Revisiting the Phase‐Equilibrium Diagram of the Ca3(PO4)2–CaMg(SiO3)2 System

As Si and Mg stimulate osteogenesis and mineralizing mechanisms, compositions belonging to the system Ca3(PO4)2–CaMg(SiO3)2, able to release these elements in situ, are promising candidates for preparing ceramic bone implants. An exact knowledge of the phase relationships in the system is required to design materials with controlled phase composition and properties. Thus, ceramic cylinders containing Ca3(PO4)2 (40, 45, 80, 90, 95, 99, and 100 wt%) and CaMg(SiO3)2 (balance to 100 wt%) were prepared and equilibrated at temperatures ranging from 1100° to 1500°C. Phase composition of the resulting ceramics was studied by X-ray diffraction, thermal analysis, reflected-light optical microscopy, and scanning electron microscopy with attached energy-dispersive spectroscopy. Two invariant points were established. An eutectic point, previously reported, was confirmed at 1300°±5°C and 37 wt% of Ca3(PO4)2, where CaMg(SiO3)2 and β-Ca3(PO4)2 solid solutions coexist. A new peritectic point, corresponding to the reaction β-Ca3(PO4)2 solid solution↔α-Ca3(PO4)2 solid solution+liquid was found at 1440°±10°C and ≈70 wt% of Ca3(PO4)2. Two new biphasic domains, where β- and α-, and α- and α′-Ca3(PO4)2 solid solutions coexist, respectively, are also proposed. The results showed that, at the eutectic temperature, the β-Ca3(PO4)2 solid solution extends up to ≈17 wt% CaMg(SiO3)2. It was also found that the Si/Mg molar ratio in Ca3(PO4)2 solid solutions decreases with regard to pure CaMg(SiO3)2, which might be explained through the incongruence of the dissolution process.