Calcite Single Crystals Research Articles

Conversion of sea urchin spines to Mg-substituted tricalcium phosphate for bone implants

The skeleton of sea urchin spines is composed of large single crystals of Mg-rich calcite, which have smooth, continuously curved surfaces and form a three-dimensional fenestrated mineral network. Spines of the echinoids Heterocentrotus trigonarius and Heterocentrotus mammillatus were converted by the hydrothermal reaction at 180 °C to bioresorbable Mg-substituted tricalcium phosphate (β-TCMP). Due to the presence of Mg in the calcite lattice, conversion to β-TCMP occurs preferentially to hydroxyapatite formation. The converted β-TCMP still maintains the three-dimensional interconnected porous structures of the original spine. The main conversion mechanism is the ion-exchange reaction, although there is also a dissolution–reprecipitation process that forms some calcium phosphate precipitates on the surfaces of the spine network. The average fracture strength of urchin spines and converted spines (β-TCMP) in the compression tests are 42 and 23 MPa, respectively. In vivo studies using a rat model demonstrated new bone growth up to and around the β-TCMP implants after implantation in rat femoral defects for 6 weeks. Some new bone was found to migrate through the spine structural pores, starting from the outside of the implant through the pores at the edge of the implants. These results indicate good bioactivity and osteoconductivity of the porous β-TCMP implants.

Mechanodestruction of minerals at the crack tip (Overview): 1. Experiment

The kinetics of the amount of volatiles emitted from synthetic and natural calcite single crystals while cleaving and grinding are determined using time-of-flight mass spectrometry. It is revealed that the CO2 emission is a product of mechano destruction rather than the release of impurities trapped in the crystal. Irradiation of calcite increases the CO2 emission. A diffusion–desorption model is suggested to explain the kinetics of volatile emission from cleaved surfaces and experimental results on the emission of volatile products during the mechano destruction of various single crystals are summarized. The effects of several sources of systematic errors are discussed. The yield of volatiles is correlated with the speed of the brittle crack propagation and the physicochemical properties of minerals.

Theoretical Investigation of Electron Paramagnetic Resonance Spectra and Local Structure Distortion for Mn2+ Ions in CaCO3:Mn2+ System: A Simple Model for Mn2+ Ions in a Trigonal Ligand Field

This paper reports on a novel application of a ligand field model for the detection of the local molecular structure of a coordination complex. By diagonalizing the complete energy matrices of the electron-electron repulsion, the ligand field and the spin-orbit coupling for the d5 configuration ion in a trigonal ligand field, the local distortion structure of the (MnO6)10- coordination complex for Mn2+ ions doped into CaCO3, have been investigated. Both the second-order zero-field splitting parameter b(0)2 and the fourth-order zero-field splitting parameter b(0)4 are taken simultaneously in the structural investigation. From the electron paramagnetic resonance (EPR) calculations, the local structure distortion, DeltaR=-0.169 A to -0.156 A, Deltatheta=0.996 degrees to 1.035 degrees for Mn2+ ions in calcite single crystal, DeltaR=-0.185 A to -0.171 A, Deltatheta=3.139 degrees to 3.184 degrees for Mn2+ ions in travertines, and DeltaR=-0.149 A to -0.102 A, Deltatheta=0.791 degrees to 3.927 degrees for Mn2+ ions in shells are determined, respectively. These results elucidate a microscopic origin of various ligand field parameters which are usually used empirically for the interpretation of EPR and optical absorption experiments. It is found that the theoretical results of the EPR and optical absorption spectra for Mn2+ ions in CaCO3 are in good agreement with the experimental findings. Moreover, to understand the detailed physical and chemical properties of the doped CaCO3, the theoretical values of the fourth-order zero-field splitting parameters b(0)4 for Mn2+ ions in travertines and shells are reported first.

Tip-Induced Calcite Single Crystal Nanowear

AbstractWear behavior of freshly cleaved single crystal calcite (CaCO3) was investigated by continuous scanning using the Hysitron Triboindenter in ambient environment as a function of scanning frequency (1 Hz – 3 Hz) and contact load (2 µN – 8 µN). At lower loads below 4 µN, initiation of the ripples takes place at the bottom of the surface slope, which continue to propagate up the slope as scanning progresses. The orientation of these ripple structures is perpendicular to the long scan direction. As the number of scans increases, ripples become fully developed, and their height and periodicity increase with the number of scans. At 6 µN normal load, tip-induced wear occurs as the tip begins removing the ripple structures with increased number of scan cycles. As the contact load increased further, ripples did not initiate and only tip-induced wear occurred on the surface, and saturated after 20 scans. At 1 Hz frequency wear takes place as material slides towards the scan edges when the tip moves back and forth. Material removal rate increased with contact load and it is observed that the number of scans required to create a new surface is inversely proportional to the contact load. Possible mechanisms responsible for the formation of ripples at higher frequencies are attributed to the slope of the surface, piezo hysteresis, system dynamics, or a combination of effects. The wear regime is due to abrasive wear. Single crystal calcite hardness of 2.8±0.3 GPa and elastic modulus of 75±4.9 GPa were measured using nanoindentation and used to determine the wear mode.

Porous calcite single crystals grown from a hydrogel medium

This paper describes the internal structure of calcite (CaCO3) crystals grown in an agarose hydrogel and demonstrates that the gel-grown calcite crystals, like biogenic calcite crystals, incorporate the organic matrix, resulting in internal structures with pores on the order of 100's of nanometers.

Spiers Memorial Lecture : Lessons from biomineralization: comparing the growth strategies of mollusc shell prismatic and nacreous layers in Atrina rigida

The mollusc shell prismatic layer of Atrina rigida is composed of an assemblage of large and relatively perfect single calcite crystals, embedded in an organic matrix. A key to elucidating basic mechanisms of mineralization is understanding the structures of the matrix, the mineral and the relations between them. The matrix that envelopes each prism (the inter-prismatic matrix) is composed mainly of glycine-rich proteins, while the matrix inside each prism (intra-crystalline matrix) is composed of a network of chitin fibers. Prisms grow by deposition of mineral particles on the chitin fibers. The mineral particles are associated with highly acidic proteins from the Asprich family, which presumably stabilize an amorphous mineral precursor. We infer that once in contact with the already formed crystalline prism, the particles crystallize by epitaxial nucleation. In nacre, sheets of beta-chitin are interspaced by silk-like proteins in a hydrated gel-like state. beta-Chitin forms a scaffold onto which the acidic proteins are adsorbed. Some of these are organized into a crystal nucleation site, where nucleation of aragonite, supposedly from colloidal amorphous calcium carbonate particles, is induced. Comparing the mechanisms of growth of the nacreous and prismatic layers can help to understand the underlying strategies of formation of mineralized structures.

Coprecipitation of chromate with calcite: Batch experiments and X-ray absorption spectroscopy

Batch experiments, combined with in situ spectroscopic methods, are used to examine the coprecipitation of Cr(VI) with calcite, including partitioning behavior, site-specific distribution of Cr on the surface of calcite single crystals, and local coordination of Cr(VI) in the calcite structure. It is found that the concentration of Cr incorporated in calcite increases with increasing Cr concentration in solution. The calculated apparent partition coefficient, K d ∗ , is highest at low Cr solution concentration, and decreases to a constant value with increasing Cr solution concentration. DIC images of the ( 1 0 1 ¯ 4 ) surface of calcite single crystals grown in the presence of CrO 4 2 - exhibit well-defined growth hillocks composed of two pairs of symmetrically nonequivalent vicinal faces, denoted as + and −, which reflect the orientation of structurally nonequivalent growth steps. Micro-XRF mapping of the Cr distribution over a growth hillock shows preferential incorporation of Cr into the—steps, which is considered to result from differences in surface structure geometry. XANES spectra confirm that incorporated Cr is hexavalent, and no reduction of Cr(VI) in the X-ray beam was observed up to 2 days at room temperature. EXAFS fit results show the incorporated Cr(VI) has the expected first shell of 4 O at ∼1.64 ± 0.01 Å, consistent with CrO 4 2 - . Best fit results show that the second shell is split with ∼2.5 Ca at ∼3.33 ± 0.05 and ∼2.2 Ca at ∼3.55 ± 0.05 Å, which confirms the incorporation of chromate into calcite. Consideration of possible local coordination indicates that significant distortion or disruption is required to accommodate CrO 4 2 - in the calcite structure.

Basal slip and texture development in calcite: new results from torsion experiments

The deformation behavior of calcite has been of longstanding interest. Through experiments on single crystals, deformation mechanisms were established such as mechanical twinning on \({\mathbf{e}} = \{ \ifmmode\expandafter\bar\else\expandafter\=\fi{1}018\} {\left\langle {40\ifmmode\expandafter\bar\else\expandafter\=\fi{4}1} \right\rangle }\) in the positive sense and slip on \({\mathbf{r}} = \{ 10\ifmmode\expandafter\bar\else\expandafter\=\fi{1}4\} {\left\langle {20\ifmmode\expandafter\bar\else\expandafter\=\fi{2}\ifmmode\expandafter\bar\else\expandafter\=\fi{1}} \right\rangle }\) and \({\mathbf{f}} = \{ \ifmmode\expandafter\bar\else\expandafter\=\fi{1}012\} {\left\langle {0\ifmmode\expandafter\bar\else\expandafter\=\fi{2}2\ifmmode\expandafter\bar\else\expandafter\=\fi{1}} \right\rangle },\) both in the negative sense. More recently it was observed that at higher temperatures \({\mathbf{f}}\{ \ifmmode\expandafter\bar\else\expandafter\=\fi{1}012\} {\left\langle {10\ifmmode\expandafter\bar\else\expandafter\=\fi{1}1} \right\rangle }\) slip in both senses becomes active and, based on slip line analysis, it was suggested that \({\mathbf{c}}(0001){\left\langle {11\ifmmode\expandafter\bar\else\expandafter\=\fi{2}0} \right\rangle }\) slip may occur. So far there had been no direct evidence for basal slip, which is the dominant system in dolomite. With new torsion experiments on calcite single crystals at 900 K and transmission electron microscopy, this study identifies \((0001){\left\langle {11\ifmmode\expandafter\bar\else\expandafter\=\fi{2}0} \right\rangle }\) slip unambiguously by direct imaging of dislocations and diffraction contrast analysis. Including this slip system in polycrystal plasticity simulations, enigmatic texture patterns observed in compression and torsion of calcite rocks at high temperature can now be explained, resolving a long-standing puzzle.

Infrared emission of single-crystal calcite under broadband short-wavelength excitation

Our results demonstrate that broadband excitation of an oriented calcite single crystal with the radiation from a cw xenon lamp (incident power density of ∼1 W/cm2) gives rise to secondary IR emission from the crystal surface. We have measured the IR emission spectrum of a calcite single crystal in two orientations in the range 500–1500 cm−1 with a resolution of ∼1 cm−1 using a Fourier transform IR spectrometer. The spectrum shows sharp lines arising from IR-active (polar) vibrational transitions in the calcite crystal and can be interpreted in trems of a two-photon decay of a nonpolar vibrational state (A1g mode) effectively excited through broadband visible pumping. We analyze the ways of raising the efficiency of the conversion of primary (visible) to secondary IR radiation. In particular, this might be achieved using photonic crystals and an excitation frequency near the maximum in their density of states. The new type of IR emission spectroscopy, taking advantage of broadband visible or UV excitation of molecular vibrations in bulk dielectric inorganic materials or materials infiltrated into the pore space of globular photonic crystals, appears to have considerable promise.

Fast diffusion along mobile grain boundaries in calcite

Experimental measurements of grain boundary diffusion are usually conducted on static boundaries, despite the fact that grain boundaries deep in the Earth are frequently mobile. In order to explore the possible effect of boundary mobility on grain boundary diffusion rates we have measured the uptake of 44Ca from a layer of 44Ca-enriched calcite powder during the static recrystallization of a single crystal of calcite at 900°C. A region about 500 μm wide adjacent to the powder layer is heterogeneously enriched in 44Ca, and complex zoning patterns, including sharp steps in composition and continuous increases and decreases in 44Ca content, are developed. In metamorphic rocks, these would normally be interpreted in terms of changes in pressure or temperature, Rayleigh fractionation, or episodic fluid infiltration. These explanations cannot apply to our experiments, and instead the zoning patterns are interpreted as being due to variations in grain boundary migration rate. We have applied an analytical model which allows the product of grain boundary diffusion coefficient and grain boundary width (D GB δ) to be calculated from the grain boundary migration rate and the compositional gradient away from the powder layer. The value of D GB δ in the mobile grain boundaries is at least five orders of magnitude greater than the published value for static boundaries under the same conditions. In order to allow the scale of chemical equilibrium (and hence textural evolution) to be predicted under both experimental and geological conditions, we present quantitative diffusion-regime maps for static and mobile boundaries in calcite, using both published values and our new values for grain boundary diffusion in mobile boundaries. Enhanced diffusion in mobile boundaries has wide implications for the high temperature rheology of Earth materials, for geochronology, and for interpretations of the length- and time-scales of chemical mass-transport. Moreover, zones of anomalously high electrical conductivity in the crust and mantle could be regions undergoing recrystallization such as active shear zones, rather than regions of anomalous mineralogy, water- or melt-content as is generally suggested.

Macroporous inorganic solids from a biomineral template

A range of macroporous inorganic solids, with unique, sponge-like structures were synthesised by templating sea urchin skeletal plates. Although composed of a single crystal of calcite, a sea urchin plate exhibits a bicontinuous morphology with pores of diameter 10–15 μm. As both the solid and porous fractions of the plate exhibit identical form and dimensions, filling the porous network with an alternative material and dissolving away the CaCO 3 generates a cast with identical morphology to the original plate. The versatility of this approach is demonstrated in this paper, which provides both a review of previously published work, and a description of the application to a number of new systems. Macroporous metals, including gold and nickel were synthesised either from pre-prepared particles or via electroless deposition, while silica and titania were generated by sol–gel techniques. Sponge-like polymer membranes, formed as casts of the sea urchin plates, were also used as an environment in which to precipitate a range of single crystals with complex morphologies: CaCO 3, SrSO 4, PbSO 4, PbCO 3, NaCl and CuSO 4·5H 2O. These experiments demonstrate that single crystals with intricate forms are not restricted to the realms of biology, but that shape constraint provides an extremely general route for morphological control.

Scanning-Induced Growth on Single Crystal Calcite with an Atomic Force Microscope

We report observations of localized growth on the (1014) surface of single-crystal CaCO3 in supersaturated solutions while scanning with the tip of an atomic force microscope (AFM). At low contact forces, AFM scanning strongly enhances deposition along preexisting steps. This enhancement increases rapidly with increasing solution supersaturation, and is capable of filling in multilayer etch pits to produce defect-free surfaces at the resolution of the AFM. Attempts to achieve similar deposition rates in the absence of scanning require high supersaturations that produce three-dimensional crystal nuclei, which are important defects. Localized deposition produced by drawing the AFM tip back and forth across step edges can produce monolayer deposits extending well over a micron from the scanned area. These tip-induced deposits provide convincing evidence for the importance of ledge diffusion in calcite crystal growth.

Additive-Mediated Crystallization of Complex Calcium Carbonate Superstructures in Reverse Microemulsions

Morphologically complex forms of calcium carbonate were produced within 24 h in unstirred mixtures of Ca(AOT)2 (AOT = bis(2-ethylhexyl)sulfosuccinate) reverse micelles and carbonate-containing NaAOT microemulsions (w = 10). The structures form by controlled aggregation of surfactant-coated amorphous calcium carbonate primary particles, which resulted in micrometer-sized doughnut-shaped structures consisting of densely packed layers of platelike aragonite crystals. These superstructures were progressively in-filled to produce polycrystalline multilayered spindle-shaped particles several micrometers in length. Addition of the crystallization additive sodium polyphosphate at concentrations exceeding 1 g L-1 to the carbonate-containing NaAOT microemulsions inhibited aragonite crystallization, with the consequence that single crystals of calcite with bundlelike filamentous texture along with platelike vaterite single crystals were produced. Energy-dispersive X-ray analysis indicated that AOT and polyphosphate ions were strongly associated with the crystals even after extensive washing. Unlike the aragonite superstructures, which became decorated within a few days with epitaxially oriented rodlike outgrowths of calcite, the platelike vaterite and filamentous calcite crystals remained unchanged even after 7 days. The results demonstrate that encapsulation of crystallization additives such as polyphosphate in the water droplets of reverse microemulsions can be used to influence nucleation and growth processes over a range of length scales and suggest that this novel approach could be of interest in crystal science in general.

Structural Characterization of the Transient Amorphous Calcium Carbonate Precursor Phase in Sea Urchin Embryos

Sea urchin embryos form their calcitic spicular skeletons via a transient precursor phase composed of amorphous calcium carbonate (ACC). Transition of ACC to calcite in whole larvae and isolated spicules during development has been monitored using X-ray absorption spectroscopy (XAS). Remarkably, the changing nature of the mineral phase can clearly be monitored in the whole embryo samples. More detailed analyses of isolated spicules at different stages of development using both XAS and infrared spectroscopy demonstrate that the short-range order of the transient ACC phase resembles calcite, even though infrared spectra show that the spicules are mostly composed of an amorphous mineral phase. The coordination sphere is at first distorted but soon adopts the octahedral symmetry typical of calcite. Long-range lattice rearrangement follows to form the calcite single crystal of the mature spicule. These studies demonstrate the feasibility of real-time monitoring of mineralized-tissue development using XAS, including the structural characterization of transient amorphous phases at the atomic level.

Cathodoluminescence study of americium incorporation into calcite single crystals

ABSTRACTIn order to study americium incorporation into calcite, CaCO3, under conditions of crystal growth, two samples of single crystal Am-doped calcite were synthesized and studied by cathodoluminescence (CL) spectroscopy in comparison with undoped and Eu-doped artificial calcite. Americium contents in calcite crystals were (in kBq/g): 1) 6.9; 2) 1.9(E+4). The CL emission of undoped and Am-Eu-doped calcitesamples was characterized by three broad bands at 2.03; 2.47 and 2.96 eV. Weak CL lines related to typical transitions 5D07F1,2,4 of Eu3+ and Am3+ions were observed at 1.68; 1.99, 2.06 eV and 1.60; 1.98 eV, respectively. Degrading of calcite structure under irradiation has been studied using CL emission of high power electron beam.

Crystal structure analysis of [Ca(O3SC18H37)2(DMSO)2], a lamellar coordination polymer and its relevance for model studies in biomineralization

Single crystals of a one-dimensional Ca coordination polymer of the surfactant octadecyl sulfonate (C(18)H(37)SO(3)(-)) have been grown from hot DMSO solution. The X-ray structure analysis of the compound [Ca(O(3)SC(18)H(37))(2)(DMSO)(2)] (1) shows a lamellar interdigitated arrangement of hydrophobic tails of the amphiphilic ligands. Each Ca ion is coordinated by four different sulfonate groups, and its nearly octahedral coordination environment is completed by two dimethyl sulfoxide (DMSO) ligands. The octadecyl sulfonate ligand coordinates to Ca ions in a micro(2)-bridging mode, which contrasts to information from literature suggesting a micro(3)-bridging coordination mode. Since the growth of highly oriented calcite single crystals underneath Langmuir monolayers of this particular surfactant is often regarded as textbook example of a heteroepitaxy ("template") mechanism in biomineralization, we present a critical discussion of the crystal structure of the title compound in this context.

Growth of single crystals in structured templates

Biology provides numerous examples of calcite single crystals with unique morphologies which are completely at odds with the internal lattice structure of the crystal. We have recently demonstrated that similar structures can be produced synthetically, using a suitable mould to template calcite crystal morphologies. We here investigate the generality of templating approaches to produce single crystals with complex morphologies. A variety of contrasting crystals were selected, and precipitated in polymer membranes with sponge-like structures, under a range of growth conditions. The structures of the product crystals were investigated using single crystal X-ray diffraction, and the morphology using scanning electron microscopy. True single crystal or “oligocrystalline” products were obtained for SrSO4, PbSO4, PbCO3, NaCl and CuSO4.5H2O, demonstrating that calcite is in no way unique in its ability to be moulded into complex morphologies. The principal requirement for templating single crystals is simply that the “natural” size of the crystal under the given growth conditions must exceed the length scale of the template.

Framboidal vaterite aggregates and their transformation into calcite: A morphological study

The morphology of vaterite aggregates of framboidal structure built up of smaller particles is described. It is shown that the transformation into calcite occurs partly within the framboidal vaterite aggregate, leading to much higher transformation rates compared to the transformation of solid non-framboidal vaterite spherulites. The transformation from inside the framboidal vaterite leads to the formation of single crystal calcite rhombs as observed for the transformation of non-framboidal vaterite spherulites.

Crystallization on Surfaces of Well-Defined Topography

Single crystals of calcite with regular patterned surfaces comprising close-packed arrays of hemispherical cavities or domes were produced by crystallization on colloidal monolayers or PDMS replicas of these monolayers, respectively. Perfect replication of the substrate topography was achieved for all colloidal particles, irrespective of their size and surface chemistry when the substrate geometry permitted unrestricted ion flow to the growing crystal. This work demonstrates that crystallization within a mould provides a very general route to producing single crystals with curved surfaces and unusual morphologies and that such patterning can be applied from the micro- to the nanoscale.

Synthesis of Calcite Single Crystals with Porous Surface by Templating of Polymer Latex Particles

Monodispersed copolymer latex particles functionalized by surface carboxylate groups are used as effective colloidal templates for the controlled crystallization of calcium carbonate in solution. After template removal, well-defined, calcite single crystals exhibiting a rhombohedral morphology and uniform surface pores are obtained. The surface pore size of the calcite single crystals can be readily adjusted through introduction of latex particles with varied sizes. The effects of the surface functional groups of the latexes, the concentration of the latex dispersion and CaCl2 solution and the temperature during the CaCO3 crystallization have been investigated. A three-step mechanism has been proposed for the formation of the composite particles consisting of calcite single crystals and latex particles, and it has been demonstrated that this synthesis strategy is potentially extendable to the synthesis of regularly shaped composite particles consisting of single crystals of other inorganic materials, such...