The mixed alkali effect in lithium sodium silicate glasses is strongly affected by the morphology of the sample. Properties studied include electrical conductivity, thermal expansion coefficient, glass transformation and dilatometric softening temperature and transformation range viscosity. A sharp transition region caused by phase separation exists between properties of glasses which exhibit the traditional mixed alkali effect and those that do not. The mixed alkali effect in electrical conductivity does not result in a minimum in conductivity unless the alkali rich phase exhibits a connected morphology. Under some circumstances mixed alkali glasses have a maximum in conductivity. The mixed alkali effect is also controlled by the morphology for phase separated glasses in which the alkali rich phase exhibits a connected structure.
