Local structure of channel ions in carbonate apatite

Refinement of the single-crystal X-ray diffraction structure of a type A carbonate apatite [CAp; Ca-10(PO4)(6-y)(CO3)(x+(3/2)y) (OH)(2-2), 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, R-w = 0.020) and confirming the earlier structure assignment. The carbonate ion is located in the apatite channel at Z approximate to 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; P6(3)/m) is also improved. Channel carbonate ions in CAp are canted, rotated and displaced to optimize Ca2-O bond distances. The rotation of the A I 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. (c) 2005 Elsevier Ltd. All rights reserved.