The short and medium range structure of a NaFeSi2O6 (NFS) glass has been investigated by high-resolution neutron diffraction with Fe isotopic substitution, combined with Empirical Potential Structure Refinement (EPSR) simulations. The majority (similar to 60%) of Fe is 4-coordinated (Fe-) and corresponds only to ferric iron, Fe3+, with a distance d(4) (Fe3+-O) = 1.87 +/- 0.01 angstrom. This is at variance with the 3D-structure predicted by glass stoichiometry. The existence of a majority of Fe-(3+) sites illustrates a glass structure that differs from the structure of crystalline NaFeSi2O6, which contains only octahedral Fe3+. The EPSR modeling of glass structure shows that Fe-(3+) is randomly distributed in the silicate network and shares corner with silicate tetrahedra. The network-forming behavior of Fe-(3+), coupled with the presence of Na+ ions acting as charge-compensators, is at the origin of peculiar physical properties of Fe-bearing glasses, such as the increase of the elastic modulus of sodium silicate glasses with increasing Fe-concentration. Our data provide also direct evidence for 5-coordinated Fe, with an average distance d(5 Fe-O) = 2.01 +/- 0.01 angstrom. This second Fe population concerns both Fe2+ and Fe3+. 5-Coordinated Fe atoms tend to segregate by sharing mainly edges. The direct structural evidence of the dual role of ferric iron in NFS glass provides support for understanding the peculiar properties of NFS glass, such as magnetic, optical, electronic or thermodynamic properties. (C) 2008 Elsevier B.V. All rights reserved.