Calcite Single Crystals Research Articles

Biomineralization of sea urchin teeth

The sea urchin tooth, which is composed almost entirely of Mg-enriched CaCO3, is of particular interest as a model for the study of biomineralization process due to its amazing mechanical toughness and hardness. Our recent work on the formation process, the crystal composition and orientation, and the mechanical properties of sea urchin tooth are summarized in this paper. First, transmission electron microscopy images and electron diffraction patterns, as well as crystal overgrowth experiments, show that the highly convoluted primary plate-lamellar needle complex grows into a single crystal of calcite from a transient amorphous precursor phase in the sea urchin tooth. Amorphous calcium carbonate exists in the center of both the primary plates and the needles, even though the surfaces are already well crystallized. Second, X-ray photoelectron emission spectromicroscopy demonstrates that the needles, primary plates, and polycrystalline matrix crystals are all aligned. And there are two alternating crystal orientations in the stone part of the sea urchin tooth. Microbeam X-ray diffraction patterns further prove the existence of the two crystal orientations in sea urchin tooth. The c axes of calcite in the two oriented crystals are only a few degrees from each other. Third, the mechanical properties of sea urchin tooth grinding tip were studied by nanoindentation. The polycrystalline matrix has a higher elastic modulus and hardness than single crystalline needles and plates. It is proposed that the grinding capability of the tooth can be attributed to the small and uniform sizes of the polycrystalline crystals, their high Mg contents, and the two co-orientations of single crystals and polycrystalline structure. The improved understanding of the biomineralization process of sea urchin tooth and the relations between their structures and mechanical properties may shed light on the design of mechanical grinding and cutting tools with tunable properties.

Rosette‐Shaped Calcite Structures at Surfaces: Mechanistic Implications for CaCO3 Crystallization

Biomineralization is believed to be achieved by the intimate cooperation of soluble macromolecules and an insoluble matrix at the specific inorganic-organic interface. It has been reported that positively charged matrices play an important role in controlling the structure of CaCO(3) at surfaces, although detailed mechanisms remain unclear. In this work, we studied the transformation from amorphous CaCO(3) to calcite crystals on surfaces by using thin films of poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) and its quaternized form. The positively charged PDMAEMA film was found to possess unique properties for CaCO(3) crystallization: individually separated, single calcite crystals were formed on the PDMAEMA film in the absence of poly(acrylic acid) (PAA), while circularly fused calcite crystals were formed in the presence of PAA. The circularly fused (rosette-shaped) calcite crystals could be changed from a completely packed rosette to a ring-shaped, hollow structure by tuning the crystallization conditions. A number of factors, such as reaction time, amount of (NH(4))(2)CO(3), concentration of PAA, and charge of matrix-films, were varied systematically, and we now propose a mechanism based on these observations.

Colloidal‐Crystal‐Assisted Patterning of Crystalline Materials

Colloidal crystals have shown great potential as versatile templates for the fabrication of patterned micro- and nanostructures with complex architectures and novel properties. The patterning of functional crystalline materials in two and three dimensions is essential to the realization of their applications in many technologically important fields. This article highlights some recent progress in the fabrication of 2D and 3D patterned crystalline materials with the assistance of colloidal crystals. By combining a bioinspired synthetic strategy based on a transient amorphous phase with a colloidal-crystal templating method, unique 3D ordered macroporous (3DOM) calcite single crystals can be created. Moreover, patterned arrays of regular ZnO nanopillars with controlled size, shape, and orientation can be fabricated via a facile wet chemical approach by using masks derived from monolayer colloidal crystals (MCC).

Determination of the elastic modulus of highly porous samples by nanoindentation: a case study on sea urchin spines

Nanoindenation studies were carried out on single crystal calcite and on sea urchin spines from Heterocentrotus mammillatus, Phyllacanthus imperialis, and Prinocidaris baculosa. Unlike dense calcite single crystals resin embedded porous sea urchin spine segments showed a strong dependence of the indentation modulus, but not the indentation hardness, on the local porosity. This implies that the sampled volume for the indentation modulus in nanoindentation with forces down to 15 mN is not nanoscopic but extends approximately 50 μm around the indentation spot. Only for indentation depths ≪100 nm more or less mount-unaffected values of the indentation modulus could be found. The Voigt model for composite materials (calcite/resin) was found to be applicable for the dependency of the indentation modulus on the porosity. This is attributed to the network type of porosity and opens strategies for the control of stiffness in porous networks.

Blending Rudists with Technology; Non-destructive Examination of the Internal and External Structures of Rudists Using High Quality Scanning and Digital Imagery

Certain formations within the Cretaceous of Texas include zones of silicified rudists. Such diagenetic alteration, combined with the timing of alteration, has preserved many structural features of the rudist suitable for computed tomography (CT scanning). The excellent density differences between those skeletal structures (silica) and the internal sediment (calcitic mud) or later crystallization (calcite), fulfill a fundamental requirement for such analysis. The Jackson School of Geosciences at The University of Texas provides access to a high resolution computed tomography scanning system. This equipment, not identical to medical scanners, provides higher resolution and sophisticated software to analyze the imagery. Several rudist specimens have been scanned, including Caprinuloidea perfecta, from the Lower Cretaceous Edwards Formation (Albian) and Bayeloidea clivi from the Albian (formerly considered Turonian) of Mexico. Other rudist specimens are preserved in a different manner. For example, structures may be replaced with single crystal calcite with external skeletal intricacies especially well preserved. Elsewhere the record is one of internal molds or casts, with a reduction of available detail. Cost, diagenesis, and size determine the mode of specimen imagery used in an NSF sponsored project to image and conserve the type and figured specimens in the collection of the Texas Natural Science Center, Non-vertebrate Paleontology. This paper describes how such imagery can be used to further many aspects of rudist research. CT scanning produces serial sections of minute thickness without destroying a valuable specimen, in contrast to preparing physical serial sections, a more traditional method of analytical study. High resolution digital imagery (35 mm) provides images of great clarity which can be analyzed further and can be used as a surrogate for physical peels of rare or fragile specimens. Both these methods of imagery can be made available on-line and can be studied by scientists throughout the world.

In Situ Atomic Force Microscopy Study on the Crystallization of Calcium Carbonate Modulated by Poly(vinyl alcohol)s

The effect of poly(vinyl alcohol) (PVA) on the crystallization of calcium carbonate is reported. Crystallization was initiated by flowing a supersaturated solution over a single crystal of calcite,...

Swift heavy ion irradiation induced phase transformation in calcite single crystals

Ion irradiation induced phase transformation in calcite single crystals have been studied by means of Raman and infrared spectroscopy using 120 MeV Au 9+ ions. The observed bands have been assigned according to group theory analysis. For higher fluence of 5×10 12 ion/cm 2, an extra peak on either side of the 713 cm −1 peak and an increase in the intensity of 1085 cm −1 peak were observed in Raman studies. FTIR spectra exhibit extra absorption bands at 674, 1589 cm −1 and enhancement in bands at 2340 and 2374 cm −1 was observed. This might be due to the phase transformation from calcite to vaterite. The damage cross section ( σ ) for all the Raman and FTIR active modes was determined. The increase of FWHM, shift in peak positions and appearance of new peaks indicated that calcite phase is converted into vaterite.

Hydroxyapatite synthesis from biogenic calcite single crystals into phosphate solutions at ambient conditions

Hydroxyapatite is one of the most important bone substitute biomaterials. Here, it has been successfully overgrown on biogenic seed crystals at ambient conditions. Single crystals of calcite from Atrina rigida, Paracentrotus lividus and Heterocentrotus mammillatus have been soaked in phosphate solution with different concentrations and pHs for 2 months. X-ray powder diffraction, scanning electron microscopy and Fourier transform infrared spectroscopy have been used to characterize soaking precipitates. The results show that the conversion of calcite to hydroxyapatite occurs to an extent which depends on composition and morphology of seed crystals, and starting concentration and pH of phosphate solutions. In the same experimental conditions, synthetic calcite single crystals did not convert to hydroxyapatite. The morphological observations suggest for hydroxyapatite formation, a mechanism that involves a superficial dissolution of calcite and a subsequently overgrowth of hydroxyapatite. Moreover, the final architectural assembly of the hydroxyapatite crystals resembles the shape of the starting biogenic seed crystals.

Elasticity of Single-Crystal Calcite by First-Principles Calculations

Elasticity of Single-Crystal Calcite by First-Principles Calculations

Substrate-directed formation of calcium carbonate fibres

This article describes the formation of fibres of calcium carbonate on a range of substrates in the presence of a polyacid diblock copolymer, and it is demonstrated that the morphologies and structures of the fibres can be controlled by judicious selection of both the substrate and the reaction conditions. While fibres precipitated on single crystals of calcite and aragonite proved to be single crystals of calcite, those formed on glass and mica were amorphous and frequently displayed remarkable helical morphologies. Investigation of the fibre growth mechanism(s) suggests that all formed via the self-assembly of polymer-stabilised precursor units. Particles forming the calcite single-crystal fibres were elongated calcite crystallites, and fibre formation was interpreted in terms of an oriented assembly process, where anisotropy in the precursor unit morphology and surface distribution of copolymer chains promoted one-dimensional assembly. In contrast, the particles that formed amorphous fibres were spherical and amorphous, with fibre formation being attributed to polarisation of these copolymer-rich particles. This work therefore demonstrates that directional aggregation processes can be applied to both amorphous and crystalline units, opening up the possibility of using block copolymers to control the morphologies of a very wide range of materials.

Surface‐Induced Regulation of Podosome Organization and Dynamics in Cultured Osteoclasts

Bone is continuously repaired and remodeled through the well-coordinated activity of osteoblasts, which form new bone, and osteoclasts, which resorb it. How osteoclasts sense the properties of the bone surface remains unclear. By combining light and electron microscopy, we compared osteoclast behavior on three distinct surfaces: glass, calcite single crystals, and bone. Podosomes, the basic units of the adhesion structure, and their organization into superstructures were found to be common to cells that were attached to all three substrates, whereas the structure of the resorption organelle, the so-called "ruffled border," markedly differed. Moreover, the integrity, stability, and dynamic behavior of the adhesion superstructures also fundamentally differed, depending on the substrate. We conclude that osteoclasts sense the local properties of the underlying substrate and respond to these signals, both locally and globally.

Transformation mechanism of amorphous calcium carbonate into calcite in the sea urchin larval spicule

Sea urchin larval spicules transform amorphous calcium carbonate (ACC) into calcite single crystals. The mechanism of transformation is enigmatic: the transforming spicule displays both amorphous and crystalline properties, with no defined crystallization front. Here, we use X-ray photoelectron emission spectromicroscopy with probing size of 40-200 nm. We resolve 3 distinct mineral phases: An initial short-lived, presumably hydrated ACC phase, followed by an intermediate transient form of ACC, and finally the biogenic crystalline calcite phase. The amorphous and crystalline phases are juxtaposed, often appearing in adjacent sites at a scale of tens of nanometers. We propose that the amorphous-crystal transformation propagates in a tortuous path through preexisting 40- to 100-nm amorphous units, via a secondary nucleation mechanism.

Magnetic fabrics of the Morcles Nappe complex

Anisotropy of magnetic susceptibility (AMS) measured with high fields, at ambient and low temperatures (77 K), has been used to separate ferromagnetic ( sensu lato), paramagnetic and diamagnetic sub-fabrics for deformed limestones located in a nappe structure from southwest Switzerland. The magnetic fabrics are dominantly controlled by paramagnetic and diamagnetic minerals, and ferromagnetic minerals contribute significantly only at two of the eight study locations. Separation of the magnetic sub-fabrics is possible in the root zone and overturned limb of the Morcles Nappe, which contains a high ratio of diamagnetic matrix calcite to paramagnetic secondary phases. Each of the sub-fabrics displays a relationship with structural cleavage and the crystallographic preferred orientation (CPO) of calcite in the root zone and overturned limb. The maximum axis of the paramagnetic AMS ellipsoid ( k max) lies close to the pole to cleavage, with the intermediate ( k int) and minimum ( k min) axes near the cleavage plane. Conversely, the diamagnetic k min lies close to the pole to cleavage with k max and k int axes near the cleavage plane. Slight offsets in the poles to cleavage and in the principal AMS directions are observed. They are interpreted to be the consequence of a second phase deformation event, affecting only the CPO but not the macroscopic structural elements. The diamagnetic sub-fabric reflects the CPO of calcite, produced through recrystallization during nappe deformation. Iron-rich calcite appears to control the paramagnetic sub-fabric. Susceptibility ellipsoids approach oblate shapes for the diamagnetic sub-fabric, with the most extreme shapes reaching susceptibility differences (Δ k = k max − k min) close to that of single crystal calcite, whereas the paramagnetic sub-fabric is represented mainly by prolate shape ellipsoids. The low-field AMS ellipsoids show mixed oblate and prolate shapes, without consistency within or between sites. It is evident that, when the magnetic sub-fabrics of the root zone and overturned limb are separated, their individual shapes and distribution of principal directions mirror the developed CPO.

Hierarchical fibre composite structure and micromechanical properties of phosphatic and calcitic brachiopod shell biomaterials — an overview

AbstractBrachiopods are a phylum of shell-forming sessile marine invertebrates which have existed since the early Cambrian. Two very different biomaterial design strategies for their shells evolved early in their history. Both strategies use hybrid fibre composites, however, one is based on mineral fibres embedded in ~2 wt.% of organic biopolymer sheaths and the inorganic fibres are calcite single crystals. In the second strategy the fibres are biopolymers and are reinforced with Ca-phosphate nanoparticles to form a fibrous nanocomposite. Here the organic component (chitin) dominates. The Ca-phosphate nanoparticle-reinforcement strategy is not unlike that in vertebrate bone, however, the microscale structure is laminated with alternating laminae which have a different degree of mineralization.The calcitic shells feature an outer compact layer of calcite micro- and nanoparticles protecting the inner fibrous layer from the outside. Transmission electron microscopy of the fibrous layer reveals intercrystalline and intracrystalline biopolymers. The calcitic shell material is stiff with nano-indentation E-moduli of 63±8 GPa and relatively hard (Vickers microhardness up to 400 HV 0.0005/10 and nanohardness 4±0.5 GPa). Compared to inorganic calcite the microhardness is doubled and the nanohardness increases by 60%. We attribute this increased hardness to intracrystalline biopolymers. The nano-indentation E-moduli of the chitinophosphatic shells range from 3 to 55 GPa as a result of the varying degree of mineralization between their laminae, and similarly their nanohardness varies between 0.1 and 3 GPa. For brachiopods burrowing inside the sediment, the alternation of non-mineralized laminae with thin, more strongly mineralized laminae provides abrasion-resistance, hardness and longitudinal stiffness while it preserves the flexibility provided by the organic component for bending movements.

Crystallization of biogenic Ca-carbonate within organo-mineral micro-domains. Structure of the calcite prisms of the PelecypodPinctada margaritifera(Mollusca) at the submicron to nanometre ranges

AbstractAtomic force microscopy (AFM) and transmission electron microscopy (TEM) were used to investigate the fine structure of the calcite prisms from the pearl-oyster shellPinctada margaritifera. The AFM analysis shows that the prisms are made of densely packed circular micro-domains (in the 0.1 μm range) surrounded by a dense cortex. The TEM images and diffraction patterns allow the internal structure of the micro-domains to be described. Each of them is enriched in Ca-carbonate. Hosted in distinct regions of each prism, some are fully amorphous, and some others fully crystallized as subunits of a large calcite single crystal. At the border separating the two regions, micro-domains display a crystallized core and an amorphous rim. Such a border probably marks out an arrested crystallization front having propagated through a previously bio-controlled architecture of the piling of amorphous micro-domains. Compared to recent data concerning the stepping mode of growth of the calcite prisms and the resulting layered organization at the μm-scale, these results give unexpected views regarding the modalities of biocrystallization.

Damage creation in swift heavy ion-irradiated calcite single crystals: Raman and Infrared study

Raman and Infrared studies were carried out on pristine and 100 MeV Ag 8+ ion irradiated calcite single crystals in the fluence range 1 × 10 11 to 1 × 10 13 ions/cm 2. Raman and Infrared modes were assigned according to factor theory analysis. It is observed that the intensities of the Raman and infrared bands decrease with increase of ion fluence. The decrease of these bands is attributed to breakage of carbonate ions and other details are discussed.

Bioinspired Fabrication of 3D Ordered Macroporous Single Crystals of Calcite from a Transient Amorphous Phase

Filling the gaps: Unique 3D ordered macroporous (3DOM) calcite single crystals with controlled orientation and well-defined nanopatterns are fabricated by introducing amorphous CaCO3 into a colloidal crystal template of polymer spheres (see picture). Such a bioinspired strategy suggests a route to functional single-crystalline materials and sheds light on biomineralization mechanisms.

Pseudomorphic replacement of single calcium carbonate crystals by polycrystalline apatite

AbstractDuring chemical weathering and natural hydrothermal reactions, apatite can form by replacing calcium carbonates. In hydrothermal experiments in which aragonite and calcite single crystals have been reacted with phosphate solutions, the carbonates are replaced by polycrystalline hydroxylapatite (HAP). In both cases the crystals have retained their overall morphology while their compositions have changed significantly. The HAP appears to have a crystallographic relationship to the parent carbonate crystals. The textural relationships are consistent with an interface-coupled dissolution-precipitation mechanism. Structural relationships and relative molar volumes and solubilities appear to be factors that greatly affect replacement reactions.

Stimulated globular scattering of laser radiation in photonic crystals: Temperature dependences

The characteristics of stimulated globular scattering such as the frequency shift, threshold, and conversion efficiency are studied in photonic crystals (synthetic opal matrices and opal nanocomposites) at different temperatures. The results are compared with the study of stimulated Raman scattering in calcite single crystals. In both cases, a decrease in temperature from +20° C to −196° C resulted in an increase in the energy of stimulated scattering energy and its redistribution into the higher-order components.

Mineral Deposition and Crystal Growth in the Continuously Forming Teeth of Sea Urchins

AbstractThe early stages of formation of the crystalline elements in the continuously forming sea urchin teeth were studied using polarized light microscopy, SEM, TEM and calcite overgrowth. Transient amorphous calcium carbonate (ACC) is initially deposited in the needles and subsequently transforms into a single crystal of calcite. ACC exists in the center of both the primary plates and the needles, even though the surfaces are already well crystallized. TEM images and electron diffraction patterns, as well as calcite crystal overgrowth shows that the highly convoluted primary plate‐lamellar needle complex grows into a single crystal of calcite. The calcite crystal c axis is approximately parallel to the slightly curved surface of the primary plate. The needles grow along the unusual [102] direction of calcite. This study demonstrates that highly complex shaped single crystals are produced using a transient amorphous precursor phase. The beneficial properties of the single crystals are then combined with those of a polycrystalline matrix to yield a sophisticated composite functional material. This strategy could conceivably be applied to the production of synthetic materials.