Duncan McConnell

Crystal chemistry of hydroxyapatite. Its relation to bone mineral.

Abstract The properties of a natural hydroxyapatite with Ca P = 1·75 and a synthetic hydroxyapatite with Ca P = 1·67 are considered with respect to (i) the non-stoichiometric properties of most synthetic calciumdashphosphate precipitates, (ii) the crystal chemistry of carbonate-containing mineral apatites (francolite and dahllite) and (iii) the interpretation of analyses of teeth and bones. It is concluded that “tetrahedral hydroxyls” substitute for phosphate groups, that their occurrence is not necessarily dependent upon the presence of carbonate groups within the structure and that some structural positions, which would normally contain (OH) or F, may contain H2O. The assigned structural formula for the mineral from northern Mexico is Ca10[F1·2(OH)1·5(H2O)0·3] [(PO4)5·7(H4O4)0·3]. The effects of isomorphic substitution, as well as the sizes of the crystallites, on the characteristics of X-ray diffraction patterns are considered. It is concluded that some carbonate apatites are certainly not hexagonal, and others, including those of tooth and bone mineral, may be merely pseudohexagonal. Inasmuch as “line broadening” cannot be distinguished, in general, from incipient “line splitting”, investigations which purport to determine the “degree of crystallinity” are not based on sound crystallographic premises. The size of the unit cell of an isomorphic variant involving merely fluorine, carbonate, and structurally bound hydrogen cannot be predicted because the effects are not independently assessable.

Relation between the Inorganic Chemistry and Biochemistry of Bone Mineralization.

In vitro experiments with saliva resulted in precipitation of a mineral substance (dahllite or carbonate hydroxyapatite) which is comparable in composition and crystal structure to oral calculus. Similar mineral substances were produced from synthetic solutions containing sodium phosphate and calcium chloride (in addition to a buffer) in the presence of carbonic anhydrase and available carbon dioxide. It is concluded that the carbonate ion is essential to precipitation of bone mineral and that the principal biochemical catalyst in vivo is carbonic anhydrase. Bacteria are not essential to the precipitation, but they probably play a secondary role in connection with the formation of oral calculus, urinary calculus, and so forth.

INORGANIC CONSTITUENTS IN THE SHELL OF THE LIVING BRACHIOPOD LINGULA

Lingula, a living brachiopod, has a shell composed of organic substances and an inorganic substance which is crystallochemically identical with francolite, a carbonate fluorapatite. X-ray powder diffraction photographs are essentially identical for central and marginal portions of the shell and resemble those of synthetic carbonate hydroxyapatite and human dentin (Pl. 1, fig. 1). These microcrystalline substances evidently are composed of a single inorganic crystalline phase. Fossil inarticulate brachiopods may have been accumulators of fluorine as well as phosphorus.

Dating of Fossil Bones by the Fluorine Method: Fluorine analysis by indirect methods is not a reliable means of determining age

In the light of the discussion presented here, we may conclude that fluorine dating is subject to many uncertainties. While the geochemical and geological uncertainties have been recognized (24), numerous crystalchemical concepts concerning the nature of the carbonate-apatite minerals have been almost completely disregarded.

Biologic Precipitation of Fluorite

X-ray diffraction patterns show that the statoliths of marine mysid crustaceans are composed of fluorite, and that this mineral is also a principal phase of the gizzard plates of some tectibranch gastropods. A phosphatic phase is also indicated by chemical analyses in the gizzard plates, but its crystallochemical characterization has not been feasible by x-ray diffraction. The occurrence of fluorite in mysid statoliths confirms the earlier interpretations based on insufficient documentation. Fixation of fluorine in hard tissues of marine invertebrates is extensive in the shelf seawaters and minor in the bathyal zone of the oceans.

Recent advances in the investigation of the crystal chemistry of dental enamel

Abstract In view of detailed investigations carried out by the writer and several other individuals, the conclusion is inescapable that dental enamel is composed of a single “mineral” phase which is carbonate hydroxyapatite (dahllite). There is no valid reason for supposing that the crystal structure or crystal chemistry of bone or dentine is significantly different. Even in bovine bone no appreciable isotopic exchange with C14 was observed by Francois (1958), so the concept of adsorption of carbonate on the surfaces of the crystallites can be relegated to unrealistic speculation. The writers structural theory (1952) is essentially confirmed.

Mechanical Failure of Amalgam Restorations with Zinc Phosphate and Zinc Oxide/Eugenol Cement Bases

As a guide to operative practice it was desirable to obtain pertinent information on the mechanical performance of base materials and Class I amalgam restorations. Specifically, the question was: Under what circumstances, if any, can the interaction product of zinc oxide and eugenol be substituted for a zinc phosphate cement base? Our investigations, then, have been concerned with the mechanical failure of two different types of Class I cavity preparations that have been filled with amalgam with or without these two kinds of underbase materials. Our tests have been designed to simulate actual mechanical forces of mastication, insofar as practicable, in connection with laboratory tests.

Aluminum-rich apatite.

More than 25 atomic percent of aluminum can susbtitute for other cations in the structure of apatite [A10(XO4) 6Z2]. Such a synthetic product was obtained by expelling volatile constituents (H2O and F) from morinite during thermal treatment. Infrared absorption spectra, chemical analysis, and x-ray powder diffraction demonstrate that the aluminum has two coordination numbers, and more than twice as much aluminum substitutes for calcium (A position) as for phosphorus (X position).

ARAGONITE IN HALIMED A AND TYDEMANIA (ORDER SIPHON ALES)12

The mineral component of the marine green algal genus Halimeda is the orthorhombic calcium carbonate (aragonite); its presence appears to be a generic characteristic. Tydemania expeditionis also precipitates aragonite in contradistinction to species of the red alga Corallina wliich precipitate calcium carbonate of the hexagonal form (calcite). The analyses are based on x‐ray diffraction methods. Although other inorganic substances arc present, the amounts are minor and probably represent contaminants. Specimens that are to be studied for mineral components should not be stored in formalin.

Carbonic Anhydrase and the Precipitation of Apatite

On theoretical grounds it is unlikely that the catalytic action of the enzyme carbonic anhydrase would be required for the precipitation of apatite in vitro. The presence of carbonic anhydrase in either active or inactivate form did not initiate precipitation in a metastable calcifying solution. It is unlikely that carbonate (or bicarbonate) ions are essential for the precipitation of apatite in vitro or in vivo.