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Epitactic site

In this context an epitactic site is something which helps crystal formation. Collagen is thought to provide epitactic sites for hydroxyapatite formation by mimicking the charge structure of the crystal...remember? As far as calculus formation is concerned, epitactic sites are likely provided by some component (probably a bacterium) of dental plaque.

Once the first crystals have formed they will grow and instigate the formation of more crystals themselves by acting as epitactic sites. The term "epitaxy" is defined as the growth of a thin layer on the surface of a crystal so that the layer has the same structure as the underlying crystal.