The thermal behavior of the bovine bone mineral and synthetic stoichiometric hydroxyapatite was investigated by X-ray diffraction. The bone samples in solid (planar oriented pieces) and in powder form were examined to elucidate how the microstructural and textural properties of bone mineral are modified under heating. As could be expected, the thermal behavior of the bone mineral depends not only on the structural distortions, but also on the crystal habit, texture and ordering of biocrystals in tissue. The temperature growth of biogenic apatite crystals, unlike synthetic hydroxyapatite, is seen to be nonmonotonic and multi-staged. At 600 to 700°C the biomineral crystallites grow rapidly due to disappearance of the mosaic structure as the lattice imperfections are annealed. After heating between 700°C and 900°C the bone mineral appears to be composed of roughly equidimensional ≥200 nm crystals. The further growth of the crystals in the range from 900 to 1300°C occurs by the mass transport mechanism, supporting the idea that the bone mineral is not a discrete aggregation of crystals, but rather a continuous mineral phase with direct crystal-crystal bonding. Estimates are presented to show the important role of the surface mass transport mechanism in the growth of apatite crystals. The material obtained by heating a cortical bone fragment between 900°C and 1300°C turns out to be composed of two crystal types: crystals oriented along the bone axis (major morphology) and those of differing shape and orientation (minor morphology). The heating-induced variations in the longitudinal and transverse dimensions of differing-morphology crystals are found to be coherent. Small amounts of CaO, MgO and other crystalline phases are seen to be formed in the bone mineral under heating.

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