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Iron-bearing Melts
335
Chapter II
Iron-bearing Melts I1. Structure Iron oxides are major components in natural and industrial silicate melts and glasses. It is the only major component existing in both di- and trivalent form under most conditions. Despite their importance and abundance, however, our knowledge of the structural behavior of iron oxides is less complete than for other major components in silicate melts and glasses such as A1203. The equilibria between Fe 3+and Fe 2+involve interaction with oxygen. Redox relations of iron and the structural behavior of Fe 3+and Fe 2§ in silicate melts do, therefore, depend on the structure. Conversely, variations in redox behavior affect the silicate melt structure. In this Chapter, we will first discuss existing structural data relevant to the oxygen coordination polyhedra of Fe 3+ and Fe 2+ before integrating that information with our understanding of silicate melt structure. 11.1. Ferric Iron The similar electric charge and ionic radii of Fe 3+and A13+[Whittaker and Muntus, 1970] have led to the suggestion that the structural position of these two cations is similar in silicate melts and glasses [Waft, 1977; Mysen and Virgo, 1978]. This analogy is not, however, borne out by the roles of Fe 3+ and AI 3+ in silicate crystals. Here, tetrahedrally coordinated A13+is common. However, tetrahedrally coordinated Fe 3+in crystals is rare. In fact, alumino- and ferrisilicate glasses are not structurally similar either. X-ray radial distribution functions of analogous glasses such as NaA1Si308 and NaFeSi308 [Henderson et al., 1984] reveal differences in average bond distances (Fig. 4.11; Table 11.1). The T-O bond lengths (T=A1,Si) in the aluminosilicate glasses [Taylor and Brown, 1979] are shorter than those (T=Fe3§ in ferrisilicate glasses [Henderson et al., 1984]. Bond lengths in NaA1SiO 4 glass, however, are similar to those in NaFeSi308 and KFeSi308 glass (Table 11.1). In other words, with similar T-O distances the proportion of A1 in aluminosilicates is greater than the proportion of Fe 3+ in ferrisilicates. 11. la. Bond Length The Fe3+-O bond lengths in NaFeSi308 and KFeSi30 8glasses are, nevertheless, consistent with Fe 3+ in tetrahedral coordination because a 1.60 * Si-O bond length and a 1.91 * [41Fe3+-O bond length [Brese and O'Keefe, 1991] yield an average bond length of 1.68/k.
336
Chapter 11
Table 11.1 Bonding characteristics of glasses from radial distribution functions [Henderson et al., 1984] Composition
T-O, ~
T-T, ~
1.60
3.10
151
NaA1Si308a NaA1SIzO6 NaA1SiO4a
1.63 1 64 1.67
3.12 3.13 3.17
146 145 143
NaFeSi308 KFeSi308
1.70 1.70
3.20 3.30
140 152
SiO~
9
a
T-O-T angle, degree
"From Taylor and Brown [1979] This distance is, within error, identical to the measured T-O distances (T=Fe,Si) for NaFeSi308 and KFeSi308 glasses [Henderson et al., 1984]. Additional information on Si-O and Fe3+-O bond distances in ferrisilicate glasses has been obtained with neutron diffraction measurements and molecular dynamics simulations along the join (Na20)0.3o(SiO2)0.7-(Na20)0.3.(Fe203)0.7 (Figs. 11.1 and 11.2). In Fig. 11.1, the correlation function for a glass with 13 mol % of the (Na20)0.3e(Fe203)0.7 component is compared with a simulated acmite glass spectrum with Fe 3+constrained to 6-fold oxygen coordination. The Fe-O distance in the glass is considerably shorter (-- 1.91 ,~) than in the simulated acmite glass spectrum (2.05.A,) and is consistent with Fe 3+in 4-fold coordination with oxygen [Holland et al., 1999].
I
I
1
I
=<
acmite /
o,
o
I
o v
0 8
0 ,~
.u0
/
"~ /
f\
J "-~
,/
I\1
\1
J .J~... v .I,,r-
0 C tO
o,
= 9
o, o
,..,..
@ !11,,.. I,.-.
0
0
i12 I03 ~ 2~310
~,~=,0 ~ , ! F e , O ~
I
1
!
2
13 ,,~SiO, ~, 2/0.57
I
1
3
Radial distance, A
Figure 11.1 - Correlation functions from neutron diffraction spectrum of (Na20)03.(Fe203)o.~3.(SiO2)0.57 glass and from simulated acmite with Fe 3§ in 6-fold coordination with oxygen [Holland et al., 1999; Johnson et al., 1999].
Iron-bearing Melts II. Structure
337
23t 2.2
[]
t
[]
[]
Na-O
xx
.<
D
2.0 r
"O m
1.9
9
9
Fe-O
0 I 10
0 I 15
si-o 0 i 20
"13
r
Figure 11.2 - Si-O, Na-O, and Fe-O distances in glasses along the join (Na20)03"(SIO2)0.7- (Na20)03" (Fe203)07 as a function of (NazO)0.3e(Fe203)o.7 content [Holland et al., 1999].
0 i 5
1.6 0
(Na20)o.3*(SiO2)0.7
mol %
(Na20)o.3,(Fe203)0.7
The Fe-O distance in glasses along the (Na20)o.3.(SiO2)o.7-(Na20)o.3o(Fe203)o.Tjoin does, however, decrease with increasing ferric iron content (Fig. 11.2). Extrapolation of these bond distances to lower Fe 3+ concentrations suggests that the Fe3+-O distance may, in fact, approach that of Fe 3§ in coordination polyhedra with more than 4 oxygen. In other words, the structural position of Fe 3+ may depend on the iron content of silicate glasses and melts.
I
I
I
I
0.63
0.32
-O
0.30
Figure 11.3 - Isomer shift of Fe 3+, ISFe3+, relative of Fe metal at 298 K, from MOssbauer spectra of glasses along the nominal joins Na2Si~Os-Fe~O _ _ 3 (open symbols) and SiO2-NaFeO ~ (closed symbols) as a function of iron oxide (Fe~O3) added. Numbers on individual points denote the Fe3+/ZFe of these glasses from MOssbauer spectroscopy [Virgo et al., 1983; Dingwell and Virgo, 1988].
"~ E 0.28 ._ eo~ 0.26 E o
0.81
0.89
O _
9
0 0.82
0.95 0.85
0.24
O
0.95
O
O
1.00
9 0.98
0.22
O
1.00
e 0.20
i 10
i 20
i 30
w t % iron oxide added
i 40
Chapter 11
338 I
I
I
I
I
I
zx
I /k
1.3
(b 1.1
<3
0
E E
0
o
o
0
~~
o-~
[]
Fe2+ [ ] 4-,o,,,
<~ 5-fold 0 6-fold /k 8-fold
0 [] []
-
0.9
0 O?2]oOQ~ 0
%
[]
[] []
(D
E o.7 offl
0.5
eOOg'o"'. 0.3 -
9
t 5-fold
|.' 0.2
I
0.2
Fe3+ _ _ 9 4-fold
o 9
9
0.7
6-fold
w, I
I
1.2
1.7
I 2.2
I 2.7
I 3.2
I 3.7
Figure 11.4 - Isomer shift of Fe 3+ (closed symbols) and Fe 2§ (open symbols) for Fe-bearing silicate crystals [Bums, 1994].
Quadrupole splitting, mm/s
ll. l b. Oxygen Coordination The structural interpretation of neutron and x-ray diffraction data of Fe3+-rich alkali silicate and aluminosilicate glasses (Figs, 4.11, 11.1, and 11.2) is supported by results from 57Fe MOssbauer resonant absorption spectroscopy. 11.~For example, for quenched melts (glasses) along the nominal joins SiO2-NaFeO 2 [Dingwell and Virgo, 1988] and NazSizOs-Fe203 [Virgo et al., 1983], the isomer shift of Fe 3+ (relative to Fe metal), ISFe3+, ranges from about 0.32 to about 0.21 mm/s (Fig. 11.3). ~1.2From the relationships between ISF,3+ and oxygen coordination in crystalline materials (Fig. 11.4), this range is consistent with Fe 3+ in 4-fold coordination. As illustrated in Fig. 11.5, there is, however, a distribution of isomer shift and quadrupole splitting in silicate glasses [Wivel and Morup, 1981; Alberto et al., 1996; Rossano et al., 1999; Galoisy et al., 2000; Wilke et al., 2002]. This distribution indicates that there is a range in Fe3+-O bond distances and O-Fe3+-O bond angles in silicate glasses and melts. Variable Fe3+-O bond distance can also be inferred from changes in ISFe3+ [Jackson et al., 1993; Johnson et al., 1999]. The isomer shifts of Fe 3+ are, for example, negatively correlated with the iron content of highly oxidized glasses (Fig. 11.3) and can be used to
I1 ~The hyperfine parameters (isomer shift and quadrupole splitting) from M0ssbauer spectra depend on the temperature at which a spectrum is obtained [e.g.,Bancroft, 1973; Mitra, 1992]. For the isomer shift of Fe 3§ this dependence is approximately 5o 10.4 mm/s K [Alberto, 1995; Johnson et al., 1999]. Unless otherwise stated in this Chapter, we will refer to hyperfine parameters recorded at room temperature (298 K). ~2 These joins are referred to as "nominal" because, even though the glasses were formed by quenching melts equilibrated with air, there is a small fraction of iron as Fe z+in these samples (see numbers on individual symbols in Fig. 11.3).
339
Iron-bearing Melts II. Structure I
1.2 E E
I
I
~_.~.~
Fe2+
0.8
CO Figure 11.5 - Example of isomer shift and quadrupole splitting distribution in Fe-bearing CaOSiO~ glass [Alberto et al., 1996].
o O.4 E ~~--~f~-
~--~- - -~7~,---..-- ~ ~
0.0 0
Fe3+
I
I
I
1
2
3
z
Quadrupole splitting, mm/s
relate iron content to Fe3+-O distance. This can be accomplished by combining neutron diffraction data of glasses to obtain Fe3+-O bond length and 57Fe resonant absorption spectroscopy for isomer shift. An approximately linear relationship between bond length, dF~3+_o, and ISFe3+is obtained as follows [Johnson et al., 1999]: (11.1)
dFe3+_O-- 1.58 + 1.301SFe3+.
From this relationship, the isomer shift range in Fig. 11.3 corresponds t o riFe3+ - o-values ranging from 1.99/~,, for the lowest iron oxide content (2.2 mol % as Fez9 in Na2Si205 glass, to 1.89 ,~, for the Na2Si205 sample with the highest iron oxide content (13.4 mol %). For glasses along the nominal SiO2-NaFeO 2join (closed symbols in Fig. 11.3), the average bond distance ranges between 1.99 and 1.86 A. In agreement with x-ray and neutron diffraction
I
0.32
9
0.30
.
.
.
.
o
9 9
I= 0.28 ,_ r 0.26 E r
I
9
E
Figure 11.6 - Isomer shift of F e 3§ (relative to Fe metal at 298 K) from MOssbauer spectra of glasses containg 5 wt % iron oxide as Fe,O 3 _ in the systems Na,O-_ A1,O3-SiO~ (NAS) and CaO-AI~O3-SiO~ (CAS). The nominal NBO/T of each Fe-free series is 0.65. Melts were equilibrated with air at 1550~ [Mysen et al., 1985; Mysen and Virgo, 1989].
I
Q
9
9 O O
0.24 0.22
O CAS, Fe3+/EFe=0.66+0.03 9 NAS, Fea+/~Fe=0.78+0.03
0.20
0.10
I
0.20
i
0.30 AI/(AI+Si)
1
0.40
340
Chapter 11 I
I
I
I
>.,, (-. (1) E
-T""
1
12000
....I
Figure 11.7 - Luminescence spectra of glasses (excited with the 21,839 cm -~ line of violet laser) in the systsem Na20-SiO 2 with 0.5 mol % iron oxide added as Fe203 and equilibrated with air at 1375~176 Lower spectrum: Na/Si =1.22; upper spectrum: Na/Si - 5.67 [Fox et al., 1982].
I
14000 16000 18000 Frequency, cm -1
data (Figs. 11.1 and 11.2), the M6ssbauer data are thus consistent with a dependence of the geometry of Fe3+-O polyhedra on iron content. Aluminum content is another variable affecting melt structure (see Chapter 9) and potentially, therefore, the local structure near Fe 3+. This has been observed in alkali aluminosilicate glasses where there is a negative correlation of Al/(Al+Si) with the isomer shift of Fe 3* (Fig. 11.6). For NazO-AlzO3-SiOz-Fe-O glasses in Fig. 11.6, for example, the dF~3+_o,calculated with equation (11.1), varies between 1.93 ,~ and 1.88/k as AI/(AI+Si) increases from --0.1 to --0.4. This decrease may be related to the diminishing T-O bond
I
I
I
I
.--..,
(.(1) (.i/
I\ j - I 800
\
980 k I 900
1 1000
Frequency, cm -1
~ 11 O0
Figure 11.8 - Difference Raman spectrum (heavy line) between Na2Oo 1.5SiO 2 glass (dashed line) and N2Oe1.5SiO2glass with 6.3 mol % iron oxide added as Fe203 formed after equilibration with air at 1375~176 [Fox et al., 1982].
341
Iron-bearing Melts II. Structure
a
I
I
MgSi03
0.35
1.3
I
I
Si03 -
E
c~ 1.2 .=_ *"
E E
7
. i ,
~ r
~-
ffl
0 I 1
I 2
Z/r2,A -2
_
I
t~ 0.25
CaSi03
/
SrSi03
1.0
BaSi03
of f J
1.1
--'~ o
E 0.30 - ~-~"w SrSi03
I
b
E
I 3
9 BaSi03 0.9 I
1
I
2 Z/r2,A"2
I
3
Figure 11.9 - (a) Isomer shift and (b) quadrupole splitting of Fe 3§ (relative to Fe metal at 298 K) from M6ssbauer spectra of several glasses. Each sample contains 5 wt % iron oxide as Fe203. Glasses formed by quenching of melts equilibrated with air at 1550-1625~ [Mysen et al., 1984]. strength with increasing A1/(AI+Si) (Table 4.1) and possibly also to the fact that the Q~-speciation in peralkaline aluminosilicate melts is affected by A1/(AI+Si) [Mysen et al., 2003]. Note, however, that there is no correlation between ISFe3+ and A1/(AI+Si) for equivalent melt compositions in the system CaO-A1203-SiO 2 (Fig. 11.6). Furthermore, the isomer shifts of Fe 3§ are higher in the CAS than in the NAS system, a difference similar to that in Al-free melt and glass systems. This relationship indicates, therefore, that geometric differences (bond length and angle distortions) between FeOa-tetrahedra in Al-free systems remains in Al-bearing glasses and melts. Variable Fe3+-O distances in FeO 4 tetrahedra may, in fact, be because there exist more than one type of tetrahedron in silicate melts and glasses. Different tetrahedra have been documented in silicate glasses and melts. In simple Na20-SiO2-Fe203 glasses, Fox et al. [ 1982] found that Fe 3§ is in tetrahedral coordination from Raman and luminescence spectroscopic data. Ferric iron may exist in more than one type of tetrahedron because there are bands near 14,200 and 16,200 cm -1 in the luminescence spectra (Fig. 11.7) and bands near 890 and 980 cm -1 in the Raman spectra, the intensities of which are correlated with Fe 3§ content. (Fig. 11.8). The relative abundance of these tetrahedra appears to be a function of the Na/Si-ratio [Fox et al., 1982]. The existence of two different FeOa-tetrahedra is also consistent with interpretations of the 57Fe M0ssbauer spectra of glasses in the same system [Burkhard, 2000]. The structural role of ferric iron in metal oxide ferrisilicate glasses depends not only on total ferric iron content, but also on the metal cation. In 57Fe M0ssbauer spectroscopy, both the isomer shift and the quadrupole splitting values of Fe 3§ are positively correlated with the ionization potential of the metal cation, Z/r ~ (Fig.ll.9). For the alkaline earth metasilicate compositions in Fig. 11.9, the ISFe3+of less than --0.3 mm/s (at 298K) for
Chapter 11
342 0.6 + 03
I
I
I
I
I
0.4--
I.I_
Figure 11.10 - Relationship between the proportion of 4- and 6-fold coordinated Fe 3§ and temperature of equilibration for a (CaO)0.40 (SiO2)0.40(Fe203)0.2 (nominal) glass after melt equlibration with air at the temperatures indicated [Nagata and Hayashi, 2001 ].
0.2-+ 03
u..
m
O.O-
v(--
--
-0.2
-
-0.4 5.2
I 5.4
I I 5.6 5.8 104/-!", K-1
I 6.0
I 6.2
BaSiO 3, SrSiO 3, and CaSiO 3 are consistent with Fe 3+ in 4-fold coordination with oxygen, whereas the ISFe3+ (at 298 K) of Fe-bearing MgSiO 3 glass is near the lower end of the range observed in minerals with octahedral Fe 3+ [Burns, 1994]. Another interpretation of the isomer shift trends in Fig. 11.9a is that Fe 3+ exists in both 4-fold and 6-fold coordination in all the melts. The increase of ISFe3+with Z/r 2 of the metal cation could reflect increasing fraction of Fe 3+ in 6-fold coordination, a concept consistent with results from other iron-bearing alkaline earth and mixed alkali/alkaline earth silicate melts [Dingwell and Brearley, 1988; Hannoyer et al., 1992; Nagata and Hayashi, 2001; Burkhard, 2000]. That there may be more than one coordination state of Fe 3+ has also been proposed, for example, for mixed alkali/alkaline earth silicate glasses [Levy et al., 1976; Nagata and H a y a s h i , 2 0 0 1 ] . By u s i n g Mt~ssbauer s p e c t r o s c o p y to c h a r a c t e r i z e a (CaO)o.4o(SiO2)o.4o(Fe203)0.2 glass formed from a melt equilibrated with air between 1608
I
I
0.6
I
I
I 0
E 0.5 E ==..m t'(,O !__
0.4
E 0 r
0.3 I
0.2
I
i
0.4
0.6
Fe3+/EFe
O
J
0.8
Figure 11.11 - Isomer shift of Fe 3§ (relative to Fe metal at 298 K) from M6ssbauer spectra as a function of the Fe3+/ZFe of Na20-A1203-SiO 2 glasses containing 5 wt % iron oxide as Fe203. Melts were equilibrated at 1550~ with air and CO-CO, gas mixtures between 10.0.68and 10.9bar. The Fe-ffee nominal NBO/T of each series is 0.65 [Mysen and Vugo, 1989].
Iron-bearing Melts II. Structure
343
and 1858 K, Nagata and Hayashi [2001] found that the [4]Fe3+/[6lFe3+ ratio is negatively correlated with temperature (Fig. 11.10). The temperature-dependence yields an enthalpy of 68 kJ/mol for this coordination change, but it is not clear whether this transformation is actually governed by temperature, or whether the data of Fig. 11.10 result from the effect of temperature on the iron redox ratio. This interpretation of 57Fe M/3ssbauer data is also consistent with oxygen coordination numbers inferred from Fe3+-O bond length information. If we assume that equation (11.1), derived for alkali silicate glasses, can also be applied to alkaline earth compositions, the IaFe3+ interval in Fig. l l.9a corresponds to dFe3+_oranging from about 1.96 = for Staur01ite _ e2+ Fe 3+ in BaSiO 3 melt to 2.03 A for Fe 3+ in MgSiO 3 melt. From b o n d - v a l e n c e calculations [Brese and O'Keefe, 1991 ], dFe3+_o= 2.03 A would be consistent with Fe 3+ dominantly in 6-fold coordination, whereas the 1.96 --- value is near or slightly above that expected for Fe 3+ in 4-fold coordination. A mixture of 4- and 9 o Fayalite 6-fold coordinated Fe 3+ might yield average dFe3+_o-values in between. In o analogy with other transition metals [e.g., Calas et al., 2002 ], one might also surmise that a 5-fold coordination could exist, but no data currently available appear consistent with such an intermediate coordination state of Fe 3+in I I I I I silicate melts and glasses [Tangeman and Glass, Lange, 1998]. As a matter of fact, a relationship between ISFe3+and Fe3+/EFe shown in Fig. 11.11 suggests that the redox ratio may be an important factor controlling the coordination state of Fe 3+ [Virgo and Mysen, 1985]. For a sodium aluminosilicate, the /SFe3+ increases 7092 7094 7096 7098 71 O0 from about 0.25 mm/s to about 0.58 Photon energy, eV mm/s in the Fe3+/ZFe-range between 0.79 Figure 11.12- Pre-edge from x-ray absoprtion and 0.2 (isomer shift data from 298K spectra of staurolite, fayalite, and a Fe-bearing MOssbauer spectra). This led Virgo and glass [Calas and Petiau, 1983]. Mysen [1985] to propose that iron-
Chapter 11
344
cO 0 (.-
i
i
,.=.= =i==.
i
i
\\
tO
,i.
\\
,m ~== .4,=..
//
"0 I
I
1.8
1.6
Figure 11.13 - Pair distribution function for FeO in a CaFeSi206 glass, quenched from a melt equilibrated atfoz= 10.7bar at 1075 K, from molecular dynamics (MD) simulation and EXAFS analysis [Rossano et al., 2000].
~~k\ klVIDsi~ ulation
I
I
2.0 2.2 Distance, A
2.4
2.6
complexes locally resembling an inverted spinel structure form in these melts and that the relative stability of such complexes is governed by the Fe3+/ZFe of the melt. This structural model is also consistent with magnetic [O'Horo and Levy, 1978] and thermodynamic [Kress and Carmichael, 1988] data. 11.2. Ferrous Iron
In analogy with the crystal chemistry of ferromagnesian silicate crystals, it is often assumed that Fe z+ occupies structural positions similar to Mg in silicate melts and glasses. From this reasoning Fe z+ is a network-modifying cation perhaps in octahedral coordination with oxygen. That assumption, however, is not always supported by experimental data. For example, in an x-ray absorption study of a 2FeOo4MgOo4CaOoSiO 2 glass, Calas and Petiau [ 1983] concluded that the oxygen coordination number around Fe z+ might be closer to 4 than to 6. They suggested that similar x-ray absorption spectra of staurolite, which has 4-fold coordinated Fe z+ [Smith, 1968], and that of the 2FeOo4MgOo4CaOoSiO 2
I
I
I
I
I
I
or c~ ..Q O <
,~
(NaAISi308)o 5(CaMgSi206)o5 I
700
I
I
1
!
1300 1900 Wavelength, nm
I
2500
Figure 11.14- Optical absorption spectra of Fe2§ in an NaA1Si308 glass containing 3.08 wt % FeO and in an (NaA1Si3Os)05(CaMgSi206)05 glass containing 1.98 wt % FeO. Glasses quenched from melts equilibrated at 1200-1500~ with CO,-H2 gas mixture [Keppler, 1992].
Iron-bearing Melts II. Structure
345
glass (Fig. 11.12) are consistent with at least a large fraction o f F e 2+ in 4-fold coordination with oxygen. The energy overlap with the spectrum of fayalite, FezSiO 4 (Fig. 11.12), which has F e z+ in 6-fold coordination [Smyth, 1975], was rationalized as a crystal field effect in the fayalite spectrum. Whether or not there are crystal field effects in the glass is not known. ll.2a. Oxygen Coordination A suggestion by Calas and Petiau [1983] of 4-fold coordinated Fe z+ in an alkaline earth silicate glass has recently gained support from a study that combined x-ray absorption (EXAFS) and molecular dynamics simulation (MD) of a CaFeSi20 6 glass (Fig. 11.13). Here, Rossano et al. [2000] concluded that there are 2 different oxygen-coordination spheres around Fe z+. One is a distorted tetrahedron (4-fold coordination) and the other a trigonal bipyramid (5-fold coordination). Optical absorption spectroscopy is another method suited to examine the structural role of transition metals in silicate melts [e.g., Wong and Angell, 1976]. In Fig. 11.14 are shown the spectra obtained by Keppler [ 1992] for FeZ+-bearing glasses of NaA1Si308+3 wt % FeO and (NaA1Si308)0.5(CaMgSi206)0.5+2 wt % FeO composition. Both spectra are dominated by a broad peak with a maximum near 1100 nm, which can be assigned to F e z+ in 6-fold coordination, as deduced from comparisons with optical absorption spectra of FeZ+-bearing minerals. Similar assignments of F e z+ f r o m optical spectra of other Fe2+-rich glasses have been made by Bell and Mao [1974] and Nolet et al. [1979]. There are, however, two bands in the absorption spectra. The weaker band near 1900 nm in the spectrum of (NaA1Si3Os)0.5(CaMgSi206)05 + 2 wt % FeO glass (Fig. 11.14) could result from transitions in a distorted octahedron or, alternatively, to tetrahedrally c o o r d i n a t e d F e z+. Keppler [ 1992] suggested that the former assignment is the most likely. Calas and Petiau [19831, however, assigned the band near 1900 nm in absorption spectra of other FeZ+-bearing glasses to F e z+ in tetrahedral coordination because the halfwidth of this band is much smaller than that of the 1100 nm band. That notwithstanding, Keppler [1992] concluded that the argument based on halfwidth differences of the 1100 and 1900 cml bands is not necessarily correct because different iron-oxygen bonds exist in strongly distorted polyhedra. Thus, there is no reason why the amplitudes of their vibrations should be similar. The hyperfine parameters (isomer shift and quadrupole splitting) of Fe z+ from 57Fe M6ssbauer resonant absorption spectroscopy have also been be used to distinguish between the possible 4, 5, and 6 oxygen coordination numbers. In crystalline ferromagnesian silicates there is, however, some overlap in the isomer shift values for I41Fe2+ and t61Fe2+ (Fig. 11.4). The interpretation of the isomer shift becomes even more difficult when the possibility of Fe z+ in 5-fold coordination is included [Waychunas et al., 1988]. The variations in hyperfine parameters for 4- and 5-fold coordination of Fe 2§are not necessarily large. The uncertainties associated with interpretation of the M6ssbauer spectra are compounded by the possibility that the values of hyperfine parameters derived from the
346
Chapter 11
d~
,
cO
Figure 11.15 - Example of 2-doublet fit of Lorentzian lines to 57Fe resonant absorption M6ssbauer spectrum of a glass in the system CaO-MgO-FeO-A1203-SiO2formed by quenching a melt equilibrated at 1400~ at fo~_ = 10-6bar. In this fit, the component peaks of the two doublets, AA'and BB', were constrained to have equal area and equal full width at half height [MOssbauer spectrum from Mysen and Dubinsky, 2004].
m
Q. 0 ..Q c~ c c~ 0 (D
n--
I -3.8
-1.9
0.0
1.9
3.8
Velocity, mm/s absorption envelope may depend on the method used to deconvolute the spectra. Broadened Lorentzian curves are often fitted to the spectra of glasses. Furthermore, when dealing with the Fe 2§ absorption doublet of reduced Fe-bearing silicate glasses (Fig. 11.15), the asymmetry of the Fe 2+absorption doublet can result in a fit that is statistically better with two Fe 2§ doublets than with a single one [e.g., Mysen et al., 1985; Dyar et al., 1987; Wang et al., 1993]. Two absorption doublets may imply 2 different Fe2+-O polyhedra in the glasses. In the example in Fig. 11.15, the isomer shifts of 1.1 and 1.25 mm/s from M6ssbauer spectra recorded at 298 K point to Fe 2+in 6-fold coordination with oxygen in both polyhedra. Such a solution is not at all unique, however, as other fits could be made consistent with a splitting of Fe 2+between tetrahedral and octahedral coordination. In the latter case, the high-velocity component would fit at lower velocity for the AA' than for the BB' doublet in Fig. 11.15 [Dyar et al., 1987]. The underlying problem in M6ssbauer spectroscopy of glasses is that rather than modeling the line shape of the absorption envelope, we should model the distribution of the hyperfine field because its distribution is in direct response to the structure around the Fe nucleus. Fitting of the hyperfine parameter distribution is, therefore, a more appropriate approach to the deconvolution of the M0ssbauer absorption envelope [Alberto, 1995; Rossano et al., 1999; Galoisy et al., 2000; Wilke et al., 2002]. In reduced Fe-bearing Casilicate glasses, fitting the Fe 2+ hyperfine parameter distribution (Fig. 11.16) yields maximum ISFe2+between 1.1 and 1.2 mm/s (marked "[6]Fe2+''. 11.16). There is also a smaller intensity maximum near 0.9 mm/s (marked "I41Fe2+". 11.16). Alberto et al. [1996] interpreted the results of Fig. 11.16 (and those obtained for other Ca-silicate glasses) in
347
Iron-bearing Melts II. Structure I
I
I
I
I
1
1.4
Figure 11.16 - Example of 2dimensional fit to hyperfine parameter distribution (isomer shift and quadrupole splitting) of a CaOSiO2 glass (Ca/Si=0.7) with 5 wt % FeO formed by quenching of melt equilibrated at 1550~ atfo2 = 10-9bar. The positions marked NFe2+and [6]Fe2+ indicate the location of maximum in distribution for 4- and 6-fold coordinated ferrous iron [Alberto, 1995].
E 1.2 E r (/) (I)
E o 1.0 CO
0.8 0
1 2 3 Quadrupole splitting, mm/s
terms of Fe 2§ predominantly in 6-fold coordination along with less than 10 % of total Fe 2§ in 4-fold coordination. In summary, the experimental data on the structural role of Fe z§ in silicate glasses are not always conclusive. Interpretations of optical absorption spectra favor Fe 2§ in 6-fold coordination, whereas x-ray absorption techniques commonly have been interpreted in terms of a smaller number of oxygens in the Fe2+-O polyhedra. In general, MOssbauer spectra have been found consistent with predominantly 6-fold coordination of Fe 2§ but even this interpretation is not universal. The difficulty is compounded by the fact that different structural probes have been used to characterize the structural position of Fe 2§ in the same sample in only a few studies.
11.3. Ferric and Ferrous Iron in Silicate Melts at High Temperature Application of glass structure data to Fe-bearing melts relies on the assumption that quenching does not significantly affect the structural position of Fe 3§ or Fe 2§ Resolution of this question requires examination of relations between quenching rate and redox relations or, even better, structural studies of melts at high temperature. There exist some data on redox and structural relations of iron oxides in glass as a function of temperature and quenching rate [Dyar and Birnie, 1984; Dyar et al., 1987]. From superliquidus temperatures to the glass transition, quenching at rates of the order of hundreds of degrees per second do not the affect redox ratio of the original melt. Dyar et al. [ 1987] did suggest, however, that the isomer shift of Fe 2§from M6ssbauer spectroscopy
348
Chapter 11 I
I
I
_
I
I
t.i-i
tO)
i
t
400
i
i
Figure 11.17 - Unpolarized Raman spectra of glass (25~ and supercooled melt (771 ~ of Na2Oe3SiO2o0.5Fe203 [Wang et al., 1993].
F
800 1200 Raman shift, cm -1
may depend on quenching rate although these were not quantified. If so, oxygen coordination polyhedra of Fe might differ in molten and glassy silicates. Diffraction techniques and vibrational spectroscopy have been used to examine the structure of Fe-bearing melts at high temperature, but only for a relatively small number of compositions. An oxidized and a reduced sample of a Fe-bearing sodium trisilicate were i n v e s t i g a t e d by Wang et al. [1993]. From M 6 s s b a u e r analysis of an NazOe3SiO/e0.5Fe/O 3 glass formed from melt equilibrated in air at 77 I~ all iron was found to be oxidized. The Raman spectra of this glass at 25~ and of the supercooled liquid at 771~ show considerable similarity (Fig. 11.17). Wang et al. [1993] concluded that Fe 3§ is in 4-fold coordination in both the glass and the melt. They also observed, however, an intensity growth near 1060-1070 cm -1 in the high-temperature Raman spectrum, but could not determine whether this was due to some changes in Qn-abundance as the temperature was increased across the glass transition or, more simply, to temperaturedependent Raman cross-sections.
I
I
I
I
I r tt3 0
.R
tt3
cM
e-
=
25~
!
400
I
I
800
I
I
1200
Raman shift, cm -1
Figure 11.18 - Unpolarized Raman spectra of glass (25~ and supercooled melt (676~ of NazO.3SiOzeFeO [Wang et al., 1993].
349
Iron-bearing Melts II. Structure I
<
2.1
r-
2.0
I
I
I
Fe-O
._~ E .,9,o
Figure 11.19 - Si-O and Fe-O distances in FeO-SiO~ melts at 1250~176 from x-ray diffraction [Waseda and Toguri, 1978].
1.7
1.6
0 FeO
si-o I
o
o i
o
u J
20
o--o J
40 mol %
SiO 2
By annealing the same material under a hydrogen atmosphere, Wang et al. [1993] obtained a glass of composition Na2Oo3SiO2oFeO with all the iron as FeO. Its NBO/Si would be 1.3 with the assumption that all Fe 2+is a network-modifier. The Raman spectra at room temperature (glass) and at 676~ (supercooled liquid under a hydrogen atmosphere) are plotted in Fig. 11.18. Both spectra are nearly identical to that of Fe-free Na2Ool.5SiO 2 glass, a glass whose NBO/Si is also 1.3 [Furukawa et al., 1981]. Hence, Wang et al. [ 1993] concluded that the Raman spectra of the reduced glass and melt are consistent with Fe 2+ serving in a network-modifying role. Unfortunately, the structural interpretation by Wang et al. [ 1993] conflicts with those of near-edge x-ray absorption (XANES) spectra of glass and melt of a nearly identical composition [Waychunas et al., 1988]. Even though the M0ssbauer spectra of glasses in the latter study are similar to those reported by Wang et al. [ 1993], Waychunas et al. [ 1988] concluded that the XANES spectra of melts at 850~ are best interpreted in terms of Fe 2+ in 4-fold coordination. This conclusion would agree with that of Waseda and Toguri [1978] and Waseda et al. [1980] who found, from x-ray absorption studies of FeO-SiO 2 and FeO-Fe203-SiO z melts, that the Fe-O bond lengths (between 2.04 and 2.08 ,~) are those expected for Fe2+in 4-fold coordination (Fig. 11.19). From the bond valence treatment, of Brese and O'Keefe [ 1991 ], Fe-O bond lengths in this range could, however, be considered consistent with those expected for Fe 2+ in a coordination state higher than 4. Thus, the conclusion that Fe 2§is in 4-fold coordination with oxygen might not be as firm as originally suggested by Waseda and Toguri [1978] and Waseda et al. [1980]. In summary, (i) the Raman data for Na2Oo3SiO2o0.5Fe203 and Na2Oo3SiO2oFeO glasses and melts indicate significantly different roles of Fe 3§ and Fe z+ in the silicate melt structure (Figs. 11.17 and 11.18). (ii) The Raman spectra of Na2Oo3SiO2oFeO glass and melts are similar to Raman spectra of Na2Oo 1.5SIO 2 glass, which suggest that Fe z+ is indeed, a network-modifying cation. (iii) The Fe2+-O bond lengths from the hightemperature FeO-SiO 2 melt x-ray data (Fig. 11.19) are consistent with 6-fold coordinated Fe z+. (iv) In contrast, Fe 3+ is in 4-fold coordination in oxidized melts and glasses.
Chapter 11
350
11.4. Iron in Silicate Melts and Glasses at High Pressure
0.8
,,o
0.6 O.4
u..
0.2 0.0 0 E E
1
2
3
i
i
i
4 I
0.9
b-
.=_ 9"='-
0.7
t~
.9.0 o
0.5
"o
O
E E
0.3 0
1
2
I 3
0.6 -
~
~
u ~
I
1
4 _
0.5 0.4
t,-
~-
0.3
E o
0.2
C
0
1
I
I
2 3 Pressure, GPa
4
Figure 11.20 - (a) Redox ratio of iron, (b) quadrupole splitting, and (c) and isomer shift of Fe3§from MOssbauer spectra (relative to Fe metal at 298 K) of Na2Si205 + 7.5 mol % (nominal) Fe~_O3quenched from 1400~ at the pressures indicated [Mysen and Virgo, 1985].
The similar ionic radii of Fe 3+ and A13+ and their similar formal electrical charge have led to suggestions in early studies that Fe 3+ might be used as a proxy for A13+ to investigate the effect of pressure on the structural role of A13+in silicate melts [Waft, 1977]. On this basis, Mysen and Virgo [1978] examined the influence of pressure on melts along the ( n o m i n a l ) j o i n NaA1Si206-NaFe23+Si206 with the aid of MOssbauer spectroscopy. They found, however, that the main effect of pressure to several GPa pressure was to reduce Fe 3+ to Fe 2+. The redox ration of iron likely depends on pressure because the mollar volumes of FeO and Fe203 differ (see section 10.4b). This effect was investigated further by Mysen and Virgo [1985] with melts along the nominal join NazSizOs-Fe203. They reported 57Fe M6ssbauer spectroscopic data of glasses f o r m e d by t e m p e r a t u r e quenching melts in equilibrium with air at pressures of up to 4 GPa. They not only observed that Fe3+/ZFe decreases with increasing pressure (Fig. 11.20a), but also that the hyperfine parameters of Fe 3+ are pressure sensitive (Fig.11.20b and c). The rapid increase in isomer shift from about 0.3 mm/s, at ambient pressure and 298 K, to slightly less than 0.6 mm/s for glasses f o r m e d at 3 and 4 GPa points to a transformation of Fe 3+ from tetrahedral to perhaps octahedral coordination. It was suggested that this coordination change was in response to decreasing Fe3+/ZFe as the
Iron-bearing Melts II. Structure
351
pressure increased. There is no evidence in the M0ssbauer spectra of these glasses to suggest that the structural role of Fe 2+ varies with pressure.
11.5. Redox Relations and Melt Polymerization Reduction of network-forming Fe 3+ to network-modifying Fe 2+ may be expressed with the equation [Holmquist, 1966]: 4 FeO 2- r
4 Fe 2+ +
0 2 q-
6 0 2-.
(11.2)
In this equation, the oxygen anion, 0 2-, is the link between the redox reaction and the silicate network via a schematic reaction of the type: 2 Q4 + O2- r
2 Q3.
(11.3)
In equation (11.2), the NBO/Si of Q4 structural units equals 0, and that of Q3 equals 1. Reduction of tetrahedral Fe 3§to octahedral Fe 2+does, therefore, result in depolymerization of the melt: 4 FeO2- + 12 Q4 r
12 Q3 + 4 Fe 2+ + 02.
(11.4)
It follows from equation (11.4) that parameters affecting the iron redox ratio also influence melt polymerization and NBO/T (Fig. 11.21). By increasing the abundance of Fe203 in a silicate melt at ambient pressure, for example, silicate polymerization increases (Fig. 11.21 a). The Fe3+/ZFe ratio can also control the silicate polymerization. This ratio, in turn, varies with temperature, pressure, oxygen fugacity, and melt composition. These parameters will, therefore, also affect polymerization of Fe-bearing silicate melts. At constant iron content and oxygen fugacity, increasing temperature results in decreasing Fe3+/ZFe (see Chapter 10). Increasing temperature will, therefore, cause equilibria (11.3) and (11.5) to shift to the right thus depolymerizing Fe3+-containing silicate melts (Fig. ll.21b). Two trends are shown, however, in Fig. ll.21a. 11.3 The dashed line is NBO/T trajectory with Fe 3+in 4-fold coordination at all temperatures ([4lFe3+).The "actual" trend depicts gradual transformation of"t41Fe3+ to [6lEe3+ due to increasing temperature and decrease in Fe3+/ZFe, which, in turn, induces the coordination change of ferric iron [Mysen and Virgo, 1989]. As a result of the coordination transformation of Fe 3+,
113The melt in Fig. ll.21B is a peralkaline Na20-A1203-SiO2melt with A1/(AI+Si) = 0.334 and Na/(Al+Si)=0.65.
Chapter 11
352 I
1.00
I
I
a
I
-
I
I
I
I
b
actual
0.95 0.7 O
//////j
0.90
133
Z
0.85 0.5 0.80
0
I
I
I
4
8
12
k
I
I
I
I
I
I
1600
Temperature, ~
Iron added as Fe20 3, mol % I
I
1400
1200
/
I
I
I
I
I
actual 1.2 F
d
actual 0.7
~
t
~
""
""
--
"" 1
1
1
m Z
1.0
/
0.6
-
0.9 1
0
1
2
3
Pressure, GPa
4
0
!
-4
!
I
I
-8 -log fo2 (bar)
Figure 11.21 - Relationships between degree of polymerization of silicate network, NBO/T, and structure and redox relations of iron. (a) NazSi2Os+Fe203 (nominal mol % as indicated) glass quenched from melt equilibrated at 1400~ with air [Mysen and Virgo, 1985]. (b) Sodium aluminosilicate glass with AI/(AI+Si) -- 0.334, Na/(Na+A1) = 0.65 and 5 wt % iron oxide added as FezO3 quenched from melt equilibrated with air at the temperatures indicated [Mysen and Virgo, 1989]. (c) Glass of nominal composition Na2Si205+7.5 mol % Fe203, quenched from 1400~ at the pressure indicated [Mysen and Virgo, 1985]. (d) Same glass as in (b) quenched from melt equilibrated with CO-CO 2 gas mixture to control fo 2 at 1550~ [Mysen and Virgo, 1989].
depolymerization of the melt occurs. This depolymerization can be described by the reaction:
[41FeO2- + 2 Q4 r
[6lFe3+ + 2 Q3.
(11.5)
Iron-bearing Melts II. Structure
353
P r e s s u r e increases also cause r e d u c t i o n of F e 3+ to F e 2+ and, therefore, depolymerization of the melt structure. For NazSi205 melt with 7.5 mol % iron oxide added (Fig. 11.2 lc), the NBO/T is more sensitive to pressure ("actual" curve) than expected if the coordination state of Fe 3+were not affected by pressure (dashed line in Fig. 11.21 c). In the latter case, pressure-induced melt depolymerization is that described by equation (11.4) only. However, as pressure can also induce coordination transformation of Fe 3+ from 4-fold to 6-fold, equations (11.4) and (11.5) describe the circumstances. The resulting NBO/T trajectory is marked "actual" in Fig. 11.2 lc. Decreasing oxygen fugacity affects the degree of polymerization in the same manner as pressure and temperature (Fig. 11.21 d). For the melt composition in Fig. 11.21 d, the two depolymerization curves ("actual" and "[41Fe3+") meet at 1550~ at an oxygen fugacity between 10.8 and 10 -9 bar, where all iron is Fe 2+ [Mysen and Virgo, 1989]. Changes in melt polymerization, NBO/T, obviously also affect Q" speciation, but experimental data are lacking to determine the relationships between iron content, redox ratio of iron, structural roles of Fe 2+and Fe 3+, and Qn speciation. These relationships will depend on whether tetrahedra!ly coordinated Fe 3+substitutes for SP + in the structure or if it forms isolated complexes. In the former case, partitioning of Fe 3+ between individual Qn-species may affect speciation in a manner conceptually similar to that observed for AI 3+ in peralkaline aluminosilicate melts (Fig. 9.25), thus, possibly driving the general Qn-speciation reaction (Stebbins, 1987), 2 Qn r
Qn+l + Qn-1,
(11.6)
to the right. The situation is, however, likely more complicated because Fe 2+, with its large ionization potential, tends to favor bonding with oxygen in the least polymerized of available Qn-species [Q,-I in equation (11.6)]. This tendency would tend to drive equilibrium to the right with increasing pressure for Fe-bearing silicate melts.
11.5. Summary Remarks 1. The structural role of ferric iron in Fe3+-rich silicate melts and glasses is mainly that of a network-former. It depends, however, on the redox ratio of iron so that Fe 3+ may be a network-modifying cation in melts with low Fe3+/EFe. 2. The oxygen coordination number around Fe 2+in melts is less clear. All evidence taken together indicates, however, that ferrous iron is dominantly a network-modifier, but it is possible that the structural role of Fe 2+depends on silicate composition. 3. Given the above premises, increasing ferric iron content of a silicate melt induces melt polymerization, whereas increasing Fe 2+probably causes depolymerization. Thus, any variable affecting the redox state of iron (pressure, temperature, bulk composition, total iron content, and oxygen fugacity) will also affect polymerization and Qn-speciation of the melt.
354
Chapter 11
References
Alberto H. V. (1995) Structural characterization of CaO-SiO2-TiO2-Fe-O glasses by Raman and MOssbauer spectroscopy. Ph.D., Univ. Coimbra. Alberto H. V., Pinto da Cunha J. L., Mysen B. O., Gil J. M., and de Campos N. A. (1996) Analysis of MOssbauer spectra of silicate glasses using a two-dimensional Gaussian distribution of hyperfine parameters. J. Non-Cryst. Solids 194, 48-57. Bancroft G. M. (1973) M6ssbauer Spectroscopy: An Introduction for Inorganic Chemists and Geochemists. McGraw Hill. New York. Bell E M. and Mao H. K. (1974) Crystal-field spectra of Fe z+ and Fe 3§ in synthetic basalt glass as a function of oxygen fugacity. Carnegie Instn. Wash. Year Book 73, 496-497. Brese N. E. and O'Keefe M. (1991) Bond-valence parameters for solids. Acta Cryst. B 47, 192-197. Burkhard D. J. M. (2000) Iron-bearing silicate glasses at ambient conditions. J. Non-Cryst. Solids 275, 175-188. Burns R. G. (1994) Mineral M0ssbauer spectroscopy: Correlations between chemical shift and quadrupole splitting parameters. Hyperfine Interact. 91, 739-745. Calas G. and Petiau J. (1983) Coordination state of iron in oxide glasses through high-resolution K-edge spectra: Information from pre-edge. Solid State Comm. 48, 625-629. Calas G., Cormier L., Galoisy L., and Jollivet P. (2002) Structure-property relationships in multicomponent oxide glasses. C. R. Chimie 5, 831-843. Dingwell D. B. and Brearley M. (1988) Melt densities in the CaO-FeO-Fe203-SiO 2 system and the compositional dependence of the partial molar volume of ferric iron in silicate melts. Geochim. Cosmochim. Acta 52, 2815-2825. Dingwell D. B. and Virgo D. (1988) Viscosities of melts in the Na20-FeO-Fe~O3-SiO~- systems and factors controlling relative viscosities in fully polymerized melts. Geochim. Cosmochim. Acta 52, 395-404. Dyar M. D. and Birnie D. E (1984) Quench media effects on iron partitioning in a lunar glass. J. Non-Cryst. Solids 67, 397-411. Dyar M. D., Naney M. T., and Swanson S. E. (1987) Effect of quench methods on Fe3+/Fe2*ratios: A MOssbauer and wet chemical study. Amen Mineral. 72, 792-800. Fox K. E., Furukawa Y., and White W. B. (1982) Transition metal ions in silicate melts. Part 2. Iron in sodium silicate glasses. Phys. Chem. Glasses 23, 169-178. Furukawa T., Fox K. E., and White W. B. (1981) Raman spectroscopic investigation of the structure of silicate glasses. III. Raman intensities and structural units in sodium silicate glasses. J. Chem. Phys. 153, 3226-3237. Galoisy L., Cormier L., Rossano S., Ramos A., Calas G., Gaskell E, and Le Grand M. (2000) Cationic ordering in oxide glasses: The example of transition elements. Mineral. Mag. 64, 409-424. Hannoyer B., Lenglet M., Dtirr J., and Cortes J. (1992) Spectroscopic evidence of octahedral iron (III) in soda-lime silicate glasses and crystals. J. Non-Cryst. Solids 151, 209-216. Henderson G. S., Fleet M. E., and Bancroft G. M. (1984) An X-ray scattering study of vitreous KFeSi308 and NaFeSi308 and reinvestigation of vitreous S i O 2 using quasicrystalline modeling. J. Non-Cryst. Solids 68, 333-349. Holland D., Mekki A., Gee I. A., McConville C. F., Johnson J. A., Johnson C. E., Appleyard E, and Thomas M. (1999) The structure of sodium iron silicate glass - a multi-technique approach. J. Non-Cryst. Solids 253, 192-202. Holmquist S. (1966) Ionic formulation of redox equilibria in glass melts. J. Amer. Ceram. Soc. 49, 228-229.
Iron-bearing Melts II. Structure
355
Jackson W. E., Mustre de Leon J., Brown G. E., Waychunas G. A., Conradson S. D., and Combes J.-M. (1993) High-temperature XAS study of Fe2SiO4 liquid: Reduced coordination of ferrous iron. Science 262, 229-232. Johnson J. A., Johnson C. E., Holland D., Mekki A., Appleyard R, and Thomas M. E (1999) Transition metal ions in ternary sodium silicate glasses: A Mrssbauer and neutron study. J. Non-Cryst. Solids 246, 104-114. Keppler H. (1992) Crystal field spectra and geochemistry of transition metal ions in silicate melts and glasses. Amen. Mineral. 77, 62-75. Kress V. C. and Carmichael I. S. E. (1988) Stoichiometry of the iron oxidation reaction in silicate melts. Amer. Mineral. 73, 1267-1274. Levy R. A., Lupis C. H., and Flinn E A. (1976) Mrssbauer analysis of the valence and coordination of iron cations in SiO2-Na20-CaO glasses. Phys. Chem. Glasses 17, 94-103. Mitra A. (1992) Applied MOssbauer Spectroscopy. Theory and Practice for Geochemists and Archaeologists, Pergamon Press. New York. Mysen B. O. and Virgo D. (1978) Influence of pressure, temperature, and bulk composition on melt structures in the system NaA1Si206-NaFe3§ . Amer. J. Sci. 278, 1307-1322. Mysen B. O. and Virgo D. (1985) Iron-bearing silicate melts: relations between pressure and redox equilibria. Phys. Chem. Minerals 12, 191-200. Mysen B. O. and Virgo D. (1989) Redox equilibria, structure, and properties of Fe-bearing aluminosilicate melts: Relationships between temperature, composition, and oxygen fugacity in the system Na20-A1203-SiOz-Fe-O. Amer. Mineral. 74, 58-76. Mysen B. O. and Dubinsky E. (2004) Mineral/melt element partitioning and melt structure. Geochim. Cosmochim. Acta 68, 1617-1634. Mysen B. O., Virgo D., and Seifert E A. (1984) Redox equilibria of iron in alkaline earth silicate melts: Relationships between melt structure, oxygen fugacity, temperature and properties of iron-bearing silicate liquids. Amer. Mineral. 69, 834-848. Mysen B. O., Virgo D., Neumann E. R., and Seifert E A. (1985) Redox equilibria and the structural states of ferric and ferrous iron in melts in the system CaO-MgO-AlzO3-SiO2: Relations between redox equilibria, melt structure and liquidus phase equilibria. Amer. Mineral. 70, 317-322. Mysen B. O., Lucier A., and Cody G. D. (2003) The structural behavior of A1§ in peralkaline melts and glasses in the system Na20-A1203-SiO 2. Amer. Mineral. 88, 1668-1678. Nagata K. and Hayashi M. (2001) Structural relaxation of silicate melts containing iron oxide. J. Non-Cryst. Solids 282, 1-6. Nolet D. A., Burns R. G., Flamm S. L., and Besancon J. R. (1979) Spectra of Fe-Ti silicate glasses: Implications to remote-sensing of planetary surfaces. Proc. lOth ~ Planet. Sci. Conf., 1775-1786. O'Horo M. P. and Levy R. A. (1978) Effect of melt atmosphere on the magnetic properties of a [(SIO2)45 (CaO)5516s [Fe203135glass. J. Appl. Phys. 49, 1635-1637. Rossano S., Balan E., Morin G., Bauer J.-P., Calas G., and Brouder C. (1999) 57Fe Mrssbauer spectroscopy of tektites. Phys. Chem. Mineral. 26, 530-538. Rossano S., Ramos A. Y., and Delaye J. M. (2000) Environment of ferrous iron in CaFeSi206 glass: Contributions of EXAFS and molecular dynamics. J. Non-Cryst. Solids 273, 48-52. Smith J. V. (1968) The crystal structure of staurolite. Amer. Mineral. 53, 1139-1155. Smyth J. R. (197 5) High temperature crystal chemistry of fayalite. Amer Mineral. 60, 1092-1097. Stebbins J. E (1987) Identification of multiple structural species in silicate glasses by 295i NMR. Nature 330, 465-467. Tangeman J. A. and Lange R. A. (1998) The effect of AP +, Fe 3§ and T i4§ on the configurational heat capacities of sodium silicate liquids. Phys. Chem. Minerals 26, 83-99.
356
Chapter 11
Taylor M. and Brown G. E. (1979) Structure of mineral glasses. II. The SiO2-NaA1SiO 4 join. Geochim. Cosmochim. Acta 43, 1467-1475. Virgo D. and Mysen B. O. (1985) The structural state of iron in oxidized vs. reduced glasses at 1 atm: A 57Fe M6ssbauer study. Phys. Chem. Minerals 12, 65-76. Virgo D., Mysen B. O., and Danckwerth R D. (1983) The coordination of Fe 3§ in oxidized versus reduced sodium aluminosilicate glasses: A 57Fe M6ssbauer study. Carnegie Instn. Washington Year Book 82, 309-313. Waft H. S. (1977) The structural role of ferric iron in silicate melts. Can. Mineral. 15, 198-199. Wang Z., Cooney T. F., and Sharma S. K. (1993) High temperature structural investigation of Na2Oe0.5Fe203e3SiO2 and Na2OeFeOe3SiO 2 melts and glasses. Contrib. Mineral. Petrol. 115, 112-122. Waseda Y. and Toguri J. M. (1978) The structure of the molten FeO-SiO 2 system. Metall. Trans. B. 9, 595-601. Waseda Y., Shiraishi Y., and Toguri J. M. (1980) The structure of the molten FeO-Fe203-SiO2 system by X-ray diffraction. Trans. Jap. Inst. Metall. 21, 51-62. Waychunas G. A., Brown G. E., Ponader C. W., and Jackson W. E. (1988) Evidence from x-ray absorption for network-forming Fe 2§ in molten alkali silicates. Nature 332, 251-253. Whittaker E. J. W. and Muntus R. (1970) Ionic radii for use in geochemistry. Geochim. Cosmochim. Acta 34, 945-957. Wilke M., Behrens H., Burkhard D. J. M., and Rossano S. (2002) The oxidation state of iron in silicic melt at 500 MPa water pressure. Earth Planet. Sci. Lett. 189, 55-67. Wivel C. and Morup S. (1981) Improved computational procedure for evaluation of overlapping hyperfine parameter distributions in M6ssbauer spectra. J. Phys. E. Sci. Instr. 14, 605-610. Wong J. and Angell C. A. (1976) Glass Structure by Spectroscopy. Marcel Dekker, New York.
Chapter II
Iron-bearing Melts I1. Structure Iron oxides are major components in natural and industrial silicate melts and glasses. It is the only major component existing in both di- and trivalent form under most conditions. Despite their importance and abundance, however, our knowledge of the structural behavior of iron oxides is less complete than for other major components in silicate melts and glasses such as A1203. The equilibria between Fe 3+and Fe 2+involve interaction with oxygen. Redox relations of iron and the structural behavior of Fe 3+and Fe 2§ in silicate melts do, therefore, depend on the structure. Conversely, variations in redox behavior affect the silicate melt structure. In this Chapter, we will first discuss existing structural data relevant to the oxygen coordination polyhedra of Fe 3+ and Fe 2+ before integrating that information with our understanding of silicate melt structure. 11.1. Ferric Iron The similar electric charge and ionic radii of Fe 3+and A13+[Whittaker and Muntus, 1970] have led to the suggestion that the structural position of these two cations is similar in silicate melts and glasses [Waft, 1977; Mysen and Virgo, 1978]. This analogy is not, however, borne out by the roles of Fe 3+ and AI 3+ in silicate crystals. Here, tetrahedrally coordinated A13+is common. However, tetrahedrally coordinated Fe 3+in crystals is rare. In fact, alumino- and ferrisilicate glasses are not structurally similar either. X-ray radial distribution functions of analogous glasses such as NaA1Si308 and NaFeSi308 [Henderson et al., 1984] reveal differences in average bond distances (Fig. 4.11; Table 11.1). The T-O bond lengths (T=A1,Si) in the aluminosilicate glasses [Taylor and Brown, 1979] are shorter than those (T=Fe3§ in ferrisilicate glasses [Henderson et al., 1984]. Bond lengths in NaA1SiO 4 glass, however, are similar to those in NaFeSi308 and KFeSi308 glass (Table 11.1). In other words, with similar T-O distances the proportion of A1 in aluminosilicates is greater than the proportion of Fe 3+ in ferrisilicates. 11. la. Bond Length The Fe3+-O bond lengths in NaFeSi308 and KFeSi30 8glasses are, nevertheless, consistent with Fe 3+ in tetrahedral coordination because a 1.60 * Si-O bond length and a 1.91 * [41Fe3+-O bond length [Brese and O'Keefe, 1991] yield an average bond length of 1.68/k.
336
Chapter 11
Table 11.1 Bonding characteristics of glasses from radial distribution functions [Henderson et al., 1984] Composition
T-O, ~
T-T, ~
1.60
3.10
151
NaA1Si308a NaA1SIzO6 NaA1SiO4a
1.63 1 64 1.67
3.12 3.13 3.17
146 145 143
NaFeSi308 KFeSi308
1.70 1.70
3.20 3.30
140 152
SiO~
9
a
T-O-T angle, degree
"From Taylor and Brown [1979] This distance is, within error, identical to the measured T-O distances (T=Fe,Si) for NaFeSi308 and KFeSi308 glasses [Henderson et al., 1984]. Additional information on Si-O and Fe3+-O bond distances in ferrisilicate glasses has been obtained with neutron diffraction measurements and molecular dynamics simulations along the join (Na20)0.3o(SiO2)0.7-(Na20)0.3.(Fe203)0.7 (Figs. 11.1 and 11.2). In Fig. 11.1, the correlation function for a glass with 13 mol % of the (Na20)0.3e(Fe203)0.7 component is compared with a simulated acmite glass spectrum with Fe 3+constrained to 6-fold oxygen coordination. The Fe-O distance in the glass is considerably shorter (-- 1.91 ,~) than in the simulated acmite glass spectrum (2.05.A,) and is consistent with Fe 3+in 4-fold coordination with oxygen [Holland et al., 1999].
I
I
1
I
=<
acmite /
o,
o
I
o v
0 8
0 ,~
.u0
/
"~ /
f\
J "-~
,/
I\1
\1
J .J~... v .I,,r-
0 C tO
o,
= 9
o, o
,..,..
@ !11,,.. I,.-.
0
0
i12 I03 ~ 2~310
~,~=,0 ~ , ! F e , O ~
I
1
!
2
13 ,,~SiO, ~, 2/0.57
I
1
3
Radial distance, A
Figure 11.1 - Correlation functions from neutron diffraction spectrum of (Na20)03.(Fe203)o.~3.(SiO2)0.57 glass and from simulated acmite with Fe 3§ in 6-fold coordination with oxygen [Holland et al., 1999; Johnson et al., 1999].
Iron-bearing Melts II. Structure
337
23t 2.2
[]
t
[]
[]
Na-O
xx
.<
D
2.0 r
"O m
1.9
9
9
Fe-O
0 I 10
0 I 15
si-o 0 i 20
"13
r
Figure 11.2 - Si-O, Na-O, and Fe-O distances in glasses along the join (Na20)03"(SIO2)0.7- (Na20)03" (Fe203)07 as a function of (NazO)0.3e(Fe203)o.7 content [Holland et al., 1999].
0 i 5
1.6 0
(Na20)o.3*(SiO2)0.7
mol %
(Na20)o.3,(Fe203)0.7
The Fe-O distance in glasses along the (Na20)o.3.(SiO2)o.7-(Na20)o.3o(Fe203)o.Tjoin does, however, decrease with increasing ferric iron content (Fig. 11.2). Extrapolation of these bond distances to lower Fe 3+ concentrations suggests that the Fe3+-O distance may, in fact, approach that of Fe 3§ in coordination polyhedra with more than 4 oxygen. In other words, the structural position of Fe 3+ may depend on the iron content of silicate glasses and melts.
I
I
I
I
0.63
0.32
-O
0.30
Figure 11.3 - Isomer shift of Fe 3+, ISFe3+, relative of Fe metal at 298 K, from MOssbauer spectra of glasses along the nominal joins Na2Si~Os-Fe~O _ _ 3 (open symbols) and SiO2-NaFeO ~ (closed symbols) as a function of iron oxide (Fe~O3) added. Numbers on individual points denote the Fe3+/ZFe of these glasses from MOssbauer spectroscopy [Virgo et al., 1983; Dingwell and Virgo, 1988].
"~ E 0.28 ._ eo~ 0.26 E o
0.81
0.89
O _
9
0 0.82
0.95 0.85
0.24
O
0.95
O
O
1.00
9 0.98
0.22
O
1.00
e 0.20
i 10
i 20
i 30
w t % iron oxide added
i 40
Chapter 11
338 I
I
I
I
I
I
zx
I /k
1.3
(b 1.1
<3
0
E E
0
o
o
0
~~
o-~
[]
Fe2+ [ ] 4-,o,,,
<~ 5-fold 0 6-fold /k 8-fold
0 [] []
-
0.9
0 O?2]oOQ~ 0
%
[]
[] []
(D
E o.7 offl
0.5
eOOg'o"'. 0.3 -
9
t 5-fold
|.' 0.2
I
0.2
Fe3+ _ _ 9 4-fold
o 9
9
0.7
6-fold
w, I
I
1.2
1.7
I 2.2
I 2.7
I 3.2
I 3.7
Figure 11.4 - Isomer shift of Fe 3+ (closed symbols) and Fe 2§ (open symbols) for Fe-bearing silicate crystals [Bums, 1994].
Quadrupole splitting, mm/s
ll. l b. Oxygen Coordination The structural interpretation of neutron and x-ray diffraction data of Fe3+-rich alkali silicate and aluminosilicate glasses (Figs, 4.11, 11.1, and 11.2) is supported by results from 57Fe MOssbauer resonant absorption spectroscopy. 11.~For example, for quenched melts (glasses) along the nominal joins SiO2-NaFeO 2 [Dingwell and Virgo, 1988] and NazSizOs-Fe203 [Virgo et al., 1983], the isomer shift of Fe 3+ (relative to Fe metal), ISFe3+, ranges from about 0.32 to about 0.21 mm/s (Fig. 11.3). ~1.2From the relationships between ISF,3+ and oxygen coordination in crystalline materials (Fig. 11.4), this range is consistent with Fe 3+ in 4-fold coordination. As illustrated in Fig. 11.5, there is, however, a distribution of isomer shift and quadrupole splitting in silicate glasses [Wivel and Morup, 1981; Alberto et al., 1996; Rossano et al., 1999; Galoisy et al., 2000; Wilke et al., 2002]. This distribution indicates that there is a range in Fe3+-O bond distances and O-Fe3+-O bond angles in silicate glasses and melts. Variable Fe3+-O bond distance can also be inferred from changes in ISFe3+ [Jackson et al., 1993; Johnson et al., 1999]. The isomer shifts of Fe 3+ are, for example, negatively correlated with the iron content of highly oxidized glasses (Fig. 11.3) and can be used to
I1 ~The hyperfine parameters (isomer shift and quadrupole splitting) from M0ssbauer spectra depend on the temperature at which a spectrum is obtained [e.g.,Bancroft, 1973; Mitra, 1992]. For the isomer shift of Fe 3§ this dependence is approximately 5o 10.4 mm/s K [Alberto, 1995; Johnson et al., 1999]. Unless otherwise stated in this Chapter, we will refer to hyperfine parameters recorded at room temperature (298 K). ~2 These joins are referred to as "nominal" because, even though the glasses were formed by quenching melts equilibrated with air, there is a small fraction of iron as Fe z+in these samples (see numbers on individual symbols in Fig. 11.3).
339
Iron-bearing Melts II. Structure I
1.2 E E
I
I
~_.~.~
Fe2+
0.8
CO Figure 11.5 - Example of isomer shift and quadrupole splitting distribution in Fe-bearing CaOSiO~ glass [Alberto et al., 1996].
o O.4 E ~~--~f~-
~--~- - -~7~,---..-- ~ ~
0.0 0
Fe3+
I
I
I
1
2
3
z
Quadrupole splitting, mm/s
relate iron content to Fe3+-O distance. This can be accomplished by combining neutron diffraction data of glasses to obtain Fe3+-O bond length and 57Fe resonant absorption spectroscopy for isomer shift. An approximately linear relationship between bond length, dF~3+_o, and ISFe3+is obtained as follows [Johnson et al., 1999]: (11.1)
dFe3+_O-- 1.58 + 1.301SFe3+.
From this relationship, the isomer shift range in Fig. 11.3 corresponds t o riFe3+ - o-values ranging from 1.99/~,, for the lowest iron oxide content (2.2 mol % as Fez9 in Na2Si205 glass, to 1.89 ,~, for the Na2Si205 sample with the highest iron oxide content (13.4 mol %). For glasses along the nominal SiO2-NaFeO 2join (closed symbols in Fig. 11.3), the average bond distance ranges between 1.99 and 1.86 A. In agreement with x-ray and neutron diffraction
I
0.32
9
0.30
.
.
.
.
o
9 9
I= 0.28 ,_ r 0.26 E r
I
9
E
Figure 11.6 - Isomer shift of F e 3§ (relative to Fe metal at 298 K) from MOssbauer spectra of glasses containg 5 wt % iron oxide as Fe,O 3 _ in the systems Na,O-_ A1,O3-SiO~ (NAS) and CaO-AI~O3-SiO~ (CAS). The nominal NBO/T of each Fe-free series is 0.65. Melts were equilibrated with air at 1550~ [Mysen et al., 1985; Mysen and Virgo, 1989].
I
Q
9
9 O O
0.24 0.22
O CAS, Fe3+/EFe=0.66+0.03 9 NAS, Fea+/~Fe=0.78+0.03
0.20
0.10
I
0.20
i
0.30 AI/(AI+Si)
1
0.40
340
Chapter 11 I
I
I
I
>.,, (-. (1) E
-T""
1
12000
....I
Figure 11.7 - Luminescence spectra of glasses (excited with the 21,839 cm -~ line of violet laser) in the systsem Na20-SiO 2 with 0.5 mol % iron oxide added as Fe203 and equilibrated with air at 1375~176 Lower spectrum: Na/Si =1.22; upper spectrum: Na/Si - 5.67 [Fox et al., 1982].
I
14000 16000 18000 Frequency, cm -1
data (Figs. 11.1 and 11.2), the M6ssbauer data are thus consistent with a dependence of the geometry of Fe3+-O polyhedra on iron content. Aluminum content is another variable affecting melt structure (see Chapter 9) and potentially, therefore, the local structure near Fe 3+. This has been observed in alkali aluminosilicate glasses where there is a negative correlation of Al/(Al+Si) with the isomer shift of Fe 3* (Fig. 11.6). For NazO-AlzO3-SiOz-Fe-O glasses in Fig. 11.6, for example, the dF~3+_o,calculated with equation (11.1), varies between 1.93 ,~ and 1.88/k as AI/(AI+Si) increases from --0.1 to --0.4. This decrease may be related to the diminishing T-O bond
I
I
I
I
.--..,
(.(1) (.i/
I\ j - I 800
\
980 k I 900
1 1000
Frequency, cm -1
~ 11 O0
Figure 11.8 - Difference Raman spectrum (heavy line) between Na2Oo 1.5SiO 2 glass (dashed line) and N2Oe1.5SiO2glass with 6.3 mol % iron oxide added as Fe203 formed after equilibration with air at 1375~176 [Fox et al., 1982].
341
Iron-bearing Melts II. Structure
a
I
I
MgSi03
0.35
1.3
I
I
Si03 -
E
c~ 1.2 .=_ *"
E E
7
. i ,
~ r
~-
ffl
0 I 1
I 2
Z/r2,A -2
_
I
t~ 0.25
CaSi03
/
SrSi03
1.0
BaSi03
of f J
1.1
--'~ o
E 0.30 - ~-~"w SrSi03
I
b
E
I 3
9 BaSi03 0.9 I
1
I
2 Z/r2,A"2
I
3
Figure 11.9 - (a) Isomer shift and (b) quadrupole splitting of Fe 3§ (relative to Fe metal at 298 K) from M6ssbauer spectra of several glasses. Each sample contains 5 wt % iron oxide as Fe203. Glasses formed by quenching of melts equilibrated with air at 1550-1625~ [Mysen et al., 1984]. strength with increasing A1/(AI+Si) (Table 4.1) and possibly also to the fact that the Q~-speciation in peralkaline aluminosilicate melts is affected by A1/(AI+Si) [Mysen et al., 2003]. Note, however, that there is no correlation between ISFe3+ and A1/(AI+Si) for equivalent melt compositions in the system CaO-A1203-SiO 2 (Fig. 11.6). Furthermore, the isomer shifts of Fe 3§ are higher in the CAS than in the NAS system, a difference similar to that in Al-free melt and glass systems. This relationship indicates, therefore, that geometric differences (bond length and angle distortions) between FeOa-tetrahedra in Al-free systems remains in Al-bearing glasses and melts. Variable Fe3+-O distances in FeO 4 tetrahedra may, in fact, be because there exist more than one type of tetrahedron in silicate melts and glasses. Different tetrahedra have been documented in silicate glasses and melts. In simple Na20-SiO2-Fe203 glasses, Fox et al. [ 1982] found that Fe 3§ is in tetrahedral coordination from Raman and luminescence spectroscopic data. Ferric iron may exist in more than one type of tetrahedron because there are bands near 14,200 and 16,200 cm -1 in the luminescence spectra (Fig. 11.7) and bands near 890 and 980 cm -1 in the Raman spectra, the intensities of which are correlated with Fe 3§ content. (Fig. 11.8). The relative abundance of these tetrahedra appears to be a function of the Na/Si-ratio [Fox et al., 1982]. The existence of two different FeOa-tetrahedra is also consistent with interpretations of the 57Fe M0ssbauer spectra of glasses in the same system [Burkhard, 2000]. The structural role of ferric iron in metal oxide ferrisilicate glasses depends not only on total ferric iron content, but also on the metal cation. In 57Fe M0ssbauer spectroscopy, both the isomer shift and the quadrupole splitting values of Fe 3§ are positively correlated with the ionization potential of the metal cation, Z/r ~ (Fig.ll.9). For the alkaline earth metasilicate compositions in Fig. 11.9, the ISFe3+of less than --0.3 mm/s (at 298K) for
Chapter 11
342 0.6 + 03
I
I
I
I
I
0.4--
I.I_
Figure 11.10 - Relationship between the proportion of 4- and 6-fold coordinated Fe 3§ and temperature of equilibration for a (CaO)0.40 (SiO2)0.40(Fe203)0.2 (nominal) glass after melt equlibration with air at the temperatures indicated [Nagata and Hayashi, 2001 ].
0.2-+ 03
u..
m
O.O-
v(--
--
-0.2
-
-0.4 5.2
I 5.4
I I 5.6 5.8 104/-!", K-1
I 6.0
I 6.2
BaSiO 3, SrSiO 3, and CaSiO 3 are consistent with Fe 3+ in 4-fold coordination with oxygen, whereas the ISFe3+ (at 298 K) of Fe-bearing MgSiO 3 glass is near the lower end of the range observed in minerals with octahedral Fe 3+ [Burns, 1994]. Another interpretation of the isomer shift trends in Fig. 11.9a is that Fe 3+ exists in both 4-fold and 6-fold coordination in all the melts. The increase of ISFe3+with Z/r 2 of the metal cation could reflect increasing fraction of Fe 3+ in 6-fold coordination, a concept consistent with results from other iron-bearing alkaline earth and mixed alkali/alkaline earth silicate melts [Dingwell and Brearley, 1988; Hannoyer et al., 1992; Nagata and Hayashi, 2001; Burkhard, 2000]. That there may be more than one coordination state of Fe 3+ has also been proposed, for example, for mixed alkali/alkaline earth silicate glasses [Levy et al., 1976; Nagata and H a y a s h i , 2 0 0 1 ] . By u s i n g Mt~ssbauer s p e c t r o s c o p y to c h a r a c t e r i z e a (CaO)o.4o(SiO2)o.4o(Fe203)0.2 glass formed from a melt equilibrated with air between 1608
I
I
0.6
I
I
I 0
E 0.5 E ==..m t'(,O !__
0.4
E 0 r
0.3 I
0.2
I
i
0.4
0.6
Fe3+/EFe
O
J
0.8
Figure 11.11 - Isomer shift of Fe 3§ (relative to Fe metal at 298 K) from M6ssbauer spectra as a function of the Fe3+/ZFe of Na20-A1203-SiO 2 glasses containing 5 wt % iron oxide as Fe203. Melts were equilibrated at 1550~ with air and CO-CO, gas mixtures between 10.0.68and 10.9bar. The Fe-ffee nominal NBO/T of each series is 0.65 [Mysen and Vugo, 1989].
Iron-bearing Melts II. Structure
343
and 1858 K, Nagata and Hayashi [2001] found that the [4]Fe3+/[6lFe3+ ratio is negatively correlated with temperature (Fig. 11.10). The temperature-dependence yields an enthalpy of 68 kJ/mol for this coordination change, but it is not clear whether this transformation is actually governed by temperature, or whether the data of Fig. 11.10 result from the effect of temperature on the iron redox ratio. This interpretation of 57Fe M/3ssbauer data is also consistent with oxygen coordination numbers inferred from Fe3+-O bond length information. If we assume that equation (11.1), derived for alkali silicate glasses, can also be applied to alkaline earth compositions, the IaFe3+ interval in Fig. l l.9a corresponds to dFe3+_oranging from about 1.96 = for Staur01ite _ e2+ Fe 3+ in BaSiO 3 melt to 2.03 A for Fe 3+ in MgSiO 3 melt. From b o n d - v a l e n c e calculations [Brese and O'Keefe, 1991 ], dFe3+_o= 2.03 A would be consistent with Fe 3+ dominantly in 6-fold coordination, whereas the 1.96 --- value is near or slightly above that expected for Fe 3+ in 4-fold coordination. A mixture of 4- and 9 o Fayalite 6-fold coordinated Fe 3+ might yield average dFe3+_o-values in between. In o analogy with other transition metals [e.g., Calas et al., 2002 ], one might also surmise that a 5-fold coordination could exist, but no data currently available appear consistent with such an intermediate coordination state of Fe 3+in I I I I I silicate melts and glasses [Tangeman and Glass, Lange, 1998]. As a matter of fact, a relationship between ISFe3+and Fe3+/EFe shown in Fig. 11.11 suggests that the redox ratio may be an important factor controlling the coordination state of Fe 3+ [Virgo and Mysen, 1985]. For a sodium aluminosilicate, the /SFe3+ increases 7092 7094 7096 7098 71 O0 from about 0.25 mm/s to about 0.58 Photon energy, eV mm/s in the Fe3+/ZFe-range between 0.79 Figure 11.12- Pre-edge from x-ray absoprtion and 0.2 (isomer shift data from 298K spectra of staurolite, fayalite, and a Fe-bearing MOssbauer spectra). This led Virgo and glass [Calas and Petiau, 1983]. Mysen [1985] to propose that iron-
Chapter 11
344
cO 0 (.-
i
i
,.=.= =i==.
i
i
\\
tO
,i.
\\
,m ~== .4,=..
//
"0 I
I
1.8
1.6
Figure 11.13 - Pair distribution function for FeO in a CaFeSi206 glass, quenched from a melt equilibrated atfoz= 10.7bar at 1075 K, from molecular dynamics (MD) simulation and EXAFS analysis [Rossano et al., 2000].
~~k\ klVIDsi~ ulation
I
I
2.0 2.2 Distance, A
2.4
2.6
complexes locally resembling an inverted spinel structure form in these melts and that the relative stability of such complexes is governed by the Fe3+/ZFe of the melt. This structural model is also consistent with magnetic [O'Horo and Levy, 1978] and thermodynamic [Kress and Carmichael, 1988] data. 11.2. Ferrous Iron
In analogy with the crystal chemistry of ferromagnesian silicate crystals, it is often assumed that Fe z+ occupies structural positions similar to Mg in silicate melts and glasses. From this reasoning Fe z+ is a network-modifying cation perhaps in octahedral coordination with oxygen. That assumption, however, is not always supported by experimental data. For example, in an x-ray absorption study of a 2FeOo4MgOo4CaOoSiO 2 glass, Calas and Petiau [ 1983] concluded that the oxygen coordination number around Fe z+ might be closer to 4 than to 6. They suggested that similar x-ray absorption spectra of staurolite, which has 4-fold coordinated Fe z+ [Smith, 1968], and that of the 2FeOo4MgOo4CaOoSiO 2
I
I
I
I
I
I
or c~ ..Q O <
,~
(NaAISi308)o 5(CaMgSi206)o5 I
700
I
I
1
!
1300 1900 Wavelength, nm
I
2500
Figure 11.14- Optical absorption spectra of Fe2§ in an NaA1Si308 glass containing 3.08 wt % FeO and in an (NaA1Si3Os)05(CaMgSi206)05 glass containing 1.98 wt % FeO. Glasses quenched from melts equilibrated at 1200-1500~ with CO,-H2 gas mixture [Keppler, 1992].
Iron-bearing Melts II. Structure
345
glass (Fig. 11.12) are consistent with at least a large fraction o f F e 2+ in 4-fold coordination with oxygen. The energy overlap with the spectrum of fayalite, FezSiO 4 (Fig. 11.12), which has F e z+ in 6-fold coordination [Smyth, 1975], was rationalized as a crystal field effect in the fayalite spectrum. Whether or not there are crystal field effects in the glass is not known. ll.2a. Oxygen Coordination A suggestion by Calas and Petiau [1983] of 4-fold coordinated Fe z+ in an alkaline earth silicate glass has recently gained support from a study that combined x-ray absorption (EXAFS) and molecular dynamics simulation (MD) of a CaFeSi20 6 glass (Fig. 11.13). Here, Rossano et al. [2000] concluded that there are 2 different oxygen-coordination spheres around Fe z+. One is a distorted tetrahedron (4-fold coordination) and the other a trigonal bipyramid (5-fold coordination). Optical absorption spectroscopy is another method suited to examine the structural role of transition metals in silicate melts [e.g., Wong and Angell, 1976]. In Fig. 11.14 are shown the spectra obtained by Keppler [ 1992] for FeZ+-bearing glasses of NaA1Si308+3 wt % FeO and (NaA1Si308)0.5(CaMgSi206)0.5+2 wt % FeO composition. Both spectra are dominated by a broad peak with a maximum near 1100 nm, which can be assigned to F e z+ in 6-fold coordination, as deduced from comparisons with optical absorption spectra of FeZ+-bearing minerals. Similar assignments of F e z+ f r o m optical spectra of other Fe2+-rich glasses have been made by Bell and Mao [1974] and Nolet et al. [1979]. There are, however, two bands in the absorption spectra. The weaker band near 1900 nm in the spectrum of (NaA1Si3Os)0.5(CaMgSi206)05 + 2 wt % FeO glass (Fig. 11.14) could result from transitions in a distorted octahedron or, alternatively, to tetrahedrally c o o r d i n a t e d F e z+. Keppler [ 1992] suggested that the former assignment is the most likely. Calas and Petiau [19831, however, assigned the band near 1900 nm in absorption spectra of other FeZ+-bearing glasses to F e z+ in tetrahedral coordination because the halfwidth of this band is much smaller than that of the 1100 nm band. That notwithstanding, Keppler [1992] concluded that the argument based on halfwidth differences of the 1100 and 1900 cml bands is not necessarily correct because different iron-oxygen bonds exist in strongly distorted polyhedra. Thus, there is no reason why the amplitudes of their vibrations should be similar. The hyperfine parameters (isomer shift and quadrupole splitting) of Fe z+ from 57Fe M6ssbauer resonant absorption spectroscopy have also been be used to distinguish between the possible 4, 5, and 6 oxygen coordination numbers. In crystalline ferromagnesian silicates there is, however, some overlap in the isomer shift values for I41Fe2+ and t61Fe2+ (Fig. 11.4). The interpretation of the isomer shift becomes even more difficult when the possibility of Fe z+ in 5-fold coordination is included [Waychunas et al., 1988]. The variations in hyperfine parameters for 4- and 5-fold coordination of Fe 2§are not necessarily large. The uncertainties associated with interpretation of the M6ssbauer spectra are compounded by the possibility that the values of hyperfine parameters derived from the
346
Chapter 11
d~
,
cO
Figure 11.15 - Example of 2-doublet fit of Lorentzian lines to 57Fe resonant absorption M6ssbauer spectrum of a glass in the system CaO-MgO-FeO-A1203-SiO2formed by quenching a melt equilibrated at 1400~ at fo~_ = 10-6bar. In this fit, the component peaks of the two doublets, AA'and BB', were constrained to have equal area and equal full width at half height [MOssbauer spectrum from Mysen and Dubinsky, 2004].
m
Q. 0 ..Q c~ c c~ 0 (D
n--
I -3.8
-1.9
0.0
1.9
3.8
Velocity, mm/s absorption envelope may depend on the method used to deconvolute the spectra. Broadened Lorentzian curves are often fitted to the spectra of glasses. Furthermore, when dealing with the Fe 2§ absorption doublet of reduced Fe-bearing silicate glasses (Fig. 11.15), the asymmetry of the Fe 2+absorption doublet can result in a fit that is statistically better with two Fe 2§ doublets than with a single one [e.g., Mysen et al., 1985; Dyar et al., 1987; Wang et al., 1993]. Two absorption doublets may imply 2 different Fe2+-O polyhedra in the glasses. In the example in Fig. 11.15, the isomer shifts of 1.1 and 1.25 mm/s from M6ssbauer spectra recorded at 298 K point to Fe 2+in 6-fold coordination with oxygen in both polyhedra. Such a solution is not at all unique, however, as other fits could be made consistent with a splitting of Fe 2+between tetrahedral and octahedral coordination. In the latter case, the high-velocity component would fit at lower velocity for the AA' than for the BB' doublet in Fig. 11.15 [Dyar et al., 1987]. The underlying problem in M6ssbauer spectroscopy of glasses is that rather than modeling the line shape of the absorption envelope, we should model the distribution of the hyperfine field because its distribution is in direct response to the structure around the Fe nucleus. Fitting of the hyperfine parameter distribution is, therefore, a more appropriate approach to the deconvolution of the M0ssbauer absorption envelope [Alberto, 1995; Rossano et al., 1999; Galoisy et al., 2000; Wilke et al., 2002]. In reduced Fe-bearing Casilicate glasses, fitting the Fe 2+ hyperfine parameter distribution (Fig. 11.16) yields maximum ISFe2+between 1.1 and 1.2 mm/s (marked "[6]Fe2+''. 11.16). There is also a smaller intensity maximum near 0.9 mm/s (marked "I41Fe2+". 11.16). Alberto et al. [1996] interpreted the results of Fig. 11.16 (and those obtained for other Ca-silicate glasses) in
347
Iron-bearing Melts II. Structure I
I
I
I
I
1
1.4
Figure 11.16 - Example of 2dimensional fit to hyperfine parameter distribution (isomer shift and quadrupole splitting) of a CaOSiO2 glass (Ca/Si=0.7) with 5 wt % FeO formed by quenching of melt equilibrated at 1550~ atfo2 = 10-9bar. The positions marked NFe2+and [6]Fe2+ indicate the location of maximum in distribution for 4- and 6-fold coordinated ferrous iron [Alberto, 1995].
E 1.2 E r (/) (I)
E o 1.0 CO
0.8 0
1 2 3 Quadrupole splitting, mm/s
terms of Fe 2§ predominantly in 6-fold coordination along with less than 10 % of total Fe 2§ in 4-fold coordination. In summary, the experimental data on the structural role of Fe z§ in silicate glasses are not always conclusive. Interpretations of optical absorption spectra favor Fe 2§ in 6-fold coordination, whereas x-ray absorption techniques commonly have been interpreted in terms of a smaller number of oxygens in the Fe2+-O polyhedra. In general, MOssbauer spectra have been found consistent with predominantly 6-fold coordination of Fe 2§ but even this interpretation is not universal. The difficulty is compounded by the fact that different structural probes have been used to characterize the structural position of Fe 2§ in the same sample in only a few studies.
11.3. Ferric and Ferrous Iron in Silicate Melts at High Temperature Application of glass structure data to Fe-bearing melts relies on the assumption that quenching does not significantly affect the structural position of Fe 3§ or Fe 2§ Resolution of this question requires examination of relations between quenching rate and redox relations or, even better, structural studies of melts at high temperature. There exist some data on redox and structural relations of iron oxides in glass as a function of temperature and quenching rate [Dyar and Birnie, 1984; Dyar et al., 1987]. From superliquidus temperatures to the glass transition, quenching at rates of the order of hundreds of degrees per second do not the affect redox ratio of the original melt. Dyar et al. [ 1987] did suggest, however, that the isomer shift of Fe 2§from M6ssbauer spectroscopy
348
Chapter 11 I
I
I
_
I
I
t.i-i
tO)
i
t
400
i
i
Figure 11.17 - Unpolarized Raman spectra of glass (25~ and supercooled melt (771 ~ of Na2Oe3SiO2o0.5Fe203 [Wang et al., 1993].
F
800 1200 Raman shift, cm -1
may depend on quenching rate although these were not quantified. If so, oxygen coordination polyhedra of Fe might differ in molten and glassy silicates. Diffraction techniques and vibrational spectroscopy have been used to examine the structure of Fe-bearing melts at high temperature, but only for a relatively small number of compositions. An oxidized and a reduced sample of a Fe-bearing sodium trisilicate were i n v e s t i g a t e d by Wang et al. [1993]. From M 6 s s b a u e r analysis of an NazOe3SiO/e0.5Fe/O 3 glass formed from melt equilibrated in air at 77 I~ all iron was found to be oxidized. The Raman spectra of this glass at 25~ and of the supercooled liquid at 771~ show considerable similarity (Fig. 11.17). Wang et al. [1993] concluded that Fe 3§ is in 4-fold coordination in both the glass and the melt. They also observed, however, an intensity growth near 1060-1070 cm -1 in the high-temperature Raman spectrum, but could not determine whether this was due to some changes in Qn-abundance as the temperature was increased across the glass transition or, more simply, to temperaturedependent Raman cross-sections.
I
I
I
I
I r tt3 0
.R
tt3
cM
e-
=
25~
!
400
I
I
800
I
I
1200
Raman shift, cm -1
Figure 11.18 - Unpolarized Raman spectra of glass (25~ and supercooled melt (676~ of NazO.3SiOzeFeO [Wang et al., 1993].
349
Iron-bearing Melts II. Structure I
<
2.1
r-
2.0
I
I
I
Fe-O
._~ E .,9,o
Figure 11.19 - Si-O and Fe-O distances in FeO-SiO~ melts at 1250~176 from x-ray diffraction [Waseda and Toguri, 1978].
1.7
1.6
0 FeO
si-o I
o
o i
o
u J
20
o--o J
40 mol %
SiO 2
By annealing the same material under a hydrogen atmosphere, Wang et al. [1993] obtained a glass of composition Na2Oo3SiO2oFeO with all the iron as FeO. Its NBO/Si would be 1.3 with the assumption that all Fe 2+is a network-modifier. The Raman spectra at room temperature (glass) and at 676~ (supercooled liquid under a hydrogen atmosphere) are plotted in Fig. 11.18. Both spectra are nearly identical to that of Fe-free Na2Ool.5SiO 2 glass, a glass whose NBO/Si is also 1.3 [Furukawa et al., 1981]. Hence, Wang et al. [ 1993] concluded that the Raman spectra of the reduced glass and melt are consistent with Fe 2+ serving in a network-modifying role. Unfortunately, the structural interpretation by Wang et al. [ 1993] conflicts with those of near-edge x-ray absorption (XANES) spectra of glass and melt of a nearly identical composition [Waychunas et al., 1988]. Even though the M0ssbauer spectra of glasses in the latter study are similar to those reported by Wang et al. [ 1993], Waychunas et al. [ 1988] concluded that the XANES spectra of melts at 850~ are best interpreted in terms of Fe 2+ in 4-fold coordination. This conclusion would agree with that of Waseda and Toguri [1978] and Waseda et al. [1980] who found, from x-ray absorption studies of FeO-SiO 2 and FeO-Fe203-SiO z melts, that the Fe-O bond lengths (between 2.04 and 2.08 ,~) are those expected for Fe2+in 4-fold coordination (Fig. 11.19). From the bond valence treatment, of Brese and O'Keefe [ 1991 ], Fe-O bond lengths in this range could, however, be considered consistent with those expected for Fe 2+ in a coordination state higher than 4. Thus, the conclusion that Fe 2§is in 4-fold coordination with oxygen might not be as firm as originally suggested by Waseda and Toguri [1978] and Waseda et al. [1980]. In summary, (i) the Raman data for Na2Oo3SiO2o0.5Fe203 and Na2Oo3SiO2oFeO glasses and melts indicate significantly different roles of Fe 3§ and Fe z+ in the silicate melt structure (Figs. 11.17 and 11.18). (ii) The Raman spectra of Na2Oo3SiO2oFeO glass and melts are similar to Raman spectra of Na2Oo 1.5SIO 2 glass, which suggest that Fe z+ is indeed, a network-modifying cation. (iii) The Fe2+-O bond lengths from the hightemperature FeO-SiO 2 melt x-ray data (Fig. 11.19) are consistent with 6-fold coordinated Fe z+. (iv) In contrast, Fe 3+ is in 4-fold coordination in oxidized melts and glasses.
Chapter 11
350
11.4. Iron in Silicate Melts and Glasses at High Pressure
0.8
,,o
0.6 O.4
u..
0.2 0.0 0 E E
1
2
3
i
i
i
4 I
0.9
b-
.=_ 9"='-
0.7
t~
.9.0 o
0.5
"o
O
E E
0.3 0
1
2
I 3
0.6 -
~
~
u ~
I
1
4 _
0.5 0.4
t,-
~-
0.3
E o
0.2
C
0
1
I
I
2 3 Pressure, GPa
4
Figure 11.20 - (a) Redox ratio of iron, (b) quadrupole splitting, and (c) and isomer shift of Fe3§from MOssbauer spectra (relative to Fe metal at 298 K) of Na2Si205 + 7.5 mol % (nominal) Fe~_O3quenched from 1400~ at the pressures indicated [Mysen and Virgo, 1985].
The similar ionic radii of Fe 3+ and A13+ and their similar formal electrical charge have led to suggestions in early studies that Fe 3+ might be used as a proxy for A13+ to investigate the effect of pressure on the structural role of A13+in silicate melts [Waft, 1977]. On this basis, Mysen and Virgo [1978] examined the influence of pressure on melts along the ( n o m i n a l ) j o i n NaA1Si206-NaFe23+Si206 with the aid of MOssbauer spectroscopy. They found, however, that the main effect of pressure to several GPa pressure was to reduce Fe 3+ to Fe 2+. The redox ration of iron likely depends on pressure because the mollar volumes of FeO and Fe203 differ (see section 10.4b). This effect was investigated further by Mysen and Virgo [1985] with melts along the nominal join NazSizOs-Fe203. They reported 57Fe M6ssbauer spectroscopic data of glasses f o r m e d by t e m p e r a t u r e quenching melts in equilibrium with air at pressures of up to 4 GPa. They not only observed that Fe3+/ZFe decreases with increasing pressure (Fig. 11.20a), but also that the hyperfine parameters of Fe 3+ are pressure sensitive (Fig.11.20b and c). The rapid increase in isomer shift from about 0.3 mm/s, at ambient pressure and 298 K, to slightly less than 0.6 mm/s for glasses f o r m e d at 3 and 4 GPa points to a transformation of Fe 3+ from tetrahedral to perhaps octahedral coordination. It was suggested that this coordination change was in response to decreasing Fe3+/ZFe as the
Iron-bearing Melts II. Structure
351
pressure increased. There is no evidence in the M0ssbauer spectra of these glasses to suggest that the structural role of Fe 2+ varies with pressure.
11.5. Redox Relations and Melt Polymerization Reduction of network-forming Fe 3+ to network-modifying Fe 2+ may be expressed with the equation [Holmquist, 1966]: 4 FeO 2- r
4 Fe 2+ +
0 2 q-
6 0 2-.
(11.2)
In this equation, the oxygen anion, 0 2-, is the link between the redox reaction and the silicate network via a schematic reaction of the type: 2 Q4 + O2- r
2 Q3.
(11.3)
In equation (11.2), the NBO/Si of Q4 structural units equals 0, and that of Q3 equals 1. Reduction of tetrahedral Fe 3§to octahedral Fe 2+does, therefore, result in depolymerization of the melt: 4 FeO2- + 12 Q4 r
12 Q3 + 4 Fe 2+ + 02.
(11.4)
It follows from equation (11.4) that parameters affecting the iron redox ratio also influence melt polymerization and NBO/T (Fig. 11.21). By increasing the abundance of Fe203 in a silicate melt at ambient pressure, for example, silicate polymerization increases (Fig. 11.21 a). The Fe3+/ZFe ratio can also control the silicate polymerization. This ratio, in turn, varies with temperature, pressure, oxygen fugacity, and melt composition. These parameters will, therefore, also affect polymerization of Fe-bearing silicate melts. At constant iron content and oxygen fugacity, increasing temperature results in decreasing Fe3+/ZFe (see Chapter 10). Increasing temperature will, therefore, cause equilibria (11.3) and (11.5) to shift to the right thus depolymerizing Fe3+-containing silicate melts (Fig. ll.21b). Two trends are shown, however, in Fig. ll.21a. 11.3 The dashed line is NBO/T trajectory with Fe 3+in 4-fold coordination at all temperatures ([4lFe3+).The "actual" trend depicts gradual transformation of"t41Fe3+ to [6lEe3+ due to increasing temperature and decrease in Fe3+/ZFe, which, in turn, induces the coordination change of ferric iron [Mysen and Virgo, 1989]. As a result of the coordination transformation of Fe 3+,
113The melt in Fig. ll.21B is a peralkaline Na20-A1203-SiO2melt with A1/(AI+Si) = 0.334 and Na/(Al+Si)=0.65.
Chapter 11
352 I
1.00
I
I
a
I
-
I
I
I
I
b
actual
0.95 0.7 O
//////j
0.90
133
Z
0.85 0.5 0.80
0
I
I
I
4
8
12
k
I
I
I
I
I
I
1600
Temperature, ~
Iron added as Fe20 3, mol % I
I
1400
1200
/
I
I
I
I
I
actual 1.2 F
d
actual 0.7
~
t
~
""
""
--
"" 1
1
1
m Z
1.0
/
0.6
-
0.9 1
0
1
2
3
Pressure, GPa
4
0
!
-4
!
I
I
-8 -log fo2 (bar)
Figure 11.21 - Relationships between degree of polymerization of silicate network, NBO/T, and structure and redox relations of iron. (a) NazSi2Os+Fe203 (nominal mol % as indicated) glass quenched from melt equilibrated at 1400~ with air [Mysen and Virgo, 1985]. (b) Sodium aluminosilicate glass with AI/(AI+Si) -- 0.334, Na/(Na+A1) = 0.65 and 5 wt % iron oxide added as FezO3 quenched from melt equilibrated with air at the temperatures indicated [Mysen and Virgo, 1989]. (c) Glass of nominal composition Na2Si205+7.5 mol % Fe203, quenched from 1400~ at the pressure indicated [Mysen and Virgo, 1985]. (d) Same glass as in (b) quenched from melt equilibrated with CO-CO 2 gas mixture to control fo 2 at 1550~ [Mysen and Virgo, 1989].
depolymerization of the melt occurs. This depolymerization can be described by the reaction:
[41FeO2- + 2 Q4 r
[6lFe3+ + 2 Q3.
(11.5)
Iron-bearing Melts II. Structure
353
P r e s s u r e increases also cause r e d u c t i o n of F e 3+ to F e 2+ and, therefore, depolymerization of the melt structure. For NazSi205 melt with 7.5 mol % iron oxide added (Fig. 11.2 lc), the NBO/T is more sensitive to pressure ("actual" curve) than expected if the coordination state of Fe 3+were not affected by pressure (dashed line in Fig. 11.21 c). In the latter case, pressure-induced melt depolymerization is that described by equation (11.4) only. However, as pressure can also induce coordination transformation of Fe 3+ from 4-fold to 6-fold, equations (11.4) and (11.5) describe the circumstances. The resulting NBO/T trajectory is marked "actual" in Fig. 11.2 lc. Decreasing oxygen fugacity affects the degree of polymerization in the same manner as pressure and temperature (Fig. 11.21 d). For the melt composition in Fig. 11.21 d, the two depolymerization curves ("actual" and "[41Fe3+") meet at 1550~ at an oxygen fugacity between 10.8 and 10 -9 bar, where all iron is Fe 2+ [Mysen and Virgo, 1989]. Changes in melt polymerization, NBO/T, obviously also affect Q" speciation, but experimental data are lacking to determine the relationships between iron content, redox ratio of iron, structural roles of Fe 2+and Fe 3+, and Qn speciation. These relationships will depend on whether tetrahedra!ly coordinated Fe 3+substitutes for SP + in the structure or if it forms isolated complexes. In the former case, partitioning of Fe 3+ between individual Qn-species may affect speciation in a manner conceptually similar to that observed for AI 3+ in peralkaline aluminosilicate melts (Fig. 9.25), thus, possibly driving the general Qn-speciation reaction (Stebbins, 1987), 2 Qn r
Qn+l + Qn-1,
(11.6)
to the right. The situation is, however, likely more complicated because Fe 2+, with its large ionization potential, tends to favor bonding with oxygen in the least polymerized of available Qn-species [Q,-I in equation (11.6)]. This tendency would tend to drive equilibrium to the right with increasing pressure for Fe-bearing silicate melts.
11.5. Summary Remarks 1. The structural role of ferric iron in Fe3+-rich silicate melts and glasses is mainly that of a network-former. It depends, however, on the redox ratio of iron so that Fe 3+ may be a network-modifying cation in melts with low Fe3+/EFe. 2. The oxygen coordination number around Fe 2+in melts is less clear. All evidence taken together indicates, however, that ferrous iron is dominantly a network-modifier, but it is possible that the structural role of Fe 2+depends on silicate composition. 3. Given the above premises, increasing ferric iron content of a silicate melt induces melt polymerization, whereas increasing Fe 2+probably causes depolymerization. Thus, any variable affecting the redox state of iron (pressure, temperature, bulk composition, total iron content, and oxygen fugacity) will also affect polymerization and Qn-speciation of the melt.
354
Chapter 11
References
Alberto H. V. (1995) Structural characterization of CaO-SiO2-TiO2-Fe-O glasses by Raman and MOssbauer spectroscopy. Ph.D., Univ. Coimbra. Alberto H. V., Pinto da Cunha J. L., Mysen B. O., Gil J. M., and de Campos N. A. (1996) Analysis of MOssbauer spectra of silicate glasses using a two-dimensional Gaussian distribution of hyperfine parameters. J. Non-Cryst. Solids 194, 48-57. Bancroft G. M. (1973) M6ssbauer Spectroscopy: An Introduction for Inorganic Chemists and Geochemists. McGraw Hill. New York. Bell E M. and Mao H. K. (1974) Crystal-field spectra of Fe z+ and Fe 3§ in synthetic basalt glass as a function of oxygen fugacity. Carnegie Instn. Wash. Year Book 73, 496-497. Brese N. E. and O'Keefe M. (1991) Bond-valence parameters for solids. Acta Cryst. B 47, 192-197. Burkhard D. J. M. (2000) Iron-bearing silicate glasses at ambient conditions. J. Non-Cryst. Solids 275, 175-188. Burns R. G. (1994) Mineral M0ssbauer spectroscopy: Correlations between chemical shift and quadrupole splitting parameters. Hyperfine Interact. 91, 739-745. Calas G. and Petiau J. (1983) Coordination state of iron in oxide glasses through high-resolution K-edge spectra: Information from pre-edge. Solid State Comm. 48, 625-629. Calas G., Cormier L., Galoisy L., and Jollivet P. (2002) Structure-property relationships in multicomponent oxide glasses. C. R. Chimie 5, 831-843. Dingwell D. B. and Brearley M. (1988) Melt densities in the CaO-FeO-Fe203-SiO 2 system and the compositional dependence of the partial molar volume of ferric iron in silicate melts. Geochim. Cosmochim. Acta 52, 2815-2825. Dingwell D. B. and Virgo D. (1988) Viscosities of melts in the Na20-FeO-Fe~O3-SiO~- systems and factors controlling relative viscosities in fully polymerized melts. Geochim. Cosmochim. Acta 52, 395-404. Dyar M. D. and Birnie D. E (1984) Quench media effects on iron partitioning in a lunar glass. J. Non-Cryst. Solids 67, 397-411. Dyar M. D., Naney M. T., and Swanson S. E. (1987) Effect of quench methods on Fe3+/Fe2*ratios: A MOssbauer and wet chemical study. Amen Mineral. 72, 792-800. Fox K. E., Furukawa Y., and White W. B. (1982) Transition metal ions in silicate melts. Part 2. Iron in sodium silicate glasses. Phys. Chem. Glasses 23, 169-178. Furukawa T., Fox K. E., and White W. B. (1981) Raman spectroscopic investigation of the structure of silicate glasses. III. Raman intensities and structural units in sodium silicate glasses. J. Chem. Phys. 153, 3226-3237. Galoisy L., Cormier L., Rossano S., Ramos A., Calas G., Gaskell E, and Le Grand M. (2000) Cationic ordering in oxide glasses: The example of transition elements. Mineral. Mag. 64, 409-424. Hannoyer B., Lenglet M., Dtirr J., and Cortes J. (1992) Spectroscopic evidence of octahedral iron (III) in soda-lime silicate glasses and crystals. J. Non-Cryst. Solids 151, 209-216. Henderson G. S., Fleet M. E., and Bancroft G. M. (1984) An X-ray scattering study of vitreous KFeSi308 and NaFeSi308 and reinvestigation of vitreous S i O 2 using quasicrystalline modeling. J. Non-Cryst. Solids 68, 333-349. Holland D., Mekki A., Gee I. A., McConville C. F., Johnson J. A., Johnson C. E., Appleyard E, and Thomas M. (1999) The structure of sodium iron silicate glass - a multi-technique approach. J. Non-Cryst. Solids 253, 192-202. Holmquist S. (1966) Ionic formulation of redox equilibria in glass melts. J. Amer. Ceram. Soc. 49, 228-229.
Iron-bearing Melts II. Structure
355
Jackson W. E., Mustre de Leon J., Brown G. E., Waychunas G. A., Conradson S. D., and Combes J.-M. (1993) High-temperature XAS study of Fe2SiO4 liquid: Reduced coordination of ferrous iron. Science 262, 229-232. Johnson J. A., Johnson C. E., Holland D., Mekki A., Appleyard R, and Thomas M. E (1999) Transition metal ions in ternary sodium silicate glasses: A Mrssbauer and neutron study. J. Non-Cryst. Solids 246, 104-114. Keppler H. (1992) Crystal field spectra and geochemistry of transition metal ions in silicate melts and glasses. Amen. Mineral. 77, 62-75. Kress V. C. and Carmichael I. S. E. (1988) Stoichiometry of the iron oxidation reaction in silicate melts. Amer. Mineral. 73, 1267-1274. Levy R. A., Lupis C. H., and Flinn E A. (1976) Mrssbauer analysis of the valence and coordination of iron cations in SiO2-Na20-CaO glasses. Phys. Chem. Glasses 17, 94-103. Mitra A. (1992) Applied MOssbauer Spectroscopy. Theory and Practice for Geochemists and Archaeologists, Pergamon Press. New York. Mysen B. O. and Virgo D. (1978) Influence of pressure, temperature, and bulk composition on melt structures in the system NaA1Si206-NaFe3§ . Amer. J. Sci. 278, 1307-1322. Mysen B. O. and Virgo D. (1985) Iron-bearing silicate melts: relations between pressure and redox equilibria. Phys. Chem. Minerals 12, 191-200. Mysen B. O. and Virgo D. (1989) Redox equilibria, structure, and properties of Fe-bearing aluminosilicate melts: Relationships between temperature, composition, and oxygen fugacity in the system Na20-A1203-SiOz-Fe-O. Amer. Mineral. 74, 58-76. Mysen B. O. and Dubinsky E. (2004) Mineral/melt element partitioning and melt structure. Geochim. Cosmochim. Acta 68, 1617-1634. Mysen B. O., Virgo D., and Seifert E A. (1984) Redox equilibria of iron in alkaline earth silicate melts: Relationships between melt structure, oxygen fugacity, temperature and properties of iron-bearing silicate liquids. Amer. Mineral. 69, 834-848. Mysen B. O., Virgo D., Neumann E. R., and Seifert E A. (1985) Redox equilibria and the structural states of ferric and ferrous iron in melts in the system CaO-MgO-AlzO3-SiO2: Relations between redox equilibria, melt structure and liquidus phase equilibria. Amer. Mineral. 70, 317-322. Mysen B. O., Lucier A., and Cody G. D. (2003) The structural behavior of A1§ in peralkaline melts and glasses in the system Na20-A1203-SiO 2. Amer. Mineral. 88, 1668-1678. Nagata K. and Hayashi M. (2001) Structural relaxation of silicate melts containing iron oxide. J. Non-Cryst. Solids 282, 1-6. Nolet D. A., Burns R. G., Flamm S. L., and Besancon J. R. (1979) Spectra of Fe-Ti silicate glasses: Implications to remote-sensing of planetary surfaces. Proc. lOth ~ Planet. Sci. Conf., 1775-1786. O'Horo M. P. and Levy R. A. (1978) Effect of melt atmosphere on the magnetic properties of a [(SIO2)45 (CaO)5516s [Fe203135glass. J. Appl. Phys. 49, 1635-1637. Rossano S., Balan E., Morin G., Bauer J.-P., Calas G., and Brouder C. (1999) 57Fe Mrssbauer spectroscopy of tektites. Phys. Chem. Mineral. 26, 530-538. Rossano S., Ramos A. Y., and Delaye J. M. (2000) Environment of ferrous iron in CaFeSi206 glass: Contributions of EXAFS and molecular dynamics. J. Non-Cryst. Solids 273, 48-52. Smith J. V. (1968) The crystal structure of staurolite. Amer. Mineral. 53, 1139-1155. Smyth J. R. (197 5) High temperature crystal chemistry of fayalite. Amer Mineral. 60, 1092-1097. Stebbins J. E (1987) Identification of multiple structural species in silicate glasses by 295i NMR. Nature 330, 465-467. Tangeman J. A. and Lange R. A. (1998) The effect of AP +, Fe 3§ and T i4§ on the configurational heat capacities of sodium silicate liquids. Phys. Chem. Minerals 26, 83-99.
356
Chapter 11
Taylor M. and Brown G. E. (1979) Structure of mineral glasses. II. The SiO2-NaA1SiO 4 join. Geochim. Cosmochim. Acta 43, 1467-1475. Virgo D. and Mysen B. O. (1985) The structural state of iron in oxidized vs. reduced glasses at 1 atm: A 57Fe M6ssbauer study. Phys. Chem. Minerals 12, 65-76. Virgo D., Mysen B. O., and Danckwerth R D. (1983) The coordination of Fe 3§ in oxidized versus reduced sodium aluminosilicate glasses: A 57Fe M6ssbauer study. Carnegie Instn. Washington Year Book 82, 309-313. Waft H. S. (1977) The structural role of ferric iron in silicate melts. Can. Mineral. 15, 198-199. Wang Z., Cooney T. F., and Sharma S. K. (1993) High temperature structural investigation of Na2Oe0.5Fe203e3SiO2 and Na2OeFeOe3SiO 2 melts and glasses. Contrib. Mineral. Petrol. 115, 112-122. Waseda Y. and Toguri J. M. (1978) The structure of the molten FeO-SiO 2 system. Metall. Trans. B. 9, 595-601. Waseda Y., Shiraishi Y., and Toguri J. M. (1980) The structure of the molten FeO-Fe203-SiO2 system by X-ray diffraction. Trans. Jap. Inst. Metall. 21, 51-62. Waychunas G. A., Brown G. E., Ponader C. W., and Jackson W. E. (1988) Evidence from x-ray absorption for network-forming Fe 2§ in molten alkali silicates. Nature 332, 251-253. Whittaker E. J. W. and Muntus R. (1970) Ionic radii for use in geochemistry. Geochim. Cosmochim. Acta 34, 945-957. Wilke M., Behrens H., Burkhard D. J. M., and Rossano S. (2002) The oxidation state of iron in silicic melt at 500 MPa water pressure. Earth Planet. Sci. Lett. 189, 55-67. Wivel C. and Morup S. (1981) Improved computational procedure for evaluation of overlapping hyperfine parameter distributions in M6ssbauer spectra. J. Phys. E. Sci. Instr. 14, 605-610. Wong J. and Angell C. A. (1976) Glass Structure by Spectroscopy. Marcel Dekker, New York.