It is well known that the viscosity of the molten FeO–SiO₂ system exhibits a marked hump near the fayalite composition. In order to ascertain this phenomenon, the viscosity and density of the molten FeO–SiO₂ system were measured with special emphasis on the behavior of the fayalite melt. The viscosity measurement was carried out by a rotating cylinder method and the density measurement was made by an Archimedian method using two bobs. The experimental results revealed small but sharp humps on both of the viscosity and expansivity vs the composition curves near the fayalite composition.
The origin of these phenomena may be attributed to the presence of the fayalite cluster in the melt. Then, the amount and size of such cluster are estimated by the applications of Einstein’s equation derived for a viscosity of a colloidal solution and some thermodynamical relations. The calculated results show that the amount of the cluster having a size of (Fe₂SiO₄)_11∼12 is about 15% in volume at the melting point, decreases with increasing temperature, and vanishes at temperatures between 1800–2000°C. It is discussed also that the feature of the molar volume of fayalite melt can be explained by the same cluster model derived from the viscosity data.