Abstract

Apatites in a fluorine-free chemical system have been synthesized hydrothermally at 650 °C and 1.5 kbar along the hydroxylapatite–britholite-(Y) join [i.e., Ca10(PO4)6(OH)2 – Ca4Y6(SiO4)6(OH)2 join], from oxide mixtures at nominal Si-contents of 0, 0.5, 1, 2, 3, 4, 5.5, and 6 pfu. The hexagonal apatite unit-cell volume decreases by 1.5% from the phosphate to the silicate end-member. A single 31P MAS NMR resonance is recognized at 2.8 ppm in hydroxylapatite. An additional broad line centered around 1.5 ppm is present for Y-rich compositions. Although the 31P MAS NMR spectra could not be fully assigned, the 2.8 ppm resonance that persists even in Si-rich compositions must include the contribution of P atoms involved in P-O-Y bonds. 1H NMR spectroscopy shows that the H content decreases by around 50% from the phosphate to the silicate end-member although all the compounds were synthesized hydrothermally at 650 °C. In addition to the expected YSiCa–1P–1 sub-stitution, a second substitution vector, Y□Ca–1H–1, is inferred. It is proposed that the resulting proton vacancies influence neighbor H atoms to give rise to a 1H line at 4.9 ppm. Beside the 4.9 ppm resonance, four other resonances at 0.2, 1.2, 1.5, and 2.0 ppm, are encountered along the series. In the apatite columns, OH groups are bonded to three cations from the Ca2 site and form (Ca2)3OH groups. In the proposed assignment model, the 0.2 ppm line, the only resonance present in the hydroxylapatite spectrum, is readily attributed to protons from (Ca)3OH groups. The 1.2 and 1.5 ppm resonances are assigned to (Ca2Y)OH groups whereas the 2.0 ppm line represents the contribution of protons from both (CaY2)OH and (Y)3OH groups.

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