Anhydrous Amorphous Calcium Carbonate Research Articles

Bioinspired Stabilization of Amorphous Calcium Carbonate by Carboxylated Nanocellulose Enables Mechanically Robust, Healable, and Sensing Biocomposites

Nature builds numerous structurally complex composites with fascinating mechanical robustness and functionalities by harnessing biopolymers and amorphous calcium carbonate (ACC). The key to successfully mimicking these natural designs is efficiently stabilizing ACC, but developing highly efficient, biodegradable, biocompatible, and sustainable stabilizing agents remains a grand challenge since anhydrous ACC is inherently unstable toward crystallization in the wet state. Inspired by the stabilized ACC in crustacean cuticles, we report the efficient stabilization ability of the most abundant biopolymer-cellulose nanofibrils (CNFs) for ACC. Through the cooperative stabilizing effect of surface carboxyl groups and a rigid segregated network, the CNFs exhibit long-term stability (more than one month) and achieved a stabilization efficiency of 3.6 and 4.4 times that of carboxymethyl cellulose (CMC) and alginate, respectively, even higher than poly(acrylic acid). The resulting CNF/ACC dispersions can be constructed into transparent composite films with the high strength of 286 MPa and toughness up to 28.5 MJ/m3, which surpass those of the so far reported synthetic biopolymer-calcium carbonate/phosphate composites. The dynamic interfacial interaction between nanocomponents also provides the composite films with good self-healing properties. Owing to their good wet stability, the composite films present high humidity sensitivity for monitoring respiration and finger contact.

Elasticity of amorphous calcium carbonate at high pressure and its dependence on the H2O content: A Brillouin scattering study to 20 GPa

Sound velocities and elastic properties of three synthetic samples of amorphous calcium carbonate (ACC), with different water contents up to 18 wt%, have been determined at high pressures, up to 20 GPa, by Brillouin spectroscopy. The experimental results show a smooth pressure-dependence of both longitudinal, vp, and transverse, vs, velocities in compression and decompression. Our results do not show any anomaly at 10 GPa, where a structural phase transition has been reported for a hydrous ACC. Instead, the anomalous behaviour of the isentropic bulk modulus, KS, and its isothermal pressure derivative, KS′, obtained in this study, with respect to the isothermal bulk modulus, KT, suggest that ACC undergoes a continuous structural evolution upon compression. The presence of 18 wt% H2O leads to a lowering of the bulk modulus up to 39% and of the shear modulus up to 38%. The values of the bulk and shear moduli for anhydrous ACC are Ka = 54(2) GPa, and Ga = 22(1) GPa, respectively. If 10 wt% CaCO3 is present in subducted carbonated oceanic crust, the phase transition from aragonite to anhydrous ACC, observed at pressures between 4 and 10 GPa and T>1000 K, would lead to a seismically detectable discontinuity in the acoustic velocities (−2.8% for vp, −4.4% for vs at 4.8 GPa).

Amorphous calcium carbonate (ACC) in fresco mural paintings

A fresco painting has a calcium carbonate binder produced as result of the carbonation process of a lime paste. The reaction environmental conditions are similar to those of bio-mineral formation where an amorphous calcium carbonate (ACC) phase has been found to be the precursor of other CaCO3 polymorphs following the sequence: hydrated ACC, anhydrous ACC, vaterite, aragonite, and, finally, stable calcite. In order to determine whether ACC is also formed during the fresco drying process, as well as, the final surface calcium carbonate phases present, a series of laboratory mock-ups, replicating as close as possible each of the paint strata and submitted to the same atmospheric conditions have been designed and studied. The results indicate that the continuous supply of water and reagents by the lime and sand mortar promote the ACC formation even at the completion of water evaporation and that the high pH of the medium does not favour the formation of calcium carbonate polymorphs other than calcite. In fact, the results demonstrate that the presence of an ACC layer in the mural painting surface confirms that the original painting technique used was fresco. Finally, the presence of an ACC layer is important for stablishing adequate cleaning (the solubility of hydrated ACC is superior to those of calcite) and conservation protocols (ACC is susceptible to suffer dehydration) for the fresco paintings.

Anhydrous amorphous calcium carbonate (ACC) is structurally different from the transient phase of biogenic ACC.

Amorphous calcium carbonate (ACC) is an important precursor phase of biogenic calcite. In this work, an in situ Ca L-edge X-ray absorption spectroscopic study has been carried out to monitor the phase transformation process of hydrated ACC from room temperature to 773 K in the presence of water vapor pressure at 0.4 mbar. The L2,3 crystal field splittings of the near edge X-ray absorption fine structure (NEXAFS) spectra acquired for hydrated and anhydrous ACC are indistinguishable. The transformation process from anhydrous ACC to calcite is greatly facilitated by the presence of water moisture. Our data acquired for nano-calcite are in close resemblance to those reported for "type 2" ACC in sea urchin larval spicules. We suggest that "type 2 ACC" or the "transient phase of ACC" is a disordered calcium carbonate phase with a nascent calcitic structure at the nanometer length scale.

Growth and regrowth of adult sea urchin spines involve hydrated and anhydrous amorphous calcium carbonate precursors

In various mineralizing marine organisms, calcite or aragonite crystals form through the initial deposition of amorphous calcium carbonate (ACC) phases with different hydration levels. Using X-ray PhotoEmission Electron spectroMicroscopy (X-PEEM), ACCs with varied spectroscopic signatures were previously identified. In particular, ACC type I and II were recognized in embryonic sea urchin spicules. ACC type I was assigned to hydrated ACC based on spectral similarity with synthetic hydrated ACC. However, the identity of ACC type II has never been unequivocally determined experimentally. In the present study we show that synthetic anhydrous ACC and ACC type II identified here in sea urchin spines, have similar Ca L2,3-edge spectra. Moreover, using X-PEEM chemical mapping, we revealed the presence of ACC-H2O and anhydrous ACC in growing stereom and septa regions of sea urchin spines, supporting their role as precursor phases in both structures. However, the distribution and the abundance of the two ACC phases differ substantially between the two growing structures, suggesting a variation in the crystal growth mechanism; in particular, ACC dehydration, in the two-step reaction ACC-H2O → ACC → calcite, presents different kinetics, which are proposed to be controlled biologically.

Interplay between Calcite, Amorphous Calcium Carbonate, and Intracrystalline Organics in Sea Urchin Skeletal Elements

Biomineralization processes in living organisms result in the formation of skeletal elements with complex ultrastructures. Although the formation pathways in sea urchin larvae are relatively well known, the interrelation between calcite, amorphous calcium carbonate (ACC), and intracrystalline organics in adult sea urchin biominerals is less clear. Here, we study this interplay in the spines and test plates of the Paracentrotus lividus sea urchins. Thermogravimetric analysis coupled with differential scanning calorimetry or mass spectrometry measurements, nuclear magnetic resonance technique, and high-resolution powder X-ray diffraction show that pristine spines and test plates are composed of Mg-rich calcite and comprise about 1.2 to 1.6 wt % organics, 10 wt % of anhydrous ACC and less than 0.2 wt % of water. Anhydrous ACC originates from incomplete crystallization of a precursor ACC phase during biomineralization and is associated with intracrystalline organics at the molecular level. Molecular interacti...

The Water–Amorphous Calcium Carbonate Interface and Its Interactions with Amino Acids

Amorphous calcium carbonate is often the first phase to precipitate from solution during the mineralization of calcium carbonate, before the formation of one of the crystalline polymorphs. In vivo, this phase is believed to be essential for the manufacture of minerals displaying nonequilibrium morphologies. The precipitation of this, usually transient, phase and its subsequent transformation into one of the crystalline polymorphs can be controlled by organic molecules. Here, we present a series of molecular dynamics simulations that explore the amorphous calcium carbonate–water interface, the attachment of amino acids onto both hydrous and anhydrous amorphous calcium carbonate, and their effect on the surface. The results show that surface ions have a different coordination number distribution from bulk ions and can diffuse up to two orders of magnitude faster than their bulk counterparts, suggesting that crystallization is much more likely to occur in this region. All the amino acids investigated bind to the amorphous calcium carbonate surfaces. However, acidic amino acids have a clear preference for the surface of amorphous CaCO3·H2O. The favored mode of interaction of the amino acids is through amine and/or guanidine moieties. The important ramifications of the results for our understanding of protein–mineral interactions are discussed.

Amorphous calcium carbonate particles form coral skeletons

Do corals form their skeletons by precipitation from solution or by attachment of amorphous precursor particles as observed in other minerals and biominerals? The classical model assumes precipitation in contrast with observed "vital effects," that is, deviations from elemental and isotopic compositions at thermodynamic equilibrium. Here, we show direct spectromicroscopy evidence in Stylophora pistillata corals that two amorphous precursors exist, one hydrated and one anhydrous amorphous calcium carbonate (ACC); that these are formed in the tissue as 400-nm particles; and that they attach to the surface of coral skeletons, remain amorphous for hours, and finally, crystallize into aragonite (CaCO3). We show in both coral and synthetic aragonite spherulites that crystal growth by attachment of ACC particles is more than 100 times faster than ion-by-ion growth from solution. Fast growth provides a distinct physiological advantage to corals in the rigors of the reef, a crowded and fiercely competitive ecosystem. Corals are affected by warming-induced bleaching and postmortem dissolution, but the finding here that ACC particles are formed inside tissue may make coral skeleton formation less susceptible to ocean acidification than previously assumed. If this is how other corals form their skeletons, perhaps this is how a few corals survived past CO2 increases, such as the Paleocene-Eocene Thermal Maximum that occurred 56 Mya.

Formation of amorphous calcium carbonate and its transformation into mesostructured calcite

Amorphous calcium carbonate (ACC) is a key precursor of crystalline CaCO3 biominerals and biomimetic materials. Despite recent extensive research, its formation and amorphous-to-crystalline transformation are not, however, fully understood. Here we show that hydrated ACC nanoparticles form after spinodal liquid–liquid phase separation and transform via dissolution/(re)precipitation into poorly hydrated and anhydrous ACC nanoparticles that aggregate, forming a range of 1D, 2D and 3D structures. The formation of these structures appears to be achieved by oriented attachment (OA), facilitated by the calcite medium-range order of ACC nanoparticles. Both electron irradiation processes in the TEM and under humid air exposure at room temperature of the latter ACC structures result in pseudomorphs of single crystalline mesostructured calcite. While the high-vacuum/e-beam heating leads to solid-state transformation, the transformation in air occurs via an interface-coupled dissolution/precipitation mechanism. Our results differ significantly from the currently accepted model, which considers that the low T ACC-to-calcite transformation in air and during biomineralization is a solid-state process. These results may help to better understand how calcite biominerals form after ACC and offer the possibility of biomimetically preparing single crystalline calcite structures after ACC by tuning pH2O at room temperature.

Oxygen spectroscopy and polarization-dependent imaging contrast (PIC)-mapping of calcium carbonate minerals and biominerals.

X-ray absorption near-edge structure (XANES) spectroscopy and spectromicroscopy have been extensively used to characterize biominerals. Using either Ca or C spectra, unique information has been obtained regarding amorphous biominerals and nanocrystal orientations. Building on these results, we demonstrate that recording XANES spectra of calcium carbonate at the oxygen K-edge enables polarization-dependent imaging contrast (PIC) mapping with unprecedented contrast, signal-to-noise ratio, and magnification. O and Ca spectra are presented for six calcium carbonate minerals: aragonite, calcite, vaterite, monohydrocalcite, and both hydrated and anhydrous amorphous calcium carbonate. The crystalline minerals reveal excellent agreement of the extent and direction of polarization dependences in simulated and experimental XANES spectra due to X-ray linear dichroism. This effect is particularly strong for aragonite, calcite, and vaterite. In natural biominerals, oxygen PIC-mapping generated high-magnification maps of unprecedented clarity from nacre and prismatic structures and their interface in Mytilus californianus shells. These maps revealed blocky aragonite crystals at the nacre-prismatic boundary and the narrowest calcite needle-prisms. In the tunic spicules of Herdmania momus, O PIC-mapping revealed the size and arrangement of some of the largest vaterite single crystals known. O spectroscopy therefore enables the simultaneous measurement of chemical and orientational information in CaCO3 biominerals and is thus a powerful means for analyzing these and other complex materials. As described here, PIC-mapping and spectroscopy at the O K-edge are methods for gathering valuable data that can be carried out using spectromicroscopy beamlines at most synchrotrons without the expense of additional equipment.

Time‐Resolved Evolution of Short‐ and Long‐Range Order During the Transformation of Amorphous Calcium Carbonate to Calcite in the Sea Urchin Embryo

AbstractUse of amorphous precursors is a widespread strategy in biomineralization. In sea urchin embryos, controlled transformation of amorphous calcium carbonate (ACC) to calcite results in smoothly curving and branching single crystals. However, the mechanism of the disorder‐to‐order transformation remains poorly understood. Here, the use of strontium as a probe in X‐ray absorption spectroscopy (XAS) greatly facilitates investigation of the evolution of order. In pulse‐chase experiments, embryos incorporate Sr2+ from Sr‐enriched seawater into small volumes of the growing endoskeleton. During the chase, the Sr‐labeled mineral matures under physiological conditions. Based on Sr K‐edge spectra of cryo‐frozen whole embryos, it is proposed that the transformation occurs in three stages. The initially deposited calcium carbonate has short‐range order resembling synthetic hydrated ACC. Within 3 h, the short‐range order of calcite is established. Between 3 h and 24 h, the short‐range order does not change, while long‐range order increases. These results refute the notion that organisms imprint the local order of the final crystal on ACC. Furthermore, it is proposed that the intermediate is more similar to disordered calcite than to anhydrous ACC. Pulse‐chase experiments in conjunction with heavy element labeling have great potential to improve understanding of phase transformations during biomineralization.

Dehydration-Induced Amorphous Phases of Calcium Carbonate

Amorphous calcium carbonate (ACC) is a critical transient phase in the inorganic precipitation of CaCO3 and in biomineralization. The calcium carbonate crystallization pathway is thought to involve dehydration of more hydrated ACC to less hydrated ACC followed by the formation of anhydrous ACC. We present here computational studies of the transition of a hydrated ACC with a H2O/CaCO3 ratio of 1.0 to anhydrous ACC. During dehydration, ACC undergoes reorganization to a more ordered structure with a significant increase in density. The computed density of anhydrous ACC is similar to that of calcite, the stable crystalline phase. Compared to the crystalline CaCO3 phases, calcite, vaterite, and aragonite, the computed local structure of anhydrous ACC is most-similar to those of calcite and vaterite, but the overall structure is not well described by either. The strong hydrogen bond interaction between the carbonate ions and water molecules plays a crucial role in stabilizing the less hydrated ACC compositions compared to the more hydrated ones, leading to a progressively increasing hydration energy with decreasing water content.

Thermal Dehydration of Monohydrocalcite: Overall Kinetics and Physico-geometrical Mechanisms

Monohydrocalcite (CaCO(3)·H(2)O: MHC) is similar in composition and synthetic conditions to hydrated amorphous calcium carbonate (ACC), which is focused recently as a key intermediate compound of biomineralization and biomimetic mineralization of calcium carbonate polymorphs. Detailed comparisons of the physicochemical property and reactivity of those hydrated calcium carbonates are required for obtaining fundamental information on the relevancy of those compounds in the mineralization processes. In the present study, kinetics of the thermal dehydration of spherical particles of crystalline MHC was investigated in view of physico-geometrical mechanism. The reaction process was traced systematically by means of thermogravimetry under three different modes of temperature program. A distinguished induction period for the thermal dehydration and cracking of the surface product layer on the way of the established reaction were identified as the characteristic events of the reaction. By interpreting the kinetic results in association with the morphological changes of the reactant particles during the course of reaction, it was revealed that nucleation and crystal growth of calcite regulate the overall kinetics of the thermal dehydration of MHC. In comparison with the thermal dehydration of hydrated ACC, which produces anhydrous ACC as the solid product, the kinetic characteristics of the thermal dehydration of MHC were discussed from the viewpoint of physico-geometry of the component processes.

Transformation and crystallization energetics of synthetic and biogenic amorphous calcium carbonate

Amorphous calcium carbonate (ACC) is a metastable phase often observed during low temperature inorganic synthesis and biomineralization. ACC transforms with aging or heating into a less hydrated form, and with time crystallizes to calcite or aragonite. The energetics of transformation and crystallization of synthetic and biogenic (extracted from California purple sea urchin larval spicules, Strongylocentrotus purpuratus) ACC were studied using isothermal acid solution calorimetry and differential scanning calorimetry. Transformation and crystallization of ACC can follow an energetically downhill sequence: more metastable hydrated ACC → less metastable hydrated ACC ⇒ anhydrous ACC ∼ biogenic anhydrous ACC ⇒ vaterite → aragonite → calcite. In a given reaction sequence, not all these phases need to occur. The transformations involve a series of ordering, dehydration, and crystallization processes, each lowering the enthalpy (and free energy) of the system, with crystallization of the dehydrated amorphous material lowering the enthalpy the most. ACC is much more metastable with respect to calcite than the crystalline polymorphs vaterite or aragonite. The anhydrous ACC is less metastable than the hydrated, implying that the structural reorganization during dehydration is exothermic and irreversible. Dehydrated synthetic and anhydrous biogenic ACC are similar in enthalpy. The transformation sequence observed in biomineralization could be mainly energetically driven; the first phase deposited is hydrated ACC, which then converts to anhydrous ACC, and finally crystallizes to calcite. The initial formation of ACC may be a first step in the precipitation of calcite under a wide variety of conditions, including geological CO(2) sequestration.

Thermal behaviors of amorphous calcium carbonates prepared in aqueous and ethanol media

Two different samples of amorphous calcium carbonate (ACC) hydrates were prepared respectively by mixing aqueous solutions of CaCl2 and Na2CO3-NaOH and by allowing the diffusion of (NH4)2CO3 sublimate into ethanol solution of CaCl2. Thermal behaviors of the synthetic ACCs were investigated comparatively by means of thermoanalytical techniques complimented by powder X-ray diffraction, FTIR spectrometry and microscopic observations. The anhydrous ACCs produced by the thermal dehydration of the respective samples were crystallized to calcite in different ways. The sample prepared in aqueous medium was crystallized at around 600 K in a single step. Crystallization in two separated steps at around 600 and 825 K was observed for the sample prepared in ethanol medium. Characteristics of the crystallization processes were discussed from thermodynamic and kinetic points of view.