Alejandro B. Rodríguez-Navarro

Thermal decomposition of calcite: Mechanisms of formation and textural evolution of CaO nanocrystals

Abstract Field emission scanning electron microscopy (FESEM), two-dimensional X-ray diffraction (2DXRD), and transmission electron microscopy coupled with selected area electron diffraction (TEMSAED) analyses of the reactant/product textural relationship show that the thermal decomposition of Iceland spar single crystals according to the reaction CaCO3(s) → CaO(s) + CO2(g) is pseudomorphic and topotactic. This reaction begins with the formation of a mesoporous structure made up of up to four sets of oriented rod-shaped CaO nanocrystals on each rhombohedral cleavage face of the calcite pseudomorph. The four sets formed on (101̅4)calcite display the following topotactic relationships: (1) (12̅10)calcite//(110)CaO; (2) (1̅104)calcite┴ (110)CaO; (3) (1̅018)calcite//(110)CaO; and (4) (01̅14)calcite┴(110)CaO; with [841]calcite//[11̅0]CaO in all four cases. At this stage, the reaction mechanism is independent of PCO2 (i.e., air or high vacuum). Strain accumulation leads to the collapse of the mesoporous structure, resulting in the oriented aggregation of metastable CaO nanocrystals (~5 nm in thickness) that form crystal bundles up to ~1 μm in cross-section. Finally, sintering progresses up to the maximum T reached (1150 °C). Oriented aggregation and sintering (plus associated shrinking) reduce surface area and porosity (from 79.2 to 0.6 m2/g and from 53 to 47%, respectively) by loss of mesopores and growth of micrometer-sized pores. An isoconversional kinetic analysis of non-isothermal thermogravimetric data of the decomposition of calcite in air yields an overall effective activation energy Eα = 176 ± 9 kJ/ mol (for α > 0.2), a value which approaches the equilibrium enthalpy for calcite thermal decomposition (177.8 kJ/mol). The overall good kinetic fit with the F1 model (chemical reaction, first order) is in agreement with a homogeneous transformation. These analytical and kinetic results enable us to propose a novel model for the thermal decomposition of calcite that explains how decarbonation occurs at the atomic scale via a topotactic mechanism, which is independent of the experimental conditions. This new mechanistic model may help reinterpret previous results on the calcite/CaO transformation, having important geological and technological implications.

Influence of the microstructure on the shell strength of eggs laid by hens of different ages

1. The eggshell is a bioceramic material constructed of columnar calcite crystals preferentially oriented with their c -axis perpendicular to the shell surface. 2. The influence of microstructure (crystal size, shape and crystallographic orientation of crystal grains) on the mechanical properties of eggshells (shell strength) was investigated using eggs from hens of different ages. 3. There was a strong correlation between crystallographic texture and the strength of the eggshell in the case of eggs laid by young hens. The strength of eggshells increased as the preferential orientation of the crystals constituting the eggshell decreased. 4. By comparing two age populations, the effect of hen age on eggshell properties was evaluated. In general, eggshells from aged hens had a lower breaking strength (less than half that of those laid by young hens) and showed a greater variability in their structural properties such as thickness, grain morphology and crystallographic texture. 5. Texture analysis revealed that shells from eggs laid by aged hens have two preferred crystal orientations, after (001) and (104), compared with mainly one, after (001), in eggs laid by young hens. 6. These observed changes in eggshell properties could be due to changes in the organic matrix of the eggshell associated with ageing of the hens.

Long-term effects of lead poisoning on bone mineralization in vultures exposed to ammunition sources

Long-lived species are particularly susceptible to bioaccumulation of lead in bone tissues. In this paper we gain insights into the sublethal effects of lead contamination on Egyptian vultures (Neophron percnopterus). Our approach was done on the comparison of two populations (Canary Islands and Iberian Peninsula) differing in exposures to the ingestion of lead ammunition. Blood lead levels were higher in the island population (Canary Islands range: 5.10-1780 microg L(-1) n=137; Iberian Peninsula range: 5.60-217.30 microg L(-1) n=32) showing clear seasonal trends, peaking during the hunting season. Moreover, males were more susceptible to lead accumulation than females. Bone lead concentration increased with age, reflecting a bioaccumulation effect. The bone composition was significatively altered by this contaminant: the mineralization degree decreased as lead concentration levels increased. These results demonstrate the existence of long-term effects of lead poisoning, which may be of importance in the declines of threatened populations of long-lived species exposed to this contaminant.

The eggshell: structure, composition and mineralization

The calcareous egg is produced by all birds and most reptiles. Current understanding of eggshell formation and mineralization is mainly based on intensive studies of one species - the domesticated chicken Gallus gallus. The majority of constituents of the chicken eggshell have been identified. In this article we review eggshell microstructure and ultrastructure, and the results of recent genomic, transcriptomic and proteomic analyses of the chicken eggshell matrix to draw attention to areas of current uncertainty such as the potential role of amorphous calcium carbonate and the specific nature of the molecules that initiate (nucleate) mammillary cone formation and terminate palisade layer calcification. Comparative avian genomics and proteomics have only recently become possible with the publication of the Taeniopygia guttata (zebra finch) genome. Further rapid progress is highly anticipated with the soon-to-be-released genomes of turkey (Meleagris gallopavo) and duck (Anas platyrhynchos). These resources will allow rapid advances in comparative studies of the organic constituents of avian eggshell and their functional implications.

XRD2DScan: new software for polycrystalline materials characterization using two-dimensional X-ray diffraction

XRD2DScan is a Windows application for displaying and analyzing two-dimensional X-ray diffraction patterns collected with an area detector. This software allows users to take full advantage of diffractometers that are equipped with an area detector but that cannot readily process the information contained in diffraction patterns from polycrystalline materials. XRD2DScan has many capabilities for generating different types of scans (2θ scan, ψ scan, d spacing versus ψ angle), which allows users to extract the maximum amount of information from two-dimensional patterns. Analyses of multiple data files can be fully automated using batch processing. The use of the software is illustrated through several examples.

Influence of lysozyme on the precipitation of calcium carbonate: a kinetic and morphologic study

Several mechanisms have been proposed to explain the interactions between proteins and mineral surfaces, among them a combination of electrostatic, stereochemical interactions and molecular recognition between the protein and the crystal surface. To identify the mechanisms of interaction in the lysozyme- calcium carbonate model system, the effect of this protein on the precipitation kinetics and morphology of calcite crystals was examined. The solution chemistry and morphology of the solid were monitored over time in a set of time-series free-drift experiments in which CaCO3 was precipitated from solution in a closed system at 25°C and 1 atm total pressure, in the presence and absence of lysozyme. The precipitation of calcite was preceded by the precipitation of a metastable phase that later dissolved and gave rise to calcite as the sole phase. With increasing lysozyme concentration, the nucleation of both the metastable phase and calcite occurred at lower calcite, indicating that lysozyme favored the nucleation of both phases. Calcite growth rate was not affected by the presence of lysozyme, at least at protein concentrations ranging from 0 mg/mL to 10 mg/mL. Lysozyme modified the habit of calcite crystals. The degree of habit modification changed with protein concentration. At lower concentrations of lysozyme, the typical rhombohedral habit of calcite crystals was modified by the expression of {110} faces, which resulted from the preferential adsorption of protein on these faces. With increasing lysozyme concentration, the growth of {110}, {100}, and finally {001} faces was sequentially inhibited. This adsorption sequence may be explained by an electrostatic interaction between lysozyme and calcite, in which the inhibition of the growth of {110}, {100}, and {001} faces could be explained by a combined effect of the density of carbonate groups in the calcite face and the specific orientation (perpendicular) of these carbonate groups with respect to the calcite surface. Overgrowth of calcite in the presence of lysozyme demonstrated that the protein favored and controlled the nucleation on the calcite substrate. Overgrowth crystals nucleated epitaxially in lines which run diagonal to rhombohedral {104} faces. Copyright

Changes in eggshell mechanical properties, crystallographic texture and in matrix proteins induced by moult in hens.

1. The effect of moult on eggshell mechanical properties, on composition and concentrations of organic matrix components and on eggshell microstructure was investigated. The observed changes were studied to understand the role of organic matrix and eggshell microstructure in eggshell strength. 2. Moult was induced by zinc oxide (20 g zinc/kg diet) in 53 ISA Brown laying hens at 78 weeks of age. No difference was observed for egg or eggshell weights after moult. In contrast, moult improved the shell breaking strength (28·09 vs 33·71 N). 3. After moult, there was a decrease in the average size of calcite crystals composing the eggshell and in their heterogeneity, whereas crystal orientation remained basically the same. 4. After moulting, the total protein concentration in eggshell increased slightly. The comparisons of SDS-PAGE profiles of the organic matrix constituents extracted before and after moulting showed changes in staining intensity of certain bands. After moult, bands associated with main proteins specific to eggshell formation (OC-116 and OC-17) showed higher staining intensity, while the intensity of the egg white proteins (ovotransferrin, ovalbumin and lysozyme) decreased. ELISA confirmed the decrease in ovotransferrin after moult. Its concentration was inversely correlated with breaking strength before moult. 5. These observations suggest that changes in eggshell crystal size could be due to changes in organic matrix composition. These changes may provide a mechanism for the improvement in shell solidity after moulting.

Geometrical and crystallographic constraints determine the self-organization of shell microstructures in Unionidae (Bivalvia: Mollusca)

Unionid shells are characterized by an outer aragonitic prismatic layer and an inner nacreous layer. The prisms of the outer shell layer are composed of single–crystal fibres radiating from spheruliths. During prism development, fibres progressively recline to the growth front. There is competition between prisms, leading to the selection of bigger, evenly sized prisms. A new model explains this competition process between prisms, using fibres as elementary units of competition. Scanning electron microscopy and X–ray texture analysis show that, during prism growth, fibres become progressively orientated with their three crystallographic axes aligned, which results from geometric constraints and space limitations. Interestingly, transition to the nacreous layer does not occur until a high degree of orientation of fibres is attained. There is no selection of crystal orientation in the nacreous layer and, as a result, the preferential orientation of crystals deteriorates. Deterioration of crystal orientation is most probably due to accumulation of errors as the epitaxial growth is suppressed by thick or continuous organic coats on some nacre crystals. In conclusion, the microstructural arrangement of the unionid shell is, to a large extent, self–organized with the main constraints being crystallographic and geometrical laws.

Development of preferred orientation in polycrystalline AlN thin films deposited by rf sputtering system at low temperature

The development of preferred orientation in AlN thin films deposited on silica glass substrates by rf sputtering at low substrate temperature ( c -axis perpendicular to the substrate surface. At higher pressure (>15 mTorr), a spreading in the incidence angle of the arriving particles, due to gas phase collisions, favors the formation of AlN crystal twinning. A change in the preferred orientation of the films from (0001) to (10 1 1) for deposition rates above 1.8 A/s is observed.

Model of texture development in polycrystalline films growing on amorphous substrates with different topographies

A two-dimensional computer model is presented to study the development of microstructure and crystallographic texture in polycrystalline films growing on amorphous substrates, which often exhibit strong textures. In particular, the analysis focuses on the influence of growth parameters such as the crystal habit, the relative growth rate along different crystal directions, the nucleation density on the substrate surface and the substrate topography. The simulation explains the development of fiber and in-plane textures in polycrystalline films growing from randomly oriented nuclei. These textures result from a geometric selection of the orientation of crystals, mainly due to differences in growth rates along different crystal directions. The calculated distribution of crystal orientations and its evolution with time are in agreement with textures observed in films deposited under growth conditions, corresponding to zone 2 of the structure zone model. The growth of crystals composing of a polycrystalline film is mainly constrained by the growth of adjacent crystals and the geometry of the environment where crystal growth takes place. For instance, average lateral crystal size scales with time as a power law, the exponent varying as a function of the curvature and/or topography of the substrate.