Hydroxyapatite: Hexagonal or Monoclinic?

Bioapatite, the major constituent of mineralized tissues in mammalian bones and teeth, has been modeled to the hexagonal hydroxyapatite phase. Monoclinic hydroxyapatite, synthesized before only at very high temperature, is the thermodynamically most stable phase and is expected to exist also in hard tissues. In this work, hydroxyapatite nanobelts are produced by hydrolysis of brushite crystals and are identified to be the monoclinic phase based on electron microscopy and electron diffraction techniques. This is the first report of fabricating monoclinic hydroxyapatite crystals at low temperature. As the structural differences between hexagonal or monoclinic hydroxyapatite are very subtle, the success of this characterization also shows the great potential of electron microscopy and electron diffraction techniques for precise phase identification.