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International audienceWe propose a microscopic model of the dominant carbonate for phosphate substitution in fluor- apatite. A well-crystallized sedimentary fluorapatite sample containing ~2.3 ± 0.8 wt% of carbonate was investigated using Fourier transform infrared spectroscopy (FTIR) and ¹³C and ¹⁹F magic angle spinning nuclear magnetic resonance (MAS NMR). About 75% of the carbonate groups replace the phosphate group ("B-site"), whereas a lesser contribution from carbonate groups located in the structural channels ("A-site") is observed. Beside the dominant ¹⁹F NMR signal of channel ions at ~ -102 ppm, an additional signal corresponding to ~8% of fluoride ions is observed at -88 ppm. ¹⁹F double quantum-single quantum (DQ-SQ) MAS NMR and ¹³C{¹⁹F} frequency-selective Rotational Echo DOuble Resonance (REDOR) experiments prove that this additional signal corresponds to iso- lated fluoride ions in the apatite structure, located in close proximity of substituted carbonate groups. Density functional theory (DFT) calculations allow us to propose a composite carbonate-fluoride tetrahedron defect model accounting for these experimental observations. The planar carbonate ion lies in the sloping face of the tetrahedron opposite a fluoride ion occupying the remaining vertex, together replacing the tetrahedral phosphate ion. This "francolite-type" defect leads to a diagnostic narrow IR absorption band at 864 cmĐ¹ that could be used as a guide to, e.g., detect the incipient transformation of fossil bone and teeth samples

ABSTRACT. Over the past decade, radiocarbon dating of the carbonate contained in the mineral fraction of calcined bones has emerged as a viable alternative to dating skeletal remains in situations where collagen is no longer present. However, anomalously low δ 13 C values have been reported for calcined bones, suggesting that the mineral fraction of bone is altered. Therefore, exchange with other sources of carbon during heating cannot be excluded. Here, we report new results from analyses on cremated bones found in archaeological sites in Africa and the Near East, as well as the results of several experiments aiming at improving our understanding of the fate of mineral and organic carbon of bone during heating. Heating of modern bone was carried out at different temperatures, for different durations, and under natural and controlled conditions, and the evolution of several parameters (weight, color, %C, %N, δ 13 C value, carbonate content, crystallinity indexes measured by XRD and FTIR) was monitored. Results from archaeological sites confirm that calcined bones are unreliable for paleoenvironmental and paleodietary reconstruction using stable isotopes. Experimental results suggest that the carbon remaining in bone after cremation likely comes from the original inorganic pool, highly fractionated due to rapid recrystallization. Therefore, its reliability for 14 C dating should be seen as close to that of tooth enamel, due to crystallographic properties of calcined bones.

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