La2O3 was substituted for CaO in a glass of composition 2.5CaO . 2SiO(2) and the effect of this substitution on thermal properties and non-isothermal behavior was studied. The glass samples were submitted to thermal analysis (DTA) and X-ray diffraction analysis. The trends with the composition of the glass transformation, T-g, and the activation energy of crystal growth, E-c, are the result of a type of competition between Ca2+ and La3+ ions each to achieve their preferred coordination. The lanthanum ion does not succeed in realizing its own average coordination. As a result T-g=810+/-5 degrees C and the activation energy for crystal growth was E-c=650+/-25 kJ/mol, regardless of composition. Devitrification involved a mechanism of surface nucleation: however, because the samples softened and sintered before devitrifying, surface nuclei behaved as bulk nuclei. In all the devitrified samples CaO . SiO2 crystals form together with a secondary calcium silicate phase, while Ca3La6(SiO4)(6) crystals form when congruent to 40% molar weight CaO is substituted. The pattern for the La2O3 free glass shows that the alpha CaO . SiO2 is also formed in the temperature range 900-1000 degrees C, which should be stable only above 1125 degrees C as reported in the phase diagram.