Synthetic Calcite Single Crystals Research Articles

Shape change of calcite single crystals to accommodate interfacial curvature: Crystallization in presence of Mg2+ ions and agarose gel-networks

Synthetic calcite single crystals, due to their strong crystal habit, tend to grow into characteristic rhombohedra. In the nature, biogenic calcite crystals form composites together with biomacromolecular materials, spurring investigations of how the growing calcite single crystals change their habit to satisfy the curvature of the organic phase. In this work, we examine calcite crystallization on a flat surface of glass slide and a curved surface of polystyrene (PS) sphere. The crystals exhibit tiny contact area onto the glass substrate that is averagely only 15% of their projected area on the substrate. In sharp contrast, the contact area greatly increase to above 75% of the projected area, once magnesium ions or agarose gel networks are introduced into the crystallization media. Furthermore, the calcite crystals form rough and step-like interfaces with a curved surface. However, the interfaces become smooth and curved as the crystals grow in presence of magnesium ions or agarose gel networks. The discrepancy between the interfacial structures implies kinetic effects of the additives on the crystallization around the surfaces. This work may provide implications for understanding the formation mechanisms of single-crystal composite materials.

Unraveling the internal microstructure of biogenic and bioinspired calcite single crystals

Abstract

Micelles in a crystal

Inclusion of organic molecules in inorganic crystals is thought to enhance their mechanical properties, yet obtaining high occlusion levels has been a challenge. It is now shown that synthetic calcite single crystals incorporating a significant amount of copolymer micelles have mechanical properties similar to biogenic calcite crystals.

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.

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.

Structure and morphology of synthetic magnesium calcite

The X-ray crystal structure refinements of two synthetic calcite single crystals containing 3.1 and 5.7 mol% magnesium were carried out. The structure of the second crystal was also refined at –80 °C. No significant structural difference from biogenic magnesium calcite was found. Magnesium within the crystals of calcite forms a solid solution over the compositional range investigated. The principal axes of vibration ellipsoids at –80 °C resemble very closely those of calcite suggesting that the large thermal parameters in magnesium calcite reflect increased thermal vibrations more than positional disorder. The crystals of magnesium calcite express new rhombohedral faces due to the interaction of magnesium with growing crystals. The role of magnesium in the morphology of single crystal and globular aggregates of magnesium calcite is discussed in view of its possible relevance in both biomineralization and crystal growth.

Surface structural controls on compositional zoning of SO 2−4 and SeO 2−4 in synthetic calcite single crystals

Coprecipitation experiments show that structural characteristics of growth surfaces on synthetic calcite single crystals are a primary control on SO 2− 4 and SeO 2− 4 incorporation. Electron probe microanalyses of sections through sectorally zoned crystals show that SO 2− 4 concentrations in 1014 sectors are 50% higher than in 0112 sectors. Electron probe and synchrotron X-ray fluorescence microanalyses within 1014 sectors document a twofold to threefold difference in SO 2− 4 and SeO 2− 4 contents between subsectors associated with nonequivalent vicinal faces of growth hillocks. This differential incorporation of SO 2− 4 and SeO 2− 4 documents path-dependent, nonequilibrium partitioning behavior. Models of nearest-neighbor and second-nearest-neighbor coordination environments of nonequivalent kink sites reveal differences in site sizes and geometries. Vicinal faces that consist of steps with larger and geometrically less constrained kink sites have greater SO 2− 4 and SeO 2− 4 incorporation, whereas vicinal faces with smaller, more constrained kink sites always have less SO 2− 4 and SeO 2− 4 incorporation. This correlation requires surface-structural, i.e., crystallographic, controls on SO 2− 4 and SeO 2− 4 incorporation and, therefore, is consistent with SO 2− 4 and SeO 2− 4 substitution for CO 2− 3. Our results clearly show that consideration of surface structural controls on trace ion partitioning is essential for a better understanding of trace ion studies in carbonate geochemistry and of crystal surface processes.

Synthetic calcite single crystals for optical device

Hydrotermal synthesis of calcite single crystals; crystalline morphologie, defects and optical properties.

New type of compositional zoning in calcite: Insights into crystal-growth mechanisms

Compositional of trace elements is observed in time-equivalent regions within 10 \(\overline{1}\) 4 growth sectors of natural and synthetic calcite single crystals, and has been called intrasectoral zoning to distinguish it from conventional sectoral zoning. Differential interference contrast microscopy shows that {101 \(\overline{1}\) 4} faces grow by the spiral mechanism. Cathodoluminescence microscopy and electron probe microanalyses reveal that trace element partitioning differences on {101 \(\overline{1}\) 4} faces correspond exactly to crystallographically controlled orientations of growth steps. Effective partitioning of Mg and Mn varies up to a factor of four between time-correlative growth steps spreading in different directions. Effective Sr partitioning varies by approximately 20% and is inversely correlated with enrichment trends of Mg and Mn. Preferential incorporation of different aqueous species at different growth steps may provide an explanation for morphological and chemical variation in natural calcites.

Electron-hole centers in natural and synthetic calcite single crystals containing phosphorus

Electron-hole centers in natural and synthetic calcite single crystals containing phosphorus