Phase relations around the apatite phase in the Nd_2O_3-SiO_2 system have been reinvestigated by the slow-cooling floating zone technique and conventional solid state reactions. Nd_3(SiO_4)_6 believed to have an apatite structure and to melt congruently does not exist as a pure phase, whereas Nd_<9.33>(SiO_4)O_2 having an apatite structure is found to melt congruently. Solid solubility of the apatite phase is also discussed.
