Carbonate ions in apatites: infrared investigations in the upsilon 4 CO3 domain

Fourier transform infrared (IR) spectroscopic investigations of precipitated carbonate apatites in the upsilon 4 CO3 domain reveal the existence of five bands at 757, 740, 718, 692, 670 cm-1 which can be assigned to several distinct environments of the carbonate ion in the apatite structure. In order to identify these environments precisely, fluoridated and pure type A carbonate apatites (i.e., with carbonate ions in monovalent anionic sites) were examined. The bands at 670 and 757 cm-1 were attributed to type A carbonate and their relative intensity was found to increase when the carbonate content of the apatite diminished or when samples were heated at 400 degrees C. Fluoridated apatites show only two bands, close to 718 and 692 cm-1, corresponding to type B carbonate ions (carbonate in trivalent anionic sites). The band at 740 cm-1 was revealed by heating the samples to 400 degrees C. This is due to OH ions' hydrogen bonded to fluoride and to carbonate ions in an undetermined apatite site. Despite the low intensity of IR bands, investigations in the upsilon 4 CO3 domain appear complementary to those in other carbonate vibrational domains and could be useful for a more precise identification of bone mineral.