Structure and substitutions in fluorapatite

Fluorapatite, Ca10(PO4)6F2. is a widely spread form of calcium phosphate present particularly in biological material. Human hard tissues contain crystals structurally related to apatite. Fluoride can be found in various natural sources and is also used for its beneficial action in caries prevention. Fluorapatite belongs to the spatial group P6(3/m) (C(6h)2) and consists of 3 ions: F-, Ca2+, PO4(3-). In the present paper, we have carried out a crystallographic study of the fluorapatite structure and of the changes induced by the substitutions. The fluorapatite structure and the presence of a large number of ionic bonds make fluorapatite a very suitable host for many substituents, some of them harmless for the human organism, some not. According to the substitution site, we can describe four types of substitution. The F- substitution, also called Type A substitution, is the main one, and the best known. Only the Ca2+ substitution implies changes in the crystal structure. However, some questions remain, in particular for the PO4(3-) substitution, which is the main substitution present in the biological calcium phosphates.