An X-ray Diffraction (XRD) and X-ray Fluorescence (XRF) investigation in human and animal fossil bones from Holocene to Middle Triassic

We examined by Rietveld refinement of X-ray Diffraction patterns a series of 61 human and animal fossil bones in an age range from present time to Middle Triassic (around 245 Ma). This approach, supplemented by elemental analysis according to X-ray Fluorescence, has permitted to obtain a quantitative evaluation of the mineralogical phases in the specimens, thus allowing to reconstruct the mineralization process involved. Concerning the apatite phase, after adopting a monoclinic geometry for the unit cell, the method permits to determine with fair degree of precision the unit cell volume, which is found to decrease in relatively short geological times as a function of the fluorine substitution process for the hydroxyl group –OH. After excluding the role of a high-temperature fire treatment to the bones, it is found that a certain linear correlation may exist over the geological time scale involved between the age of the specimen and the average crystallite size. This observation permits to use the XRD pattern as an evaluation of the age of the fossil, although the stratigraphic observations, where available, remain as the main reliable source for dating such biomaterials. The uncertainty related to this age estimation may be hardly better than 15–20% in absolute, because of various assumptions involved in the XRD methodology used and in the variability of the burial environment that may be subjected to discontinuous changes over the very long time of deposition.