X-Ray Structures

Following the determination of the crystal structures of fluorapatite

and hydroxylapatite by Na´ray-Szabo´30 and Mehmel,105 respectively,

there has been a steady interest in the structural details of nat-

ural and synthetic apatites,70,83,106-114 with hydroxylapatite, fluor-

apatite, synthetic carbonate hydroxylapatite (CHAP) and synthetic

carbonate fluorapatite (CFAP) recognized as proxymaterials for bio-

logical apatite. Direct study of biological apatites proved challenging

using X-ray and neutron diffraction techniques due to the nanocrys-

talline grain size and low degree of crystallinity of the natural

materials. These problems were compounded when interest turned

to the role of carbonate, a critical minor component, in the structure

of biological apatite.14,15,68,77,84,115,116 These earlier studies, and

numerous others, have been reviewed in detail elsewhere.8-10

In the modern era, the structural role of carbonate in hy-

droxylapatite and fluorapatite has been investigated extensively by

X-ray powder, X-ray single-crystal, and neutron powder diffraction

methods,40,84,86,87,88,97,117-119 as well as by infrared, Raman and

nuclear magnetic resonance spectroscopy,21,41,120-125 and theoret-

ical simulations.126,127 Although it was established in the earlier

studies15 that the carbonate ion can be accommodated both in the

c-axis structural channel (type A carbonate) and as a substituent for the phosphate group (type B carbonate), more detailed structure

analysis of CHAP and CFAP precipitated from aqueous solution, as

well as francolite from phosphorites and biological apatites, has

been frustrated by numerous problems, including: (1) the limited

substitution of carbonate, especially of type B carbonate; (2) small,

nanoscale crystal size, (3) fragility and reactivity of nanocrystals

extracted from bone tissue, (4) poor degree of crystallinity; and (5)

weak and overlapped electron density of carbonate atoms.