcite via a dissolution-recrystallization reaction. The latter is complemented by ion-by-ion crystal growth and associated with impurity release. These observations are integrated into a new model of the biocrystallization of earthworm-produced carbonate granules which highlights the sensibility of this process to environmental chemical changes, its potential impact on the bioavailability of contaminants as well as the threat that chemical pollution poses to the normal development of its early stages. STATEMENT OF SIGNIFICANCE: Understanding the mechanisms of nucleation, stabilization and aggregation of amorphous calcium carbonate (ACC) and factors controlling its further transformation into crystalline phases is fundamental for elucidation of biogenic mineralization. Some species of earthworms are natural workbench to understand the biogenic ACC, stabilization and the transformation mechanisms, because they create millimeter-sized calcareous granules from amorphous calcium carbonate, which crystallize to a more stable mineral phase (mostly calcite). This study undergoes into the mechanisms of ACC stabilization by the incorporation of trace elements, as manganese, and the ulterior precipitation of calcareous granules by a coupled process of amorphous particle attachment and ion-by-ion growth. The study points to sensibility of this process to environmental chemical changes.

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.

Origin of the biphase nature and surface roughness of biogenic calcite secreted by the giant barnacle Austromegabalanus psittacus

The calcite grains forming the wall plates of the giant barnacle Austramegabalanus psittacus have a distinctive surface roughness made of variously sized crystalline nanoprotrusions covered by extremely thin amorphous pellicles. This biphase (crystalline-amorphous) structure also penetrates through the crystal’s interiors, forming a web-like structure. Nanoprotrusions very frequently elongate following directions related to the crystallographic structure of calcite, in particular, the <− 441> directions, which are the strongest periodic bond chains (PBCs) in calcite. We propose that the formation of elongated nanoprotrusions happens during the crystallization of calcite from a precursor amorphous calcium carbonate (ACC). This is because biomolecules integrated within the ACC are expelled from such PBCs due to the force of crystallization, with the consequent formation of uninterrupted crystalline nanorods. Expelled biomolecules accumulate in adjacent regions, thereby stabilizing small pellicle-like volumes of ACC. With growth, such pellicles become occluded within the crystal. In summary, the surface roughness of the biomineral surface reflects the complex shape of the crystallization front, and the biphase structure provides evidence for crystallization from an amorphous precursor. The surface roughness is generally explained as resulting from the attachment of ACC particles to the crystal surface, which later crystallised in concordance with the crystal lattice. If this was the case, the nanoprotrusions do not reflect the size and shape of any precursor particle. Accordingly, the particle attachment model for biomineral formation should seek new evidence.

Preparation and characterization of a composite coating composed of polycaprolactone (PCL) and amorphous calcium carbonate (ACC) particles for enhancing corrosion resistance of magnesium implants

Magnesium (Mg) and its alloys serve as potential biodegradable implants material because of their excellent biocompatibility and appropriate biomechanical compatibility. However, the high degradation rate in vivo limits the practical applications of magnesium based materials. To improve the corrosion resistance of Mg alloy, a uniform composite coating of polycaprolactone (PCL) and amorphous calcium carbonate (ACC) particles was prepared on AZ60 by dip-coating. Results of electrochemical measurements and immersion tests in SBF solution suggested that the composite coating remarkably improved the corrosion resistance of AZ60 alloy. The increase in corrosion resistance by the composite coating protection can be ascribed to two factors: (i) an initial barrier to delay the ingress of SBF solution, (ii) the gradual dissolution of ACC particles and release of phosphate ions to form a Mg phosphate layer, which protects the Mg substrate. In addition, the microstructure and composition for the composite coating after immersion showed that massive Mg modified ACC and apatite precipitates formed on the surface, which is indicative of its superior biomineralization capability. In summary, this PCL/ACC composite is a promising coating material for tailoring the degradation behavior of magnesium for orthopedic applications.

Polymer-Functionalised Nanograins of Mg-Doped Amorphous Calcium Carbonate via a Flow-Chemistry Approach.

Calcareous biominerals typically feature a hybrid nanogranular structure consisting of calcium carbonate nanograins coated with organic matrices. This nanogranular organisation has a beneficial effect on the functionality of these bioceramics. In this feasibility study, we successfully employed a flow-chemistry approach to precipitate Mg-doped amorphous calcium carbonate particles functionalized by negatively charged polyelectrolytes—either polyacrylates (PAA) or polystyrene sulfonate (PSS). We demonstrate that the rate of Mg incorporation and, thus, the ratio of the Mg dopant to calcium in the precipitated amorphous calcium carbonate (ACC), is flow rate dependent. In the case of the PAA-functionalized Mg-doped ACC, we further observed a weak flow rate dependence concerning the hydration state of the precipitate, which we attribute to incorporated PAA acting as a water sorbent; a behaviour which is not present in experiments with PSS and without a polymer. Thus, polymer-dependent phenomena can affect flow-chemistry approaches, that is, in syntheses of functionally graded materials by layer-deposition processes.

Synthesis of ultra-small hollow silica nanoparticles using the prepared amorphous calcium carbonate in one-pot process

The ultra-small hollow silica nanoparticles were synthesized using the prepared amorphous calcium carbonate (ACC) particles as a template. The ACC particles were firstly prepared by carbonation method, which procedure was conducted in the methanol solvent to form the Ca(OCH3)2 layers on the ACC particles. An effect of methanol concentration on the morphology of ACC particles was also investigated. The prepared ACC particles were directly coated by silica through adding tetraethoxysilane (TEOS) into the methanol solvent. Hence, the ACC-silica core-shell particles were obtained since the ACC particles have a positive charge and interact with hydrolyzed TEOS. The ACC particles could be stabilized through the reaction between methanol and calcium ions when the methanol concentration was increased over than 40 vol%.

Elucidating the accelerated carbonation products of calcium silicates using multi-technique approach

This article features an investigation of the accelerated carbonation products of four calcium silicate phases, namely monoclinic-tricalcium silicate (3CaO·SiO2 or C3S), γ-dicalcium silicate (2CaO·SiO2 or γ-C2S), tricalcium disilicate (3CaO·2SiO2 or C3S2, rankinite), and monocalcium silicate (CaO·SiO2 or CS, wollastonite). During the carbonation reaction, the calcium silicate minerals form calcium carbonate and Ca-modified silica gel. These reaction products were examined using thermogravimetric analysis (TGA), X-ray powder diffraction (XRD), 29Si magic angle spining (MAS) nuclear magnetic resonance (NMR), 13C {1H} cross polarized (CP)/MAS NMR, and dynamic vapor sorption (DVS) techniques. Along with the crystalline forms of CaCO3 (i.e., calcite, vaterite, and aragonite), amorphous calcium carbonate (ACC) was also found to be present in the carbonated calcium silicate systems. Based on the experimental results, it is proposed that the ACC particles were stabilized by the deposition of silica layers on their surfaces. Presence of ACC also affected the pore size distribution of the matrixes, which eventually influenced the diffusion based carbonation rate of the matrixes. Moreover, the degree of carbonation of CS was found to be most effective compared to other calcium silicates in terms of the percentage of the phase reacted for the experimental conditions used in this study.

Nucleation and Growth of Mg-Calcite Spherulites Induced by the BacteriumCurvibacter lanceolatusStrain HJ-1

Calcite spherulites have been observed in many laboratory experiments with different bacteria, and spherulitic growth has received much interest in mineralogy research. However, the nucleation and growth mechanism, as well as geological significance of calcite spherulites in solution with bacteria is still unclear. Herein, spherulites composed of an amorphous core, a Mg-calcite body and an organic film were precipitated by the Curvibacter lanceolatus HJ-1 bacterial strain in a solution with a molar Mg/Ca ratio of 3. Based on the results, we provide a possible mechanism for the biomineralization of Mg-calcite spherulites. First, amorphous calcium carbonate particles are deposited and aggregated into a stable sphere-like core in combination with organic molecules. The core then acts as the nucleus of spherulitic radial growth. Finally, the organic film grows on the surface of Mg-calcite spherulites as a result of bacterial metabolism and calcification. These findings provide insight into the growth mode and crystallization of biogenic spherulites during biomineralization, and are of significance in the application of novel biological materials.

Using Confinement To Study the Crystallization Pathway of Calcium Carbonate

Studies of crystallization in small volumes have shown that constraining the space in which precipitation occurs can provide an effective means of studying the pathways by which crystallization occurs. This article describes an investigation into the precipitation of calcium carbonate in a crossed-cylinder apparatus, where the confinement of low solution concentrations between unfunctionalized glass surfaces delivers a dramatic increase in the lifetime of metastable phases. This phenomenon allows the direct observation of amorphous calcium carbonate (ACC) in very dilute systems where it is virtually undetectable in bulk solution, thus confirming the predictions of a recent titration study which showed that the solubility product of ACC is an order of magnitude lower than previously believed. The aggregation of ACC particles is also seen to significantly decrease with increasing confinement, suggesting that the stability of ACC in confinement is related to its aggregation state and restricted mobility. Fin...

A proto-particular path to corals

Corals![Figure][1] False color image of a Stylophora pistillata coral skeleton CREDIT: PUPA GILBERT, CHANG-YU SUN, CAYLA STIFLER, UNIVERSITY OF WISCONSIN–MADISON, AND MATTHEW MARCUS, ADVANCE LIGHT SOURCE, LAWRENCE BERKELEY NATIONAL LABORATORY Coral skeletons form less directly than has long been thought. The classical picture of skeleton formation, involving simple inorganic precipitation of aragonite (a form of calcium carbonate) from a calcifying fluid, is now being supplanted by a body of research that identifies biologically controlled processes occurring within cells as the ones responsible. Mass et al. present spectromicroscopic evidence from Stylophora pistillata coral showing that amorphous calcium carbonate particles first form within the coral's organic tissue and then attach to the surface of the coral skeleton, where they soon crystallize into aragonite. This allows corals to grow faster than through ion-by-ion growth from solution and may make them less vulnerable to the harmful consequences of ocean acidification than has been assumed. Proc. Natl. Acad. Sci. U.S.A. 10.1073/pnas.1707890114 (2017). [1]: pending:yes

Ammonium-Carbamate-Rich Organogels for the Preparation of Amorphous Calcium Carbonates

Amine-CO2 chemistry is important for a range of different chemical processes, including carbon dioxide capture. Here, we studied how aspects of this chemistry could be used to prepare calcium carbonates. Chemically crosslinked organogels were first prepared by reacting hyperbranched polyethylene imine (PEI) dissolved in DMSO with carbon dioxide. The crosslinks of the organogel consisted of ammonium-carbamate ion pairs as was shown by IR spectroscopy. These carbamate-rich organogels were subsequently subjected to aqueous solutions of calcium acetate, and amorphous calcium carbonate (ACC) precipitated. The ACC did not crystalize during the mixing for up to 20 h, as was shown by a combination of IR spectroscopy, X-ray diffraction, scanning electron microscopy, and thermal analysis. Some PEI had been included or adsorbed on the ACC particles. Traces of calcite were observed in one sample that had been subjected to water in a work-up procedure.

Amorphous polyphosphate/amorphous calcium carbonate implant material with enhanced bone healing efficacy in a critical-size defect in rats

In this study the effect of amorphous calcium carbonate (ACC) microparticles and amorphous calcium polyphosphate (polyP) microparticles (termed aCa-polyP-MP) on bone mineral forming cells/tissue was investigated in vitro and in vivo. The ACC particles (termed ACC-P10-MP) were prepared in the presence of Na-polyP. Only the combinations of polyP and ACC microparticles enhanced the proliferation rate of human mesenchymal stem cells (MSCs). Gene expression studies revealed that ACC causes an upregulation of the expression of the cell membrane-associated carbonic anhydrase IX (CA IX; formation of ACC), while the transcript level of the alkaline phosphatase (ALP; liberation of orthophosphate from polyP) changes only relatively little. In contrast, aCa-polyP-MP primarily induces ALP expression. If both components are applied together a strong stimulation of expression of both marker genes is observed. In order to investigate whether ACC also enhances bone regeneration induced by polyP in vivo, the particles were encapsulated into PLGA (poly(d,l-lactide-co-glycolide)) microspheres (diameter ~800 μm) and implanted into rat critical-size calvarial defects. The studies revealed that animals that received aCa-polyP-MP microspheres showed an increased rate of regeneration compared to β-tri-calcium phosphate (β-TCP) controls. This effect is even accelerated if microspheres with both aCa-polyP-MP and ACC-P10-MP (1 : 1 weight ratio) are applied, resulting in an almost complete restoration of the defect area after 12 weeks. qRT-PCR analyses of tissue sections through the regeneration zone with microspheres containing both aCa-polyP-MP and ACC-P10-MP revealed a significantly higher upregulation of expression of the marker genes compared to each of the components alone. The Young’s moduli for microspheres containing aCa-polyP-MP (1.74 MPa) and aCa-polyP-MP/ACC-P10-MP (2.38 MPa) were markedly higher compared to β-TCP-controls (0.63 mPa). Our results show that the combined application of ACC and Ca-polyP (both in the amorphous state) opens new strategies for the development of regenerative implants for the reconstruction of bone defects.

Formation of hollow bone-like morphology of calcium carbonate on surfactant/polymer templates

Novel hollow, bone-like structures of Precipitated Calcium Carbonate (PCC) are fabricated, for the first time, starting from naturally occurring dolomite. The hollow, bone-like structures are prepared by precipitating calcium carbonate on self-assembled poly(acrylic acid)/cetyltrimethylammonium chloride (PAA/CTAC) template. Fourier Transform Infrared Spectroscopy (FT-IR), X-ray diffraction (XRD), Transmission Electron Microscopy (TEM) and Field Emission Scanning Electron Microscopic (FE-SEM) studies reveal that the bone-like structure is composed of Amorphous Calcium Carbonate (ACC) nanoparticles in the center and calcite nanoparticles at the edges. Bone-like PCC particles are in particle length of 2–3μm and particle width of 1μm. The internal hollow structures of bone-like particles are observed from TEM images. As identified by FE-SEM images, the bone-like structure has been formed through the crystal growth of initially formed ACC nanoparticles. The ACC particles are stabilized in the center while the calcite crystals have been grown from the ACC toward the edges of the structure to form a bone-like morphology. We also propose a possible mechanism for the formation of hollow bone-like PCC in this study. The fabricated hollow, bone-like PCC has potential applications in the preparation of release systems such as drugs, cosmetics and pigments.

The Effect of Aspartic Acid and Glycine on Amorphous Calcium Carbonate (ACC) Structure, Stability and Crystallization

The effect of organic molecules on CaCO3 crystallization, in particular on the formation of the initial amorphous calcium carbonate (ACC) phase, is poorly understood despite this knowledge being crucial for designing biomimetic compounds with specific function, strength and stability. We monitored ACC crystallization in the presence of varying concentrations of aspartic acid (ASP) and glycine (GLY). We observed an increase in ACC lifetime with increasing amino acid concentrations and showed that the amino acid molecules sorbed onto the ACC particles. However, little if any difference in composition and atomic structure or the so formed ACC was observed. Similarly, the crystallization pathway of ACC via vaterite and calcite although delayed, was only slightly affected by the added amino acids. The only exemption was at the highest tested ASP concentration where ACC formation was inhibited, The calcite crystals that formed in the presence of ASP had rounded edges and rough surfaces, features that are not observed for the pure, inorganic calcite or calcite formed in the presence of GLY. Overall, the results suggest that the amino acids affected ACC lifetime through the inhibition of crystal nucleation and growth, more so in the presence of ASP than GLY.

Effects of Magnesium Ions and Water Molecules on the Structure of Amorphous Calcium Carbonate: A Molecular Dynamics Study

Molecular dynamics simulations were conducted to elucidate the effects of Mg(2+) and H2O additives on the structure of amorphous calcium carbonate (ACC). New potential parameters for Mg(2+) ions were developed. The distribution function of the angle formed by three nearest-neighbor atoms was introduced to analyze the short-range local structure of ACC. The simulation indicated that ACC had a weakly ordered local structure resembling the local structure of a CaCO3 crystal. The local structure of pure ACC resembled that of vaterite. The formation of the vaterite-like local structure was hindered by Mg(2+) ions, whereas H2O molecules did not significantly influence the structure of ACC when the fraction of H2O molecules was low. However, when the fraction of H2O was high, the formation of a monohydrocalcite-like local structure was promoted. The effects of the additives on the structure of ACC were verified using the size of the additives and the interaction between the additives and CaCO3. The simulated structure of ACC was compared with the structure of CaCO3 crystals nucleated through the formation of ACC particles in real systems.

Precipitation and characterization of hollow calcite nanoparticles

AbstractA method for preparation of significant amount of hollow rhombohedral calcite nanoparticles, based on carbonation of calcium hydroxide suspension, is described. The mineralogical and morphological analyses of the precipitate confirmed the existence of exclusively stable polymorphic modification, calcite, with the mean particle size of about 100 nm and the diameter of the holes observed at the surfaces that are about 50 nm. The analysis of carbonation kinetics pointed out to a complex mechanism of hollow particles formation at high initial supersaturation, that assumed nucleation of amorphous precursor calcium carbonate phase and its solution mediated transformation into nanosized crystalline calcite. The holes obtained at the calcite surfaces are most probably the imprints remained after the dissolution of amorphous calcium carbonate particles.

Toward the understanding of the interfacial dairy fouling deposition and growth mechanisms at a stainless steel surface: A multiscale approach

The microstructures of two dairy fouling deposits obtained at a stainless steel surface after different processing times in a pilot plate heat exchanger were investigated at different scales. Electron-Probe Micro Analysis, Time-of-Flight Secondary Ion Mass Spectrometry, Atomic Force Microscopy, and X-Ray Photo-electron Spectroscopy techniques were used for this purpose. The two model fouling solutions were made by rehydrating whey protein in water containing calcium or not.Results on samples collected after 2h processing show that the microstructure of the fouling layers is completely different depending on calcium content: the layer is thin, smooth, and homogeneous in absence of calcium and on the contrary very thick and rough in presence of calcium.Analyses on substrates submitted to 1min fouling reveal that fouling mechanisms are initiated by the deposit of unfolded proteins on the substrate and start immediately till the first seconds of exposure with no lag time. In presence of calcium, amorphous calcium carbonate nuclei are detected in addition to unfolded proteins at the interface, and it is shown that the protein precedes the deposit of calcium on the substrate. Moreover, it is evidenced that amorphous calcium carbonate particles are stabilized by the unfolded protein. They are thus more easily trapped in the steel roughnesses and contribute to accelerate the deposit buildup, offering due to their larger characteristic dimension more roughness and favorable conditions for the subsequent unfolded protein to depose.

Amorphous Calcium Carbonate Stabilized by a Flexible Biomimetic Polymer Inspired by Marine Mussels

Organisms make use of amorphous materials as a certain precursor to form insoluble complex biominerals in the presence of various organic matrices. In this article, we demonstrate that amorphous calcium carbonate (ACC) nanoparticles can be highly stabilized by a polymerized dopamine (DA), a biomimetic molecule inspired by adhesive proteins in mussels. The cross-linking polydopamine (PDA, a mussel mimicking polymer) flexible chains are adhesively associated with the ACC particles to form ACC@PDA core–shell spheres. We are able to modulate the thickness of PDA shells in the range of a few to several tens of nanometers by adjusting the additive DA amount. The thickness of this mussel mimicking PDA shell significantly influences the stability of ACC nanoparticles; the thicker the PDA shell is, the more stable the ACC core is. The obtained ACC–PDA hybrid particles have high stability, partly because the complexing interaction of Ca2+ ions with PDA and encapsulating PDA networks inhibit ACC dissolution and reta...

Micromolding of Calcium Carbonate Using a Bio‐Inspired, Coacervation‐Mediated Process

Based on a novel approach that takes into account the coacervation of calcium and poly(acrylic acid) (PAA), we were able to biomimetically produce molded micropatterned parts from amorphous calcium carbonate (ACC) particles. We studied the time‐ and concentration‐dependent growth of Ca2+/PAA coacervate droplets using dynamic light scattering (DLS) and turbidity measurements. Applying these results for the generation of high amounts of unstable ACC particles, we were able to produce slurries that could be molded into micropatterned casts. The obtained slurries contained both micrometer sized ACC particles and smaller nano‐sized particles. When both types of particles were used for molding, materials with a high surface roughness could be produced, while the micropatterns of the molds could not be reproduced properly. However, by removing the bigger particles from the slurry using only the smaller, unstable, ACC particles, good reproduction of the micropatterns could be achieved, yielding smooth surfaces with a high surface area. The processing route represents a versatile platform for the bottom‐up preparation of micropatterned ceramics on the basis of calcium carbonate.

Solid state crystallization of amorphous calcium carbonate nanoparticles leads to polymorph selectivity

While amorphous calcium carbonate (ACC) has been widely exploited as a precursor to crystalline calcium carbonate in solution, the synthetic potential of crystallising ACC in the solid state has received little attention. Building on our recent results which demonstrate that additives occluded within ACC can facilitate its crystallisation in the solid state, we here demonstrate that annealing of ACC nanoparticles precipitated within Na-AOT (sodium bis(2-ethylhexyl)sulfosuccinate) reverse microemulsions not only leads to remarkable control over the size and morphology of the particles, but that polymorph selectivity can also be achieved through selection of the thermal profile. The ACC precipitated within the microemulsions crystallised at just 70 °C on incubation at this temperature for 12 hours and non-aggregated, sub-micron crystals of either calcite or vaterite – which are often difficult to access using standard synthetic protocols – were produced according to the heating rate employed. The observed low-temperature crystallisation was attributed to surfactant molecules entrapped with the ACC particles during their formation, where these impurities act to facilitate nucleation of a crystalline phase within the ACC. These results therefore highlight the potential of using thermal treatment of dry ACC as a route to achieving control over calcium carbonate formation that is sometimes difficult to achieve in solution.