Thermal expansion behaviour of glass ceramics derived from prophyric sands

Abstract The thermal expansion curves are reported for “as-quenched” and “heat-treated” glasses obtained by adding MgO (8–16%) and TiO 2 (4%) to porphyric sands of composition: SiO 2 = 72.40%; Al 2 O 3 = 13.85%; K 2 O = 4.60%; Na 2 O = 3.00%; Fe 2 O 3 = 2.15%; CaO = 1.30%; MgO = 0.84%; TiO 2 = 0.20%; loss on calcination = 2.06%. The variations in the glass transition temperature T g , the thermal expansion coefficient α, and the dialatometric softening temperature M g , are discussed. The experimental results can be explained by assuming that Na 2 O, K 2 O 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.