J. C. Elliott

Bio: J. C. Elliott is an academic researcher from Queen Mary University of London. The author has contributed to research in topics: Enamel paint & Demineralization. The author has an hindex of 23, co-authored 41 publications receiving 4333 citations.

Structure and chemistry of the apatites and other calcium orthophosphates

Abstract: 1. General Chemistry of the Calcium Orthophosphates. Introduction. Monocalcium phosphates (monohydrate and anhydrous). Octacalcium phosphate. Dicalcium phosphate dehydrate, brushite. Dicalcium phosphate anhydrous, monetite. Anhydrous tricalcium phosphates and whitlockite. Tetracalcium phosphate. Amorphous calcium phosphates. 2. Fluorapatite and chlorapatite. Introduction. Structures. Substitutions in apatites. Preparation of powders. Growth of single crystals. Infrared and Raman spectra. Other physical and chemical studies. 3. Hydroxapatite and Nonstoichiometric Apatites. Introduction. Structure of hydroxyapatite. Preparation of stoichiometric hydroxyapatite powders. Preparation of other apatites with hydroxyl ions. Growth of hydroxyapatite single crystals. Special analytical methods. Structure of calcium-deficient hydroxyapatites. Kinetics of nucleation and crystal growth. Solubility and interfacial phenomena. Reactions in solution. Infrared and Raman spectroscopy. NMR spectroscopy. Other physical and chemical studies. 4. Mineral, synthetic and biological carbonate apatites. Introduction. Francolite and dahllite. A-type carbonate apatite, Caio(PO4)6CO3. Synthetic high temperature B-type carbonate apatites. Carbonate apatites from aqueous systems. Biological apatites. Electron spin resonance of X-irradiated carbonate apatites. Summary. Appendix. Calculated X-ray diffraction patterns of the calcium orthophosphates. References. Index.

Calcium Phosphate Biominerals

Abstract: ### Tissues and minerals On a quantitative basis, the most important of the calcium phosphates (Table 1⇓) is an apatite closely related to hydroxylapatite (HAP). This is better described as an impure carbonate-containing apatite (CO3Ap) and forms the inorganic component of bones and teeth. Table 1⇓ also includes calcium phosphates (including two pyrophosphates) that occur in pathological mineralizations and those that are used for the repair of mineralized tissues. Unlike the well-controlled process of normal mineralization in bones and teeth (see later), pathological mineralizations are usually poorly controlled with the result that several calcium phosphates may occur together. In addition, their crystallographic orientations are often random. View this table: Table 1. Occurrence of calcium phosphates. The solubility isotherms of the calcium phosphates in the system Ca(OH)2-H3PO4-H2O at 37°C are shown in Figure 1⇓. Monetite occurs in the phase diagram, although its occurrence in normal or pathological calcifications has never been confirmed, one contributing reason being that its nucleation and growth is more difficult than brushite under biological conditions, so brushite forms in preference, even though brushite is less stable. β-Ca3(PO4)2 is a high-temperature phase that does not precipitate directly in aqueous systems; however, it is sufficiently stable in water for a solubility product to be determined so that an isotherm can be calculated, which is the origin of the isotherm in Figure 1⇓. However, if the aqueous system contains Mg2+ (1 or 2 mmol/l), the solubility product of “Ca3(PO4)2” is dramatically reduced (more than a thousand-fold) with the result that the structurally related whitlockite can easily precipitate (Hamad and Heughebaert 1986, LeGeros et al. 1989). Fe2+ ions have a similar effect. Whitlockite has superficially the same X-ray diffraction (XRD) …

Rietveld refinement of the crystallographic structure of human dental enamel apatites

Abstract: Rietveld refinements using 12 sets of X-ray diffraction powder data from milligram samples of human dental enamel provide detailed information about the structure and composition of enamel apatite. The principal difference in atomic parameters between enamel apatite and Holly Springs hydroxylapatite is in O2, which is reflected in a reduction in the P-O2 bond length of 0.085 A and PO4 volume by 3.6%. Modeling the hexad axis scattering with a single OH - ion gives a 0.089 A shift of the ion further away from the mirror plane at z = 1 ⁄ 4. The known distributed electron density along the hexad axis in enamel has been confirmed by direct comparison with synthetic hydroxylapatite. Although the CO3 2- ion position could not be determined directly, evidence for partial replacement of PO4 3- by CO3 2- ions came from an 8% diminution of the P site occupancy compared with that in stoichiometric hydroxylapatite. The observed reduction in the P-O2 bond length and PO 4 volume in enamel is also consistent with this substitution. The loss of negative charge caused by CO 3 2- replacing PO4 3- ions and loss of OH - ions is compensated by loss of Ca 2+ ions from Ca2 sites. The calculated density from the X-ray results is 3.021 g/cm 3 , in agreement with deductions from previous chemical analyses.

X-ray microtomography: nondestructive three-dimensional imaging for in vitro endodontic studies.

Abstract: Article shows the application of a laboratory x-ray microtomography system, a miniaturized form of conventional computerized axial tomography, to the study of root canal morphologic characteristics and changes in the course of root canal treatment in extracted teeth. After reconstruction of the three-dimensional images, the IDL software package (Research Systems, Inc., Colorado) was used to obtain cross-sectional slices of the tooth and three-dimensional views of rendered surfaces of constant mineral density. The root canal systems and changes in these were imaged at a resolution (cubic voxel side-length) of approximately 40 microns.

Processing and properties of hydroxyapatite-based biomaterials for use as hard tissue replacement implants

Abstract: This paper reviews the past, present, and future of the hydroxyapatite (HAp)-based biomaterials from the point of view of preparation of hard tissue replacement implants. Properties of the hard tissues are also described. The mechanical reliability of the pure HAp ceramics is low, therefore it cannot be used as artificial teeth or bones. For these reasons, various HAp-based composites have been fabricated, but only the HAp-coated titanium alloys have found wide application. Among the others, the microstructurally controlled HAp ceramics such as fibers/whiskers-reinforced HAp, fibrous HAp-reinforced polymers, or biomimetically fabricated HAp/collagen composites seem to be the most suitable ceramic materials for the future hard tissue replacement implants.

Biological and medical significance of calcium phosphates.

Abstract: The inorganic part of hard tissues (bones and teeth) of mammals consists of calcium phosphate, mainly of apatitic structure. Similarly, most undesired calcifications (i.e. those appearing as a result of various diseases) of mammals also contain calcium phosphate. For example, atherosclerosis results in blood-vessel blockage caused by a solid composite of cholesterol with calcium phosphate. Dental caries result in a replacement of less soluble and hard apatite by more soluble and softer calcium hydrogenphosphates. Osteoporosis is a demineralization of bone. Therefore, from a chemical point of view, processes of normal (bone and teeth formation and growth) and pathological (atherosclerosis and dental calculus) calcifications are just an in vivo crystallization of calcium phosphate. Similarly, dental caries and osteoporosis can be considered to be in vivo dissolution of calcium phosphates. On the other hand, because of the chemical similarity with biological calcified tissues, all calcium phosphates are remarkably biocompatible. This property is widely used in medicine for biomaterials that are either entirely made of or coated with calcium phosphate. For example, self-setting bone cements made of calcium phosphates are helpful in bone repair and titanium substitutes covered with a surface layer of calcium phosphates are used for hip-joint endoprostheses and tooth substitutes, to facilitate the growth of bone and thereby raise the mechanical stability. Calcium phosphates have a great biological and medical significance and in this review we give an overview of the current knowledge in this subject.

Oxidation of Alcohols with Molecular Oxygen on Solid Catalysts

Abstract: ion from a primary OH group of glyc-erol. 223,231 A similar mechanism was proposed manyyears ago for alcohol oxidation on Pt/C, involving asecond step, the transfer of a hydride ion to the Ptsurface (Scheme 11). 8,87,237 We consider it more feasible that the rate-deter-mining step is the cleavage of the C-H bond at theR-carbon atom. A similar mechanism is now generallyaccepted for Au electrodes (Scheme 12). 238 Despite thestructural differences between Au nanoparticles andan extended Au electrode surface, there are alsosimilarities, such as the critical role of aqueousalkaline medium and the absence of deactivation dueto decomposition products (CO and C x H y frag-ments). 239,240 An important question is the nature of active siteson Au nanoparticles. Electrooxidation of ethanol onAu nanoparticles supported on glassy carbon re-quired the partial coverage of Au surface by oxides. 241 Another analogy might be the model proposed for COoxidation. 219,242,243 According to this suggestion, theactive site consists of an ensemble of metallic Auatoms and a cationic Au

Evidence for life on Earth before 3,800 million years ago.

Abstract: It is unknown when life first appeared on Earth. The earliest known microfossils (approx. 3,500 Myr before present) are structurally complex, and if it is assumed that the associated organisms required a long time to develop this degree of complexity, then the existence of life much earlier than this can be argued. But the known examples of crustal rocks older than approx. 3,500 Myr have experienced intense metamorphism, which would have obliterated any fragile microfossils contained therein. It is therefore necessary to search for geochemical evidence of past biotic activity that has been preserved within minerals that are resistant to metamorphism. Here we report ion-microprobe measurements of the carbon-isotope composition of carbonaceous inclusions within grains of apatite (basic calcium phosphate) from the oldest known sediment sequences a approx. 3,800 Myr-old banded iron formation from the Isua supracrustal belt, West Greenland, and a similar formation from the nearby Akilia island that is possibly older than 3,850 Myr. The carbon in the carbonaceous inclusions is isotopically light, indicative of biological activity; no known abiotic process can explain the data. Unless some unknown abiotic process exists which is able both to create such isotopically light carbon and then selectively incorporate it into apatite grains, our results provide evidence for the emergence of life on Earth by at least 3,800 Myr before present.