Refinement of the single-crystal X-ray diffraction structure of a type A carbonate apatite [CAp; Ca 10(PO 4) 6– y (CO 3) x +(3/2) y (OH) 2−2 x , x = 0.75 , y = 0.0 ; space group P 3 ¯ ] 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≈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 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.