HR: 0800h
AN: V31D-0682    [Abstracts]
TI: Crystal Chemistry of Carbonate Apatites from High-Pressure Synthesis
AU: * Fleet, M E
EM: mfleet@uwo.ca
AF: University of Western Ontario, Department of Earth Sciences, London, ON N6A 5B7, Canada
AU: Liu, X
EM: xliu258@uwo.ca
AF: University of Western Ontario, Department of Earth Sciences, London, ON N6A 5B7, Canada
AB: Relatively large (50-200 μm) crystals of carbonate-bearing hydroxylapatite (CHAP) and fluorapatite (CFAP) have been grown from carbonate-rich melts at 1-3 GPa and used to determine structural details beyond the resolution of Rietveld powder diffraction methods, using X-ray single-crystal structure and FTIR spectroscopy. The new information includes the structural location of the channel (type A) and phosphate group (type B) carbonate ions in various composition series, as well as the location of the excess fluoride anion in francolite, substitution mechanisms, and identification of the hydrogen carbonate (bicarbonate) ion as a new apatite channel species. For equivalent conditions of synthesis, the uptake of A-B carbonate is greater for Na-bearing CHAP (up to 2 carbonate ions pfu) than Na-bearing CFAP (about 0.4 pfu). The Na cation and A and B carbonate ions are locally coupled in ratios of 1:1:1 in CHAP and 1:1:2 in CFAP, to minimize the effects of charge compensation and spatial accommodation. An extensive data base of type A and B site occupancies reveals that the amount of A carbonate in type A-B CHAP and CFAP is considerably under represented by the relative band areas for asymmetric stretching (ν3) and out-of-plane bending (ν2) of carbonate in FTIR spectra. The weaker absorption intensity and shift to higher wavenumbers of type A bands indicates that the carbonate ion is bound more weakly in the apatite channel than in the interior of the crystal structure. Thus literature spectra for apatites, and especially for apatite biomineralisation, showing dominant amounts of B carbonate should be re-evaluated.
DE: 0419 Biomineralization
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3630 Experimental mineralogy and petrology
DE: 3934 Optical, infrared, and Raman spectroscopy
DE: 3954 X-ray, neutron, and electron spectroscopy and diffraction
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting