Isotopic substitution in neutron diffraction combined with structural simulations (Empirical Potential Structure Refinement, EPSR), has been employed to probe the sites occupied by Fe3+ and Fe2+ in an oxidized NaFeSi2O6 glass. This study reveals the presence of two populations of Fe3+ sites. 70% of Fe3+ is in tetrahedral sites (dFe3+-O=1.866±0.001 Å), as the remainder ferric iron and ferrous iron (~12% of total Fe) are 5-coordinated. 4-coordinated Fe3+ would act as a network former, while 5-coordinated Fe tends to segregate and would act as a modifier. The presence of this species, although minority, could explain the variations of some physical properties of Fe-bearing glasses. The substitution of Fe for Al was studied along the join NaFeSi2O6-NaAlSi2O6 using neutron diffraction combined with EPSR simulations. Whatever the Fe (and Al) content, the structural behaviour of Fe, Al3+ and Si4+ are unchanged. Al (4-coordinated) is randomly distributed in the silicate network, which is in agreement with a network former behaviour. As for Fe, the two populations observed in the NaFeSi2O6 end-member, and the structural behaviour we have assigned to them are preserved along the join. X-ray absorption spectroscopy near edge structure has been used for the first time at Al L2,3-edges to study the evolution of Al environment in those glasses. Finally, Mössbauer spectroscopy allowed the determination of redox ratio along the join.
