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Fetot ratio from XAS spectra at the Fe K-edge
Author’s Accepted Manuscript Accurate Fe3+/Fetotratio from XAS spectra at the Fe K-edge Federico Galdenzi, Giancarlo Della Ventura, Giannantonio Cibin, Salvatore Macis, Augusto Marcelli www.elsevier.com/locate/radphyschem
PII: DOI: Reference:
S0969-806X(18)30934-4 https://doi.org/10.1016/j.radphyschem.2018.12.008 RPC8088
To appear in: Radiation Physics and Chemistry Received date: 25 October 2018 Revised date: 30 November 2018 Accepted date: 9 December 2018 Cite this article as: Federico Galdenzi, Giancarlo Della Ventura, Giannantonio Cibin, Salvatore Macis and Augusto Marcelli, Accurate Fe 3+/Fetotratio from XAS spectra at the Fe K-edge, Radiation Physics and Chemistry, https://doi.org/10.1016/j.radphyschem.2018.12.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Accurate Fe3+/Fetot ratio from XAS spectra at the Fe K-edge 1,2
1,2
3
4,5
Federico Galdenzi , Giancarlo Della Ventura , Giannantonio Cibin , Salvatore Macis , Augusto Marcelli
2,5
1
Department Sciences, University of Roma Tre, Rome, Italy 2 INFN-LNF, P.O. Box 13, 00044 Frascati (Rome), Italy 3 Diamond Light Source, Harvell Science and Innovation Campus, Didcot, UK 4 Università di Tor Vergata, Via della Ricerca Scientifica 1, 00133 5 RICMASS, Rome International Center for Materials Science Superstripes, Rome, Italy
Abstract A new approach to extract accurate information on the Fe oxidation state has been tested on three different Fe-rich amphiboles annealed at 194 °C and 520 °C. The method is based on the analysis of the preedge structure of the X-ray absorption spectra at the Fe-K edge. The pre-edge band is deconvolved into four Gaussian components where the integrated intensities of the pairs of lower- and higher-energy bands are 3+ related to the ferrous to ferric ion ratio. These Fe /Fetot ratios have been compared with those obtained with independent techniques (X-ray diffraction and Mossbauer spectroscopy) on the same samples. The experimental data allowed establishing a calibration curve that is in good agreement with a recent statistical analysis based on spectroscopic data for the same type of minerals. The method could be extended to the 3+ 2+ analysis of other complex systems containing both Fe and Fe in octahedral coordination or to study the dynamics of iron under non-ambient conditions. 1.
Introduction
Iron is one of the most abundant elements on the Earth having variable oxidation state that is extremely sensitive to the environment of formation of Fe-bearing minerals. Development of methods for the 3+ 2+ microanalysis of Fe /Fe thus remains a high priority in both Material and Geological Sciences, since this information can provide key insights in parameters like temperature, pressure, redox state, f O2, pH, etc. In the last two decades, also due to the increasing worldwide availability of beamlines dedicated to X-ray Absorption 3+ Spectroscopy (XAS), a growing number of studies has been dedicated to the determination the Fe /Fetot ratio in minerals and glasses. Bajt et al. [1] noticed that the pre-edge peak energy increased between minerals with a different Fe valence and pointed out that the energy shift could be used to measure Fe valence. Petit et al. [2] used the total integrated area of different contributions obtained by fitting the pre-edge structures, considering also the different iron speciation and coordination. Wilke et al. [3] later observed that changes in the pre-edge peak intensity were a function of coordination. More recently, Darby Dyar et al. [4] presented a multivariate analysis of polarized pre-edge XAS spectra, in combination with Mössbauer data for a large set of Fe-bearing amphiboles, a group of important rock-forming silicates having an extremely complex crystalchemistry [5]. We recently investigated the oxidation behaviour of a synthetic potassic-ferro-richterite 2+ [Na(NaCa)Fe 5Si8O22(OH)2] annealed up 750° C by using simultaneous X-ray absorption spectroscopy and Xray diffraction experiments with synchrotron radiation. [6] A key issue of this study using powder samples has been the analysis of the pre-edge features to characterize all local electronic and structural changes associated 3+ to the variation of the Fe /Fetot ratio in this material where at room temperature iron atoms are in octahedral sites. In this work we describe our approach, based on an accurate description of the 1s ->3d/4s transitions of Fe atoms in distorted octahedral coordination, extending the results presented in [6]. To this purpose, we compare the results obtained on three amphiboles: the synthetic potassic-ferro-richterite (hereafter PFR) 2+ 3+ studied in [6], a riebeckite (hereafter RI) with ideal composition Na2(Fe 3Fe 2)Si8O22(OH)2 whose crystal chemistry has been examined in Ref.s [7,8] and a synthetic ferri-clino-ferroholmquistite (hereafter FCF), with 2+ 3+ composition Li2(Fe 3Fe 2)Si8O22(OH)2 [9]. We performed experiments vs. temperature since all these materials undergo a deprotonation process where part of ferrous irons oxidizes to ferric iron [10,11]. The 3+ experiments have been aimed at following the evolution of the Fe /Fetot ratio that, according to previous studies [8,10,11] is correlated to the temperature conditions. Besides the characterization of the thermal 3+ process, the obtained results allow establishing a calibration curve for monitoring the Fe /Fetot ratio for this type of silicate systems. We underline that polarization effects are not considered in this study because we investigated exclusively powders, i.e. randomly oriented crystallites.
2. Methodology The three studied samples are described in the supplementary materials. All XAS measurements were performed at B18, the XAS beamline at the Diamond light source (Oxford, UK), in transmission mode in the temperature range from Room Temperature (RT) to 750° C. Data were collected in continuous scan mode using a double Si (111) crystals monochromator. Samples were prepared as pellets consisting of ~10 mg of amphibole powder mixed with 150 mg of boron nitride. Additional information on the experimental methods is available in Ref. [6]. All XAS spectra were normalized at the edge step using the Athena program [12]. To extract the iron pre-edge contributions for iron in the octahedral site we compared two methods: four Gaussian curves [13] or four pseudo-Voigt functions with a 50:50 Gaussian-Lorentzian contribution, each couple representing the contribution of ferrous and ferric ions, respectively [14], as detailed below. The extraction of the background in the pre-edge region was based on the use of an arctangent function. Additional information is provided in the supplementary materials. Four contributions have been considered to account for the pre-edge structures arising from electronic transitions from the 1s > 3d-4s orbitals. In brief, 2+ the pre-edge region of a compound containing octahedral Fe is expected to show three peaks corresponding to the T1g and T2g states while a fourth predicted transition, A2g, is not visible being a low probability two3+ electron transition. For Fe in octahedral coordination, two electronic transitions are expected, i.e., those for 3+ octahedral Fe in T2g and Eg state [15]. In our approach, we decomposed the pre-edge by using two 2+ 3+ contributions separated in energy for Fe and Fe . For each ion we thus considered two Gaussian curves for 2+ 3+ the Fe from the core level to the T1g and T2g orbitals, and two others for Fe (from 1s to T2g and Eg), as schematically shown in Fig. 1, left panel.
Figure 1 – (Left) Schematic representation of the electronic transitions contributing to the pre-edge structure (left) of an iron atom in an octahedral coordination. The corresponding contributions used to fit the pre-edge structure at the Fe K-edge of the riebeckite spectrum collected at 194 °C are shown in the right panel.
We treated the experimental data by using a routine built to this purpose. Preliminary tests showed that Gaussian curves were the better choice to fit the pre-edge transitions compared to pseudo-Voigt profiles [6]. During refinement, each component was left free to vary its energy position in a range of ± 0.75 eV from the initial value, and the FWHM was constrained to be constant at ~1.5 eV [6]; the intensities were left free to 3+ vary. The final integrated intensities (area) were used to calculate the Fe /Fetot ratio following the formula
2+
3+
where A1, A2 and A3, A4 are the areas of the two Fe and Fe contributions, respectively. 3.
Results and discussion
From the result of the spectra decompositions we extracted the weight of each iron species in the three samples, measured at the temperature of 194 °C, and 520 °C and labelled, respectively, LT and HT. For the fit at HT we hypothesized that the T increase does not change significantly the absolute energy position of each Gaussian component, and thus we reduced the range of variability of the four components to ±0.2 eV. In Fig. 2 we compare all pre-edge structures and their fits. The immediate information is that there is a clear variation 3+ of the spectra vs. temperature with an increase of the weight of the Fe species. It is important to underline here that the residual of the fit is extremely low, except for the pre-edge spectra at HT, where a residual
2
difference appears around 7118 eV. This feature could be due to the extraction of the background, a feature that is particularly critical at the rising edge region. However, in these samples the discrepancy could be also related to an additional weak contribution of iron in a distorted structural site (M4) that has not been considered in the fitting process and that might be present at high temperature [11]. The possible presence of this weak contribution does not represent a critical issue for our analysis and will be discussed in a forthcoming publication devoted to the evolution vs. temperature of these materials [6]. Data obtained by fitting the pre-edge region are compared in Table 1 with Mössbauer data already available for these samples [9,10,11]. The comparison shows that while similar results are obtained for potassic-ferrorichterite, differences are observed for the other two samples. In a recent paper Darby Dyar et al. [4] showed 3+ that a multivariate analysis of the XAS spectra provides Fe /Fetot values consistent with values obtained by Mössbauer spectroscopy. Moreover, the calibration curves provided in their work (see Fig. 5 in ref. 4) suggests 3+ 2+ that local structural and electronic changes associated to the site occupancy by Fe vs. Fe are clearly 3+ detectable by XAS spectra and can be reliably used to measure the Fe /Fetot ratio.
Figure 2 - Pre-edge features for the samples investigated at the temperature of 194 °C (top) and at 520 °C (bottom). The red curves are the contributions associated to Fe2+ while the green curves are the contributions associated to Fe3+.
Following the approach of [4] we plotted our XAS-derived data vs. Mössbauer data collected for the same samples in a similar way (Fig. 3). Although based on a limited number of points, our calibration curve is in excellent agreement with that provided in [4]. Therefore, the results we obtained suggest that the method is effective in providing the iron oxidation state in amphiboles and could be possibly extended to other complex systems containing iron in octahedral sites with a high degree of confidence. Table 1 - Fe3+/Fetot calculated using the method discussed in section 2 compared with data from Mössbauer spectra (right). σ is the standard deviation of the Fe3+/Fetot ratio obtained by XAS and Mössbauer data. XAS Mössbauer PFR RI FCF PFR [10] RI [11] FCF [9] Fe3+/Fetot σ Fe3+/Fetot σ Fe3+/Fetot σ Fe3+/Fetot σ Fe3+/Fetot σ Fe3+/Fetot σ 194 °C 0,36 ±0,04 0,44 ±0,02 0,45 ±0,02 0,29 ±0,01 0,37 ±0,02 0,40 ±0,02 520 °C 0,46 ±0,03 0,62 ±0,06 0,64 ±0,1 0,48 ±0,02 0,75 ±0,04 N/A
As outlined above, the analysis that we just discussed has been performed on powdered samples where x-ray pleochroism effects can be neglected, and is thus valid only when working in these conditions. We anticipate that we did preliminary tests on single-crystals and indeed observed a clear effect of the sample orientation
3
on the peak intensity in the pre-edge region; this issue, however, needs to be investigated in detail and is behind the scope of this paper. 4.
Conclusions
In this work we analysed the pre-edge structure of XAS spectra collected for three amphiboles heated up to 520 °C. After an accurate procedure of background subtraction, the pre-edge structure was fitted using four 2+ 3+ contributions representative of the Fe and Fe configurations of iron in an octahedral geometry. The 3+ Fe /Fetot ratio was evaluated from the relative integrated intensities of the fitted components. Previous 3+ statistical work on similar compounds shows that Fe /Fetot values from XAS may differ from those provided by Mössbauer up to the 20 % [4,6]. Combining the results from the pre-edge analysis and Mössbauer spectroscopy we obtained the calibration curve in Figure 3; our curve is in excellent agreement with that provided in [4], based on a large data set of polarized XAS from amphiboles spanning a wide range of compositions. Our work demonstrates, thanks to a good energy resolution and the high S/N ratio of the XAS spectra in the pre-edge region, that the extraction of both electronic and structural information from the preedge is feasible.
Figure 3 – Relationship between the Fe3+ content from the pre-edge analysis (in % of total iron) versus the Mössbauer data for the three investigated samples. The calibration curve obtained from our data (black line) is compared with blue and red curves from Ref. [4] 3+
The same approach can be used also to investigate the iron dynamics following the changes of the Fe /Fetot ratio as discussed in [6]. Work is in progress to extend this approach to other systems where iron may be simultaneously present in different local geometrical coordination. References [1] [2] [3] [4]
[5] [6] [7] [8]
Bajt, S., Sutton, S.R., and Delaney, J.S. Microanalysis of iron oxidation states in silicates and oxides using X-ray absorption near edge structure (XANES). Geoch. Cosmoch. Acta, 58, 5209–5214 (1994). Petit, F. Farges, M. Wilke, V. Solè, Determination of the iron oxidation state in Earth materials using XANES pre-edge information, J. Synch. Rad, 8, 952-954 (2001) M. Wilke, G.M. Partzsch, R. Bernhardt, D. Lattard, Determination of the iron oxidation state in basaltic glasses using XANES at the Kedge, Chemical Geology 213, 71-87 (2004) M. Darby Dyar, E.A. Breves, M.E. Gunter, A. Lanzirotti, J.M. Tucker, C.J. Carey, S.E. Pee, E.B. Brown, R. Oberti, M. Lerotic, and J.S. Delaney, Use of multivariate analysis for synchrotron micro-XANES analysis of iron valence state in amphiboles, Am. Mineral. 101, 1171–1189 (2016) F. C. Hawthorne, R. Oberti, Amphiboles: Crystal Chemistry, Reviews in Mineralogy and Geochemistry, 67, 1-54 (2007) G. Della Ventura, F. Galdenzi, G. Cibin, R. Oberti, W. Xu, S. Macis, A. Marcelli, Iron oxidation dynamics vs. temperature of synthetic potassic-ferro-richterite: a XANES investigation, Phys. Chem. Chem. Phys. (2018) DOI: 10.1039/c8cp04249 U. Susta, G. Della Ventura, F.C. Hawthorne, Y.A. Abdu, M.C. Day, B. Mihailova, R. Oberti, The Crystal Chemistry of riebeckite, ideally Na2Fe32+Fe23+Si8O22(OH)2: a multidisciplinary study, Mineralogical Magazine (2018) G. Della Ventura, B. Mihailova, U. Susta, M. C. Guidi, A. Marcelli, J. Schulter, R. Oberti, The dynamics of Fe oxidation in riebeckite: A model for amphiboles, American Mineralogist, Vol 103 (2018)
4
[9] [10] [11]
[12] [13]
[14]
[15]
G. Iezzi, G. Della Ventura, G. Pedrazzi, J.L. Robert, R. Oberti, Synthesis and characterisation of ferri-clinoferroholmquistite, Li2(Fe32+Fe23+)Si8O22 (OH)2, Eur. J. Mineral (2003) R. Oberti, M. Boiocchi, M. Zema, G. Della Ventura Synthetic Potassic-Ferro-Richterite:1. Composition, Crystal Structure Refinement, and HT Behaviour by in Operando Single-Crystal X-Ray Diffraction, The Canad. Mineral., 54, 353-369 (2016). R. Oberti, M. Boiocchi, M. Zema, F.C. Hawthorne, G.J. Redhammer, U. Susta, G. Della Ventura, The high-temperature behaviour of riebeckite: expansivity, deprotonation, selective Fe oxidation and a novel cation disordering scheme for amphiboles, Eur. J. Mineral, (2018) B. Ravel and M. Newville, ATHENA, ARTEMIS, HEPHAESTUS: data analysis for X-ray absorption spectroscopy using IFEFFIT, J. Synchr. Rad. 12, 537–541 (2005) doi:10.1107/S0909049505012719 M.S. Platunov, N.V. Kazak, Yu.V. Knyazev, L.N. Bezmaternykh, E.M. Moshkina, A.L. Trigub, A.A. Veligzhanin, Y.V. Zubavichus, L.A. Solovyov, D.A. Velikanov, S.G. Ovchinnikov, Effect of Fe-substitution on the structure and magnetism of single crystals Mn2-xFexBO4. J. Cryst. Growth 475, 239-246 (2017) N. Bolfan-Casanova, M. Muñoz, C. McCammon, E. Deloule, A. Fêrot, S. Demouchy, L. France, D. Andrault, S. Pascarelli, Ferric iron and water incorporation in wadsleyite under hydrous and oxidizing conditions: A XANES, Mössbauer, and SIMS study, Amer. Mineral. 97, 1483-1493 (2012) Berry, A.J., Yaxley, G.M., Woodland, A.B., and Foran, G.J. A XANES calibration for determining the oxidation state of iron in mantle garnet. Chemical Geology, 278, 31–37. (2010)
5
Highlights: 3+/
Extraction of the Fe Fetot ratio from the pre-edge structure at the Fe K-edge Fitting of the pre-edge with 4 Gaussians representative of ferric and ferrous ion Calibration line combining the pre-edge analysis and Mössbauer spectroscopy data Method suitable to investigate Fe dynamics in complex systems such as amphiboles
6
PII: DOI: Reference:
S0969-806X(18)30934-4 https://doi.org/10.1016/j.radphyschem.2018.12.008 RPC8088
To appear in: Radiation Physics and Chemistry Received date: 25 October 2018 Revised date: 30 November 2018 Accepted date: 9 December 2018 Cite this article as: Federico Galdenzi, Giancarlo Della Ventura, Giannantonio Cibin, Salvatore Macis and Augusto Marcelli, Accurate Fe 3+/Fetotratio from XAS spectra at the Fe K-edge, Radiation Physics and Chemistry, https://doi.org/10.1016/j.radphyschem.2018.12.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Accurate Fe3+/Fetot ratio from XAS spectra at the Fe K-edge 1,2
1,2
3
4,5
Federico Galdenzi , Giancarlo Della Ventura , Giannantonio Cibin , Salvatore Macis , Augusto Marcelli
2,5
1
Department Sciences, University of Roma Tre, Rome, Italy 2 INFN-LNF, P.O. Box 13, 00044 Frascati (Rome), Italy 3 Diamond Light Source, Harvell Science and Innovation Campus, Didcot, UK 4 Università di Tor Vergata, Via della Ricerca Scientifica 1, 00133 5 RICMASS, Rome International Center for Materials Science Superstripes, Rome, Italy
Abstract A new approach to extract accurate information on the Fe oxidation state has been tested on three different Fe-rich amphiboles annealed at 194 °C and 520 °C. The method is based on the analysis of the preedge structure of the X-ray absorption spectra at the Fe-K edge. The pre-edge band is deconvolved into four Gaussian components where the integrated intensities of the pairs of lower- and higher-energy bands are 3+ related to the ferrous to ferric ion ratio. These Fe /Fetot ratios have been compared with those obtained with independent techniques (X-ray diffraction and Mossbauer spectroscopy) on the same samples. The experimental data allowed establishing a calibration curve that is in good agreement with a recent statistical analysis based on spectroscopic data for the same type of minerals. The method could be extended to the 3+ 2+ analysis of other complex systems containing both Fe and Fe in octahedral coordination or to study the dynamics of iron under non-ambient conditions. 1.
Introduction
Iron is one of the most abundant elements on the Earth having variable oxidation state that is extremely sensitive to the environment of formation of Fe-bearing minerals. Development of methods for the 3+ 2+ microanalysis of Fe /Fe thus remains a high priority in both Material and Geological Sciences, since this information can provide key insights in parameters like temperature, pressure, redox state, f O2, pH, etc. In the last two decades, also due to the increasing worldwide availability of beamlines dedicated to X-ray Absorption 3+ Spectroscopy (XAS), a growing number of studies has been dedicated to the determination the Fe /Fetot ratio in minerals and glasses. Bajt et al. [1] noticed that the pre-edge peak energy increased between minerals with a different Fe valence and pointed out that the energy shift could be used to measure Fe valence. Petit et al. [2] used the total integrated area of different contributions obtained by fitting the pre-edge structures, considering also the different iron speciation and coordination. Wilke et al. [3] later observed that changes in the pre-edge peak intensity were a function of coordination. More recently, Darby Dyar et al. [4] presented a multivariate analysis of polarized pre-edge XAS spectra, in combination with Mössbauer data for a large set of Fe-bearing amphiboles, a group of important rock-forming silicates having an extremely complex crystalchemistry [5]. We recently investigated the oxidation behaviour of a synthetic potassic-ferro-richterite 2+ [Na(NaCa)Fe 5Si8O22(OH)2] annealed up 750° C by using simultaneous X-ray absorption spectroscopy and Xray diffraction experiments with synchrotron radiation. [6] A key issue of this study using powder samples has been the analysis of the pre-edge features to characterize all local electronic and structural changes associated 3+ to the variation of the Fe /Fetot ratio in this material where at room temperature iron atoms are in octahedral sites. In this work we describe our approach, based on an accurate description of the 1s ->3d/4s transitions of Fe atoms in distorted octahedral coordination, extending the results presented in [6]. To this purpose, we compare the results obtained on three amphiboles: the synthetic potassic-ferro-richterite (hereafter PFR) 2+ 3+ studied in [6], a riebeckite (hereafter RI) with ideal composition Na2(Fe 3Fe 2)Si8O22(OH)2 whose crystal chemistry has been examined in Ref.s [7,8] and a synthetic ferri-clino-ferroholmquistite (hereafter FCF), with 2+ 3+ composition Li2(Fe 3Fe 2)Si8O22(OH)2 [9]. We performed experiments vs. temperature since all these materials undergo a deprotonation process where part of ferrous irons oxidizes to ferric iron [10,11]. The 3+ experiments have been aimed at following the evolution of the Fe /Fetot ratio that, according to previous studies [8,10,11] is correlated to the temperature conditions. Besides the characterization of the thermal 3+ process, the obtained results allow establishing a calibration curve for monitoring the Fe /Fetot ratio for this type of silicate systems. We underline that polarization effects are not considered in this study because we investigated exclusively powders, i.e. randomly oriented crystallites.
2. Methodology The three studied samples are described in the supplementary materials. All XAS measurements were performed at B18, the XAS beamline at the Diamond light source (Oxford, UK), in transmission mode in the temperature range from Room Temperature (RT) to 750° C. Data were collected in continuous scan mode using a double Si (111) crystals monochromator. Samples were prepared as pellets consisting of ~10 mg of amphibole powder mixed with 150 mg of boron nitride. Additional information on the experimental methods is available in Ref. [6]. All XAS spectra were normalized at the edge step using the Athena program [12]. To extract the iron pre-edge contributions for iron in the octahedral site we compared two methods: four Gaussian curves [13] or four pseudo-Voigt functions with a 50:50 Gaussian-Lorentzian contribution, each couple representing the contribution of ferrous and ferric ions, respectively [14], as detailed below. The extraction of the background in the pre-edge region was based on the use of an arctangent function. Additional information is provided in the supplementary materials. Four contributions have been considered to account for the pre-edge structures arising from electronic transitions from the 1s > 3d-4s orbitals. In brief, 2+ the pre-edge region of a compound containing octahedral Fe is expected to show three peaks corresponding to the T1g and T2g states while a fourth predicted transition, A2g, is not visible being a low probability two3+ electron transition. For Fe in octahedral coordination, two electronic transitions are expected, i.e., those for 3+ octahedral Fe in T2g and Eg state [15]. In our approach, we decomposed the pre-edge by using two 2+ 3+ contributions separated in energy for Fe and Fe . For each ion we thus considered two Gaussian curves for 2+ 3+ the Fe from the core level to the T1g and T2g orbitals, and two others for Fe (from 1s to T2g and Eg), as schematically shown in Fig. 1, left panel.
Figure 1 – (Left) Schematic representation of the electronic transitions contributing to the pre-edge structure (left) of an iron atom in an octahedral coordination. The corresponding contributions used to fit the pre-edge structure at the Fe K-edge of the riebeckite spectrum collected at 194 °C are shown in the right panel.
We treated the experimental data by using a routine built to this purpose. Preliminary tests showed that Gaussian curves were the better choice to fit the pre-edge transitions compared to pseudo-Voigt profiles [6]. During refinement, each component was left free to vary its energy position in a range of ± 0.75 eV from the initial value, and the FWHM was constrained to be constant at ~1.5 eV [6]; the intensities were left free to 3+ vary. The final integrated intensities (area) were used to calculate the Fe /Fetot ratio following the formula
2+
3+
where A1, A2 and A3, A4 are the areas of the two Fe and Fe contributions, respectively. 3.
Results and discussion
From the result of the spectra decompositions we extracted the weight of each iron species in the three samples, measured at the temperature of 194 °C, and 520 °C and labelled, respectively, LT and HT. For the fit at HT we hypothesized that the T increase does not change significantly the absolute energy position of each Gaussian component, and thus we reduced the range of variability of the four components to ±0.2 eV. In Fig. 2 we compare all pre-edge structures and their fits. The immediate information is that there is a clear variation 3+ of the spectra vs. temperature with an increase of the weight of the Fe species. It is important to underline here that the residual of the fit is extremely low, except for the pre-edge spectra at HT, where a residual
2
difference appears around 7118 eV. This feature could be due to the extraction of the background, a feature that is particularly critical at the rising edge region. However, in these samples the discrepancy could be also related to an additional weak contribution of iron in a distorted structural site (M4) that has not been considered in the fitting process and that might be present at high temperature [11]. The possible presence of this weak contribution does not represent a critical issue for our analysis and will be discussed in a forthcoming publication devoted to the evolution vs. temperature of these materials [6]. Data obtained by fitting the pre-edge region are compared in Table 1 with Mössbauer data already available for these samples [9,10,11]. The comparison shows that while similar results are obtained for potassic-ferrorichterite, differences are observed for the other two samples. In a recent paper Darby Dyar et al. [4] showed 3+ that a multivariate analysis of the XAS spectra provides Fe /Fetot values consistent with values obtained by Mössbauer spectroscopy. Moreover, the calibration curves provided in their work (see Fig. 5 in ref. 4) suggests 3+ 2+ that local structural and electronic changes associated to the site occupancy by Fe vs. Fe are clearly 3+ detectable by XAS spectra and can be reliably used to measure the Fe /Fetot ratio.
Figure 2 - Pre-edge features for the samples investigated at the temperature of 194 °C (top) and at 520 °C (bottom). The red curves are the contributions associated to Fe2+ while the green curves are the contributions associated to Fe3+.
Following the approach of [4] we plotted our XAS-derived data vs. Mössbauer data collected for the same samples in a similar way (Fig. 3). Although based on a limited number of points, our calibration curve is in excellent agreement with that provided in [4]. Therefore, the results we obtained suggest that the method is effective in providing the iron oxidation state in amphiboles and could be possibly extended to other complex systems containing iron in octahedral sites with a high degree of confidence. Table 1 - Fe3+/Fetot calculated using the method discussed in section 2 compared with data from Mössbauer spectra (right). σ is the standard deviation of the Fe3+/Fetot ratio obtained by XAS and Mössbauer data. XAS Mössbauer PFR RI FCF PFR [10] RI [11] FCF [9] Fe3+/Fetot σ Fe3+/Fetot σ Fe3+/Fetot σ Fe3+/Fetot σ Fe3+/Fetot σ Fe3+/Fetot σ 194 °C 0,36 ±0,04 0,44 ±0,02 0,45 ±0,02 0,29 ±0,01 0,37 ±0,02 0,40 ±0,02 520 °C 0,46 ±0,03 0,62 ±0,06 0,64 ±0,1 0,48 ±0,02 0,75 ±0,04 N/A
As outlined above, the analysis that we just discussed has been performed on powdered samples where x-ray pleochroism effects can be neglected, and is thus valid only when working in these conditions. We anticipate that we did preliminary tests on single-crystals and indeed observed a clear effect of the sample orientation
3
on the peak intensity in the pre-edge region; this issue, however, needs to be investigated in detail and is behind the scope of this paper. 4.
Conclusions
In this work we analysed the pre-edge structure of XAS spectra collected for three amphiboles heated up to 520 °C. After an accurate procedure of background subtraction, the pre-edge structure was fitted using four 2+ 3+ contributions representative of the Fe and Fe configurations of iron in an octahedral geometry. The 3+ Fe /Fetot ratio was evaluated from the relative integrated intensities of the fitted components. Previous 3+ statistical work on similar compounds shows that Fe /Fetot values from XAS may differ from those provided by Mössbauer up to the 20 % [4,6]. Combining the results from the pre-edge analysis and Mössbauer spectroscopy we obtained the calibration curve in Figure 3; our curve is in excellent agreement with that provided in [4], based on a large data set of polarized XAS from amphiboles spanning a wide range of compositions. Our work demonstrates, thanks to a good energy resolution and the high S/N ratio of the XAS spectra in the pre-edge region, that the extraction of both electronic and structural information from the preedge is feasible.
Figure 3 – Relationship between the Fe3+ content from the pre-edge analysis (in % of total iron) versus the Mössbauer data for the three investigated samples. The calibration curve obtained from our data (black line) is compared with blue and red curves from Ref. [4] 3+
The same approach can be used also to investigate the iron dynamics following the changes of the Fe /Fetot ratio as discussed in [6]. Work is in progress to extend this approach to other systems where iron may be simultaneously present in different local geometrical coordination. References [1] [2] [3] [4]
[5] [6] [7] [8]
Bajt, S., Sutton, S.R., and Delaney, J.S. Microanalysis of iron oxidation states in silicates and oxides using X-ray absorption near edge structure (XANES). Geoch. Cosmoch. Acta, 58, 5209–5214 (1994). Petit, F. Farges, M. Wilke, V. Solè, Determination of the iron oxidation state in Earth materials using XANES pre-edge information, J. Synch. Rad, 8, 952-954 (2001) M. Wilke, G.M. Partzsch, R. Bernhardt, D. Lattard, Determination of the iron oxidation state in basaltic glasses using XANES at the Kedge, Chemical Geology 213, 71-87 (2004) M. Darby Dyar, E.A. Breves, M.E. Gunter, A. Lanzirotti, J.M. Tucker, C.J. Carey, S.E. Pee, E.B. Brown, R. Oberti, M. Lerotic, and J.S. Delaney, Use of multivariate analysis for synchrotron micro-XANES analysis of iron valence state in amphiboles, Am. Mineral. 101, 1171–1189 (2016) F. C. Hawthorne, R. Oberti, Amphiboles: Crystal Chemistry, Reviews in Mineralogy and Geochemistry, 67, 1-54 (2007) G. Della Ventura, F. Galdenzi, G. Cibin, R. Oberti, W. Xu, S. Macis, A. Marcelli, Iron oxidation dynamics vs. temperature of synthetic potassic-ferro-richterite: a XANES investigation, Phys. Chem. Chem. Phys. (2018) DOI: 10.1039/c8cp04249 U. Susta, G. Della Ventura, F.C. Hawthorne, Y.A. Abdu, M.C. Day, B. Mihailova, R. Oberti, The Crystal Chemistry of riebeckite, ideally Na2Fe32+Fe23+Si8O22(OH)2: a multidisciplinary study, Mineralogical Magazine (2018) G. Della Ventura, B. Mihailova, U. Susta, M. C. Guidi, A. Marcelli, J. Schulter, R. Oberti, The dynamics of Fe oxidation in riebeckite: A model for amphiboles, American Mineralogist, Vol 103 (2018)
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[9] [10] [11]
[12] [13]
[14]
[15]
G. Iezzi, G. Della Ventura, G. Pedrazzi, J.L. Robert, R. Oberti, Synthesis and characterisation of ferri-clinoferroholmquistite, Li2(Fe32+Fe23+)Si8O22 (OH)2, Eur. J. Mineral (2003) R. Oberti, M. Boiocchi, M. Zema, G. Della Ventura Synthetic Potassic-Ferro-Richterite:1. Composition, Crystal Structure Refinement, and HT Behaviour by in Operando Single-Crystal X-Ray Diffraction, The Canad. Mineral., 54, 353-369 (2016). R. Oberti, M. Boiocchi, M. Zema, F.C. Hawthorne, G.J. Redhammer, U. Susta, G. Della Ventura, The high-temperature behaviour of riebeckite: expansivity, deprotonation, selective Fe oxidation and a novel cation disordering scheme for amphiboles, Eur. J. Mineral, (2018) B. Ravel and M. Newville, ATHENA, ARTEMIS, HEPHAESTUS: data analysis for X-ray absorption spectroscopy using IFEFFIT, J. Synchr. Rad. 12, 537–541 (2005) doi:10.1107/S0909049505012719 M.S. Platunov, N.V. Kazak, Yu.V. Knyazev, L.N. Bezmaternykh, E.M. Moshkina, A.L. Trigub, A.A. Veligzhanin, Y.V. Zubavichus, L.A. Solovyov, D.A. Velikanov, S.G. Ovchinnikov, Effect of Fe-substitution on the structure and magnetism of single crystals Mn2-xFexBO4. J. Cryst. Growth 475, 239-246 (2017) N. Bolfan-Casanova, M. Muñoz, C. McCammon, E. Deloule, A. Fêrot, S. Demouchy, L. France, D. Andrault, S. Pascarelli, Ferric iron and water incorporation in wadsleyite under hydrous and oxidizing conditions: A XANES, Mössbauer, and SIMS study, Amer. Mineral. 97, 1483-1493 (2012) Berry, A.J., Yaxley, G.M., Woodland, A.B., and Foran, G.J. A XANES calibration for determining the oxidation state of iron in mantle garnet. Chemical Geology, 278, 31–37. (2010)
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Highlights: 3+/
Extraction of the Fe Fetot ratio from the pre-edge structure at the Fe K-edge Fitting of the pre-edge with 4 Gaussians representative of ferric and ferrous ion Calibration line combining the pre-edge analysis and Mössbauer spectroscopy data Method suitable to investigate Fe dynamics in complex systems such as amphiboles
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