title: Local structure of channel ions in carbonate apatite.
abstract: Refinement of the single-crystal X-ray diffraction structure of a type A carbonate apatite [CAp; Ca10(PO4)6-y(CO3)x+(3/2)y(OH)2-2x, x=0.75, y=0.0; space group P3 ] has been continued with independent positional and isotropic displacement parameters for the carbonate oxygen atoms, reducing the residual indices significantly (R=0.024, Rw=0.020) and confirming the earlier structure assignment. The carbonate ion is located in the apatite channel at z approximately 0.5, and oriented with two oxygen atoms close to the c-axis. Rigid body refinement, giving a preferred structure, used a novel procedure for defining the ideal equilateral triangular geometry of the channel carbonate ion. Resolution of the channel carbonate ions in type A-B CAp (x=0.69, y=0.57; P63/m) is also improved. Channel carbonate ions in CAp are canted, rotated and displaced to optimize Ca2-O bond distances. The rotation of the A1 carbonate in type A-B CAp is opposite to that of the channel carbonate in type A CAp, due mainly to the accommodation of a second channel carbonate ion (A2). These structures simulate the local structure of type A carbonate in hydroxyapatite of bone and dental enamel.
authors: Department of Earth Sciences, University of Western Ontario, London, Ont. Canada N6A 5B7. mfleet@uwo.ca
authors: Michael E
authors: ME
authors: Fleet
authors: Xiaoyang
authors: X
authors: Liu
mesh_terms: D001031:Apatites / Q000737:chemistry*; D001672:Biocompatible Materials / Q000737:chemistry; D002244:Carbon / Q000737:chemistry; D002254:Carbonates / Q000737:chemistry; D018360:Crystallography, X-Ray; D003743:Dental Enamel / Q000737:chemistry; D017886:Durapatite / Q000737:chemistry; D007477:Ions; D008422:Materials Testing; D008958:Models, Molecular; D008968:Molecular Conformation; D010100:Oxygen / Q000737:chemistry; D014961:X-Ray Diffraction
chemical_list: D001031:Apatites; D001672:Biocompatible Materials; D002254:Carbonates; D007477:Ions; C030782:carboapatite; D002244:Carbon; D017886:Durapatite; D010100:Oxygen