Calcite Single Crystals Research Articles

Transmission electron microscopy characterization of macromolecular domain cavities and microstructure of single-crystal calcite tooth plates of the sea urchin Lytechinus variegatus

The calcite plates and prisms in Lytechinus variegatus teeth form a complex biocomposite and employ a myriad of strengthening and toughening strategies. These crystal elements have macromolecule-containing internal cavities that may act to prevent cleavage. Transmission electron microscopy employing a small objective aperture was used to quantify several characteristics of these cavities. Cavity diameters ranged from 10 to 225 nm, the mean cavity diameter was between 50 and 60 nm, and cavities comprised approximately 20% of the volume of the crystal. Some cavities exhibited faceting and trace analysis identified these planes as being predominately of { 10 1 ¯ 4 } type. Through focus series of micrographs show the cavities were homogeneously distributed throughout the foil. The electron beam decomposed a substance within cavities and this suggests that these cavities are filled with a hydrated organic phase.

Elucidating the role of charge density on the growth of CaCO 3 crystals underneath Calix[4]arene monolayers

The amphiphilic 5,11,17,23-tetrakis-(1,1,3,3-tetramethylbutyl)-25,26,27,28-tetra(2-hydroxyethoxy)calix[4]arene ( 1) forms stable monolayers at the air–water interface. The growth of CaCO 3 crystals underneath monolayers of 1 is strongly inhibited, in contrast to the corresponding carboxylic acid derivative 5,11,17,23-tetrakis-(1,1,3,3-tetramethylbutyl)-25,26,27,28-tetra(carboxymethoxy)calix[4]arene ( 2), the monolayers of which lead to growth of preferentially oriented calcite single crystals. The growth morphology of CaCO 3 crystals is correlated with the phase behaviour and surface potential of the monolayers. The investigations demonstrate that the average charge density is the dominant factor for heterogeneous nucleation of CaCO 3 crystals at the calixarene monolayer/solution interface.

Single-contact pressure solution creep on calcite monocrystals

Abstract Pressure solution creep rates and interface structures have been measured by two methods on calcite single crystals. In the first kind of experiments, calcite monocrystals were indented at 40 °C for six weeks using ceramic indenters under stresses in the 50–200 MPa range in a saturated solution of calcite and in a calcite-saturated aqueous solution of NH 4 Cl. The deformation (depth of the hole below the indenter) is measured ex situ at the end of the experiment. In the second type of experiment, calcite monocrystals were indented by spherical glass indenters for 200 hours under stresses in the 0–100 MPa range at room temperature in a saturated aqueous solution of calcite. The displacement of the indenter was continuously recorded using a specially constructed differential dilatometer. The experiments conducted in a calcite-saturated aqueous solution of NH 4 Cl show an enhanced indentation rate owing to the fairly high solubility of calcite in this solution. In contrast, the experiments conducted in a calcite-saturated aqueous solution show moderate indentation rate and the dry control experiments did not show any measurable deformation. The rate of calcite indentation is found to be inversely proportional to the indenter diameter, thus indicating that the process is diffusion-controlled. The microcracks in the dissolution region under the indenter dramatically enhance the rate of calcite indentation by a significant reduction of the distance of solute transport in the trapped fluid phase. This result indicates that care should be taken in extrapolating the kinetic data of pressure solution creep from one mineral to another.

Preparation of Single Crystals under Hydrothermal Conditions

The capability of preparing carbonates and hydroxides that decompose in air at high temperatures is one of the advantages of hydrothermal reactions as a preparation technique for inorganic materials. In this study, new hydrothermal techniques to prepare single crystals of calcite (carbonate) and hydroxyapatite (hydroxide) were described. For hydrothermal crystal growth of calcite, organic salt solutions were used as a mineralizer for the first time and a solution of ammonium acetate was found to be an excellent mineralizer to obtain high growth rate under mild hydrothermal conditions. Single crystals of hydroxyapatite were prepared from chlorapatite single crystals by ion exchange in alkaline solutions under hydrothermal conditions. The transformation mechanism was explained by dissolution and precipitation of clusters.

Growth and characteristics of calcite single crystals

This paper deals with the experimental results of calcite ( CaCO 3 ) growth on seeds parallel to rhombohedral { 1 0 1 ¯ 1 } , pinacoidal {0 0 0 1} and prismatic { 1 1 2 ¯ 0 } faces. The process was carried out by the hydrothermal method in the system CaCO 3 – NH 4 Br – H 2 O . Autoclaves (24–1500 liters) were equipped with titanium inserts. Optical properties of the grown crystals have been investigated by means of spectrophotometry. Laser radiation initiates red luminescence whose intensity depends strongly on the radiant emittance that is probably connected with impurity absorption. The grown crystals are analogous to Iceland spar in their properties but excel them in optical homogeneity. Dispersion of refractive indices (dependence of N o and N e on the length of light wave) was measured and optical inhomogeneities indicated by refractive indices of calcite single crystals were studied.

Precipitation of Calcium Carbonate in Confinement

AbstractIn contrast to the regular morphologies displayed by synthetic single crystals, biogenic single crystals frequently exhibit unusual forms and curved surfaces. Such controlled mineralization in biology occurs within restricted volumes constructed for this purpose and there is significant evidence that some crystals with complex morphologies may form via an amorphous precursor phase. We here investigate morphological control of single crystals, using these biological mechanisms as inspiration. Calcite was precipitated within the cylindrical pores of track‐etch membranes via an amorphous calcium carbonate (ACC) precursor phase. Perfect replication of the pore channel to yield rod‐shaped single crystals of calcite was only achieved when the ACC precursor particles entirely filled the pore channels prior to crystallization, a process dependent on the pore diameter. Misshapen particles were formed in all pore diameters in the absence of an ACC phase. This system provides an ideal opportunity to carry out a systematic study into the effect of pore diameter, and the presence of an ACC precursor phase on crystal growth.

Site-specific incorporation of uranyl carbonate species at the calcite surface

Spatially resolved luminescence spectra from U(VI) co-precipitated at the (101̄4) growth surface of synthetic calcite single crystals confirm heterogeneous incorporation corresponding to the distribution of structurally non-equivalent steps composing the vicinal surfaces of spiral growth hillocks. Spectral structure from U(VI) luminescence at the “–” vicinal regions and featureless, weak luminescence at the “+” vicinal regions are consistent with previously reported observations of enrichment at the former sites during calcite growth. Luminescence spectra differ between the non-equivalent regions of the crystal, with the spectral features from the “–” vicinal region corresponding to those observed in bulk calcite samples. Subtle spectral shifts are observed from U(VI) co-precipitated with microcrystalline calcite synthesized by a different method, and all of the U(VI)-calcite sample spectra differ significantly from that of U(VI) co-precipitated with aragonite. The step-selective incorporation of U(VI) can be explained by a proposed model in which the allowed orientation for adsorption of the dominant calcium uranyl triscarbonate species is controlled by the atomic arrangement at step edges. Differences in the tilt angles of carbonate groups between non-equivalent growth steps favor adsorption of the calcium uranyl triscarbonate species at “–” steps, as observed in experiments.

Sea Urchin Spine Calcite Forms via a Transient Amorphous Calcium Carbonate Phase

The skeletons of adult echinoderms comprise large single crystals of calcite with smooth convoluted fenestrated morphologies, raising many questions about how they form. By using water etching, infrared spectroscopy, electron diffraction, and environmental scanning electron microscopy, we show that sea urchin spine regeneration proceeds via the initial deposition of amorphous calcium carbonate. Because most echinoderms produce the same type of skeletal material, they probably all use this same mechanism. Deposition of transient amorphous phases as a strategy for producing single crystals with complex morphology may have interesting implications for the development of sophisticated materials.

Synchrotron x-ray microbeam diffraction from abalone shell

Microstructured biomaterials such as mollusk shells receive much attention at present, due to the promise that advanced materials can be designed and synthesized with biomimetic techniques that take advantage of self-assembly and aqueous, ambient processing conditions. A satisfactory understanding of this process requires characterization of the microstructure not only in the mature biomaterial, but at the growth fronts where the control over crystal morphology and orientation is enacted. In this paper, we present synchrotron microbeam x-ray diffraction (XRD) and electron microscopy observations near the nacre–prismatic interface of red abalone shell. The relative orientations of calcite and aragonite grains exhibit some differences from the idealizations reported previously. Long calcite grains impinge the nacre–prismatic boundary at 45° angles, suggestive of nucleation on (104) planes followed by growth along the c axis. In the region within 100 μm of the boundary, calcite and aragonite crystals lose their bulk orientational order, but we found no evidence for qualitative changes in long-range order such as ideal powder texture or an amorphous structure factor. XRD rocking curves determined the mosaic of calcite crystals in the prismatic region to be no broader than the 0.3° resolution limit of the beamline’s capillary optics, comparable to what can be measured on geological calcite single crystals.

Single crystal structure analysis of sea urchin spine calcites: Systematic investigations of the Ca/Mg distribution as a function of habitat of the sea urchin and the sample location in the spine

The hard skeleton of sea urchin spines consists of a monolithic single crystalline magnesian calcite with typical contents of MgCO 3 ranging from 2 to 12 mol% and about 0.1 weight% of occluded organic macromolecules. These macromolecules take effect on the calcite crystal growth, forming the spine shape in a porous spongy structure crossed by filled wedges of calcite radially arranged about the long axis of the spine. The single crystal structure analysis and polarization microscopy confirm the perfect alignment of the crystallites, thus the spine is a single crystal of calcite with the spine axis along the crystallographic c-axis, in agreement with earlier experiments. The magnesium content of the skeletal parts of the sea urchin is approximately linearly related with the water temperature of the sea urchin habitat. Elemental and chemical analysis showed an inhomogeneous Mg distribution in single spines: The Mg content increases from the spine-tip to the base by about 2 mol% at average. This structural inhomogeneity leads to a solid solution hardening of the material by the statistical incorporation of Mg 2+ ions in a crystal structure. The size difference between Ca 2+ and Mg 2+ ions impedes the propagation of cracks, thus affecting the fracture behavior of the biogenic calcite, which is conchoidal in contrast to the perfect {10.4} cleavage of geological calcite. The accumulation of Mg in the spine-base most likely serves to enhance this effect and thus strengthens the spine close to the shell.

Role of marble microstructure in near-infrared laser-induced damage during laser cleaning

When marble is cleaned by nanosecond neodymium yttrium–aluminum–garnet lasers (1064 nm), strongly absorbing surface contaminants are removed at fluences substantially below the damage threshold for the much less absorptive marble substrate. Recent studies have shown, however, that unacceptable roughening of the marble surface also may occur at low fluences due to removal of individual grains. In order to elucidate this effect, we have compared the low-fluence response of marbles with two different grain sizes and single-crystal calcite, in the fluence range 0.12–1.25 J cm−2. Damage was greater in fine-grained than coarse-grained marble, and did not occur in the single-crystal calcite at these fluences. The temperature rise following defect-mediated absorption triggers thermal plasma emission and generates shock waves; the concomitant surface damage depends on the size and crystallographic orientation of the crystals. Laser irradiation anneals the defects and increases “crystallite size.” The implications for the laser-assisted cleaning of marble artworks are outlined.

Shape-constraint as a route to calcite single crystals with complex morphologies

Sponge-like polymer membranes, formed by templating sea urchin skeletal plates, were used as an environment in which to precipitate calcium carbonate. Using double diffusion conditions and under a specific range of reagent concentrations single crystals of calcite with sponge-like morphologies were precipitated, whose form was dictated by the confines of the polymer membrane. Investigation of the mechanism of formation of these particles showed that both templated calcite and vaterite particles nucleated at early times, but that the proportion of vaterite present decreased with time due to dissolution and reprecipitation as calcite. All evidence suggests that the templated calcite single crystals grew in the absence of an amorphous calcium carbonate (ACC) precursor phase, demonstrating that the presence of ACC is not a necessary prerequisite to the formation of single crystals of calcite with complex morphologies.

Interfacial electrostatics guiding the crystallization of CaCO3 underneath monolayers of calixarenes and resorcarenesElectronic supplementary information (ESI) available: representative optical and scanning electron micrographs of CaCO3 crystals grown underneath a monolayer of 1 at low surface pressure; additional crystallographic data including numbering schemes, tables and refinement details. See http://www.rsc.org/suppdata/jm/b4/b403132f/

The amphiphilic octaacids rccc-4,6,10,12,16,18,22,24-octakis-O-(carboxymethyl)-2,8,14,20-tetra(n-undecyl)resorc[4]arene (1) and 5,11,17,23,29,35,41,47-octakis(1,1,3,3-tetramethylbutyl)-49,50,51,52,53,54,55,56-octa(carboxymethoxy)calix[8]arene (2) form stable monolayers at the air–water interface which induce growth of CaCO3 crystals at the monolayer–solution interface. Uniformly (012) oriented rhombohedral calcite single crystals grow underneath a monolayer of 2, whereas crystallization under a monolayer of 1 preferentially leads to formation of acicular aggregates of aragonite crystals. While polymorph selection and orientations of the CaCO3 crystals critically depend on the average charge density of the monolayer, the molecular shape and the particular Ca coordination behaviour of the amphiphiles that form the monolayer are of minor importance. CaCO3 crystal growth underneath monolayers of macrocyclic amphiphiles is briefly reviewed and the present experimental observations are compared to previous related investigations on “template-induced” biomimetic mineralization.

Nanobacteria-like calcite single crystals at the surface of the Tataouine meteorite.

Nanobacteria-like objects evidenced at the surface of the orthopyroxenes of the Tataouine meteorite in South Tunisia have been studied by scanning and transmission electron microscopies. A method of micromanipulation has been developed to ensure that exactly the same objects were studied by both methods. We have shown that the nanobacteria-like objects are spatially correlated with filaments of microorganisms that colonized the surface of the meteoritic pyroxene during its 70 years of residence in the aridic Tataouine soil. Depressions of a few micrometers in depth are observed in the pyroxene below the carbonates, indicating preferential dissolution of the pyroxene and calcite precipitation at these locations. The nanobacteria-like small rods that constitute calcium carbonate rosettes are well crystallized calcite single crystals surrounded by a thin amorphous layer of carbonate composition that smoothes the crystal edges and induces rounded shapes. Those morphologies are unusual for calcite single crystals observed in natural samples. A survey of recent literature suggests that the intervention of organic compounds derived from biological activity is likely in their formation.

The Transient Phase of Amorphous Calcium Carbonate in Sea Urchin Larval Spicules: The Involvement of Proteins and Magnesium Ions in Its Formation and Stabilization

AbstractAmorphous calcium carbonate (ACC) is a precursor phase of calcite in the formation of the sea urchin larval spicule. The goal of this research is to study the formation and stabilization mode of this transient phase. We first characterized the mineralogy of the spicules from the sea urchin Strongylocentrotus purpuratus. We then examined the role of the macromolecules extracted from the spicules at different growth stages in the formation of transient ACC in vitro.The biogenic amorphous transient phase is shown to be both structurally and compositionally different from the known stable ACC phases. It does not contain bound water, and is thus the first dehydrated ACC phase to be detected. The macromolecules that were extracted at early stages of spicule growth, when the amorphous content of the biogenic mineral is high, induced the formation of transient ACC in vitro in the presence of magnesium ions. In contrast, the macromolecules extracted at a later stage, when the spicules are completely crystalline, induced the formation of single crystals of low magnesian calcite. We therefore deduce that the macromolecules from the sea urchin larval spicules together with magnesium ions, mediate the transient formation of ACC as a precursor to calcite. These observations may well provide novel ideas for improved materials synthesis.

Biomimetic control of crystal assembly by growth in an organic hydrogel network

Calcite aggregates are mineralized in an organic poly-acrylamide hydrogel using a counter diffusion arrangement. The particles obtained show a characteristic pseudo-octahedral morphology, which is unexpected for calcite crystals. Scanning and transmission electron microscopy reveal a microstructure composed of individual highly aligned calcite crystallites. Although the aggregates consist of independent crystallites, the X-ray diffraction patterns suggest calcite single crystals. By analogy with some biominerals, the inorganic assembly is intergrown with an organic hydrogel network. A specific model is proposed for growth of the aggregate.

Direct fabrication of large micropatterned single crystals.

Micropatterning of single crystals for technological applications is a complex, multistep process. Nature provides alternative fabrication strategies, when crystals with exquisite micro-ornamentation directly develop within preorganized frameworks. We report a bio-inspired approach to growing large micropatterned single crystals. Micropatterned templates organically modified to induce the formation of metastable amorphous calcium carbonate were imprinted with calcite nucleation sites. The template-directed deposition and crystallization of the amorphous phase resulted in the fabrication of millimeter-sized single calcite crystals with sub-10-micron patterns and controlled crystallographic orientation. We suggest that in addition to regulating the shape, micropatterned frameworks act as sites for stress and impurity release during the amorphous-to-crystalline transition. The proposed mechanisms may have direct biological relevance and broad implications in materials synthesis.

Growth of Calcite Single Crystals Underneath Monolayers of 5, 11, 17, 23‐Tetra‐t‐butyl‐25, 26, 27, 28‐tetrakis(carboxymethoxy)calix[4]arene

AbstractThe amphiphilic 5, 11, 17, 23‐tetra‐t‐butyl‐25, 26, 27, 28‐tetrakis(carboxymethoxy)calix[4]arene (1) forms stable monolayers at the air‐water interface which serve as template to induce growth of CaCO3 (calcite) single crystals. The nucleation density and the preferred orientation(s) of calcite single crystals depend on the surface pressure applied to the monolayer. Models of the pressure‐dependant aggregation of amphiphiles at the air‐water interface are derived from the crystal structures of the novel Ca coordination compounds [(Ca(C52H60O12))2Ca(H2O)2(CH3OH)3Ca(H2O)2(CH3OH)2]·12CH3OH·4H2O (2) and [Ca(C52H60O12)Ca(H2O)2.5(MeOH)0.5]·7H2O·CH3OH (3). Structural data are analyzed in terms of supramolecular packing arrangements and Ca coordination motifs of the constituting amphiphilic macrocycles.

Morphology and dynamics of calcium carbonate granules produced by different earthworm speciesThe 7th international symposium on earthworm ecology · Cardiff · Wales · 2002

Summary Granules of calcium carbonate produced by earthworms are regularly found in soil profiles, but little is known of their origins and dynamics. Pure cultures of a range of species were therefore set up, using artificial soils, and the granules recovered for examination. Species of Lumbricus, Aporrectodea, Octolasion and Allolobophora all produced granules larger than 0.125mm, but Eisenia hortensis did not. For comparison, two compost-dwelling Eisenia species were also cultured. These yielded no granules at all, suggesting that the genus as a whole does not produce granules, consistent with its lack of calciferous sacs. Granules recovered from the other 7 species have a variety of sizes and morphologies, ranging from single calcite crystals to aggregations up to 2.5mm in diameter. SEM photographs of the granules and data on granule concentrations in relation to depth are presented

Morphology and dynamics of calcium carbonate granules produced by different earthworm species: The 7th international symposium on earthworm ecology · Cardiff · Wales · 2002

Granules of calcium carbonate produced by earthworms are regularly found in soil profiles, but little is known of their origins and dynamics. Pure cultures of a range of species were therefore set up, using artificial soils, and the granules recovered for examination. Species of Lumbricus, Aporrectodea, Octolasion and Allolobophora all produced granules larger than 0.125mm, but Eisenia hortensis did not. For comparison, two compost-dwelling Eisenia species were also cultured. These yielded no granules at all, suggesting that the genus as a whole does not produce granules, consistent with its lack of calciferous sacs. Granules recovered from the other 7 species have a variety of sizes and morphologies, ranging from single calcite crystals to aggregations up to 2.5mm in diameter. SEM photographs of the granules and data on granule concentrations in relation to depth are presented