The unusual behaviour of bone-derived HA crystals may be explained by the crystalline structure of physiological HA. The analysis of bone mineral shows a Ca : P ratio ranging from 1.3 : 1 to 1.9 : 1 in comparison to the theoretical Ca : P stoichiometric ratio of 1.67 : 1.71 This deviation is mainly attributed to the carbonated groups present distorting the crystal structure of most biological HA.