The reaction leading to the formation of the Pb free phase from calcined precursor powders has been studied by means of scanning electron microscopy, X-ray diffraction and differential thermal analysis. The observation of the transformations occurring at a fixed location on the surface of a pellet has shown that the formation of the phase crystallites occurs simultaneously with a gradual decomposition of the platelets initially present in the mixture. The present investigations are discussed in relation with the formation of the Pb substituted phase. It is shown that there is a close relationship between the formation mechanisms of the Pb free and the Pb substituted phases, which both can be described as a nucleation and growth process.