The formation of the Y-4.69(SiO4)(3)O apatite phase was observed in compositions targeted at the yttrium monosilicate and disilicate phases. Through the use of high-temperature in situ X-ray diffraction, the lattice parameters of the phase were characterized as a function of temperature, and thermal expansion coefficients were calculated. Phase fractions were assessed as a function of sintering temperature, and the apatite phase fraction increased with temperature.