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technique is employed to investigate the environment of Fe 3+ /Fe 2+ cations in a sodium borosilicate glass matrix of composition 0•210Na 2 O.0•185 11 B 2 O 3 .0•605SiO 2. The neutron diffraction data were obtained using the D4c diffractometer at the Institut Laue-Langevin (ILL; Grenoble, France), and were recorded for three samples; the base glass, the base glass incorporating natural Fe 2 O 3 (12 mol%) and a similar glass containing Fe 2 O 3 enriched in 57 Fe. The data are Fourier transformed to yield the real-space total correlation function, T(r), and the first coordination shells of the Fe 3+ /Fe 2+ cations are investigated via a peak fit to the isotopic difference correlation function ∆T Fe (r). It is concluded that the iron is mainly present as Fe 3+ cations, both tetrahedrally and octahedrally coordinated by oxygen atoms, plus a small fraction (0•07±0•01) of Fe 2+ cations in octahedral coordination. The Fe 3+ tetrahedral fraction is 0•45±0•10, and appears to exist as FeØ 4 − structural units incorporated into the network of silicate chemical groupings, with their negative charge being balanced by Na + network modifying cations. The remaining Fe 3+ cations (fraction 0•48±0•10) are thought to be predominantly octahedrally coordinated and associated with BO 3 3 − orthoborate anions in FeBO 3 2 chemical groupings, which become non-stoichiometric due to the reduction of some of the Fe 3+ cations to Fe 2+ .

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