Figure 3a shows the predictions of the structural model for glassy (M_2O)_x(Al_2O_3)_y(SiO_2)_{1-x-y} or (MO)_x(Al_2O_3)_y(SiO_2)_{1-x-y} along the 50 mol% SiO_2 tie-line for p = 1.0.