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Hartman-Perdok theory enables classification of crystal faces as F, S or K faces. Only F faces should be present on growth forms as they grow slowly according to a layer mechanism. Such a classification can be quantified for ionic crystals by the calculation of attachment energies in electrostatic pointcharge models. The attachment energy of a crystal face (hkl) e l , is the energy released per mole, when a new elementary growth layer, called a slice, with a thickness of dhkl crystallizes on an existing crystal face. Theoretical growth

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Eighteen connected nets have been determined for hydroxyapatite, and 12 different F faces run parallel to at least one of these connected nets. The order of morphological importance of these F faces is determined by the slice energy,~and compared to the morphological data for natural apatites. It is found that there is a close agreement between predicted and observed morphology. It follows that the lowering of the apatite symmetry (P6 3/m -~P21/b) due to the ordering of OH is not likely to be the cause of the occurrence of plate like apatitic crystals in calcified tissues.