Spin and valence states of iron in (Mg[subscript
0.8]Fe[subscript 0.2])SiO[subscript 3] perovskite
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Grocholski, B. et al. “Spin and valence states of iron in
(Mg[subscript 0.8]Fe[subscript 0.2])SiO[subscript 3] perovskite.”
Geophys. Res. Lett. 36.24 (2009): L24303. ©2009 by the
American Geophysical Union.
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http://dx.doi.org/10.1029/2009gl041262
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GEOPHYSICAL RESEARCH LETTERS, VOL. 36, L24303, doi:10.1029/2009GL041262, 2009
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Spin and valence states of iron in (Mg0.8Fe0.2)SiO3 perovskite
B. Grocholski,1 S.-H. Shim,1 W. Sturhahn,2 J. Zhao,2 Y. Xiao,3 and P. C. Chow3
Received 13 October 2009; accepted 19 November 2009; published 18 December 2009.
[1] The spin and valence states of iron in (Mg0.8Fe0.2)SiO3
perovskite were measured between 0 and 65 GPa using
synchrotron Mössbauer spectroscopy. Samples were
synthesized in situ in the laser-heated diamond cell under
reducing conditions. The dominant spin state of iron in
perovskite is high spin at pressures below 50 GPa. Above
50 GPa, the spectra shows severe changes which can be
explained by appearance of two distinct iron sites with
similar site weightings. One site has Mössbauer parameters
consistent with high spin Fe2+, while the other has the
parameters previously interpreted as intermediate spin. The
latter intermediate-spin assignment is not unique, as similar
Mössbauer parameters have been reported for high spin
Fe2+ in almandine at ambient pressure. However, our data
do not rule out the existence of low-spin iron, which may
exist with a smaller fraction and explain the observation of
lower spin moments in the X-ray emission spectroscopy of
perovskite at high pressure. From these considerations, our
preferred interpretation is that iron in perovskite is mixed
or high spin to at least 2000 km depths in the mantle,
consistent with computational results. Our study also
reveals that reducing conditions do not inhibit the
formation of Fe3+ in perovskite at deep-mantle pressures.
Citation: Grocholski, B., S.-H. Shim, W. Sturhahn, J. Zhao,
Y. Xiao, and P. C. Chow (2009), Spin and valence states of
iron in (Mg0.8Fe0.2)SiO3 perovskite, Geophys. Res. Lett., 36,
L24303, doi:10.1029/2009GL041262.
1. Introduction
[2] The spin and valence of iron in perovskite (Pv) has
important effects on the properties of the lower mantle phase
assemblage, particularly transport properties and element
partitioning [Xu and McCammon, 2002; Auzende et al.,
2008; Goncharov et al., 2009]. The valency of iron at high
pressure is also important for understanding the redox state of
the mantle [McCammon, 2005].
[3] Previous studies using Mössbauer spectroscopy have
led to an unclear picture about the dominant spin state of
iron in Pv in the lower mantle [Jackson et al., 2005; Li et
al., 2006; McCammon et al., 2008; Lin et al., 2008]. This
situation is understandable, given the complexity of the Pv
crystal structure and the varying experimental conditions
under which Pv is synthesized. Computational studies on
the spin state of Fe2+ favor high spin (HS) to low spin (LS)
transitions above 100 GPa for iron content below 30% with
transition pressures dependent on the distribution of iron in
Pv [Zhang and Oganov, 2006; Stackhouse et al., 2007;
Bengtson et al., 2008; Umemoto et al., 2008]. The spin
pairing of iron in Fe3+ in these simulations occurs at lower
pressure, from 60 to 100 GPa [Li et al., 2005; Zhang and
Oganov, 2006; Stackhouse et al., 2007].
[4] Recent studies have identified an iron electronic
configuration with a set of Mössbauer parameters (quadrupole splitting: QS = 3.5 4.0 mm/s) that are outside the
range normally observed for HS (QS = 2 3 mm/s) or LS
Fe2+ (QS = 0 1 mm/s) [McCammon et al., 2008; Lin et al.,
2008], with this electronic configuration becoming dominant above 50 GPa. Combined with X-ray emission spectroscopy (XES) studies that indicate a decrease in average
spin moment with pressure [Li et al., 2004; Badro et al.,
2004; Lin et al., 2008], this new site has been interpreted as
Fe2+ in the intermediate spin (IS) state, with a stability field
from 40 GPa to at least 135 GPa [McCammon et al., 2008;
Lin et al., 2008]. However, computational results do not
find this electronic configuration of iron to be stable [Zhang
and Oganov, 2006; Stackhouse et al., 2007; Bengtson et al.,
2008, 2009], and no mineralogical examples of IS Fe2+ at
room pressure have been identified, while there are some
reports on IS iron in molecular complexes [see Bengtson et
al., 2009, and references therein]. We have conducted a set
of experiments designed to better control the conditions
under which Pv is synthesized and constrain the likely spin
and valence states of iron in the lower-mantle Pv phase.
2. Experimental Method
Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA.
2
Sector 3, Advanced Photon Source, Argonne National Laboratory,
Argonne, Illinois, USA.
3
HPCAT, Advanced Photon Source, Argonne National Laboratory,
Argonne, Illinois, USA.
[5] The starting material was a (Mg0.8Fe0.2)SiO3 pyroxene synthesized using the same technique as Lin et al.
[2008] under reducing conditions. The sample was 95%
57
Fe enriched. The starting material was powdered and then
pressed into platelets. Among total of five different samples,
three platelets were compressed together with a thin (2 –
3 micron) iron foil with a natural 57Fe level to provide a
reducing environment (Figure S1 of the auxiliary material),
while the other two samples were loaded without the iron
foil.4 Samples were loaded in diamond cells with 200 or
300 mm culets with argon for thermal insulation and to
minimize deviatoric stresses. Small grains of pyroxene
with the same composition were used for spacers to allow
argon to penetrate underneath and increase thermal insulation. For pressure measurements, ruby grains were
placed at the edge of the sample chamber and away from
the sample to prevent reaction with the sample during
laser heating [Mao et al., 1986].
[6] The Pv phase was synthesized at MIT using a
Nd:YLF laser between 1500 and 2000 K to ensure the iron
Copyright 2009 by the American Geophysical Union.
0094-8276/09/2009GL041262$05.00
4
Auxiliary materials are available in the HTML. doi:10.1029/
2009GL041262.
1
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Figure 1. Representative synchrotron Mössbauer spectra of Pv at different pressures. (a) Spectra collected at low (<5 GPa)
pressure. The bottom trace is the starting material and can be fit with Mössbauer parameters consistent with other pyroxenes
[Lin et al., 2008]. Pressure-quenched samples from synthesis at 50 GPa and 65 GPa are also shown. (b) High pressure SMS
from three different samples. The increased frequency of the quantum beats in the top two spectra are the result of the high QS
site (site 3) in the sample. SMS from McCammon et al. [2008] at 44 GPa to 120 ns shown for comparison.
foils did not melt and react with the silicate. Each sample was
pressurized to 37, 50, or 65 GPa and heated for 30 minute
cycles 3 or 4 times to ensure full conversion to Pv. The
synthesis of Pv was confirmed by X-ray diffraction on a
pressure quenched, iron-foil free Pv sample at the GSECARS
beamline at the Advanced Photon Source (APS). Laser
annealing was performed after any pressure increase, but
not after release of pressure from the diamond cells.
[7] Synchrotron Mössbauer spectra (SMS) were collected
at Sectors 3 and 16 of APS. Measurements of SMS on our
starting material confirms that it is Fe3+ free (Figure 1 and
Table S1). SMS collection time was about 2 hours at Sector 3
and 6 hours at HPCAT, with separate spectra collected with a
10 mm stainless steel foil to obtain relative center shifts (CS).
Details on the technique of SMS and the equivalency to
conventional Mössbauer parameters are given by Sturhahn
[2004].
[8] In order to extract Mössbauer parameters, spectral
fitting was perform using the CONUSS program [Sturhahn,
2000]. The results are shown in Figure 1 and Table S1. All
spectra are fit with a three site model. Perovskite synthesized at 37 GPa required a site to account for iron metal that
contaminated the spectrum, which has little effect on the
Mössbauer parameters of the other two sites. Decompressed
samples from 50 and 37 GPa have a magnetic site consistent
with a small amount of elemental iron either from the foils
or (possibly) due to charge disproportionation of iron during
sample synthesis [Frost et al., 2004]. We do not use these
fitting results due to relatively high c2 and instability during
spectral fitting.
3. Results
[9] Our synchrotron Mössbauer spectra of Pv consists of
1 – 2 irregularly spaced quantum beats up to 37 GPa and 2 –
3 beats at pressures above 50 GPa (Figure 1). This is in
sharp contrast with the spectra reported by Lin et al. [2008]
and McCammon et al. [2008] where evenly spaced 3 – 4
quantum beats were found (Figure 1). Quadrupole splitting
(QS) values are shown in Figure 2 along with the ranges for
different spin and valence states from previous high pres-
Figure 2. The quadrupole splitting of different iron sites at
high pressure. Different symbols represent the different
synthesis pressure. Closed and open symbols were synthesized with and without an iron foil, respectively. The QS for
pyroxene (two Fe2+ sites) are plotted at 0 GPa for
comparison (double triangles). Ranges of QS for different
valence and spin states of iron in Pv reported by high
pressure experiments are shown along the right of the figure
for comparison. M = McCammon et al. [2008], L = Li et al.
[2006], J = Jackson et al. [2005], C = Catalli et al. [2009].
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sure studies. The combination of QS and CS allows us to
identify 4 – 5 sites representing different electronic configurations of iron labeled in Figure 2. At pressures lower than
40 GPa, the Pv component can be fit with of two sites.
Site 1 is compatible with HS Fe2+ in Pv [Fei et al., 1994]
and reported in previous high pressure studies. Site 5 is a
low QS site (0.2–0.6 mm/s) traditionally associated with the
formation of HS Fe3+, but the value is also comparable to
that expected Mössbauer parameters of LS Fe2+ [Li et al.,
2006; Rouquette et al., 2008; Bengtson et al., 2009].
[10] Three distinct sites were found in the samples
synthesized above 50 GPa. Site 3 is the high QS site which
is previously interpreted as IS Fe2+ in Pv [McCammon et
al., 2008; Lin et al., 2008]. In our experiment, this high QS
site does not become the dominant feature of the spectra at
high pressure, unlike McCammon et al. [2008] and Lin et al.
[2008]: the fraction of this site remaining at 20 – 30% both
in the samples directly synthesized and laser annealed at
65 GPa, while McCammon et al. [2008] reported 100% for
this site at pressures higher than 60 GPa. A similar site
with a slightly low QS value (3.3– 3.5 mm/s) has been also
reported by Jackson et al. [2005] and Li et al. [2006]. They
also found that the site has a smaller weighting (10 – 25%),
similar to our results. McCammon et al. [2008] argued that
the lack of laser heating of Jackson et al. [2005] and Li et al.
[2006] may cause the difference. However, in our study Pv
is synthesized directly at high pressure with longer heating
duration at higher temperature combined with the use of
quasi-hydrostatic, thermally insulating argon pressure
medium. The iron content in our sample (20%) is slightly
higher than McCammon et al. [2008] (12 – 14%) and may
have a small effect on the relative sight weighting. However,
computations indicate the spin transition pressure would
not change between 0 and 30% Fe in Pv [Bengtson et al.,
2008].
[11] Site 4 has a mid-range QS value (1.0– 2.0 mm/s).
This site was not given by Jackson et al. [2005] and Li et al.
[2006], but by McCammon et al. [2008] with a smaller site
weighting (between 0 and 15%). We are uncertain as to the
details of site 4, but it may be related to site 2 (HS Fe2+)
with some degree of charge delocalization [Fei et al., 1994].
Site 1 contributes to the spectra to higher pressures in
previous studies [Jackson et al., 2005; Li et al., 2006],
while it disappears above 40 GPa in our study. The
persistence of this low-pressure feature is likely due to lack
of heating in those previous studies. This highlights the
importance of sufficient heating to ensure iron is in the
lowest energy configuration at given high pressure.
[12] We also conducted low-pressure SMS measurements
on Pv recovered from high pressure. A total of 3 sites were
identified: sites 1, 2, and 5. Site 2 has a similar QS value as
site 1. These two sites are likely related to site 1 at high
pressure, are better resolved due to the removal of deviatoric
stresses at ambient conditions, and are consistent with HS
Fe2+. Site 5 is consistent with HS Fe3+ and found to
represent 20% (Figure 2), higher than the Fe3+ free Pv that
Jeanloz et al. [1992] measured, but consistent with other
measurements of Pv decompressed from high pressure [Fei
et al., 1994].
[13] From the comparison of the samples synthesized
with and without iron foil, we found that adding the iron
foil to ensure reducing conditions does not significantly
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change the Mössbauer parameters. The site weighting for
the Fe3+-like sites (site 5) is lower for samples containing
the iron foil, but within the error bars of the measurements.
This behavior is similar for Pv synthesized in the multianvil press [Frost et al., 2004], and seems to indicate a
surprising insensitivity to redox conditions during Pv
crystallization.
[14] The fraction of site 5 ranges 25 – 43% at high pressure, which is higher than the site weighting for recovered
samples. The site fraction measured at high pressure may
contain much larger uncertainty than for the measurements on recovered samples, due to severe broadening of
the QS value and lower spectral quality. Nevertheless, the
higher fraction of site 5 appears to be systematic, as it
persists over 6 data points at different pressures. The actual
amount of Fe 3+ in our samples may be better estimated
from the spectra measured on pressure quenched samples
(Fe3+/SFe 20%), as unloading the sample at room temperature is unlikely to change the valence state of iron. The
Mössbauer parameters of LS Fe2+ are virtually indistinguishable to that of HS Fe3+ (Figure 2) and LS Fe2+ should
transform back to HS during unloading [Rouquette et al.,
2008]. If we assume our low pressure spectra give a more
accurate accounting of Fe3+ in our sample, this means up to
5– 20% of the additional low-QS Mössbauer signal at high
pressure could be due to LS Fe2+.
4. Discussion
[15] Recent studies have focused on the appearance the
high QS site necessary to fit the spectra, inferring it to be IS
Fe2+ due to Jahn-Teller distortion of the iron 3d orbitals
[McCammon et al., 2008; Lin et al., 2008]. While IS iron
has been documented in some molecular complexes [see
Bengtson et al., 2009, and references therein], it is notable
that there is no known example of IS iron in silicate and
oxide minerals at ambient pressure to our knowledge. In
addition, computational results do not find the IS state to be
stable in Pv at lower-mantle pressures [Zhang and Oganov,
2006; Stackhouse et al., 2007; Bengtson et al., 2008].
Bengtson et al. [2009] calculated the QS of IS Fe2+ in the
A site to be much lower (0.7 mm/s) than 3.5– 4.0 mm/s.
On the other hand, some examples exist in the literature of
HS Fe2+ with large QS (>3.5 mm/s), including synthetic
almandine [Murad and Wagner, 1987] and naturally
occurring garnet in eclogite [Li et al., 2005], which have
(distorted) dodecahedral coordination environments most
similar to the A site in Pv. Low spin Fe3+ also appears to
have QS up to 3.5 mm/s in recent work by Catalli et al.
[2009].
[16] The interpretation of the high QS site as IS comes in
part from X-ray emission spectroscopy (XES) of Pv [Li et
al., 2004; Badro et al., 2004; Lin et al., 2008]. These spectra
are much more sensitive to the spin state of iron in the
sample, with coordination environment and valence state
having small effects [Vanko et al., 2006]. However, it only
provides information on average spin state in the sample.
[17] The decrease in satellite peak observed for Pv at high
pressure may be due to spin pairing, with Fe going to IS or
LS. On the other hand, the production of iron metal may
also mimic the change in spectral features used to infer spin
state [Rueff et al., 2008], which combined with recent
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observation of the charge disproportionation of iron in Pv
(3Fe2+ ! 2Fe3+ + Fe0) makes this a plausible explanation
[Frost et al., 2004; Auzende et al., 2008]. The possibility
remains other factors influence the spectral shape, as half of
the intensity reduction from Li et al. [2004] occurs between
0 at 27 GPa, which should reflect in the appearance of a
significant amount of IS or LS iron by 30 GPa. This is
inconsistent with current interpretation of Mössbauer spectra in that pressure range. Other probes have also failed to
unambiguously detect spin transitions in Pv at high pressure
[Narygina et al., 2009].
[18] Another alternative is to have LS Fe2+ and/or LS
Fe3+. As discussed above, from the difference in the site
fractions of site 5, we cannot rule out the possible existence
of LS Fe2+ at high pressure. In addition, the Mössbauer
parameters of LS Fe3+ are very similar to HS Fe2+, leaving
the possibility of the existence of LS Fe3+ [Xu et al., 2001].
Indeed, LS Fe3+ may appear at much lower pressures
than LS Fe2+ according to recent studies [Li et al., 2005;
Stackhouse et al., 2007; Catalli et al., 2009].
[19] While the appearance of the high QS term is intriguing, it is important that experimental probes yield more
consistent results as in the case for the (Mg,Fe)O system
[Lin and Tsuchiya, 2008]. While our experiment does not
give a definitive answer as to the nature of Fe2+ in Pv, we
can rule out a predominantly intermediate spin iron in the
mantle at least to 2000-km depth. We find that at high
pressure Fe2+ exists in two different environments, but both
are likely high spin along with octahedrally coordinated
Fe3+ and possibly small amounts of LS Fe2+ (and LS Fe3+).
Fe3+ is produced in the similar amounts under different
redox conditions, consistent with results from lower pressure perovskite synthesis [Frost et al., 2004] and a recent
study without oxygen fugacity control [Auzende et al.,
2008; McCammon et al., 2008].
[20] For the discussion of the spin state of iron in the
mantle it is important to know the effect of temperature and
aluminum on the spin transition. Temperature will have an
effect on the population of iron in different spin configurations
[Hofmeister, 2006; Sturhahn et al., 2005]. Aluminum also
appears to alter the valence state of iron in Pv and increases
Fe3+/SFe to 60% [Frost et al., 2004]. As shown in recent
studies [Stackhouse et al., 2007; Catalli et al., 2009], the spin
state of iron could be valence-dependent: Fe3+ may undergo
spin pairing at much lower pressure. The investigation of high
temperature (in excess of 2000 K) and systems with realistic
amount of Al for the mantle (5 – 10 mol%) would be important for determining the implication for the lower mantle.
[21] Acknowledgments. The authors would like to thank J. Barr and
T. Grove for help synthesizing the starting material and acknowledge
S. Speakman, K. Catalli, and V. Prakapenka for experimental assistance.
We would like to thank the editor, anonymous reviewers, D. Morgan,
A. Bengtson, and R. Jeanloz for helpful comments. Use of Sector 3 was
partially supported by COMPRES. Portions of this work were performed at
HPCAT (Sector 16), Advanced Photon Source (APS), Argonne National
Laboratory. HPCAT is supported by DOE-BES, DOE-NNSA, NSF, and the
W.M. Keck Foundation. APS is supported by DOE-BES, under contract
DE-AC02-06CH11357. This work was supported by NSF to S.-H. S.
(EAR0738655).
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