The environment of CO₃²⁻ ions in the bone mineral of chickens of different ages and in bone fractions of different density have been investigated by resolution-enhanced Fourier Transform Infrared (FTIR) Spectroscopy. Three carbonate bands appear in thev 2 CO₃ domain at 878, 871, and 866 cm⁻¹, which may be assigned to three different locations of the ion in the mineral:​ in monovalent anionic sites of the apatitic structure (878 cm⁻¹), in trivalent anionic sites (871 cm⁻¹), and in unstable location (866 cm⁻¹) probably in perturbed regions of the crystals. The distribution of the carbonate ions among these locations was estimated by comparing the intensities of the corresponding FTIR spectral bands. The intensity ratio of the 878 and 871 cm⁻¹ bands remains remarkably constant in whole bone as well as in the fractions obtained by density centrifugation. On the contrary, the intensity ratio of the 866 cm⁻¹ to the 871 cm⁻¹ band was found to vary directly and decreased with the age of the animal. In bone of the same age, the relative content of the unstable carbonate ion was found to be highest in the most abundant density centrifugation fraction. A resolution factor of the CO₃²⁻ band (CO₃ RF) was calculated from the FTIR spectra which was shown to be very sensitive to the degree of crystallinity of the mineral. The crystallinity was found to improve rapidly with the age of the animal. The CO₃ RF in the bone samples obtained by density centrifugation from bone of the same animal was found to be essentially constant. This indicates a fairly homogeneous, crystalline state of the mineral phase. A comparison of the maturation characteristics of synthetic carbonated apatites with bone mineral indicates that a simple, passive, physicochemical maturation process cannot explain the changes observed in the mineral phase of whole bone tissue or in the density centrifugation fractions of bone during aging and maturation.