An evolution of the structure of three glasses of Na2O-Nb2O5-SiO2 system in the course of isothermal heat treatments at 660-700C and the extinction coefficient of the material were studied. Spinodal phase separation was found to be a primary process followed by the precipitation of nano-sized NaNbO3 crystalline phase. It was found that the spectral dependence of the extinction coefficient in the wavelength range (formula) corresponds to light scattering by spinodal structure at the phase of separation stage and by independent Rayleigh scatters (NaNbO3 nanocrystals) at the early stage of crystallisation. The extinction coefficient increases at the first half of the crystallisation stage and decreases. At the late stage of crystallisation and for the final glass ceramics, the extinction coefficient (formula) is 10-20 times smaller than that calculated...
