The formation process of Ba2La8(SiO4)(6)O-2 was clarified using thermogravimetry-differential thermal analysis (TG-DTA) and a high-temperature powder X-rag diffraction (HT-XRD) method, Phase changes identified from the HT-XRD data surprisingly corresponded to the weight loss and/or endothermic peaks observed in the TG-DTA curves. Raw material with the composition Ba2La8(SiO4)(6)O-2 was completely reacted at 1400 degreesC and produced only an apatite-type compound without a secondary phase. Moreover, the synthesis of Ba2+xLa8-x-(SiO4)(6)O2-delta crystals with x = 0-2 was attempted using a solid-state reaction.