Phase relations around langasite (LGS, La3Ga5SiO](4)) were studied on the basis of phase assemblage observed during calcination and crystallization process of samples of various compositions in the ternary system La2O3-Ga2O3-SiO2. A ternary compound of apatite structure, La14GaxSi9-xO39-x/2 was found for the first time. Crystallization of this compound was observed in the cooling process of molten samples of stoichiometric LGS as well as LGS single crystal, demonstrating that LGS is an incongruent-melting compound. A phase diagram was established primarily based on the crystallization sequence in the cooling process. (C) 2003 Elsevier Science B.V. All rights reserved.