Thermal study and structural characterization of biological hydroxyapatite (HA) samples were done as well as their comparison with commercial and synthetic samples in this study. The X-ray micro analyser shows that all three samples of human teeth (HT1–HT3) contain two types of HA structures with different crystallite sizes, unlike sample of bovine thigh-bone (BTB). The bone sample was composed only of one HA phase with varied porosity. The molar Ca/P ratio in biological samples was lower compared to theoretical ratio for pure HA; moreover, in the case of teeth, Ca/P ratio varyies between the centre and the periphery of the cross-sectional samples. Thermogravimetry of the biological samples showed mass decreases—three regions for the bone and four regions for the teeth. In comparison, commercial HA has only two-step weight loss and synthetic HA three-step weight loss. After the calcination up to 1280 °C all the samples of teeth transformed into whitlockite, β-(Ca,Mg)3(PO4)2 (98 wt%) and 2 wt% HA. Besides, HT3 contained further trace amount of hilgenstockite (HIL, Ca4P2O9). The sample BTB partly transited from natural HA into HIL (6 wt%) and lime, CaO (14 wt%). X-ray powder diffraction (XRD) proved occurrence of HIL (9 wt%) beside stability part HA (91 wt%) in the commercial HA after thermal treatment but the synthetic HA composed from Ca3(PO4)2 (74 wt%) and HA (26 wt%).
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