Determination of the iron oxidation state in basaltic glasses using XANES at the K-edge

Chemical Geology 213 (2004) 71 – 87 www.elsevier.com/locate/chemgeo

Determination of the iron oxidation state in basaltic glasses using XANES at the K-edge Max Wilkea,*, Georg M. Partzschb, Ralf Bernhardtb, Dominique Lattardb a

Institut fu¨r Geowissenschaften, Universita¨t Potsdam, Postfach 601553, D-14415 Potsdam, Germany b Mineralogisches Institut, Universita¨t Heidelberg, INF 236, D-69120 Heidelberg, Germany Received 3 December 2003; received in revised form 28 June 2004; accepted 31 August 2004

Abstract Fe K-edge X-ray absorption near edge structure (XANES) and Mfssbauer spectra were collected on synthetic glasses of basaltic composition and of glasses on the sodium oxide–silica binary to establish a relation between the pre-edge of the XANES at the K-edge and the Fe oxidation state of depolymerised glasses. Charges of sample material were equilibrated at ambient pressure, superliquidus temperatures and oxygen fugacities that were varied over a range of about 15 orders of magnitude. Most experiments were carried out in gas-flow furnaces, either with pure oxygen, air, or different CO/CO2 mixtures. For the most reduced conditions, thePsamples charges were enclosed together with a pellet of the IQF oxygen buffer in an evacuated silica glass ampoule. Fe3+/ Fe
100 of the samples determined by Mfssbauer spectroscopy range between 0% and 100%. Position and intensity of the pre-edge centroid position vary strongly depending on the Fe oxidation state. The pre-edge centroid position and the Fe oxidation state determined by Mfssbauer spectroscopy are nonlinearly related and have been fitted by a quadratic polynomial. Alternatively, the ratio of intensities measured at positions sensitive to Fe2+ and Fe3+, respectively, provides an even more sensitive method. Pre-edge intensities of the sample suite indicate average Fe co-ordination between 4 and 6 for all samples regardless of oxidation state. A potential application of the calibration given here opens the possibility of determining Fe oxidation state in glasses of similar compositions with high spatial resolution by use of a Micro-XANES setup (e.g., glass inclusions in natural minerals). D 2004 Elsevier B.V. All rights reserved. Keywords: Fe oxidation state; XANES; Mfssbauer spectroscopy

1. Introduction

* Corresponding author. Tel.: +49 331 977 2483; fax: +49 331 977 5060. E-mail address: [email protected] (M. Wilke). 0009-2541/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2004.08.034

For the understanding of many geological processes involving silicate melts as well as for the properties of technically used glass, a precise knowledge of the Fe oxidation state in the melt or glass is necessary. Depending on the oxidation state,

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the co-ordination chemistry of Fe in the melt changes considerably and thus, it strongly affects the physical and chemical properties of the melt or glass, e.g., melt viscosity, glass colour. While there has been already a vast amount of studies characterising the relationship of the Fe oxidation state to compositional parameters and conditions of synthesis (e.g., Johnston, 1964; Fudali, 1965; Sack et al., 1980; Kilinc et al., 1983; Mysen et al., 1985a; Paul, 1990; Kress and Carmichael, 1991; Moore et al., 1995; Gaillard et al., 2001; Wilke et al., 2002), the main issue of this communication is rather to discuss the use of X-ray absorption near edge structure (XANES) for the determination of Fe oxidation state in glass. XANES is sensitive to the oxidation state of the probed element. In the case of Fe, especially the pre-edge feature located about 10 eV before the main K-edge is very sensitive to the valence state. The pre-edge is related to 1sY3d (quadrupolar) and/or 1sY4p (dipolar) metal electronic transitions (Dra¨ger et al., 1988; Westre et al., 1997) and its position shifts toward higher energy with increasing oxidation state (White and McKinstry, 1966; Srivastava and Nigam, 1973). The preedge position has been used for the determination of Fe oxidation state in a number of crystalline compounds (Bajt et al., 1994; Heald et al., 1995; Dyar et al., 1998; Delaney et al., 1996; Dyar et al., 2001; Wilke et al., 2001; Schmid et al., 2003, among others). In silicate glasses, however, the application of this method is not straightforward due to the poorly defined co-ordination geometry of Fe in glasses and its site-to-site variation (Alberto et al., 1996; Rossano et al., 1999). Therefore, calibrations from crystalline compounds as given by Bajt et al. (1994) and Wilke et al. (2001) cannot be applied directly. Nevertheless, the method has been previously used on glasses, by using crystalline and/or glassy reference samples (Calas and Petiau, 1983; Galoisy et al., 2001; Bonnin-Mosbah et al., 2001; Berry et al., 2003). Especially, Berry et al. (2003) uses a large set of glass samples characterised in Fe oxidation state providing a high-quality calibration for the used glass composition. The advantages of the XANES over other methods are the applicability to compositions with low contents of Fe, the possibility to perform measurements using a beam focused down to a

microscopic spot size, which is useful for the analysis of heterogeneous samples, and finally, the possibility to perform measurements on silicate melts at in situ conditions. However, to reach a precision on the oxidation state of a given glass that is significantly better than F10% with this technique, a calibration for the glass composition of interest is needed. In this contribution, we reassess the variation of the XANES, especially of the pre-edge, with oxidation state for glasses of basaltic and iron-doped sodium silicate compositions. In contrast to spectra reported by Berry et al. (2003) in an analogous study, the XANES spectra were recorded with significantly higher spectral resolution, which improves the precision of the results. We present here a new calibration of the shift of the pre-edge against the oxidation state determined by Mfssbauer spectroscopy. This data set provides a basis for determining the Fe oxidation state in heterogeneous glass-bearing samples with similar glass composition, in particular by using a micro-XANES setup (e.g., partially crystallised samples, glass inclusions in crystalline phases).

2. Experimental 2.1. Starting materials Starting materials were powders of synthetic glasses, with different, either simple or complex, chemical compositions (Table 1). The composition of sample 7159V corresponds to that of a transitional alkalic basalt from Iceland (Thy and Lofgren, 1994). 7159VB differs from 7159V essentially through its lower FeOtot and its higher SiO2 and Al2O3 contents. Starting materials 7159V-P2.5 and 7159VB-P6 were obtained by doping the formers with phosphorus. Binary glasses in the system SiO2–Na2O (NS2 corresponds to Na2O
2 SiO2, NS3 to Na2O
3 SiO2) doped with a few wt.% Fe2O3 were also used for comparison to the more complex basaltic compositions. The phosphorus-free starting materials (7159V, 7159VB, NS2, NS3) were synthesized from mixtures of oxides (SiO2, TiO2, Al2O3, Fe2O3 and MgO) and carbonates (CaCO3, Na2CO3 and K2CO3). The

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Table 1 Chemical composition of the starting materials and of the products of runs with the IQF buffer in evacuated silica glass ampoules Starting compositions

Products of runs with IQF

Sample #

7159V

7159V-P2.5

7159VB

7159VB-P6

NS2

NS3

7159V-CS

7159VB-CS

NS2-CS

No. of analyses

29

20

17

17

10

10

68

49

19

SiO2 TiO2 Al2O3 FeO MgO CaO Na2O K2O P 2 O5 P

47.88 (20) 4.04 (10) 13.22 (16) 14.38 (21) 3.23 (06) 9.04 (08) 4.68 (08) 1.15 (04) – 97.62

46.99 (21) 3.92 (06) 13.29 (08) 14.50 (22) 3.17 (03) 8.86 (10) 4.48 (09) 0.98 (03) 2.54 (09) 96.21

52.98 (16) 4.42 (08) 15.05 (12) 6.26 (09) 3.62 (07) 9.98 (12) 4.90 (20) 1.25 (04) – 98.46

51.05 (21) 4.26 (10) 14.51 (13) 6.33 (09) 3.43 (05) 9.59 (11) 4.02 (14) 0.89 (03) 4.51 (14) 98.59

65.60 (35) – 0.32 (03) 6.32 (12) – – 26.90 (36) – – 99.2

81.34 (75) – – 4.13 (15) – – 14.60 (33) 0.41 (03) – 100.5

41.59 (17) 3.49 (07) 11.60 (10) 27.82 (28) 2.81 (04) 7.88 (11) 3.11 (07) 0.64 (03) – 98.9

46.98 (27) 3.90 (07) 13.44 (11) 18.31 (37) 3.16 (06) 8.91 (10) 3.39 (08) 0.75 (03) – 98.85

54.90 (103) – 0.26 (02) 28.60 (98) – – 15.70 (38) – – 99.6

mixtures were decarbonated at 900 8C in a Pt-crucible for 0.5 h. The complex compositions (7159V and 7159VB) were fused at 1500 8C in air for 5 h and poured for quenching into a steel mortar. NS2 and NS3 were fused at 1200 8C for only half an hour to minimise sodium volatilisation. In case of starting material 7159VB-P6, phosphorus was added as ammonium dihydrogen phosphate (NH4H2PO4). All materials were homogenized by repeated grinding and fusing. Phosphorus and sodium partly volatilised during the repeated fusing. The strongest loss of phosphorus (25%) occurred during the preparation of material 7159VB-P6, the strongest sodium loss (20%) during that of the NS2 glass. Starting material 7159VP2.5 was obtained by mechanical mixing and grinding of materials 7159V and 7159VP4 (i.e., the former material doped with 4 wt.% P 2O5). The final compositions of all starting materials are listed in Table 1. 2.2. High-temperature experiments To obtain glass samples with very different Fe oxidation states, experimental charges were equilibrated at ambient pressure and superliquidus temperatures under various oxygen fugacities in the range FMQ4 to FMQ+8, i.e., at values between 4 log units below to 8 log units above the fayalite–magnetite– quartz oxygen buffer. Two alternative methods were used to fix the oxygen fugacity. In most cases, the charges (about 50 mg) were placed on small loops of platinum wire (loop diameter: 2 mm, wire diameter:

0.1 mm; loop technique after, e.g., Presnall and Brenner, 1974) and equilibrated in a gas flow (oxygen, air or CO/CO2 mixtures; for the latter, cf. Deines et al., 1974). However, the most reduced conditions (FMQ4) were imposed by the presence of the iron– quartz–fayalite oxygen buffer (IQF). The sample charge and the IQF buffer were filled into two separate small iron crucibles and sealed together in an evacuated silica glass ampoule (Klemm and Snethlage, 1975). For the experiments performed with the loop technique, the Pt-loops were presaturated with iron to minimise iron loss from the melt to the Pt-wire. Presaturation was achieved by covering the loops with Fe2O3 and treating them for 24 h at the required temperatures and oxygen fugacities, followed by cleaning in hydrochloric acid. During gas-flow experiments, the oxygen fugacity was measured using a SIRO2 zirconia probe that was calibrated against the Ni–NiO and magnetite–wuestite solid-state buffers (O’Neill, 1987, 1988). All experiments were performed at temperatures between 1050 and 1350 8C in vertical drop-quench furnaces. The temperature was measured before and after each experiment with a Pt/Pt90Rh10 thermocouple that had been calibrated against the melting point of Au (1064.4 8C). At the end of the runs, the samples were quenched into distilled water. Most gas-flow experiments were conducted at 1300 to 1350 8C (Table 2). Run durations were between 5 and 12 h, which was considered to be sufficient to ensure equilibration of the redox state of the melt.

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Table 2 P Run conditions and results of Fe3+/ Fe
100 determinations from XANES, Mfssbauer spectroscopy and chemical analyses P No. T (8C) Dwell fO2 DFMQ Centroid Intensity** I(Fe3+)/ I(Fe3+)/ Fe3+/ Fe
100 2+ 3+ 2+ (h) I(Fe ) [I(Fe )+I(Fe )] Mfss. spec. Wet chem. control (eV)* Starting material 7159V RB0-20 1350 5 RB0-31 1350 7 RB0-22 1350 5 RB0-23 1350 5 RB0-1 1350 5 RB0-15 1350 7 RB0-3 1350 5 RB0-4 1350 5 7159V-CS 1180 63

air CO/CO2 CO/CO2 CO/CO2 air CO/CO2 CO/CO2 CO/CO2 IQF

6.1 2.6 1.3 0.8 6.1 2.6 1.3 0.8 4.1

7113.36 7112.90 7112.81 7112.50 7111.98

0.1856 0.1686 0.1556 0.1401 0.1068

4.530 1.370 1.080 0.656 0.273

0.819 0.579 0.520 0.396 0.215

83 (4) 42 (4) 35 (4) 16 (4) 4 (4)

Starting material 7159VB-P2.5 RB0P2.5-12 1350 5 RB0P2.5-16 1350 7 RB0P2.5-13 1350 5 RB0P2.5-14 1350 5

air CO/CO2 CO/CO2 CO/CO2

6.1 2.6 1.3 0.8

7113.29 7112.89 7112.85 7112.46

0.1775 0.1629 0.1633 0.1433

3.560 1.270 1.140 0.617

0.781 0.559 0.533 0.382

80 39 33 14

Starting material 7159VB 7159VB-O2 1200 7 RB3-1 1350 5 RB3-2 1350 7 RB3-3 1350 5 RB3-4 1350 5 RB3-4.2 1350 5 7159VB-CS 1180 65

O2 air CO/CO2 CO/CO2 CO/CO2 CO/CO2 IQF

8.4 6.1 2.6 1.3 1.3 0.8 -4.1

7113.40 7113.33 7112.62 7112.64 7112.53 7112.14 7111.98

0.2178 0.1899 0.1580 0.1437 0.1326 0.0984 0.1017

5.940 4.800 1.170 1.030 0.865 0.440 0.2860

0.855 0.828 0.540 0.508 0.464 0.305 0.222

93 (4) 79 (4) 34 (4) 29 (4) 26 (4) 7 (4) 0 (4)

Starting material 7159VB-P6 RB3P5-5 1350 5 RB3P5-6 1350 7 RB3P5-7 1350 5 RB3P5-8 1350 5

air CO/CO2 CO/CO2 CO/CO2

6.1 2.6 1.3 0.8

7113.29 7112.64 7112.47 7112.25

0.1796 0.1410 0.1314 0.1154

3.670 1.050 0.769 0.490

0.795 0.512 0.435 0.329

75 (4) 28 (4) 20 (4) 7 (4)

Starting material NS2 NS2-O2 1050 NS2-air 1320 NS2-FMQ-2 1320 NS2-CS 1050

27 7 12 66

O2 air CO/CO2 IQF

10.4 6.4 2.0 4.4

7113.56 NA 7112.25 7112.18

0.2315 NA 0.1375 0.1346

8.599 NA 0.534 0.472

0.896 NA 0.348 0.321

100 (4) 84 (4) 4 (4) 0 (4)

Starting material NS3 NS3-air 1300 NS3-FMQ-2 1300

7 12

air CO/CO2

6.6 2.0

7113.39 7112.29

0.1824 0.1371

6.636 0.581

0.869 0.367

75 (4) 4 (4)

79 40 30 12

(5) (5) (5) (5)

(4) (4) (4) (4)

* F0.05 eV. ** 5% rel. error.

According to Helgason et al. (1992), at least 4 h are required to equilibrate basaltic compositions at 1300 8C at reducing as well as at oxidising conditions. For the experiments with the IQF buffer (performed in silica glass tubes), lower temperatures (1180 or 1050 8C, Table 2) were necessary to avoid melting of

fayalite (1205 8C) within the buffer. For better comparison, the experiments in a flow of pure oxygen were performed at similar temperatures (1200 and 1050 8C). At these lower temperatures, the run durations were greatly expanded to ensure a good equilibration (up to 66 h at 1050 8C).

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3. Analytical techniques 3.1. Mo¨ssbauer spectroscopy P Fe3+/ Fe of the glasses were determined using Mfssbauer spectroscopy (Heidelberg). Samples were prepared by grinding aliquots of glass to a powder, diluting with sugar and mounting in 10-mm-diameter sample holders (with absorber thicknesses about 5 mg Fe/cm2). Room temperature 57Fe Mfssbauer spectra have been recorded in transmission mode on a constant acceleration Mfssbauer spectrometer with a nominal 1.85 GBq (50 mCi) 57Co source in a 6-Am Rh matrix. The velocity scale was calibrated relative to a 25-Am a-Fe foil. The spectra were fitted using the commercially available fitting programs NORMOS written by R.A. Brand (distributed by Wissenschaftliche Elektronik, Germany). To ensure good counting statistics, collection times for the Mfssbauer spectra were 2–8 days corresponding to 2–5 E6 counts per channel for most samples, but 14 days for the most reduced samples corresponding to 1 E7 counts per channel. Due to the peculiarities of the glass structure, Mfssbauer spectra of silicate glasses differ in two ways from those of crystalline silicates: (1)

The full width at half maximum for the doublets of glass spectra is two to three times wider than in spectra of crystalline compounds for both the contributions of ferric and ferrous iron.

(2)

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The doublet for ferrous iron is strongly asymmetric. This is caused by site-to-site distribution of ferrous iron in the glass structure (Fig. 1a and b).

The deconvolution of Mfssbauer spectra of glasses is rather complex due to the superposition of the contributions of ferric and ferrous iron (Fig. 1a and b). This is especially important for the interpretation of spectra of strongly reduced or strongly oxidised samples. In spectra of glasses dominated by Fe2+, minor amounts of ferric iron will cause an increase in intensity in the valley between the two bands and an increase in intensity and broadening of the low-velocity band. In spectra dominated by Fe3+, minor amounts of ferrous iron will cause a line broadening and an increase in intensity of the low-velocity band and in addition, a shoulder on the high-velocity band is observed. All spectra were fitted using one or more symmetric doublets of Lorentzian shape for the contribution of ferric and ferrous iron, respectively (e.g., Helgason et al., 1992; Burkhard, 2000). More advanced fitting procedures to spectra of silicate glasses are in use (Virgo and Mysen, 1985; Alberto et al., 1996; Rossano et al., 1999). These methods may better account for the structural environment of iron in the glass structure but they do not yield any significant improvement in the P determination of Fe3+/ Fe (e.g., Virgo and Mysen, 1985; Helgason et al., 1992; Wilke etP al., 2002). Thus, for the purpose of determining Fe3+/ Fe, the computationally simpler method, with a small number of

Fig. 1. Mfssbauer spectra of the most reduced (solid curves) and the most oxidised samples (dotted curves). Each spectrum was normalised to the maximum of the most intense peak for better comparison. (a) Starting composition 7159VB equilibrated in pure oxygen (DFMQ=+8.4; dotted curve) and in presence of the fayalite–iron–quartz buffer (IQF; solid curve); (b) starting composition NS2 equilibrated in pure oxygen (DFMQ=+10.4) and at fO2 corresponding to DFMQ=2.0.

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3+ lines P of Lorentzian shape, has been used. The Fe / Fe
100 values listed in Table 2 were determined from the relative area ratios of the doublets fitted to the contribution of ferric and ferrous iron, where relative uncertainties were estimated based on counting statistics and uncertainties in the fitting model. Evidence was reported that Mfssbauer spectroscopy overestimates the Fe3+ component in glasses (Ottonello et al., 2001) unless corrections are made for variable recoil free fractions of ferrous and ferric iron. However, Jayasuriya et al. (in press) were able to show that the recoil free fractions of Fe2+ and Fe3+ are approximately equal in glasses that are similar in composition to those used here. In addition, comparisons of Mfssbauer spectroscopic and wet chemical determinations of the Fe oxidation state in silicate glasses have shown no systematic deviations between the results of the two methods (e.g., Mysen et al., 1985b; Dingwell, 1991). Consequently, we did not apply any correction for the recoil free fraction in our Mfssbauer analysis.

beam intensity before and after the sample (filled with N2 and Ar, respectively). Spectra were collected on the powdered glass using the same absorber as used for Mfssbauer analysis. XANES spectra were collected from ~60 eV below to 200 eV above the Fe K-edge (7050–7300 eV), with 0.1 eV steps in the pre-edge region (7108–7116 eV). The XANES spectra were normalised in absorbance by fitting the spectral region from 7050 to 7090 eV (the region before the pre-edge) using a polynomial and subtracting it as background absorption. The spectra were then normalised for atomic absorption, based on the average absorption coefficient of the spectral region from 7200 to 7300 eV (i.e., after the edge crest). The pre-edge was extracted by fitting two Gaussian functions for the pre-edge peak and two further Gaussian functions to model the contribution of the main edge to the pre-edge feature using a part of the spectra between 7104 and 7118 eV (Fig. 2). The pre-edge information was derived by calculating the total integrated area and the centroid (area-weighted average of the pre-edge peak position) of the background subtracted pre-edge.

3.2. XANES 3.3. Wet chemical analysis XANES spectra at the Fe K-edge were collected at the Hamburger Synchrotronstrahlungslabor (HASYLAB, Hamburg, Germany) on the bending-magnet beamline A1. The storage ring operating conditions were 4.5 GeVelectron energy and 80–120 mA positron current. A Si (111) four-crystal monochromator was used, providing an energy resolution E/DE=12,200 or 0.5 eV at 6 keV (Attenkofer et al., 2000). This energy resolution is significantly smaller than the finite corehole lifetime width of the absorbing element (ca. 1.15 eV at the Fe K-edge; Krause and Oliver, 1979). In contrast, the energy resolution of a Si (111) doublecrystal monochromator is typically about E/DE=4000– 5000 (depending on the complete optics of a given beamline, of course), which is above the core-hole lifetime width in the case of Fe. For all experiments, a reference foil (usually metallic Fe) was used to provide an internal and accurate energy calibration of the monochromator for all spectra (first inflection point of the Fe K-edge set at 7111.08 eV). Reproducibility of the monochromator energy is defined by a HeidenhainR goniometer angle encoder within a tolerance of 2
105 degree. The spectra were collected in transmission mode using ionisation chambers for measuring the

Wet chemical ferrous iron determinations were carried out using a potentiometric method (Wilson, 1955). The U.S.G.S. standard BIR-1 and the geochemical standard of the geological Survey of Japan JB1-a were used as references. The retrieved values (8.13 wt.% FeO for BIR-1, 5.71 wt.% for JB1-a) compare well with the accepted values (8.34 wt.% for BIR-1, 5.78 wt.% for JB1-a). The Fe2O3 contents of the glasses were calculated from the difference between total FeOtot (retrieved with the electron microprobe) and the wet chemically determined ferrous iron contents. The wet chemical FeO values are determined within F0.25 wt.%. The relative uncertainties for the EMPA analyses of FeO tot are about 2%. P The resulting absolute uncertainties in Fe3+/ Fe
100 are F4 to F5. 3.4. Electron microprobe EMP analyses were conducted using a CAMECA SX51 (Heidelberg) and a JEOL 8800 (Museum fqr Naturkunde, Berlin). The glasses with low alkali contents (starting materials 7159V, 7159VB) were

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Fig. 2. (a) Fe K-edge XANES Spectra a reduced Fe-bearing sodium–silica glass sample acquired with different energy resolution (two-crystal vs. four-crystal monochromator). In the spectrum taken with higher energy resolution (four-crystal monochromator, solid curve) the features present in pre-edge region are much better resolved (see also text). (b) Fit model used to determine the background corrected pre-edge intensities and centroid positions. The spectrum was modelled using two Gaussian peaks for the pre-edge and two further Gaussian peaks to model the background from the main edge in this spectral region. Black squares: measured spectrum; thin line: fitted spectrum (pre-edge peaks and background); heavy line: fitted pre-edge peaks; dotted line: fitted background; grey lines: peaks defining the background.

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analysed with an acceleration voltage of 15 kV, a beam current of 15 nA and a beam diameter enlarged to 10 Am. The counting times were 10 s on the peaks and 5 s on the background. The incident beam diameter was enlarged to 10 Am. To minimise Na migration induced by the electron beam, the sodium silicate glasses (NS2, NS3) were analysed with a beam current of only 8 nA, a beam diameter of 40 Am and counting times of 5 s on peak and 2.5 s on background.

4. Results and discussion 4.1. Run products The quenched samples are purely glassy and homogeneous within the counting statistic of the electron microprobe. All synthetic basaltic glasses are brownish or black in colour due to theirrelatively high iron and titanium contents. Iron-bearing sodium– silicate glasses are yellow if oxidised, blue if reduced, and green at intermediate oxidation states. Although the loops of Pt-wire that hold the charges during the high-temperature treatment were presaturated with iron, some changes in the iron contents of the samples did occur, as already reported from similar experiments (Grove, 1981; Toplis and Carroll, 1995). In samples of the composition 7159V, only small changes in the iron contents occurred (b4% iron gain, b3% iron loss). For most samples of starting composition 7159VB, iron loss or gain are of the same order as in 7159V, except in two experiments, where iron gain was stronger (7% and 11%). In contrast, an extreme iron gain (200–400%) from the iron crucible to the sample material took place during the experiments under the most reducing conditions, with the IQF buffer (Table 1). Nevertheless, two of the corresponding run products, 7159VCS and 7159VB-CS, are homogenous within the counting statistic of the electron microprobe. In the third one, NS2-CS, the iron content varies slightly over the sample (cf. standard deviation in Table 1). 4.2. Mo¨ssbauer results As expected, a comparison of the Mfssbauer spectra of the run products shows continuous changes with

increasing oxidation. The spectra consist of two or three broad bands (Fig. 3), as usual for iron-bearing silicate glasses (e.g., Virgo and Mysen, 1985). These bands result from the superimposition of two doublets with large half-widths, one for ferrous iron, with high quadrupole splitting (QS) and high isomer shift (IS), and one for ferric iron, with low QS and low IS. The low-velocity parts of both doublets overlap. (Figs. 1 and 3). The spectra of the most oxidised samples, i.e., those equilibrated in oxygen (DFMQ=8.4 at 1200 8C, DFMQ=10.4 at 1050 8C; cf. Table 2) exhibit two broad bands and a very weak shoulder at high velocities (e.g., Fig. 3a). The doublet, that is characterised by a low QS (about 1) and a low IS (about 0.4), can be assigned to ferric iron. The shoulder at high velocities may be assigned to ferrous iron, but the ferrous iron doublet is not resolved because its low-velocity peak is completely overlaid by the ferric iron doublet. The spectra were fitted with two doublets of Lorentzian line shape representing the contribution of ferric iron and one doublet representing the contribution of ferrous iron (Fig. 3a; cf. also Burkhard, 2000). Spectra of samples equilibrated in air are quite similar to those discussed above, but the shoulder at high velocities is more pronounced and the low-velocity band is broader than

P Fig. 3. Comparison of Fe3+/ Fe
100 values determined by Mfssbauer spectroscopy with those retrieved through the combination of wet chemical and EMP analyses for products of runs on bulk composition 7159V. The corresponding run conditions are listed in Table 2 (first 8 lines).

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the high-velocity band (Fig. 3b). Both changes are due to the stronger contributions of ferrous iron. The presence of ferrous iron in silicate glasses equilibrated in air at high temperature has already been reported (e.g., Burkhard, 2000) and has been confirmed in the present study by a wet chemical ferrous iron determination on a synthetic basaltic glass (Sample RB0-20, Table 2). The spectra of samples equilibrated at intermediate oxygen fugacities (DFMQ=2.6, DFMQ=1.3) are dominated by the contributions of ferrous iron (Fig. 3c and d). They were fitted with two Lorentzian doublets for ferrous iron and one for ferric iron. In some spectra, the ferric iron contribution is still resolved and the spectrum consists of three distinct bands (Fig. 3c). As shown in Figs. 3e,f and 4, the spectra of the three most reduced samples (equilibrated at DFMQ=4.1 and 0.8) dis-

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play two broad asymmetric bands, which essentially reflect the contributions of ferrous iron. Ferric iron contributions are expected in the low-velocity band and in the bvalleyQ between the bands. Indeed, the spectrum of the charge equilibrated at DFMQ=0.8 (RB3-4.2; Fig. 3e and 4) has the lowest transmission value between the two bands, suggesting a small Fe3V+ content 3+ P (Fe / Fe
100: 7%). In contrast, the spectrum of run product 7159VB-CS (DFMQ=4.1; Fig. 3f and 4) shows no indication of ferric iron: the transmission is highest between the bands with equal areas. With the model commonly used to fit the spectra of reduced samples (2 doublets for ferrous and 1 doublet for ferric iron), a stable fit could not be achieved. Therefore, for such extremely reduced samples, the spectra were fitted using a special procedure in order to test if the sample is in fact free of ferric iron. The spectra were successively

Fig. 4. XANES spectra of a series of quenched synthetic basaltic melts P (starting composition 7159VB and 7159VB-P6) equilibrated between DFMQ=4.1 and DFMQ=8.4, corresponding to a range of Fe3+/ Fe
100 between 0 and 93. The edge position and the position of the first maximum after the edge are shifted to higher energies with increasing Fe oxidation state. The zoom shows the changes occurring in the pre-edge region.

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fitted with an increasing number of Lorentzian doublets for Fe2+. At the different stages of the fitting procedure, we tried to add a doublet for Fe3+, and checked whether a statistically significant improvement of the fit was reached. But this was not achieved at any step of the fitting procedure. Eventually, the spectrum was fitted with 6 doublets for Fe2+ (Fig. 3f), resulting in a statistically very good fit with a minor residuum, and the Fe3+ content was considered to be below the detection limit. Some wet chemical ferrous iron determinations were conducted to verify the results of the Mfssbauer 3+ P spectroscopic Fe / Fe determinations (Fig. 5). Fe3+/ P
Fe 100 values determined by the combination of wet chemical ferrous iron determination and electronmicroprobe analysis are consistent with those retrieved by Mfssbauer spectroscopy within the uncertainties. As reported earlier (Mysen et al., 1985b; P Dingwell, 1991; Partzsch et al., in press), the Fe3+/ Fe
100 of silicate glasses may be analysed with similar confidence by Mfssbauer spectroscopy.

Fig. 5. Pre-edge parameters of all samples plotted in the variogram after Wilke et al. (2001). Grey fields designate pre-edge parameters for the Fe co-ordination and oxidation state indicated. Dashed lines between fields indicate the variation of pre-edge parameters assuming binary mixtures of respective end-members. Ticks on curves refer to the percentage of mixtures. Black circles: 7159V & 7159V-P2.5; open circles: 7159VB & 7159VB-P6; open squares: NS2; open triangles: NS3. Grey solid curve shows the variation calculated from parameters for completely reduced and oxidised end-members, respectively (see also text).

5. XANES results 5.1. Effect of monochromator energy resolution Fig. 2 shows the effect of the energy resolution of the used monochromator on the XANES of a reduced NS3 glass containing 5 wt.% Fe2O3. While the structure visible around the main crest and after the edge remains unchanged, severe differences in the preedge region are evident. The high-resolution scan reveals two distinct peaks that are merged in the lowresolution scan yielding only a broad asymmetric peak. Thus, low spectrometer resolution may potentially lead to significant loss in information. In addition, the preedge peak of the high-resolution scan is separated much better from the main edge, enabling a much safer modelling of the pre-edge background, which is needed to determine the pre-edge intensity and centroid. 5.2. XANES of glasses Depending on the Fe oxidation state, the XANES varies in the position of the edge and in the structure of the main crest (Fig. 6). The edge position moves towards lower energies for reduced samples. The structure at the main crest is significantly different for oxidised and reduced samples. These spectral differences reflect the difference in the structural environment of Fe2+ and Fe3+, respectively. The pre-edges show a structure consisting of two overlapping peaks separated by ca. 1.5 to 2 eV depending on Fe oxidation state (Fig. 6 and 7). In contrast to pre-edges from crystalline model compounds (Wilke et al., 2001), the peaks are broadened owing site-to-site distribution of the iron co-ordination in glasses. The different relative intensities of the two contributions reflect the different oxidation states of the samples. The pre-edge centroid position depends strongly on the Fe oxidation state, whereas the preedge intensity is mostly influenced by the Fe coordination geometry. Low intensities refer to geometries with a center of symmetry (e.g., octahedra); high intensities refer to non-centrosymmetric geometries (e.g., tetrahedra). Using pre-edges of model compounds with known Fe-site geometry and oxidation state, a variogram for the estimation of oxidation state and Fe co-ordination was constructed (Wilke et al., 2001). In comparison to the variogram originally published by Wilke et al.

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Fig. 6. Room temperature Mfssbauer spectra of quenched synthetic basaltic melts (starting material 7159VB) equilibratedP at various fO2, from very oxidised (a) to very reduced conditions (f). The corresponding experimental conditions are listed in Table 2. The Fe3+/ Fe values are those estimated from the Mfssbauer spectra (cf. Table 2 and Mfssbauer spectroscopy section). Dotted curves: fitted doublets for Fe3+; solid curves: fitted doublets for Fe3+.

(2001), the centroid position for ferrous iron is shifted to lower energies in the present version (Fig. 8). This corrects a bias in the original position due to underestimated ferric iron contents in some of the model compounds for ferrous iron. However, as the variogram is based on binary mixtures of crystalline model compounds, it cannot be used to precisely determine the Fe oxidation state of glasses. In contrast to crystalline materials, the co-ordination of either Fe2+ or Fe3+ in glasses is not well defined and may be a mixture of up to three co-ordinations for Fe2+ and Fe3+, respectively.

The data of the analysed glasses plot along a somewhat scattered linear trend in between the variation trends given for crystalline [4]Fe and [6]Fe (Fig. 8). The intensity of the pre-edges are similar to those observed for fivefold co-ordinated Fe in crystalline model compounds, with slightly higher intensities for the most oxidised samples. Although this indicates that the average Fe co-ordination will be around five, no distinction can be made, however, between mixtures of four- and sixfold co-ordinated (Oh- and Td-symmetry) or the potential occurrence of really fivefold coordinated Fe (C3h). The average Fe co-ordination

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transition temperature (Dingwell, 1995), which is similar for all samples.

6. Using XANES for the determination of Fe oxidation state in glass 6.1. Determination of Fe oxidation state using the preedge centroid position

Fig. 7. Mfssbauer spectra of three samples equilibrated at moderately (DFMQ=0.8; run product RB3-4.2) to highly reducing (DFMQ=4.1; run products 7159V-CS and 7159VB-CS) conditions. The three spectra have very similar shapes with minor contributions of ferric iron superimposed on the ferrous iron doublet. In these samples, 7% (7159VB, FMQ-1), 4% (7159VCS) and 0% (7159VB-CS) of the iron contents are assigned to Fe3+. Each spectrum was normalised to the maximum of the most intense peak for better comparison.

indicated by the pre-edge is consistent with results from former studies on Fe co-ordination in silicate glass (e.g., Virgo and Mysen, 1985; Waychunas et al., 1988; Hannoyer et al., 1992; Rossano et al., 2000). Sodium– silicate glasses plot on a similar linear trend with slightly higher intensities than the basaltic compositions. The small difference may indicate a slight difference in the Fe–oxygen environment. Significant changes in the Fe environment should be reflected more strongly in the intensity of the pre-edge. It should be also noted that any compositional effect on the Fe coordination in the glass may be superimposed by effects related to differences in the thermal history of the samples. For experimental reasons (see also above), some of the annealing experiments, especially the most reduced and oxidised, had to be performed at lower temperature than the rest (1300–1350 8C). However, we consider the effects to be rather small because the structural state of the resulting glass should be essentially determined by the quench rate at the glass

Variations of the pre-edge of Fe in glass samples as a function of the oxidation state cannot be modelled using crystalline model compounds as already mentioned by Berry et al. P(2003). Therefore, the relationship between Fe3+/ Fe
100 and centroid position has to be calibrated empirically with samples of known oxidation state. As the local structural environment of Fe in a glass may vary with composition, the relationship established for a single composition is potentially not transferable to other compositions. In Fig. 9, the variation of the pre-edge-centroid position is shown as a function of the Fe oxidation state determined by Mfssbauer spectroscopy. The trend of the data suggests a sublinear variation at Fe3+/ P Fe
100 b50 but a clear P deviation from this trend is observed at high Fe3+/ Fe
100, in contrast to the completely linear trend found P by Berry et al. (2003). The relation between Fe3+/ Fe
100 and the centroid position can be described by a quadratic polynomial. The fit together with the parameters is depicted in Fig. 9, as well. The difference in the trend to the one in the study of Berry et al. (2003) may be related to the differences in the pre-edge-background subtraction protocol. In addition, the energy resolution of the spectrometer used in our study is much higher than that used by Berry et al. (2003), yielding betterresolved pre-edge peaks (Fig. 2) with a more robust separation of the pre-edge peaks from the background, which influences the resulting intensities and centroid positions. The centroid positions of the sodium–silicate glasses follow a trend parallel to the one of the basaltic glasses but shifted to slightly higher values. Although the observed differences are small and still within the uncertainty of the data, they already indicate that the bulk composition may have an indirect influence on the centroid positions. This influence of the glass composition on the centroid

M. Wilke et al. / Chemical Geology 213 (2004) 71–87

83

P Fig. 8. Centroid position as a function of Fe3+/ Fe
100, determined by Mfssbauer spectroscopy. Black circles: basaltic compositions, triangles: NS2 and NS3 glasses. Data of NS2 and NS3 are only shown for comparison and were not used for the fit. Heavy black line shows the quadratic function fitted to the data with the parameters indicated in the box. Grey lines outline the 95% confidence interval of the fitted function. Heavy dashed line shows the variation curve that refers to the grey line in Fig. 7 (centroid position calculated from completely reduced and oxidised end-members).

Fig. 9. Variation of extracted pre-edges with Fe oxidation state for glasses with the composition 7159VB and 7159VB-P6. The spectra P shown are the same as in Fig. 6 (range of Fe3+/ Fe
100 between 0 and 93). The peak on the high-energy side of the pre-edge decreases P with decreasing Fe3+/ Fe
100. Vertical dashed lines indicate energy ranges used for integration for determining I(Fe3+) and I(Fe2+) (see also text).

trend reflects the effects of the bulk composition on the Fe co-ordination (as discussed above), as changes in the pre-edge intensity affect also the centroid. Fe co-ordination is mostly controlled by the main network modifying cations, such as alkalis and alkali earths (e.g., Brown et al., 1995). Although this study does not show systematically the strength of such compositional effects, the dependence implies that application of XANES to precisely determine the oxidation state of Fe in glass or melt relies strongly on the availability of a calibration for the system of interest. However, the compositional differences between the basaltic and sodium silicate glasses used here are huge as compared to compositional variations among different members of natural basaltic glasses. Hence, it seems P reasonable to apply the relation between Fe3+/ Fe
100 and centroid position to natural basaltic compositions until more data are available about the effect of the glass composition. A potential way to circumvent costly calibration procedures would be to model the variation of the pre-edge centroid position by using only spectra of completely reduced and completely oxidised glasses

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of a given composition, i.e., the end-members of the variation trend. To test this possibility, we calculated the variation of the centroid position and intensity of the pre-edge by using the values determined for the oxidised and reduced end-member, respectively. The intensity value for the reduced end-member is taken from sample 7159V-CS and the intensity value for the oxidised one is interpolated from samples 7159VB-O2 and NS2-O2. The centroid positions are 7112 and 7113.5 eV, respectively. The resulting curve calculated from these values is shown on the variogram in Fig. 7 by the grey solid line. As for the other binary mixtures on that graph, this curve shows some significant curvature and clearly deviates from the linear trend of the experimentally determined pre-edge parameters of the glasses. Apparently, a linear trend on this graph cannot be produced by binary mixtures of two end-members, unless they have same intensities (which is not the case for these glasses). Still, the variation of the centroid position of the calculated curve in Fig. 8 - compares well to the one that results from the fit to the experimental data (Fig. 9), so that both calibrations can be used to determine the Fe oxidation state.

By using the ratio of the intensities of Fe3+ and Fe2+, i.e., I(Fe3+)/I(Fe2+), the parameter variation range is considerably enlarged (Fig. 10a). The observed trend strongly deviates from linearity and the data show a significant scatter for oxidised samples. The trend of

6.2. Determination of Fe oxidation state using intensity ratios The procedure to determine the Fe oxidation state proposed above is limited by the uncertainty in the centroid position (F0.05 eV). However, the intensities of the pre-edge features can be measured with a considerably higher precision. Thus, a considerable improvement can be achieved by using the ratios of the intensities measured at energy positions corresponding to the peak positions of Fe3+ and Fe2+, respectively. A similar approach was used in the procedure used by van Aken et al. (1998) to determine Fe oxidation states from electron energy-loss spectra at the L2,3 edge. The intensities were determined by integrating the extracted pre-edge spectra at two positions: (i) between 7113 and 7114 eV for the contribution of Fe3+: I(Fe3+) and (ii) between 7111.2 and 7112.2 eV for the contribution of Fe2+: I(Fe2+) (cf. Fig. 7). The position of the integration windows was set at positions where the difference of the spectra of the most oxidised and most reduced samples is largest (see Fig. 7).

Fig. 10. (a) Variation of the intensity ratio I(Fe3+)/I(Fe2+) as a function of Fe oxidation state. Dashed line shows the fit on the complete dataset. Solid line shows the linear fit to the data with P Fe3+/ Fe
100 b45. Fitted equations and parameters are shown in the inset. Black circles: all basaltic glasses; grey triangles: NS3 and NS2 glasses. (b) Variation of the intensity ratio I(Fe 3+ )/ [I(Fe3+)+I(Fe2+)] as a function of Fe oxidation state. Solid line shows the fit on the complete data set. Symbols as panel (a).

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the complete data set may be described by a fourth degree polynomial, P as shown in Fig. 10a. However, samples with Fe3+/ Fe
100 b45 plot on a welldefined linear trend with very little scatter. The large scatter in the case of oxidised samples is probably due to the very small contribution from ferrous iron to the spectra. Hence, the fit model used to extract the preedges is not able to robustly discriminate between contributions from the pre-edge background and from the pre-edge itself. This effect is already visible to a small extent in the variation of the centroid position and is just emphasized by the use of intensity ratios. A parameter that might be less sensitive to this problem can be found by plotting the intensity of Fe3+ over the sum of intensities of Fe3+ and Fe2+, i.e., I(Fe3+)/ [I(Fe3+)+I(Fe2+)]. The data plot on a slightly curved trend with only slight scatter even for oxidised samples and may be well described by a quadratic polynomial (Fig. 10b). All samples of basaltic compositions plot on the same trend on both graph in Fig. 10, further supporting that the variation in bulk composition of this suite of samples has little to no effect on the variation trend of the pre-edge. However, intensity ratios taken from the spectra of the sodium silicate glasses plot slightly but significantly outside the trend defined by the basaltic samples. This confirms the findings from Fig. 8 suggesting an effect of the bulk composition on the pre-edge and thus on the Fe co-ordination polyhedron. For the centroid position, this effect is small compared to the uncertainties as shown in Fig. 9. This implies that the use of the I(Fe3+)/[I(Fe3+)+I(Fe2+)] and our calibration for the determination of the oxidation state will probably provide reliable estimates for basaltic compositions. However, it may yield a systematic shift if applied to glass compositions that are very different from those used here.

7. Conclusions As already shown in the literature, the pre-edge of Fe K-edge XANES spectra can be used as a sensitive probe for the oxidation state of Fe in various silicate materials. The determination of the oxidation state in glass is less straightforward due to the variable local structural environment of Fe and its complex relationship to the bulk composition. In the worst case, a

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calibration of the pre-edge variation for the composition of interest seems to be unavoidable. As the calibration given here was performed on a synthetic, slightly simplified basaltic system, it should be applicable to a large variety of similar compositions occurring in natural samples. If the centroid P C of a preedge of a given glass is known, the Fe3+/ Fe
100 may be determined from the following equation: Fe3þ =

X

Fe
100 ¼ ½  0:028 þ ð0:000784 þ 0:00052
ð7112  CÞÞ0:5  =  0:00026

with 7112 eVVCV7113:5 eV Although only a few samples with different compositions were tested, these samples indicate that the influence of the glass composition on the calibration may not be too strong. Therefore, extrapolation of this calibration to other glass compositions might be safe, provided the compositional differences are not too large. However, all glasses examined here are rather depolymerised, and thus it is not clear whether the compositional influence becomes stronger for more polymerised compositions, such as dacitic or rhyolitic glasses. A more sensitive method for the determination of the oxidation state may be achieved by the use of the ratio of the pre-edge intensities for Fe3+ and Fe2+, respectively. The best correlation is obtained in a plot of I(Fe3+)/[I(Fe3+)+I(Fe2+)] as a function of the oxidation state. This ratio is certainly more sensitive to changes in the oxidation state, simply because the uncertainty is much smaller and the maximum possible value range is much larger than for the centroid position. If I(Fe3+)/[I(Fe3+)+I(Fe2+)] is known from integrating the extracted pre-edge spectra at the integration windows described above, the Fe oxidation state may be determined from the equation: Fe3þ =

X

Fe
100 ¼ ½0:01 þ ð0:0001 þ 1:32E  04


ð0:25  RÞÞ0:5 =  6:6E  05       ð 0:25VRV0:92Þ with R ¼ I Fe3þ = I Fe3þ þI Fe2þ The pre-edge intensities observed throughout the suite of samples indicate a distribution of Fe

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co-ordination over several types of polyhedra with an average centred around a co-ordination of five. Further assignments to provide more precise information on the mixture of coordination polyhedra (octahedra, tetrahedra, trigonal di-pyramids) are not possible as only an average co-ordination number may be estimated from the pre-edge intensity. The pre-edge gives a first idea on the co-ordination chemistry, but more precise information on the structural environment of Fe in the glass may be gained by including also the EXAFS region of the spectra to the analysis.

Acknowledgements The support of E. Welter (HASYLAB) during beam time is highly appreciated. We thank A. Mqller (Universit7t Potsdam) for the wet chemical FeO measurements. Critical reviews of Fabrice Gaillard and an anonymous reviewer are highly appreciated. This study was supported by the Deutsche Forschungsgemeinschaft (DFG) through projects Wi2000/1-1 and La-1164/3-1). [RR] References Alberto, H.V., Pinto da Cunha, J.L., Mysen, B.O., Gil, J.M., Ayres de Campos, N., 1996. Analysis of Mfssbauer spectra of silicate glasses using a two-dimensional Gaussian distribution of hyperfine parameters. J. Non-Cryst. solids 194, 48 – 57. Attenkofer, K., Brqggmann, U., Haack, N., Herrmann, M., Kappen, P., Welter, E., 2000. Improvements for absorption spectroscopy at beamlines A1, E4, X1. HASYLAB Annual Report, pp. 63 – 68. Bajt, S., Sutton, S.R., Delany, J.S., 1994. X-ray microprobe analysis of iron oxidation states in silicates and oxides using X-ray absorption near edge structure (XANES). Geochim. Cosmochim. Acta 58, 5209 – 5214. Berry, A.J., O’Neill, H.S.C., Jayasuriya, K.D., Cambell, S.J., Foran, G.J., 2003. XANES calibrations for the oxidation state of iron in a silicate glass. Am. Mineral. 88, 976 – 977. Bonnin-Mosbah, M., Simionovici, A.S., Me´trich, N., Duraud, J.-P., Massare, D., Dillmann, P., 2001. Iron oxidation state in silicate glass fragments and glass inclusions with a XANES microprobe. J. Non-Cryst. solids 288, 103 – 113. Brown Jr., G.E., Farges, F., Calas, G., 1995. X-ray scattering and X-ray spectroscopy studies of silicate melts. In: Stebbins, J.F., McMillan, P.F., Dingwell, D.B. (Eds.), Structure, Dynamics and Properties of Silicate Melts, Reviews in Mineralogy, vol. 32, pp. 317 – 410.

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