Oxidation states and coordination structures of iron ions in silicate melts during relaxation process and at equilibrium

Changes of oxidation states and local structural environments of iron ions in calcium silicate melts have been determined during the equilibration process by Mössbauer spectroscopy to investigate the equilibrium states of iron ions from the viewpoint of the effects of basicity, iron oxide and basic oxide concentrations. The fraction of Fe³⁺ ions to the total iron ions became constant within 2 h after the equilibration operation was started. On the other hand, it took about 20 and 6 h for the coordination structures of Fe³⁺ ions to settle down by sharing between tetrahedral and octahedral sites when the samples were melted at 1608 and 1678 K, respectively. From these results it has been considered that the structural coordination of the melts is determined by rearrangement of oxygen ions rather than diffusion of iron ions in the silicate melts. With respect to the equilibrium state of iron ions it was found that Fe³⁺ ions tend to be in octahedral symmetry at larger ratios of the number of Fe³⁺ ions to that of nonbridging oxygen ions, and to be in tetrahedral symmetry when the glass matrix is basic.