Reduction-induced inward diffusion and crystal growth on the surfaces of iron-bearing silicate glasses

We investigate the sodium inward diffusion (i.e., sodium diffusion from surface toward interior) in iron containing alkaline earth silicate glasses under reducing conditions around T_g and the induced surface crystallization. The surface crystallization is caused by formation of a silicate-gel layer first and then the growth of silica crystals on the glass surface. The type of alkaline earth cations has a strong impact on both the glass transition and the surface crystallization. In the Mg-containing glass, a quartz layer forms on the glass surface. This could be attributed to the fact that Mg²⁺ ions have stronger bonds to oxygen and lower coordination number (4~5) than Ca²⁺, Sr²⁺ and Ba²⁺ ions. In contrast, a cristobalite layer forms in Ca-, Sr- and Ba-containing glasses.