The literature on how Mg is present in tooth enamel and on the incorporation of Mg in synthetic apatites is reviewed. Then a theoretical consideration is given on the basis of ionic radii about the incorporation of Mg in F-, Cl- and OH-apatite. Finally, the results are given of experiments on the incorporation of Mg in fluor, chlor, hydroxy, and carbonate apatite. It appears that the extent of incorporation of Mg in the apatite lattice, if at all, is very limited and that Mg in tooth enamel is to a very limited extent incorporated in the apatite lattice, the main portion being surface bound or present in a separate phase.

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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.

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