**10.3.5. Apatite-mullite glass-ceramics**

Apatite-mullite glass-ceramics crystallize from the glass of generic composition SiO2-Al2O3- P2O5-CaO-CaF2 to form an osseoconductive apatite phase existing as spherulites within a mullite matrix. The formation of apatite spherulites is accompanied by a depletion zone, from which calcium, phosphate and fluorine is taken and added to the growing crystal. The depletion zone also inhibits the formation of further apatite crystals in the immediate vicinity due to the glass' compositional similarity with a mullite composition. As mullite begins to crystallize, there is interdependence between the growth of apatite and mullite. Spherulitic grain boundaries in partially devitrified apatite-mullite ceramics act as crack promoters, offering preferential paths to propagation due to the grain boundary interfacial surface energy [54],[73].
