The thermal expansion curves are reported for "as-quenched" and "heat-treated" glasses obtained by adding MgO (8-16%) and TiO2, (4%) to porphyric sands of composition : SiO2=72.40%; Al2O3=13.85%; K2O=4.60%; Na2O=3.00%; Fe2O3=2.15%; CaO= 1.30%; MgO=0.84%; TiO2=0.20%; loss on calcination = 2.06%. The variations in the glass transition temperature T-g, the thermal expansion coefficient alpha, and the dialatometric softening temperature M(g), are discussed. The experimental results can be explained by assuming that Na2O, K2O and CaO (all or a part of them) segregate preferentially with respect to MgO into the titania-rich phase. Diffusion appears to fall as the MgO content is increased.