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  Indian J Med Microbiol
 

Figure 2: Building calcium hydroxyapatite. (A) Phosphate ions (PO43–) can be imagined as single layer (cyan layer, “A”) of billiard balls confined by a hexagonal rack. In the hexagonal close-packed layer of phosphate spheres, there are twice as many interstices as spheres. Half of the spaces are leftward pointing (white); half are rightward pointing (blue). (B) All of the leftward-pointing interstices are filled with smaller spheres (red), representing Ca2+ ions (designated Ca2). (C) A second layer of billiard balls (yellow, “B”) will not stack directly on top of the A layer, but rests on the depressions over the leftward-pointing interstices filled with Ca2 ions. (D) The third layer of phosphate ions (cyan) stacks directly on top of the original A layer. Because we have not added the hydroxyl ions or most of the Ca2+ ions, it is clear that the rightward-pointing interstices of the original A layer are still visible and that the ions occupying these spaces will form a column extending up the entire height of the crystal. (E) The column interstices are occupied by Ca2+ (designated Ca1) and OH ions in a ratio of 2:1. The column interstices are filled so that each particular column is filled either with OH or Ca2+ ions, and not a mixture, so there are columns of Ca1 and OH ions running from top to bottom throughout the crystal. (F) All of the interstices of the nearly close-packed hexagonally arranged PO43– ions are filled in hydroxyapatite. Ca2 ions occupy the leftward-pointing interstices of the A layers and the rightward-pointing interstices of the B layers, which are capped by phosphate ions above and below

Figure 2: Building calcium hydroxyapatite. (A) Phosphate ions (PO<sub>4</sub><sup>3–</sup>) can be imagined as single layer (cyan layer, “A”) of billiard balls confined by a hexagonal rack. In the hexagonal close-packed layer of phosphate spheres, there are twice as many interstices as spheres. Half of the spaces are leftward pointing (white); half are rightward pointing (blue). (B) All of the leftward-pointing interstices are filled with smaller spheres (red), representing Ca<sup>2+</sup> ions (designated Ca2). (C) A second layer of billiard balls (yellow, “B”) will not stack directly on top of the A layer, but rests on the depressions over the leftward-pointing interstices filled with Ca2 ions. (D) The third layer of phosphate ions (cyan) stacks directly on top of the original A layer. Because we have not added the hydroxyl ions or most of the Ca<sup>2+</sup> ions, it is clear that the rightward-pointing interstices of the original A layer are still visible and that the ions occupying these spaces will form a column extending up the entire height of the crystal. (E) The column interstices are occupied by Ca<sup>2+</sup> (designated Ca1) and OH<sup>–</sup> ions in a ratio of 2:1. The column interstices are filled so that each particular column is filled either with OH<sup>–</sup> or Ca<sup>2+</sup> ions, and not a mixture, so there are columns of Ca1 and OH<sup>–</sup> ions running from top to bottom throughout the crystal. (F) All of the interstices of the nearly close-packed hexagonally arranged PO<sub>4</sub><sup>3–</sup> ions are filled in hydroxyapatite. Ca2 ions occupy the leftward-pointing interstices of the A layers and the rightward-pointing interstices of the B layers, which are capped by phosphate ions above and below