Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 265

The physical properties of iron‐bearing silicate melts and glasses are less well known than for iron‐free materials. Viscosity and volume data can nonetheless be rationalized in structural terms.

The partial molar volumes of FeO and Fe₂O₃, and , are sensitive to oxygen coordination. The in glass (between about 13 and 14 cm³/mol) resembles the molar volume of crystalline wüstite (FeO), which suggests that Fe²⁺ is sixfold coordinated in glasses. The ‐values are more variable, however, and depend on composition. For example, in SiO₂─NaFeO₂ glasses formed by quenching from melt equilibrated with air (and with Fe³⁺/SFe > 0.95), is between 20 and 21 cm³/mol (as FeO_1.5), whereas for equivalent Ca‐ferrisilicate melt, = 13–15 cm³/mol [11]. These latter volumes are those expected with Fe³⁺ in tetrahedral and octahedral coordination, respectively.

The viscosity of iron‐bearing silicates depends on both redox state of iron and on the coordination state of Fe²⁺ and Fe³⁺. For example, with Fe³⁺ in fourfold coordination and Fe²⁺ in sixfold coordination with oxygen, melt viscosity increases systematically with increasing Fe³⁺/SFe because silicate polymerization also increases with increasing Fe³⁺/SFe [12]. When both Fe³⁺ and Fe²⁺ are surrounded by octahedral oxygen ligands, this relationship is reversed. Given that the redox ratio in basaltic melts normally is considerably lower and the oxygen coordination number around Fe³⁺ higher than in more silicate rick melts (andesite and rhyolite, for example), decreasing Fe³⁺/Fe²⁺ in the former may result in increased melt viscosity, whereas the opposite trend obtains for the latter.