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The Effect of Fe3+ Ions on the Anionic Structure of Iron-bearing Sodium Silicate Melts

The effects of the coordination of iron ions on the anionic structure of molten iron bearing sodium silicate slags at 1 273 K have been investigated by Raman spectroscopy. The Raman spectra of Na 2 O-SiO 2 -FeO-Fe 2 O 3 system equilibrated with various oxygen pressures (pO 2 ) have been measured. The effect of the concentration of iron oxide on the Raman spectra of these systems has also been studied. Under low pO 2 conditions (CO 2 /CO 54), a new Raman band of around 900 cm -1 appears and its intensity increases with increase of pO 2 and iron oxide content. The scattering intensity at low frequency region (<550 cm -1 ) also becomes intense with increase of the iron oxide content. It is confirmed that the near 900 cm -1 band can be attributable to the network structure involving Fe 3+ ions. Fe 3+ ions in the metasilicate melts is found to conjugate the chain units and produce the sheet and monomer units. In the disilicate melts, it will conjugate the sheet units and produce the chain and three-cimensional network units.


Introduction
Numerous studies [1][2][3][4][5][6][7][8][9] have used Raman spectroscopy to investigate the structure of silicate glasses and melts, and many of them have been applied to understand properties important in metallurgical and geochemical processes. Although there are extensive literature dealing with structures and properties of silicate glasses, most of studies have been restricted to systems not containing iron oxides in spite of the practical importance of iron oxide-containing slags. The influence of the redox ratio of iron ions on the properties of silicate melts is quite complex, because the iron ion may not respond simply to variations in melt structure; rather it may affect the melt structure itself. Furthermore, iron in a melt can play a dual role because iron ions can be both tetrahedrally and octahedrally coordinated. The coordination of the iron ions is controlled by an involved interrelationship between the degree of polymerization of the silicate, the types of cation present, the amount of iron ion present and the oxygen potential.
Depending on the coordination states of iron ions, the amount of oxygen coordinated by iron ions or that of free oxygen will be changed. As a result, variation of redox ratios of iron will change the degree of polymerization of the melt and therefore the viscosity, the cation diffusion coefficients and the reactivity of slags. Therefore, understanding of the role of iron ions in silicate melts is very important for fundamental reasons and for practical uses, since commercial steelmaking slags always contain iron ions.
The study of the Fe ions in the molten silicate have been mainly concerned with the coordination of the Fe ions. Hirao et al. 10) investigated the Fe 3ϩ coordination in alkali iron silicate glasses using Mössbauer spectroscopy and they confirmed that Fe 3ϩ ions in silicate glasses tetrahedrally coordinated, which agreed with the conclusion reported by Kurkjian and Sigety 11) who studied the coordination of Fe 3ϩ in silica glasses with low iron contents. Iwamoto et al. 12) studied the coordination of Fe ions in CaO-SiO 2 -10mol% Fe 2 O 3 glasses using Mössbauer spectroscopy. Fe 2ϩ was found to be always present in octahedral sites, while Fe 3ϩ occupied both octahedral and tetrahedral sites. The same conclusion was reached by Pargamin et al. 13) Mysen et al. 3) measured the redox equilibrium and the coordination states of iron in alkali silicates under different oxygen potentials and alkaline-earth silicate melts with different cation ratios of Ca/Mg in air using Mössbauer spectroscopy. Their Mössbauer spectroscopic results indicated that, in the system Na 2 O-FeO-Fe 2 O 3 -SiO 2 , Fe 3ϩ iron was in tetrahedral coordination, and the Fe 2ϩ was octahedrally coordinated with oxygen ions. In the system CaO-MgO-FeO-Fe 2 O 3 -SiO 2 , however, both Fe 3ϩ and Fe 2ϩ were octahedrally coor-dinated. Based on these results they suggested that only alkali ions can stabilize Fe 3ϩ iron in tetrahedral coordination, but not alkaline-earth.
Mössbauer spectroscopic results can provide the coordination state of iron ions in silicate melts, but the technique is not suitable for correlating the coordination state of iron ions with the framework of silicate structure or degree of polymerization. For this purpose, Raman spectroscopy is very suitable. Mysen et al. 3) used Raman spectroscopy to study the anionic structure of alkali silicate melts on the join Na 2 SiO 3 -NaFeSi 2 O 6 at 1 400 K in air. As they measured Raman spectra along the join Na 2 SiO 3 -NaFeSi 2 O 6 , each measured sample has different Na/Si ratio and total Fe content. In this case, the change of Raman spectra was contributed from several factors. Thus it is not easy to evaluate the role of Fe ion precisely. Namely, to study the effect of Fe ions on the structure of melts, it is preferable to study the melts of constant composition in which only the Fe 2ϩ / Fe 3ϩ ratio is varied.
The present work was undertaken to investigate the role of iron ions on the framework of molten alkali silicate structures by applying Raman spectroscopy. This has been achieved for a quenched Na 2 O · 2SiO 2 -FeO-Fe 2 O 3 melts containing up to about 9 mol% Fe 2 O 3 with varying oxygen potential from about 10-12 to 0.025 MPa and on the join Na 2 O · 2SiO 2 -Fe 2 O 3 · 2SiO 2 system (constant SiO 2 of 66 mol%) and Na 2 O · SiO 2 -Fe 2 O 3 · SiO 2 system (constant SiO 2 of 50 mol%) with varying Na 2 O/Fe 2 O 3 ratio with air.

Sample Preparation
Na 2 O-SiO 2 -FeO-Fe 2 O 3 glasses for this study were made in two stages. Appropriate portions of reagent grade Na 2 CO 3 , SiO 2 , Fe 2 O 3 were ground together in an agate mortar in air and placed in a Pt crucible. The mixture was heated at 1 073 K for 3 h, and then fired in a muffle furnace at 1 273 K for 2 h. The melted mixture was quenched into a water-cooled copper flat mold and then ground to form a powder. This treatment was repeated twice to ensure homogeneity.
The initial compositions of Na 2 O-SiO 2 -Fe 2 O 3 system are presented in Table 1. The silicate melts in the present study actually contained FeO or Fe 2ϩ ion. But the slag compositions are represented by initial compositions for convenience.

Equilibration
The Fe 3ϩ /Fe 2ϩ ratio in the Na 2 O · 2SiO 2 -FeO-Fe 2 O 3 sample was fixed by controlling the CO 2 /CO ratio. CO 2 and CO gas were dried by passage through silica gel and magnesium perchlorate, and the CO was passed through ascalite to remove any residual CO 2 . The gases were mixed in the required ratios using mass flow controllers and passed at the rate of 500 ml/min into the equilibration furnace. The Pt crucible (IDϭ5 mm, heightϭ5 mm) containing about 50 mg of sample was suspended by means of a Pt suspension wire in the hot zone of the resistance furnace, controlled to Ϯ5 K.
The Na 2 O · 2SiO 2 -FeO-Fe 2 O 3 sample was equilibrated with CO-CO 2 gas mixtures of CO 2 /CO ratio ranging from 1 to 800 and in air at 1 273 K for 24 h in the resistance furnace and then the Pt crucible was quenched by rapidly lowering the Pt wire onto the water-cooled endcap. This equilibrated quenched sample was immediately used for the Raman spectroscopy measurements to avoid the absorption of water from the atmosphere. As a preliminary experiment, Raman spectra were measured by varying the equilibrating period of 1, 2, 6, 24 hrs. It was observed that the Raman spectra did not change after 2 hrs. Based on this result, all samples were equilibrated for 24 hrs. Premixed cylinder gas (Ar-10%CO-15%CO 2 ) was used directly for the equilibration.

Raman Spectroscopy Measurement
Raman spectra of quenched glasses were obtained with an automatic micro-Raman system (JASCO NRS-2000). Silicate samples in the Pt crucibles were excited by the 514.5 nm line of a coherent Ar ϩ laser (NEC GLG-2162) operating at 100 mW. The CCD detector was maintained at a constant temperature of liquid nitrogen and all the Raman spectra measurements of samples were made at room temperature. Unpolarized spectra were measured for all samples and several spectra from each sample were measured for each composition. No detectable change in Raman spectra occurred for the same composition. All spots used for micro-Raman analysis were examined for laser damage immediately following the runs and detectable damages were not observed.

The Effect of Oxygen Potential
Observed Raman spectra from Na 2 O · 2SiO 2 -4.0 mol% Fe 2 O 3 (as an initial composition) samples equilibrated with various oxygen potentials (pO 2 ) from about 0.25 to 10 Ϫ12 MPa at 1 273 K are shown in Fig. 1. These melts differ only in Fe 3ϩ /Fe 2ϩ ratio. In Fig. 1, the spectrum of Na 2 O · 2SiO 2 is also shown as a reference. In each spectrum, there are intense slightly asymmetric bands near 1 100 and 600 cm Ϫ1 and a relatively weak band in the region of 950 cm Ϫ1 . The frequencies of these bands are virtually independent of the CO 2 /CO ratio. With increasing of pO 2 (CO 2 /COϾ10), the 600 cm Ϫ1 band is skewed and the peak position of 600 cm Ϫ1 band shifts to lower frequency. This may be due to the development of unresolved other bands near 600 cm Ϫ1 with increase of CO 2 /CO ratio. The most notable change with CO 2 /CO ratio is the appearance of a new band of around 900 cm Ϫ1 at CO 2 /CO ratio Ͼ56 and the intensity of which increases with CO 2 /CO ratio. At CO 2 /CO ratio Ͻ10, the relative intensities of the bands of each sample are only slightly changed, and these spectra are qualitatively the same as that of the binary Na 2 O · 2SiO 2 . It suggests that the distribution of consisted anions in these melts (at CO 2 /CO ratioϽ10) is not much different from each other.

The Effect of Iron Oxide Content
The Raman spectra of Na 2 O · 2SiO 2 -Fe 2 O 3 system containing from 0.4 to 8.7 mol%Fe 2 O 3 equilibrated with air are shown in Fig. 2. In these samples, their basicities are almost constant. In Fig. 2, the 900 cm Ϫ1 band is not seen in the Na 2 O · 2SiO 2 -0.4 mol%Fe 2 O 3 . In the Na 2 O · 2SiO 2 -2.0 mol%Fe 2 O 3 system, the 900 cm Ϫ1 band is observed as a shoulder, and it becomes dominant with increase of Fe 2 O 3 content. These results strongly suggest that near 900 cm Ϫ1 band is closely related to the existence of Fe 3ϩ ions. It is also found that the 900 cm Ϫ1 band observed in Na 2 O · 2SiO 2 -2 mol%Fe 2 O 3 and 4 mol%Fe 2 O 3 has shifted to 920 cm Ϫ1 in Na 2 O · 2SiO 2 -8.7 mol%Fe 2 O 3 . The scattering intensity at low frequency region (Ͻ550 cm Ϫ1 ) also becomes intense with increase in the iron oxide content.

Assignments of Bands
As shown in Fig. 1, the spectra of Na 2 O · 2SiO 2 -4.0 mol%Fe 2 O 3 under reduced condition (CO 2 /CO ratioϽ 10) are almost the same as that of the Na 2 O · 2SiO 2 system. They have the same major bands of 1 100, 950 and 600 cm Ϫ1 , although their relative intensities are slightly changed. It means that the complex anions in these melts are essentially the same, although the amount of complex anions may be slightly different. Thus the introduction of Fe 2ϩ in the melt basically doesn't change the framework of © 2000 ISIJ   Na 2 O · 2SiO 2 system. Figure 3 shows the measured Raman spectra of the Na 2 O-SiO 2 binary system as a function of Na 2 O content up to metasilicate composition. The Raman spectra of the Na 2 O-SiO 2 binary system have been widely investigated and the present measured results are essentially the same as those already reported. 2,4,5) Assignments of main bands are the most generally accepted in the literature. 1,8,14) Namely, the 1 100, 950 and 850 cm Ϫ1 bands are due to symmetric Ϫ O-Si-O 0 (e.g., Sheet structure), Ϫ O-Si-O Ϫ (e.g., chain structure) and Si-O 2Ϫ (monomer unit) stretch vibration respectively. NBO(Non bonding oxygen)/Si of simple ring structure will be 2, and that of branched chain may vary from 2 to 1. The existence of ring and branched chain is still uncertain. Presumably these units did not exist at least in significant amount. 3) Therefore, just as a convenience, the structure units with NBO/Siϭ1 and 2 have been referred to as a sheet and chain in the present study. The band around 600 cm Ϫ1 is assigned to the bending motion of Si-O-Si. In Table 2, the major band corresponded to the anionic units are shown on the basis of the average NBO/Si by summarizing the published data. 1,8,14) For some weak peaks, however, assignments are still ambiguous. In this study, however, we do not examine the assignments of weak bands.
The most interesting aspect of compared the Raman spectra in Fig. 1 with those of Fig. 3 is that there is no band at around 900 cm Ϫ1 in the binary Na 2 O-SiO 2 system that is observed in the Na 2 O · 2SiO 2 -FeO-Fe 2 O 3 system under high pO 2 conditions (CO 2 /CO ratioϾ56). There have been several studies of the pO 2 dependence of Fe 3ϩ /Fe 2ϩ ratio in Na 2 O-SiO 2 -FeO-Fe 2 O 3 system. 3,[15][16][17] From all these results, it can be safely estimated that the iron ions in Na 2 O-SiO 2 -FeO-Fe 2 O 3 system equilibrated with air are mostly Fe 3ϩ . As shown in Fig. 1, the near 900 cm Ϫ1 band in Na 2 O-SiO 2 -FeO-Fe 2 O 3 system becomes dominant under oxidizing condition (CO 2 /COϾ56). Therefore, the near 900 cm Ϫ1 band can be related to Fe 3ϩ ion. Thus, differed from the case of Fe 2ϩ , the introduction of Fe 3ϩ seems to change the anionic structure significantly. However, it is not certain whether the near 900 cm Ϫ1 band is attributed to the Fe 3ϩ contained units themselves or newly produced silicate units due to the Fe 3ϩ ion introduction.
It has been well known that the major band of 900 cm Ϫ1 was not observed in alkali silicate system, but has only been explicitly observed for the alkaline earth silicate 2,3,14) and is interpreted as being due to the presence of dimmer structural units (Si 2 O 7

6Ϫ
) in the melt. The 900 cm Ϫ1 band is maximized in relative intensity near the pyrosilicate com-position of the alkaline earth silicates, 2,3,14) and is only resolved for a narrow range of compositions near the pyrosilicate. The presence of 900 cm Ϫ1 band in higher silica glass spectra was deduced from band deconvolutions, 2) and is first recognized by this method between the metasilicate and disilicate. The Si 2 O 7 6Ϫ ions may also exist in Na 2 O-SiO 2 -FeO-Fe 2 O 3 melts, but it does not exist in significant amount and only possible in compositions closed to the pyrosilicate composition. Namely, there is least possibility that 900 cm Ϫ1 band in the composition of the present study is directly related to Si 2 O 7 6Ϫ ions. If the dimmer units are produced by the increase of Fe 3ϩ , the degree of the polymerization in the melt must be decreased with Fe 3ϩ and consequently the viscosity of the melt can be decreased. Digwell and Virgo 18) measured the viscosity of Na 2 O-SiO 2 -FeO-Fe 2 O 3 melts with varying pO 2 , and found that the viscosity increased with Fe 3ϩ content. Their result 18) suggests that the introduction of Fe 3ϩ does not produce the dimmer units in the melt. Furthermore, as already mentioned, all Mössbauer spectroscopic results 3,10-13) indicated that Fe 3ϩ ion in Na 2 O-FeO-Fe 2 O 3 -SiO 2 system was in tetrahedral coordination. The tetrahedral coordination of Fe 3ϩ generally means that Fe 3ϩ ion behaves as a network former, but not network modifier. Therefore, in the present study, the silicate dimmers do not exist enough amounts to produce the 900 cm Ϫ1 band. Combined all these results, it can be confirmed that the near 900 cm Ϫ1 band in the present study is not originated from the dimmer units, but can be attributed to the network structure unit involving Fe 3ϩ ions.

Three-dimensional Network Structure
Another interesting aspect in the present study is the increase in the scattering intensity at the low frequency region with pO 2 and Fe 2 O 3 content (see Figs. 1 and 2). A three-dimensional array of SiO 2 tetrahedra causes a strong broad band near 430 cm Ϫ1 and a sharp band near 480 cm Ϫ1 . 8,19,20) These bands between 400 to 500 cm Ϫ1 are due to the vibration of Si-O 0 bond (O 0 is bridging oxygen). The broad band around 500 cm Ϫ1 is also observed in melts with a three-dimensional network structure on the join CaAl 2 O 4 -SiO 2 and NaAlO 2 -SiO 2 , and is considered characteristic of melts with a three-dimensional network unit. 20) Therefore, the increase in the intensity in the low frequency region in the present work implies that the Fe 3ϩ introduction may develop a Si-O 0 bonds or a three-dimensional network structure involving Fe 3ϩ ions.
Hirao et al. 10) measured the coordination state of Fe 3ϩ in leucite type iron silicate glasses on the join Na 2 O · 2SiO 2 -Fe 2 O 3 · 2SiO 2 ((Na 2 O) 1Ϫx · (Fe 2 O 3 ) x · 2SiO 2 , xϭ0.15-0.65) and crystals by using Mössbauer technique. Crystalline of KFeSi 2 O 6 (iron leucite), CsFeSi 2 O 6 (iron polleucite) and KFeSi 3 O 8 (iron feldspur) are known to contain Fe 3ϩ in tetrahedral coordination, 21) whereas NaFeSi 2 O 6 (acmite) crystalline contains Fe 3ϩ in octahedral coordination. 22,23) They found that the isomer shift and quadrupole splitting values for the alkali iron silicate glasses were unlike those of crystalline NaFeSi 2 O 6 , but were similar to that of cubic crystalline CsFeSi 2 O 6 . The cubic crystalline CsFeSi 2 O 6 has a complete three-dimensional network of linked tetrahedra similar to those of the various forms of silica, but involving Fe 3ϩ ions with coordination number 4 as well as silicon ions. Therefore, their results 10) also strongly suggests that the state of Fe 3ϩ in the alkali iron silicate glasses is very similar to that of Fe 3ϩ in the cubic lattice of a crystalline compound having a three-dimensional network structure and the alkali iron silicate glasses will have the three-dimensional network structure involving Fe 3ϩ . Their results suggest that the Raman spectra of CsFeSi 2 O 6 polycrystalline involving Fe 3ϩ ions will be the same to that of alkali iron silicate glasses. To confirm this, the Raman spectra of CsFeSi 2 O 6 polycrystalline and quenched alkali iron silicate glasses ((Na 2 O) 1Ϫx · (Fe 2 O 3 ) x · 2SiO 2 , xϭ0.5) are measured and results are shown in Fig.  4. CsFeSi 2 O 6 polycrystalline was prepared by mixing the reagent grade Cs 2 CO 3 , SiO 2 , Fe 2 O 3 powder and heated the mixtures at 1 073 K for 3 h, and then fired in a muffle furnace at 1 523 K for 4 h. The sintered mixture was ground to form powder and heated again to ensure homogeneity.
It is clearly shown that they have the same band of around 920 cm Ϫ1 band that is supposed to originate from the Fe 3ϩ involving network structure. This is another strong evidence that the quenched alkali iron silicate glasses also have the three-dimensional network structure. Compared with the Raman spectra of these two samples, the band corresponded to the three-dimensional network structure involving Fe 3ϩ can be the band of 920 cm Ϫ1 . Namely the 920 cm Ϫ1 can be assigned to the (Fe, Si)-O 0 stretching. (Fe, Si)-O 0 denotes the Fe rich (Fe, Si)-O-(Fe, Si) stretch vibration.
As already shown in Fig. 3, the 900 cm Ϫ1 band seems to shift to 920 cm Ϫ1 with increase of Fe 2 O 3 content. If this shift is associated to the change of the Si/Fe ratio in the three-dimensional network structural unit, the 900 cm Ϫ1 band may be correspond to the (Fe, Si)-O 0 stretching of low Fe content three-dimensional network structural unit, or close to the Si-O 0 stretching. In pure SiO 2 glass, however, the band around 900 cm Ϫ1 has not been observed. Mysen et al. 3) also observed 900 cm Ϫ1 Raman band on the join Na 2 SiO 3 -NaFeSi 2 O 6 at 1 400 K in air and it was interpreted to be consisted of two bands of the (Fe, Si)-O 0 stretch (920 cm Ϫ1 ) and highly coupled Ϫ O-(Fe, Si)-O Ϫ stretch vibration (880 cm Ϫ1 ). There is no doubt that 900 cm Ϫ1 is related to the units involving Fe 3ϩ , but there has been no clear definite evidences to determine the origin of 900 cm Ϫ1 at this moment. Tentatively we also accept their interpretation 3) as a most plausible one, but further work is necessary to firmly establish the origin of 900 cm Ϫ1 . If so, the Ϫ O-(Fe, Si)-O Ϫ stretch vibration will be dominant for the low Fe 2 O 3 content sodium silicate, but the (Fe, Si)-O 0 stretch vibration will be governing with increase of Fe 2 O 3 .
As shown in Fig. 2, relative intensity of 1 100 cm Ϫ1 band seems to decrease with increase of Fe 2 O 3 . Since the SiO 2 content was also decreased with increase of Fe 2 O 3 content, it is not certain whether the decrease of 1 100 cm Ϫ1 band is attributable to the decrease of SiO 2 content or the effects of Fe 2 O 3 . To confirm this, Raman spectra of the melts on the join Na 2 O · 2SiO 2 -Fe 2 O 3 · 2SiO 2 with constant SiO 2 of 66 mol% in air at 1 373 K as a function of Fe 2 O 3 are measured and results are shown in Fig. 5.
From Fig. 5, the intensity of 1 100 cm Ϫ1 band is found to significantly decrease with Fe 2 O 3 content, but that of 950 cm Ϫ1 seems to increase. Since the 950 cm Ϫ1 band is asymmetric and has a shoulder at about 900 cm Ϫ1 , the 950 cm Ϫ1 band may be consisted of 950 and near 900 cm Ϫ1 bands. To evaluate the increase of 950 cm Ϫ1 quantitatively,  the bands in the high frequency region (800 to 1 200 cm Ϫ1 ) of 5Na 2 O · Fe 2 O 3 · 12SiO 2 , 2Na 2 O · Fe 2 O 3 · 6SiO 2 and Na 2 O · 2SiO 2 systems are deconvoluted with the assumption that the line shape is Gaussian, the background in this region is horizontal and all bands are symmetric. 24) To carry out the deconvolution, tentatively 5 bands of near 1 100, 1 050, 950, 920 and 850 cm Ϫ1 band are used as initial values to fit inside the high frequency envelope of the spectra. The Raman spectra in the high frequency region of these systems are well reproduced by the combination of these bands. As shown in Fig. 6, near 900 cm Ϫ1 band (880 and 920 cm Ϫ1 ) increases with Fe 2 O 3 just as expected and the 950 cm Ϫ1 band also increases with Fe 2 O 3 . It means that the amount of sheet units in the melt decreases with Fe 2 O 3 but the chain units may increase, since 1 100 and 950 cm Ϫ1 band is mainly corresponded to the vibration of sheet and chain structure of silicate anions respectively 1,8,14) as indicated in Table 2. The growth of the broad bands at low frequency region (less than 550 cm Ϫ1 ) with Fe 2 O 3 is also observed in Fig. 5, and means the development of a Si-O 0 bond or a three-dimensional network structure with Fe 2 O 3 . Based on these results, it can be confirmed that the three-dimensional and chain structural units are increased but the sheet structural unit is decreased by the introduction of Fe 3ϩ ions.
In the melts of around disilicate composition, there will be three major silicate units of chain, sheet and three-dimensional network and they can be related each other with the following equation. In this disproportionate process, to maintain the local charge-balance, the chain units ( Ϫ O-Si-O Ϫ ) also must be produced along with the production of three-dimensional structure units. The measured and deconvoluted results of Raman spectra with Fe 2 O 3 simply support this disproportionate process. Accordingly, the assumption that the role of Fe 3ϩ in the disilicate melts is to conjugate sheet units and produce the three-dimensional network structure appears to be fully consistent with all the results.
From the Raman data for sodium metasilicate melt shown in Fig. 3, there will be two major silicate units of monomer, chain and small amount of sheet units and they can be related each other with the following equation.   but that of 970 cm Ϫ1 seems to decrease. It is known that the frequency of the band corresponded to the sheet structural unit changes from 1 100 to 1 050 cm Ϫ1 depend on the silicate composition. 5) Take into account of the electric charge and ionic radius of Fe 3ϩ and Si 4ϩ , the force constant of Fe-O bond is lower than those of Si-O bond. Since the frequency of Raman bands corresponding to the Fe-O bonds will therefore be lower than those of Si-O bonds, the frequency corresponded to the sheet structural unit will shift from 1 100 to 1 050 cm Ϫ1 when some Fe 3ϩ ions entering the silicate sheet unit. Namely, the 1 050 cm Ϫ1 band observed in Fig. 7 will be correspond to the vibration of sheet unit contains Fe 3ϩ ions. Therefore, the increases of the intensity of 1 050 and 850 cm Ϫ1 bands with Fe 2 O 3 content means that the sheet structural units and monomers increase with the increase of Fe 2 O 3 . Thus, just as expected, Fe 3ϩ ions in the metasilicate melts are likely to conjugate the chain units and produce the sheet and monomer units. From these results, it is confirmed that the role of Fe 3ϩ in the molten sodium silicate is to conjugate the consisted silicate anions and consequently produce much larger silicate anion structures. Namely the Fe 3ϩ in silicate melts rearranges the degree of polymerization of the melt and therefore it will significantly influence the viscosity, the cation diffusion coefficients and the reactivity of molten silicate melts. From thermodynamic view point, the role of Fe 3ϩ in the silicate melts can be interpreted as that Fe 3ϩ ion make larger the equilibrium constants of reaction (1) and (3).

Conclusions
The Raman spectra of Na 2 O-SiO 2 -FeO-Fe 2 O 3 system at 1 273 K have been measured by varying oxygen potential and Fe 2 O 3 content to investigate the effects of iron ions on the anionic structures of molten silicates.
(1) The most notable feature of the observed Raman spectra is the appearance of a new band 900-920 cm Ϫ1 at above the CO 2 /CO ratio of 56 and its intensity increases with CO 2 /CO ratio and Fe 2 O 3 content. It is also found that the scattering intensity at the low frequency region (Ͻ600 cm Ϫ1 ) is increased with pO 2 and Fe 2 O 3 content.
(2) The observed band 900-920 cm Ϫ1 is determined to be related to the network structure involving Fe 3ϩ and the 900 cm Ϫ1 band is found to shift to 920 cm Ϫ1 with increase of Fe 2 O 3 content.
(3) The band observed at around 900 cm Ϫ1 band may result from the superposition of two bands of the (Fe, Si)-O 0 stretch vibration (920 cm Ϫ1 ) and the Ϫ O-(Fe, Si)-O Ϫ stretch vibration (880 cm Ϫ1 ).
(4) The main role of Fe 3ϩ ions in the disilicate melts, such as Na 2 O · 2SiO 2 -Fe 2 O 3 system, is most likely to conjugate the silicate sheet units and create the three-dimensional network units. In the metasilicate, Fe 3ϩ ion conjugates chain units and produces sheet units.