Density functional theory calculations of solid solutions of fluor- and chlorapatites

Electronic structure calculations based on density functional theory are employed to investigate the uptake and distribution of chloride ions into the fluorapatite structure. The calculated structural parameters of the two minerals are in good agreement with the experimental hexagonal structures. Electron density contour plots show the apatite structure to be an ionic crystal, where the phosphate groups behave as polyanions. The replacement of F- ions in fluorapatite by Cl- from gaseous Cl-2 is a highly endothermic process at all partial concentrations (similar to250 kJ mol(-1)), which is in line with the high stability and dominance of the fluorapatite form of these materials. The replacement of F- ions from chlorapatite with the formation of fluorapatite is much less endothermic (<8 kJ mol-1). The most stable structure with a F--to-Cl- ratio of 1:1 contains complete columns of F- and Cl- ions in the c direction. The low calculated excess heats of solid solution of chlor- and fluorapatite (<8 kJ mol-1) are in agreement with the widespread occurrence of Cl- in both geological and biological apatites, as well as with the ease of synthesis of mixed chlorfluorapatites, and indicate that Cl- could easily be absorbed into fluorapatite when both materials are present.