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The determination of Ti, Mn and Fe oxidation states in minerals by electron energy-loss spectroscopy
Ultramicroscopy 18 (1985) 285-290 North-Holland, Amsterdam
285
THE DETERMINATION OF Ti, Mn AND Fe OXIDATION STATES IN MINERALS BY ELECTRON ENERGY-LOSS SPECTROSCOPY Max T. OTTEN, Barbara MINER, James H. RASK and Peter R. BUSECK Departments of Geology and Chemistry, Arizona State University, Tempe, Arizona 85287, USA Received 9 July 1985; presented at Symposium January 1985
Electron energy-loss spectroscopy (EELS) as a technique for determining transition-element oxidation states offers several advantages over other methods. Chemical shifts as a function of oxidation state were determined for Ti3+-Ti 4+ (2 eV), Mn2+-Mn a+ (3 eV) and Fe2+-Fe 3+ (2 eV). The edge energies remain the same for a particular oxidation state, irrespective of the sample, confirming that EELS can be applied to oxidation-state determinations. Several experimental problems were encountered: poor edge resolution at low transition-element contents, difficulties in detecting mixed oxidation states, and in situ oxidation of hydrous minerals, probably through hydrogen loss.
1. Introduction Many minerals contain 3d transition-metal cations in mixed oxidation states, reflecting the ambient oxygen fugacities during mineral formation: low oxygen fugacities for some meteorite and lunar minerals, resulting in the presence of Ti 3+ [1] (and possibly Cr2+); fugacities between the magnetite-wustite and hematite-magnetite buffers in the Earth's crust and mantle, giving varying amounts of Fe 2+ and Fe 3+ in minerals such as pyroxenes and amphiboles; and high oxygen fugacities at the Earth's surface, where Mn occurs in different oxidation states in various minerals. Apart from their usefulness as oxygen-fugacity monitors, these mixed oxidation states can have important crystal-chemical implications [2]. Several techniques are available for the determination of oxidation states, such as titration and various spectroscopic methods: M6ssbauer, optical absorption, X-ray photoelectron and X-ray absorption spectroscopy. In many cases, however, it is difficult or impossible to obtain a sufficient amount of purified mineral sample for these techniques, because only a limited quantity is available or the mineral is extremely fine-grained or inhomogeneous (the result of zoning or intergrowths). In other cases, the technique is limited; e.g.,
MOssbauer is applied to Fe only, titration determines only the total oxidation state of all transition elements present, and optical absorption cannot detect Ti 4+ and suffers from uncertainties in band assignments. The only technique available for oxidation-state determinations that avoids many of these difficulties is electron energy-loss spectroscopy (EELS). EELS is very powerful because, being performed in a transmission electron microscope (TEM), it has a high spatial resolution (100 nm or better), requires little sample material, is non-destructive, can be combined with other TEM techniques (imaging, EDS analysis, ALCHEMI) and provides information on all transition elements simultaneously. We have explored the possibilities of EELS for determining the oxidation states of Ti, Mn and Fe in minerals.
2. Experimental technique The spectrometer system consists of a Philips 400T (LaB6 filament) microscope, a Gatan 607 electron energy-loss spectrometer and a TracorNorthern TN-2000 multi-channel analyzer. The energy resolution, determined from the width of the zero-loss peak at half-height, is ca. 2 eV. For
0304-3991/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
286
M . 7". O t t e n e t al. /
Ti, M n a n d F e o x i d a t i o n
all spectra shown, the backgrounds were fitted to a power law and subtracted. In the spectrometer system used, the spectra are acquired channel by channel. The dwell time per channel is, therefore, low (typically 0.3 or 0.15 s/channel for a total 300 s collection time). The short dwell times strongly affect the resolution of the edges, as is discussed below. The spectra for Ti and Fe minerals were calibrated by loading an unknown and a standard simultaneously in the microscope sample holder, collecting replicate series of spectra (at least 30) alternating between the unknown and the standard, and selecting those spectra that had not suffered any drift as shown by the spectra of the standard collected just before and after the unknown. The spectra for Mn were calibrated by using a floating voltage spectrometer, which allows accurate determination of the energy difference between the zero-loss peak and the feature of interest.
s t a t e s in m i n e r a l s
g',3 edge Ilmenlte
FeTI03
60 -% :S 0 U
Oxygen K
dl~
40-
20-
oA
i
I
I
I
I
S76
I 644
I
I 711
I "180
E n e r g y lose (eV)
Fig. 1. Energy-loss spectrum of ilmenite showing the Ti L2,3, oxygen K and Fe L2,3 edges. The dashed lines indicate the continuation of the Ti edge underneath the oxygen edge and of the oxygen edge underneath the Fe edge.
onset, but is characterized by a complex energy-loss near-edge structure (ELNES) that changes strongly from one mineral group to another. The L2.3 edges for the four most important transition elements in minerals, Ti, Cr, Mn and
3. Experimental results L2
Readers interested in more EELS background are referred to Egerton [3], Joy [41, and Colliex [5]; we shall confine ourselves to the features of EELS relevant to the determination of the transition-element oxidation states. A typical energy-loss spectrum, in this case of ilmenite, FeTiO3, is shown in fig. 1. The spectrum contains the Ti L2,3 edge, O K edge and Fe L2,3 edge, The L2,3 edges of the two transition elements are the result of transfer of the 2p1/2 and 2p3/2 electrons to unoccupied orbitals [6]. These edges are characterized by the intense L 3 and L 2 peaks at the onset of the edges. Transfer of the 2s electrons results in the L 1 edges, which are very weak. The Ti L 1 edge is hidden underneath the oxygen K edge, while the. Fe L 1 lies off the highenergy side of the scale. Note how much weaker the Fe edge is, compared to the Ti edge, even though these elements occur in similar amounts in the ilmenite. The difference between the two edges is the result of the decrease of the scattering crosssections to higher atomic number [3]. The oxygen K edge does not have the intense peaks at the edge
I I/ o ¢.)
-10
0
10
20
30
Energy I o u (eV) relative to L$
Fig. 2. Comparison of the L2, 3 edges of Ti, Cr, M n and Fe.
287
M.T. Otten et al. / Ti, M n and Fe oxidation states in minerals
Fe, are compared in fig. 2. There are three differences among these edges: the energy separation between the L 3 and L 2 peaks increases from 5 eV for Ti to 14 eV for Fe, so that there is progressively less overlap between these peaks; the L 2 / L 3 intensity ratio decreases strongly with increasing atomic number, from roughly 1 : 1 for Ti to less than 1 : 1 0 for Fe; the L 3 peaks become increasingly asymmetric, with a shoulder at the high-energy side, for Mn and Fe. Some of the features, such as the strong peak overlap for Ti and the high-energy shoulder for Fe and Mn, severely hamper the detection of small amounts of a different oxidation state (see below). In determining oxidation states by EELS we use the chemical shift of the energy of the edges. Fig. 3 shows the shifts determined for the Ti3+-Ti 4+, MnE+-Mn 4+ and F e 2 + - F e 3+ systems. It is clear that the shifts are small and that for mixed oxidation states the edges overlap to a considerable extent. Another feature apparent from fig. 3 is that the L 2 / L 3 intensity ratios increase with oxidation state, as shown by the Ti and Mn edges. This increase is the reverse of the change from metal to oxide found by Leapman et al. [6]. The Fe L 2 peak is too weak to show a change related to oxidation state.
..[ "f i
The use of EELS to determine the oxidation states of the transition metals requires that the edge energies remain the same for a particular oxidation state, irrespective of the sample. The Ti edges of the TiO 2 and T i 2 0 a oxides used as standards (fig. 3) are compared with those of four silicate minerals in fig. 4. The Ti-omphacite (pyroxene) and the hornblende (amphibole) are terrestrial minerals and should have Ti 4+, while the fassaite (pyroxene from the Allende meteorite [1]) and the NaTiSi (synthetic pyroxene) have Ti 3+. Clearly the Ti edges of the omphacite and hornblende occur at the same energy as that of TiO 2, while those of the fassaite and NaTiSi206 coincide with that of Ti203. Spectra for a variety of Ti-, Mn- and Fe-bearing minerals confirm that EELS can indeed be used to determine oxidation states. The oxygen K edges are also shown in fig. 4. There is a strong difference in the ELNES between the simple oxides and the silicates. Nevertheless, the onset of the oxygen edge (forming a peak for the simple oxides and a shoulder for the silicates) occurs at the same energy for all spectra, except the one for the fassalte. We have assumed an energy of 530 eV for the onset of the oxygen edge [7]. After we established from a comparison of the spectra of standards and unknowns that the
206
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S
.
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Hematite Fe3 +
,
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Energy I o u (eV)
Fig. 3. Chemical shifts as a function of oxidation state for Ti3+-Ti4+ (2 eV), Mn2+-Mn4+ (3 eV) and Fe2+-Fe 3+ (2 eV).
750
M.T. Otten et aL / Ti, Mn and Fe oxidation states in minerals
288
Tt
t.tls ,,I,,
/
edge shifts ca. 3.5 eV to lower energy in going from Mn 2+ to Mn 4+ (the Mn L2, 3 edge shifts to higher energy with increasing oxidation state).
o-,-i
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i
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:
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lS~
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60
,
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44o
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Enerlw loss (eV) Fig. 4. Comparison of the Ti edges of the Ti 4+ and Ti 3+ oxides with those of silicates. The Ti-omphacite and hornblende contain Ti 4+, the fassaite and NaTiSi206 have Ti 3+. The dashed line at 530 eV shows that the onset of the oxygen edges occurs at the same energy for all minerals, except for the fassaite.
oxygen-edge onset was constant, it provided a useful calibration point for the spectra. Such a calibration point is necessary because of spectrometer drift (consecutive spectra are commonly shifted by a few eV due to drift when the spectrometer is reset after acquisition of a spectrum). The oxygen K edge does not always remain stationary, however. For MnO and MnO 2, the oxygen
4. Experimental problems
From the foregoing we conclude that EELS is indeed a powerful tool for the determination of the oxidation states of Ti, Mn and Fe in minerals. There are several problems. One is the limited sensitivity. The backgrounds of the edges always contain noise. The resolution of an edge depends on the ratio of the counts for the edge to those for the background. If this ratio is low, the edge appears noisy, i.e. when the dwell time per channel is small, the intensity of the electron beam is low or the concentration of the element in the specimen low (omphacite, hornblende and fassalte in fig. 4). The resolution can be improved by increasing the dwell time per channel, but this has an upper limit because the overall counting time becomes prohibitively long (17 rain for a 1000-channel spectrum at 1 s/channel). A major improvement would be the use of a spectrometer that counts all channels simultaneously. This would also allow a decrease in the brightness of the electron beam, which would be helpful for minerals that damage easily in the electron beam (see below). A second problem results from mixed oxidation states. The strong overlap expected for mixed oxidation-state edges requires deconvolution. Unfortunately, the peak widths are not constant. This is demonstrated by the valleys between the Ti L 2 a n d L 3 peaks, which are shallower for the simple oxides than for the silicates (fig. 4). Another problem is the difficulty in detecting two oxidation states. This is partly because of poor resolution. The fassaite (fig. 4) probably contains Ti 4÷ in addition to Ti 3÷, and there may be a shoulder on the high-energy side of its L 2 peak, but the noise precludes confirmation of the shoulder. It is also difficult to find minerals having elements with two different oxidation states in nearly equal and thus easily detectable amounts. A mixed oxidation state for Fe has been found in chromite [8] and we have found a Fe 2÷ shoulder on the L 3 peak of an
M.T. Otten et al. / Ti, Mn and Fe oxidation states in minerals
andradite garnet. Magnetite, ideally Fe 2 + Fe~ + 04, yields only a single L 3 peak at an energy intermediate between those of Fe 2+ and Fe 3+, probably because the extra electron of the Fe 2+ actually is hopping to the Fe 3+ and vice versa [9]. The third and last problem is that hydrous minerals appear to oxidize in the electron beam, probably through hydrogen loss (Fe2++ O H - ~ Fe3++ 0 2- + H 1' ). Hornblendes and biotites that should have mostly Fe 2+ invariably have their Fe L2,3 edges at the Fe 3+ position. Preparing the sample in nitrogen atmosphere (no oxidation prior to insertion into the TEM) or heating in air at 750°C (sample oxidized before insertion into the TEM) make no difference. Manganite, Mn(OH), appears to display similar behaviour, having its Mn L2,3 edge at the Mn 4+ position. There is no other evidence of beam damage: the minerals do not become amorphous, and diffraction patterns remain those of hornblende, biotite or manganite. Probably the oxidation prevents the presence of dangling bonds left behind after hydrogen removal. The oxidation through hydrogen loss may prove to be an important mechanism for stabilizing hydrous minerals in the electron microscope.
289
an internal calibration point; in other cases (Mn oxides) the oxygen edge displays a shift in the direction opposite (to lower energy with increasing oxidation state) to that of the transition-element L2,3 edge (to higher energy). Experimental problems are: poor edge resolution at low transition-metal contents, caused by the noise of the background; a difficulty in detecting mixed oxidation states due to strong overlap between edges of the different oxidation states; and in situ oxidation of hydrous minerals, probably through hydrogen loss.
Acknowledgements Dr. O.L. Krivanek, A.A. Higgs and J.C. Wheatley kindly assisted at various stages of this work. Research was supported by grant EAR-8408168 from the NSF Earth Science Division. The ASU H R E M facility is supported by NSF Regional Instrumentation Program grant DMR-8306501 and Arizona State University. We thank Dr. C. Colliex for his helpful comments.
References 5. Conclusions Electron energy-loss spectroscopy as a method for determining transition-element oxidation states in minerals offers several advantages over other techniques. EELS provides a high spatial resolution, requires little sample material, is non-destructive, can be combined with other TEM techniques (imaging, EDS analysis, A L C H E M I ) and provides data on all transition elements simultaneously. Chemical shifts were determined for Ti3+-Ti4+ (2 eV), Mn2+-Mn4+ (3 eV) and Fe2+-Fe 3+ (2 eV). The edge energies are the same for a given oxidation state, irrespective of the sample, as shown by comparison of the edges for different mineral groups. In some instances (Ti oxides, Fe oxides, silicates) the onset of the oxygen K edge provides
[1] E. Dowty and J.R. Clark, Am. Mineralogist58 (1973) 230. [2] R.G. Burns and M.D. Dyar, Geol. Soc. Am. Abstr. Progr. 16 (1984) 459. [3] R.F. Egerton,in: ScanningElectronMicroscopy/1978,Vol. I, Ed. O. Johari (SEM, AMF O'Hare, IL, 1978) p. 133. [4] D.C. Joy, in: Introduction to Analytical Electron Microscopy, Eds. J.J. Hren, J.I. Goldstein and D.C. Joy (Plenum, New York, 1979) 223. [5] C. Colliex, in: Advances in Optical and Electron Microscopy,Vol. 9, Eds. R. Barer and V.E. Cosslett(Academic Press, London, 1984) 65. [6] R.D. Leapman,L.A. Grunes and P.L. Fejes, Phys. Rev. B26 (1982) 614. [7] L.A. Grunes, R.D. Leapman, C.N. Wilker, R. Hoffmann and A.B. Kunz, Phys. Rev. B25 (1982) 7157. [8] J. Taft~ and O.L. Krivanek, Phys. Rev. Letters 48 (1982) 560. [9] W. Ki~mdigand R.S. Hargrove, Solid State Commun. 7 (1969) 223.
285
THE DETERMINATION OF Ti, Mn AND Fe OXIDATION STATES IN MINERALS BY ELECTRON ENERGY-LOSS SPECTROSCOPY Max T. OTTEN, Barbara MINER, James H. RASK and Peter R. BUSECK Departments of Geology and Chemistry, Arizona State University, Tempe, Arizona 85287, USA Received 9 July 1985; presented at Symposium January 1985
Electron energy-loss spectroscopy (EELS) as a technique for determining transition-element oxidation states offers several advantages over other methods. Chemical shifts as a function of oxidation state were determined for Ti3+-Ti 4+ (2 eV), Mn2+-Mn a+ (3 eV) and Fe2+-Fe 3+ (2 eV). The edge energies remain the same for a particular oxidation state, irrespective of the sample, confirming that EELS can be applied to oxidation-state determinations. Several experimental problems were encountered: poor edge resolution at low transition-element contents, difficulties in detecting mixed oxidation states, and in situ oxidation of hydrous minerals, probably through hydrogen loss.
1. Introduction Many minerals contain 3d transition-metal cations in mixed oxidation states, reflecting the ambient oxygen fugacities during mineral formation: low oxygen fugacities for some meteorite and lunar minerals, resulting in the presence of Ti 3+ [1] (and possibly Cr2+); fugacities between the magnetite-wustite and hematite-magnetite buffers in the Earth's crust and mantle, giving varying amounts of Fe 2+ and Fe 3+ in minerals such as pyroxenes and amphiboles; and high oxygen fugacities at the Earth's surface, where Mn occurs in different oxidation states in various minerals. Apart from their usefulness as oxygen-fugacity monitors, these mixed oxidation states can have important crystal-chemical implications [2]. Several techniques are available for the determination of oxidation states, such as titration and various spectroscopic methods: M6ssbauer, optical absorption, X-ray photoelectron and X-ray absorption spectroscopy. In many cases, however, it is difficult or impossible to obtain a sufficient amount of purified mineral sample for these techniques, because only a limited quantity is available or the mineral is extremely fine-grained or inhomogeneous (the result of zoning or intergrowths). In other cases, the technique is limited; e.g.,
MOssbauer is applied to Fe only, titration determines only the total oxidation state of all transition elements present, and optical absorption cannot detect Ti 4+ and suffers from uncertainties in band assignments. The only technique available for oxidation-state determinations that avoids many of these difficulties is electron energy-loss spectroscopy (EELS). EELS is very powerful because, being performed in a transmission electron microscope (TEM), it has a high spatial resolution (100 nm or better), requires little sample material, is non-destructive, can be combined with other TEM techniques (imaging, EDS analysis, ALCHEMI) and provides information on all transition elements simultaneously. We have explored the possibilities of EELS for determining the oxidation states of Ti, Mn and Fe in minerals.
2. Experimental technique The spectrometer system consists of a Philips 400T (LaB6 filament) microscope, a Gatan 607 electron energy-loss spectrometer and a TracorNorthern TN-2000 multi-channel analyzer. The energy resolution, determined from the width of the zero-loss peak at half-height, is ca. 2 eV. For
0304-3991/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
286
M . 7". O t t e n e t al. /
Ti, M n a n d F e o x i d a t i o n
all spectra shown, the backgrounds were fitted to a power law and subtracted. In the spectrometer system used, the spectra are acquired channel by channel. The dwell time per channel is, therefore, low (typically 0.3 or 0.15 s/channel for a total 300 s collection time). The short dwell times strongly affect the resolution of the edges, as is discussed below. The spectra for Ti and Fe minerals were calibrated by loading an unknown and a standard simultaneously in the microscope sample holder, collecting replicate series of spectra (at least 30) alternating between the unknown and the standard, and selecting those spectra that had not suffered any drift as shown by the spectra of the standard collected just before and after the unknown. The spectra for Mn were calibrated by using a floating voltage spectrometer, which allows accurate determination of the energy difference between the zero-loss peak and the feature of interest.
s t a t e s in m i n e r a l s
g',3 edge Ilmenlte
FeTI03
60 -% :S 0 U
Oxygen K
dl~
40-
20-
oA
i
I
I
I
I
S76
I 644
I
I 711
I "180
E n e r g y lose (eV)
Fig. 1. Energy-loss spectrum of ilmenite showing the Ti L2,3, oxygen K and Fe L2,3 edges. The dashed lines indicate the continuation of the Ti edge underneath the oxygen edge and of the oxygen edge underneath the Fe edge.
onset, but is characterized by a complex energy-loss near-edge structure (ELNES) that changes strongly from one mineral group to another. The L2.3 edges for the four most important transition elements in minerals, Ti, Cr, Mn and
3. Experimental results L2
Readers interested in more EELS background are referred to Egerton [3], Joy [41, and Colliex [5]; we shall confine ourselves to the features of EELS relevant to the determination of the transition-element oxidation states. A typical energy-loss spectrum, in this case of ilmenite, FeTiO3, is shown in fig. 1. The spectrum contains the Ti L2,3 edge, O K edge and Fe L2,3 edge, The L2,3 edges of the two transition elements are the result of transfer of the 2p1/2 and 2p3/2 electrons to unoccupied orbitals [6]. These edges are characterized by the intense L 3 and L 2 peaks at the onset of the edges. Transfer of the 2s electrons results in the L 1 edges, which are very weak. The Ti L 1 edge is hidden underneath the oxygen K edge, while the. Fe L 1 lies off the highenergy side of the scale. Note how much weaker the Fe edge is, compared to the Ti edge, even though these elements occur in similar amounts in the ilmenite. The difference between the two edges is the result of the decrease of the scattering crosssections to higher atomic number [3]. The oxygen K edge does not have the intense peaks at the edge
I I/ o ¢.)
-10
0
10
20
30
Energy I o u (eV) relative to L$
Fig. 2. Comparison of the L2, 3 edges of Ti, Cr, M n and Fe.
287
M.T. Otten et al. / Ti, M n and Fe oxidation states in minerals
Fe, are compared in fig. 2. There are three differences among these edges: the energy separation between the L 3 and L 2 peaks increases from 5 eV for Ti to 14 eV for Fe, so that there is progressively less overlap between these peaks; the L 2 / L 3 intensity ratio decreases strongly with increasing atomic number, from roughly 1 : 1 for Ti to less than 1 : 1 0 for Fe; the L 3 peaks become increasingly asymmetric, with a shoulder at the high-energy side, for Mn and Fe. Some of the features, such as the strong peak overlap for Ti and the high-energy shoulder for Fe and Mn, severely hamper the detection of small amounts of a different oxidation state (see below). In determining oxidation states by EELS we use the chemical shift of the energy of the edges. Fig. 3 shows the shifts determined for the Ti3+-Ti 4+, MnE+-Mn 4+ and F e 2 + - F e 3+ systems. It is clear that the shifts are small and that for mixed oxidation states the edges overlap to a considerable extent. Another feature apparent from fig. 3 is that the L 2 / L 3 intensity ratios increase with oxidation state, as shown by the Ti and Mn edges. This increase is the reverse of the change from metal to oxide found by Leapman et al. [6]. The Fe L 2 peak is too weak to show a change related to oxidation state.
..[ "f i
The use of EELS to determine the oxidation states of the transition metals requires that the edge energies remain the same for a particular oxidation state, irrespective of the sample. The Ti edges of the TiO 2 and T i 2 0 a oxides used as standards (fig. 3) are compared with those of four silicate minerals in fig. 4. The Ti-omphacite (pyroxene) and the hornblende (amphibole) are terrestrial minerals and should have Ti 4+, while the fassaite (pyroxene from the Allende meteorite [1]) and the NaTiSi (synthetic pyroxene) have Ti 3+. Clearly the Ti edges of the omphacite and hornblende occur at the same energy as that of TiO 2, while those of the fassaite and NaTiSi206 coincide with that of Ti203. Spectra for a variety of Ti-, Mn- and Fe-bearing minerals confirm that EELS can indeed be used to determine oxidation states. The oxygen K edges are also shown in fig. 4. There is a strong difference in the ELNES between the simple oxides and the silicates. Nevertheless, the onset of the oxygen edge (forming a peak for the simple oxides and a shoulder for the silicates) occurs at the same energy for all spectra, except the one for the fassalte. We have assumed an energy of 530 eV for the onset of the oxygen edge [7]. After we established from a comparison of the spectra of standards and unknowns that the
206
1
*"'/'°t
lJ
S
.
12
Hematite Fe3 +
,
"I // '°F 4S0
47S
SO0 650
/t
J 6S5
680 700
72S
Energy I o u (eV)
Fig. 3. Chemical shifts as a function of oxidation state for Ti3+-Ti4+ (2 eV), Mn2+-Mn4+ (3 eV) and Fe2+-Fe 3+ (2 eV).
750
M.T. Otten et aL / Ti, Mn and Fe oxidation states in minerals
288
Tt
t.tls ,,I,,
/
edge shifts ca. 3.5 eV to lower energy in going from Mn 2+ to Mn 4+ (the Mn L2, 3 edge shifts to higher energy with increasing oxidation state).
o-,-i
40
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t8
0
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•
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i
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:
|4 16
0
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o
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- , NaTi$+Si206,
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, ~,,' ~ / " " ' " ~ ' ~
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l
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60
,
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:
$0 IS
44o
4'io
sN
.o
s6o
;90
Enerlw loss (eV) Fig. 4. Comparison of the Ti edges of the Ti 4+ and Ti 3+ oxides with those of silicates. The Ti-omphacite and hornblende contain Ti 4+, the fassaite and NaTiSi206 have Ti 3+. The dashed line at 530 eV shows that the onset of the oxygen edges occurs at the same energy for all minerals, except for the fassaite.
oxygen-edge onset was constant, it provided a useful calibration point for the spectra. Such a calibration point is necessary because of spectrometer drift (consecutive spectra are commonly shifted by a few eV due to drift when the spectrometer is reset after acquisition of a spectrum). The oxygen K edge does not always remain stationary, however. For MnO and MnO 2, the oxygen
4. Experimental problems
From the foregoing we conclude that EELS is indeed a powerful tool for the determination of the oxidation states of Ti, Mn and Fe in minerals. There are several problems. One is the limited sensitivity. The backgrounds of the edges always contain noise. The resolution of an edge depends on the ratio of the counts for the edge to those for the background. If this ratio is low, the edge appears noisy, i.e. when the dwell time per channel is small, the intensity of the electron beam is low or the concentration of the element in the specimen low (omphacite, hornblende and fassalte in fig. 4). The resolution can be improved by increasing the dwell time per channel, but this has an upper limit because the overall counting time becomes prohibitively long (17 rain for a 1000-channel spectrum at 1 s/channel). A major improvement would be the use of a spectrometer that counts all channels simultaneously. This would also allow a decrease in the brightness of the electron beam, which would be helpful for minerals that damage easily in the electron beam (see below). A second problem results from mixed oxidation states. The strong overlap expected for mixed oxidation-state edges requires deconvolution. Unfortunately, the peak widths are not constant. This is demonstrated by the valleys between the Ti L 2 a n d L 3 peaks, which are shallower for the simple oxides than for the silicates (fig. 4). Another problem is the difficulty in detecting two oxidation states. This is partly because of poor resolution. The fassaite (fig. 4) probably contains Ti 4÷ in addition to Ti 3÷, and there may be a shoulder on the high-energy side of its L 2 peak, but the noise precludes confirmation of the shoulder. It is also difficult to find minerals having elements with two different oxidation states in nearly equal and thus easily detectable amounts. A mixed oxidation state for Fe has been found in chromite [8] and we have found a Fe 2÷ shoulder on the L 3 peak of an
M.T. Otten et al. / Ti, Mn and Fe oxidation states in minerals
andradite garnet. Magnetite, ideally Fe 2 + Fe~ + 04, yields only a single L 3 peak at an energy intermediate between those of Fe 2+ and Fe 3+, probably because the extra electron of the Fe 2+ actually is hopping to the Fe 3+ and vice versa [9]. The third and last problem is that hydrous minerals appear to oxidize in the electron beam, probably through hydrogen loss (Fe2++ O H - ~ Fe3++ 0 2- + H 1' ). Hornblendes and biotites that should have mostly Fe 2+ invariably have their Fe L2,3 edges at the Fe 3+ position. Preparing the sample in nitrogen atmosphere (no oxidation prior to insertion into the TEM) or heating in air at 750°C (sample oxidized before insertion into the TEM) make no difference. Manganite, Mn(OH), appears to display similar behaviour, having its Mn L2,3 edge at the Mn 4+ position. There is no other evidence of beam damage: the minerals do not become amorphous, and diffraction patterns remain those of hornblende, biotite or manganite. Probably the oxidation prevents the presence of dangling bonds left behind after hydrogen removal. The oxidation through hydrogen loss may prove to be an important mechanism for stabilizing hydrous minerals in the electron microscope.
289
an internal calibration point; in other cases (Mn oxides) the oxygen edge displays a shift in the direction opposite (to lower energy with increasing oxidation state) to that of the transition-element L2,3 edge (to higher energy). Experimental problems are: poor edge resolution at low transition-metal contents, caused by the noise of the background; a difficulty in detecting mixed oxidation states due to strong overlap between edges of the different oxidation states; and in situ oxidation of hydrous minerals, probably through hydrogen loss.
Acknowledgements Dr. O.L. Krivanek, A.A. Higgs and J.C. Wheatley kindly assisted at various stages of this work. Research was supported by grant EAR-8408168 from the NSF Earth Science Division. The ASU H R E M facility is supported by NSF Regional Instrumentation Program grant DMR-8306501 and Arizona State University. We thank Dr. C. Colliex for his helpful comments.
References 5. Conclusions Electron energy-loss spectroscopy as a method for determining transition-element oxidation states in minerals offers several advantages over other techniques. EELS provides a high spatial resolution, requires little sample material, is non-destructive, can be combined with other TEM techniques (imaging, EDS analysis, A L C H E M I ) and provides data on all transition elements simultaneously. Chemical shifts were determined for Ti3+-Ti4+ (2 eV), Mn2+-Mn4+ (3 eV) and Fe2+-Fe 3+ (2 eV). The edge energies are the same for a given oxidation state, irrespective of the sample, as shown by comparison of the edges for different mineral groups. In some instances (Ti oxides, Fe oxides, silicates) the onset of the oxygen K edge provides
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