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Structural Roles of Fe³⁺ and Fe²⁺

      It is sometimes assumed that Al³⁺ and Fe³⁺ occupy similar structural positions in silicate melts and glasses because of their common nominal charge and somewhat similar ionic radii. But this assumption is not necessarily warranted because Al³⁺ is dominantly in fourfold coordination in silicate crystals, whereas for the most part Fe³⁺ is in sixfold coordination with oxygen although there are exceptions to this general statement.

      In silicate glasses and melts oxygen coordination numbers vary with bulk chemical composition, total iron content, temperature, and redox conditions that existed during precursor melting. The Fe³⁺─O bond distance of ferrisilicate glass is often used as an indicator of oxygen coordination number. For example, increasing iron content in Al‐free silicates results in increasing Fe³⁺─O distance, which may be consistent with a transformation from fourfold to sixfold coordination.

Figure 11 Activity coefficient ratio of Fe²⁺ and Fe³⁺ in CaO─SiO₂ glasses formed by quenching from melt after equilibration at 1600 °C with different oxygen fugacity. The NBO/Si‐values in this figure were calculated from the Ca/Si ratio. From the relationship to oxygen fugacity, any deviation of the concentration ratio, Fe²⁺/Fe³⁺, was ascribed to changes in the activity coefficient ratio, g_Fe2+/g_Fe3+, because (g_Fe2+/g_Fe3+)(Fe²⁺/Fe³⁺) = 0.25.

      Mössbauer spectroscopy of glasses is an analytical tool with which both redox ratio of iron and coordination of Fe³⁺ and Fe²⁺ can be determined (Chapter 2.2). In alkali ferrisilicate melts equilibrated with air, Fe³⁺ typically is in fourfold coordination with oxygen. However, by replacing Na⁺ with more electronegative metals, the oxygen tetrahedra surrounding Fe³⁺ become increasingly distorted with an eventual changes to higher oxygen coordination numbers. As a result, in complex aluminosilicate compositions containing both alkalis and alkaline earths it is not unusual that Fe³⁺ exists in more than one coordination state.

      Most evidence suggests that Fe³⁺ in fourfold coordination forms oxygen tetrahedra that are isolated from those of Si⁴⁺ and Al³⁺. Furthermore, when both alkali and alkaline earths are potential charge‐balancing cations in complex systems, alkali metals tend to associate with Al³⁺, whereas alkaline earths serve to charge‐balance Fe³⁺ in tetrahedral coordination with oxygen. This means, for example, that if a rhyolite and a basalt melt equilibrated at the same oxygen fugacity, temperature, and pressure, the iron would be more oxidized in the rhyolite than in the basalt melt.

At least in FeO─SiO₂ systems the Fe²⁺─O distances are consistent with sixfold coordination although it has also been suggested that the oxygen coordination number of Fe²⁺ might be closer to 4 than to 6. Results from ⁵⁷Fe Mössbauer resonant absorption spectroscopy of iron‐bearing glasses offer additional aid to distinguish between possible oxygen coordination numbers of ferrous iron (4, 5, and 6).

      5.3 Structure–Property Relations

      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.

6 Minor Components in Silicate Glasses and Melts

      Minor components such as TiO₂ and P₂O₅ are important in natural and commercial glasses, including optical fibers and glass wool insulating materials. The structural behavior of P⁵⁺ in silicate glasses and melts is fairly well known, whereas that of Ti⁴⁺ remains more controversial, perhaps because the oxygen coordination environment surrounding Ti⁴⁺ may be a composition‐dependent variable.

      6.1 Phosphorus Substitution for Silicon

      In P₂O₅ glass, the P─O bridging bond distance (1.60 Å) is nearly identical to the Si─O distance in SiO₂ glass (1.62 Å). Additionally, there is a second double‐bonded and shorter (1.43 Å) P=O bond. These structural features remain for glasses in the P₂O₅–SiO₂ system. In the latter, Si–O–P bridges can also be detected.

      Phosphorus in metal oxide silicate and aluminosilicate glasses and melts is dissolved by formation of phosphate (PO₄) groups. Their degree of polymerization can be derived from ³¹P NMR spectra as a function of metal oxide/P₂O₅ ratio [13] in a way similar to Q ⁿ ‐species determinations in metal oxide–SiO₂ glasses (see also Chapter 2.4 and Section 3.1). In addition, there are minor contributions from Si─O─P linkages. Mixed alumino‐silico phosphate complexes are more common (Chapter 2.4).

      6.2 Multiple Roles of Ti⁴⁺

      The ionic radius of Ti⁴⁺ is nearly twice that of Si⁴⁺. It is not surprising, therefore, that Ti⁴⁺ in crystalline materials commonly occupies sixfold coordination, whereas Si⁴⁺ is in tetrahedral coordination. In glasses and melts, on the other hand, the structural behavior of Ti⁴⁺ is more complex. From partial molar volume of TiO₂, Скачать книгу