Application of the powder X-ray diffraction (XRD) approach to a sample of human bone and teeth allowed their microstructural behavior to be predicted as a function of temperature (200-1000 degrees C) and duration of burning (0,18, 36, and 60 min). This range includes most forensic scenarios, as well as most archaeological scenarios. The growth process undergone by the hydroxylapatite crystallites in the mineralogical phase of the femoral samples follows a logarithmic sigmoid trend with a characteristic temperature of approximately 850 degrees C, which was determined with the four burning period used. This can be used not only to determine whether hard tissues have been burned, but also to suggest the temperature and duration of that burning event. In the thermal treatment of 0 min, the growth rate parameter is apparently higher than in the results from increased burning periods. Thermal treatment at a given temperature for 60 min shows the equivalent growth effects for 0 min of burning, but at 100 degrees C higher. In the case of teeth, the growth phenomena induced by firing are again described with a logistic-type function with a characteristic temperature of 841 degrees C, very close to that of bones, in spite of typical fragmentation induced in the temperature range 700-750 degrees C; however, the average crystallite size of HA in untreated teeth is significantly larger than in untreated skeletal bones. Alternatively, the average size of HA crystallites from burning above 900 degrees C is larger in bones than teeth. This suggests that the two types of natural bioapatite must be compared to their specific calibration curves when the precise estimate of the temperature of a fire is desired. 6 figures, 3 tables, and 33 references