Enamel carbonate ions

In parallel developments, oxygen isotope studies based on the stable phosphate ion in calcified tissues have been found to be more successful using enamel than bone as sample material (Ayliffe et al. 1994 Bryant et al 1994), and similarly oxygen isotopes from the less stable C—0 bond in enamel carbonate seems to be more predictable (Bocherens et al. 1996). Researchers exploring the relationship between Electron Spin Resonance (ESR) and carbonate content in enamel have found that dates are mostly consistent when carbonate levels did not deviate much from biogenic levels (Grun et al. 1990 Rink and Schwarcz 1995). 

The hydroxyapatite crystals in bone and teeth are imperfect due to other anions and cations, especially magnesium, chloride, carbonate, and fluoride ions. Carbonate (C032-) is the most important. At low carbonate contents (<4% by weight), a carbonate ion replaces a phosphate ion in the crystal ( A site substitution), but at higher contents (>4% by weight) it replaces a hydroxide ion ( B site substitution). Either substitution slightly shortens and fattens the crystal ( c or a axes increase) and increases solubility. In contrast, if hydroxide ions are present, they can be replaced by fluoride, which decreases apatite solubility (Sect. 16.2.1). Crystallographic analyses indicate that, in bone and dentin, phosphate is often replaced by carbonate, whereas in enamel it is more often replaced with chloride (Cl1-). Carbonated hydroxyapatite is critical for enamel development (see Sect. 9.5.3). 

Fowler BO (1977) I. Polarized Raman spectra of apatites. II. Raman bands of carbonate ions in human tooth enamel. Mineralized Tissue Research Communications Vol 3, no. 68 Fratzl P, Fratzl-Zelman N, Klaushofer K, Vogl G, Roller K (1991) Nucleation and growth of mineral crystals in bone studied by small-angle X-ray scattering. Calcif Tissue Inti 48 407-413 Fratzl P, Schreiber S, Boyde A (1996) Characterization of bone mineral crystals in horse radius by small-angle X-ray scattering. Calcif Tissue Inti 58 341-346... 

Carbonate tons and Enamel. Elliott (1965) has used polarized infrared absorption spectra to study longitudinal sections of human tooth enamel in order to determine whether carbonate ions can substitute for hydroxyl ions in the enamel. His conclusion was that carbonate ions substitute to a very limited extent for hydroxyl ions. The evidence consisted of certain bands in the infrared spectra of enamel (Fig. 19.12) which coincide with those of the synthetic apatite in which this substitution is known to have taken place (Fig. 19.13). In a hydroxyapatite that had been reacted with carbon dioxide at 1000°C, carbonate ion had absorption bands at 878, 1463, and 1528cm , and the hydroxyapatite 3570 cm band (OH ) had disappeared (Fig. 19.13). Elliott examined enamel which had been heated at 1000°C in carbon dioxide and measured the dichroism of the out-of-plane deformations at the 879 cm" mode (Fig 19.14). From the dichroic ratio he was able to calculate that the plane of the carbonate ion is nearly parallel to the c-axis of the apatite. Elliott et al. (1948) have given the dichroic ratio applicable to this case as... 

Fig. 19.14. Polarized infrared absorption spectrum ofthe carbonate ion that has replaced hydroxyl ions in the apatite lattice. (A) A 100- i longitudinal section of enamel heated at 1100°C in air for 2 hr. (B) A 50-/t longitudinal section of enamel heated at 900 C in CO2 for 30 min. (Elliott, 1965.)...

Elliott, J. C., Holcomb, D. W., and Young, R. A. 1985. Infrared determination of the degree of substitution of hydroxyl by carbonate ions in human enamel. Calcified Tissue International 37 372-75. 

Chemical precipitation is used in porcelain enameling to precipitate dissolved metals and phosphates. Chemical precipitation can be utilized to permit removal of metal ions such as iron, lead, tin, copper, zinc, cadmium, aluminum, mercury, manganese, cobalt, antimony, arsenic, beryllium, molybdenum, and trivalent chromium. Removal efficiency can approach 100% for the reduction of heavy metal ions. Porcelain enameling plants commonly use lime, caustic, and carbonate for chemical precipitation and pH adjustment. Coagulants used in the industry include alum, ferric chloride, ferric sulfate, and polymers.10-12... 

Fluoride also brings about a change in composition in natural hydroxypatite, since it not only undergoes a simple exchange with hydroxyl ions but also promotes the formation of a phase containing less carbonate than the initial hydroxyapatite [65]. Fluoride is taken up more readily by demineralised enamel than by sound enamel [66], which means its availability causes a self-healing effect in the mineral phase of the hard tissue. 

Substitutions in the HA structure are possible. Substitutions for Ca, PO4, and OH groups result in changes in the lattice parameter as well as changes in some of the properties of the crystal, such as solubility. If the OH" groups in HA are replaced by F" the anions are closer to the neighboring Ca " ions. This substitution helps to further stabilize the structure and is proposed as one of the reasons that fluoridation helps reduce tooth decay as shown by the study of the incorporation of F into HA and its effect on solubility. Biological apatites, which are the mineral phases of bone, enamel, and dentin, are usually referred to as HA. Actually, they differ from pme HA in stoichiometry, composition, and crystallinity, as well as in other physical and mechanical properties, as shown in Table 35.7. Biological apatites are usually Ca deficient and are always carbonate substituted (COs) " for (P04). For... 

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