It is shown how certain aspects of the composition and structure of carbonated apatites depend strictly on preparation conditions, for example, excess of phosphate or calcium ions in the reaction medium, CO32- concentration, pH, ammonia added or not. Depending on those conditions, either one or the other of the two proposed mechanisms of introduction of carbonate ions into the B sites is dominant. The mechanisms are (1) replacement of a phosphate ion by a carbonate ion with the formation of three vacancies, one in a phosphate oxygen site and one each in the neighboring Ca2+ and OH- sites; and (2) replacement of a phosphate ion by a carbonate accompanied by a hydroxyl ion. Whether mechanism (1) is observed to dominate over mechanism (2), or vice versa, is accounted for by the relative concentrations of the various ions in the reaction medium. The number of vacancies is decreased by the presence of either, or both, excess calcium ions or ammonia in the reaction medium. A structural-chemical mechanism is advanced for the view that, with the smallest CO32- content, the A sites are favored but with increasing carbonate content the B sites become favored and the A-site content becomes less than it is when the total carbonate content is less.

B-type carbonated hydroxyapatites, prepared in aqueous media free of alkali ions, fix ammonium ions present in the reaction medium. A small portion of the carbonate ions introduced into the apatite structure enter by the substitution mechanism (CO3(2-), NH4+)----(PO4(3-), Ca2+). With these results for the structural incorporation of ammonium ions, differences in lattice parameters observed among specimens with the same degree of carbonation were attributed to some substitution of NH4+ for Ca2+. The fixed ammonium ions were shown to be the source of the cyanamide and cyanate ions that develop on heating. Above 500 degrees C these apatites lost both the carbonate and the cyanate and cyanamide ions.

Fourier transform infrared (IR) spectroscopic investigations of precipitated carbonate apatites in the upsilon 4 CO3 domain reveal the existence of five bands at 757, 740, 718, 692, 670 cm-1 which can be assigned to several distinct environments of the carbonate ion in the apatite structure. In order to identify these environments precisely, fluoridated and pure type A carbonate apatites (i.e., with carbonate ions in monovalent anionic sites) were examined. The bands at 670 and 757 cm-1 were attributed to type A carbonate and their relative intensity was found to increase when the carbonate content of the apatite diminished or when samples were heated at 400 degrees C. Fluoridated apatites show only two bands, close to 718 and 692 cm-1, corresponding to type B carbonate ions (carbonate in trivalent anionic sites). The band at 740 cm-1 was revealed by heating the samples to 400 degrees C. This is due to OH ions' hydrogen bonded to fluoride and to carbonate ions in an undetermined apatite site. Despite the low intensity of IR bands, investigations in the upsilon 4 CO3 domain appear complementary to those in other carbonate vibrational domains and could be useful for a more precise identification of bone mineral.

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