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.