Synthetic Calcite Crystals Research Articles

Atom Probe Tomography (APT) Characterization of Organics Occluded in Single Calcite Crystals: Implications for Biomineralization Studies

Occlusion of organic components in synthetic calcite crystals has been recently used as a model to understand the role of intra-crystalline organics in biominerals. However, the characterization of the distribution of both types of organics inside these calcite crystals is very challenging. Here, we discuss the potential of using the technique of atom probe tomography (APT) for such characterization, focusing on the analysis of chitin incorporation in single crystals. Additionally, APT has at least the same spatial resolution as TEM tomography, yet with the advantage of obtaining quantitative chemical data. Results show that chitin, either after degradation with yatalase or in the form of nanofibers, forms discrete clusters (2 to 5 nm) in association to water and hydronium molecules, rather than forming a 3D network inside crystals. Overall findings indicate that APT can be an ideal technique to characterize intra-crystalline organic components in abiogenic and biogenic carbonates to further advance our understanding of biomineralization.

CaCO3 (Calcite)/Li2CO3 (Zabuyelite) Anomalous Mixed Crystals. Sector Zoning and Growth Mechanisms

A growth model of {0001} form bearing calcite crystals has been proposed in our previous papers. A detailed PBC (periodic bond chains) study demonstrated that calcite {0001} form could not be stable, owing to its K character. The stabilization involves a 2D epitaxial growth mechanism between calcium and lithium carbonate, the model being supported by parametric fit between calcium and lithium carbonates. In this paper, we experimentally evaluate the consistency of this model. Integrated spectroscopic and imaging characterization techniques, inductively coupled plasma optical emission spectroscopy (ICP-OES), scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray powder diffraction (XRPD), cathodoluminescence (CL) and electron paramagnetic resonance (EPR) have been applied to synthetic calcite crystals grown in the presence of lithium in order to investigate whether and how lithium can be absorbed into the calcite lattice. Experiments and theoretical considerations prove that lithium enters the calcite crystals during their growth, according to strongly differentiated sector absorption mediated by 2D and 1D epitaxy on the corresponding {0001} and {101̅4} crystal forms.

Mineral-fluid partitioning and isotopic fractionation of boron in synthetic calcium carbonate

Aragonite, high-Mg calcite, and Mg-free calcite have been grown under monitored laboratory conditions to investigate mineral-fluid partitioning and isotopic fractionation of boron into carbonates. At pH ≈ 8, both polymorphs have boron isotopic compositions I6.5%0 lower than the boron isotopic composition of the experimental fluid ( δ 11 B = 0 ± 0.7ℵ. ). The isotopic composition of the synthetic minerals is identical to the calculated boron isotopic composition of the B(OH) 4 − species in the parent solutions, consistent with only B(OH) 4 − interacting with the crystal surface. The fractionation factor calculated from this isotopic offset is within analytical error of that measured between seawater and modern marine carbonates. The boron concentrations of the experimentally grown aragonite are 13.4, 85.1, and 379 ppm for total fluid boron concentrations of 0.59, 5.1, and 37.0 ppm, respectively. High-Mg calcite (HMC) coprecipitating with the aragonite has bulk boron concentrations of 5.6, 56.2, 260, and 1771 ppm for fluid boron concentrations of 0.59, 5.1, 37, and 272 ppm, respectively. Calcite boron concentrations are 1.78, 14.0, 137.7, and 1228 ppm for fluid boron concentrations of 0.45, 4.0, 44.l, and 499 ppm, respectively. Boron uptake by aragonite is greater than boron uptake by calcite grown under nearly identical conditions, while boron uptake by HMC is intermediate. Synthetic calcite crystals grown in solutions with greater than 4 ppm boron exhibit habit modification with the appearance of a subordinate form {11 2 0} in addition to the dominant {10 1 4}, the perfect cleavage rhombohedron, found in low-boron calcites. This is evidence of strong interaction of the borate with the crystal surface.

Crystal-protein interactions studied by overgrowth of calcite on biogenic skeletal elements

A key parameter in the biological control of crystal formation is the interaction of a group of acidic macromolecules with the mineral phase. Here we study protein-calcite interactions using epitaxial overgrowth of synthetic calcite crystals under conditions in which local release of occluded macromolecules from the biogenic substrate occurs. The macromolecules subsequently interact with the newly formed overgrown crystals, resulting in modified calcite morphology. This novel method provides a means of mapping crystal-protein interactions under conditions that minimally affect the conformational states of the acidic macromolecules. We show that proteins released from calcitic sponge spicules and mollusc prisms specifically interact with {001} and {01 l} faces of calcite, whereas proteins released from echinoderm skeletal elements only interact with {01 l} faces. The extent to which the overgrown crystals are affected by the proteins varies even in the same organism and within the same element, depending on the site and crystallographic orientation of the skeletal elements.

Sector zoning in natural and synthetic calcites

Sector zoning refers to a compositional difference between coeval growth sectors in a crystal and results from differences in fluid-crystal element partitioning between nonequivalent faces of the crystal. Sector zoning involving the trace elements Mg, Mn, Fe, and Sr is common in natural calcite cements, and sector zoning of Mn has been induced in synthetic calcite crystals. Differences in surface structure between nonequivalent faces are seen as the principal causes for face-dependent partitioning.

The effect of magnetic fields on calcium carbonate scale formation

A magnetic field was constructed, based on an especially designed power supply which could generate a field 83% continuous and 17% alternating. For field intensities exceeding 10 T, a strong retardation was observed on the crystallization of calcite in metastable calcium carbonate solutions, seeded with synthetic calcite crystals. The rates of crystallization, as measured with the constant supersaturation approach, decreased with increasing field intensity. The apparent order of the crystal growth of calcite was found to be n = 2 both in the presence and in the absence of the magnetic field, suggesting a surface controlled mechanism. The mechanism was not influenced by the presence of the magnetic field, which may possibly affect the dehydration step of the growth units which migrate to the active growth sites by surface diffusion.

Identification of Organic and Inorganic Activators of Photoluminescence in Calcite by Laser Microprobe Spectroscopy: ABSTRACT

High-resolution laser-excited emission spectra provide new evidence that organic activation of photoluminescence can be distinguished from trace-element activation in calcite. Ultraviolet (362 nm), blue-violet (454 nm), and green (514 nm) wavelengths of an argon ion laser were focused through a petrographic microscope to generate photoluminescence from 2 to 5-/mu/m spots. Emissions spectra from pristine, organic-rich red algal skeletons and synthetic calcite crystals spiked with manganese were measured by an ISA U-1000 spectrometer. Biogenic and synthetic calcites display distinctly different spectra. With shorter exciting wavelengths, emission spectra of biogenic calcite show a blue shift in peak maximum from 620 to 480 nm. Spectra of manganese-spiked synthetic calcite show no change in the 630-nm peak maximum with shorter exciting wavelengths. The spectral characteristics are consistent with the known photoluminescence emission of high molecular weight aromatic hydrocarbon compounds and manganese-bearing calcite, respectively. The absolute intensity of photoluminescence also shows a difference between biogenic and synthetic calcites. In biogenic calcite, the intensity increases with shorter excited wavelengths; the intensity under 362-nm excitation is five and one-half times stronger than that under 514-nm excitation. In the manganese-spiked calcite, the intensity decreases with shorter exciting wavelengths; the intensity under 514-nm excitation is five times stronger thanmore » that under 362-nm excitation. The emission patterns are consistent with the known absorbance spectra of aromatic hydrocarbons and manganese-bearing calcite. In conjunction, measurements of peak shift and intensity demonstrate that variable-wavelength laser excitation must be used to distinguish organic from inorganic photoluminescence activators in calcite.« less

Low‐angle grain boundaries in synthetic calcite crystals

AbstractSingle crystals of calcite grown in this laboratory by the solution method have been cleaved along (100) planes and have been chemically etched in 20% NH4Cl and 5% citric acid solutions. Rows of equally spaced etch pits have been observed. By etching matched cleavage faces, correspondence has been established for low angle grain boundaries. Correspondence of grain boundaries has also been observed on the two sides of a thin plate 0.68 mm thick. It is observed that the density of pits in all the corresponding rows of matched cleavage faces and on the two sides of a thin plate is the same. Correspondence in the matching of four boundaries forming a node has also been observed on the matched faces and the relation na = nb = nc + nd (where n is the pit density along the boundary line) has also been established. The implications are discussed.

Inclined Dislocations in Synthetic Calcite Crystals

AbstractA large number of perfect (100) cleavages were obtained by cleaving the laboratory grown synthetic calcite crystals. By etching such cleavages in 20% ammonium chloride solution, it is established that the pits indicate the sites of dislocations in the crystals. This is further confirmed by comparing the etch patterns on matched cleavage surfaces. Eccentricity of the pits has been attributed to the inclined nature of dislocations. Existence of parallel dislocations seems to be due to the purity of the crystal. These observations are verified by comparing etch patterns produced on opposite faces of a thin flake (0.048 mm thick) and also by successive etching and polishing a cleavage surface. It is conjectured that curl‐bottomed pits nucleate at the sites of screw dislocations in the crystal. The implications are discussed.

On some UV emissions in calcite

Abstract Two UV emission bands have been found under UV excitation in hydrothermal synthetic calcite crystals at 347 and 385 nm. Their temperature dependence can be explained in terms of transfer through the Auger effect toward a well-known center absorbing near 350 nm, which can now be described as a killer of luminescence in calcite at room temperature.