GEOPHYSICAL RESEARCH LETTERS, VOL. 29, NO. 21, 2021, doi:10.1029/2002GL015276, 2002
On protons, iron and the high-pressure behavior of ringwoodite
Annette K. Kleppe,1 Andrew P. Jephcoat,1 Joseph R. Smyth,2 Daniel J. Frost3
Received 6 April 2002; revised 28 May 2002; accepted 29 May 2002; published 7 November 2002.
[1] Raman spectra of a single crystal of hydrous Fo89ringwoodite have been measured in a diamond-anvil cell at
room temperature to 60 GPa. Strong peaks corresponding to
asymmetric and symmetric stretching vibrations of the SiO4
tetrahedra are observed at 796 and 841 cm 1 respectively.
Additional bands occur in the range 709 – 939 and 100– 250
cm 1. The SiO4 stretching modes of the spinel structure
shift continuously up to 60 GPa. Near 40 GPa new Raman
bands emerge in the region 550– 680 cm 1 gaining intensity
to the maximum pressure, a change that is reversible on
pressure release. This pressure-induced modification in the
structure appears to be related to structural complexity
INDEX
generated by both Fe and proton substitution.
TERMS: 3924 Mineral Physics: High-pressure behavior; 3934
Mineral Physics: Optical, infrared, and Raman spectroscopy; 3630
Mineralogy and Petrology: Experimental mineralogy and
petrology. Citation: Kleppe, A. K., A. P. Jephcoat, J. R.
Smyth, and D. J. Frost, On protons, iron and the high-pressure
behavior of ringwoodite, Geophys. Res. Lett., 29(21), 2021,
doi:10.1029/2002GL015276, 2002.
1. Introduction
[2] Ringwoodite [g-(Mg,Fe)2SiO4], the nominally anhydrous highest-pressure polymorph of olivine, is considered
to be the most abundant mineral in the transition zone
between 520 and 660 km depth. It can incorporate up to
2.5 wt% H2O in form of OH in its crystal structure [e.g.,
Kohlstedt et al., 1996]. While the Fe-content of ringwoodite, with an assumed natural amount of 11%, is likely to
control mantle properties (e.g., elasticity) globally, the OH
content may be particularly important in cooler regions
within, and adjacent to, subducting slabs, where the mineralogical transformations after the metastable olivine
wedge are not yet well understood.
[3] Chopelas et al. [1994] and Liu et al. [1994] reported
high-pressure Raman data of anhydrous Mg-endmember
ringwoodite to 20 and 24 GPa respectively. Recently,
Kleppe et al. [2002] and Liu et al. [2002] studied the
hydrous Mg-endmember phase with Raman spectroscopy
to 56.5 and 20 GPa respectively. Here, we report highpressure Raman spectroscopic data of hydrous ringwoodite
with an Fe-content comparable to that of the Earth’s mantle
to 60 GPa in a helium pressure-transmitting medium.
1
Department of Earth Sciences, University of Oxford, Oxford, UK.
Department of Geological Sciences, University of Colorado, Boulder,
CO, USA.
3
Bayerisches Geoinstitut, Universität Bayreuth, Bayreuth, Germany.
2
Copyright 2002 by the American Geophysical Union.
0094-8276/02/2002GL015276$05.00
2. Experiment
[4] Ringwoodite [g-(Mg0.89,Fe0.11 )2SiO 4, 1.6 wt%
H2O] was synthesized in the 5000 tonne multi-anvil press
at the Bayerisches Geoinstitut, Bayreuth, Germany from a
starting mixture of natural Fo90 olivine, En90 orthopyroxene, plus brucite, hematite, and silica to give a bulk water
content of 3 wt%. The starting material was sealed in a
welded Pt capsule run conditions were 20 GPa and 1400 C
with a heating time of 5 h. The ringwoodite crystals were
optically isotropic and deep blue in colour (Figure 1).
Mössbauer spectroscopy indicated that about 10% of the
Fe was present as ferric with the remainder as ferrous
[Smyth et al., in preparation]. The water content of
1.6 wt% was determined by SIMS and is consistent with
IR spectroscopic measurements [Bolfan-Casanova et al. in
preparation]. A single-crystal (120 100 80 mm) from
the same capsule was also characterized by X-ray diffraction. The crystal structure was refined in space group Fd3m
with a unit-cell parameter of 8.0944(6) Å.
[5] In the diamond-anvil cell experiment, a 20 mm thick,
inclusion-free, polished single-crystal fragment 30 50 mm
in size and a <10 mm ruby sphere for pressure calibration were
loaded with fluid helium as a pressure-transmitting medium
at 200 MPa [Jephcoat et al., 1987]. Unpolarized Raman
spectra to 60 GPa were recorded with a SPEX Triplemate
equipped with a back-illuminated, liquid-N2-cooled CCD
detector. The spectra were excited by the 514.5 and 48-nm
line of an Ar+ laser. The power range used was well below
the threshold at which the crystals transformed due to local
heating by the laser beam. The crystal was preserved to
the highest pressures. Due to the use of low incident laser
power, only long collection times led to an acceptable
signal-to-noise ratio. (Red, 647.09-nm excitation caused
thermal damage at the lowest possible power as the dark
blue crystals absorbed red light preferentially.) Furthermore, our observations are not generated or affected by
non-hydrostatic stresses in the sample because we used
helium as pressure-transmitting medium, and neither the
ruby nor the silicate stretching modes (with approximately
constant FWHM over the whole pressure range) gave any
evidence for a significant pressure gradient across the
sample.
3. Raman and Single-Crystal X-Ray Results at
Ambient Conditions
[ 6 ] The ambient, unpolarized Raman spectrum of
hydrous iron-bearing ringwoodite (Figure 2) contains more
than the five modes expected for the ideal cubic spinel
structure (space group Fd3m) [White and De Angelis, 1967]
and observed in hydrous and anhydrous Mg-endmember
phase [e.g. Kleppe et al., 2002; Chopelas et al., 1994]. The
two intense bands at 796 and 841 cm 1 in our spectrum
correspond to the asymmetric (T2g) and symmetric (A1g)
17 - 1
17 - 2
KLEPPE ET AL.: HIGH-PRESSURE BEHAVIOR OF RINGWOODITE
Figure 1. Single crystal fragments of hydrous Fo89ringwoodite synthesized in a multi-anvil press at the
Bayerisches Geoinstitut.
stretching vibrations of the SiO4 tetrahedra respectively, and
agree with iron-free g-Mg2SiO4 [Chopelas et al., 1994].
The three further modes of the Mg-endmember phase
(expected at 302, 372, and 600 cm 1) are much weaker
and in the present spectrum are either not observed possibly
due to use of low laser power, or hidden in the background.
On the other hand, additional modes were present as broad
and relatively intense features at low-frequencies (100– 250
cm 1). And we observed at higher frequencies, a shoulder
to the 841 cm 1 mode (at 881 cm 1) as well as two
further modes at 709 and 939 cm 1. The larger number of
bands than expected for the ideal cubic spinel structure is
not due to an impurity phase because the bands do not
correspond to known modes of likely impurity phases such
as wadsleyite or stishovite.
[7] It is interesting that the 1-bar additional modes at 709,
881 and 939 cm 1 occur in a frequency range typical for
stretching frequencies of Si2O7 groups. For example, wadsleyite, has Si2O7 stretching frequencies at 723 cm 1,
812 cm 1 and 919 cm 1 [Kleppe et al., 2001; Mernagh
and Liu, 1996]. The presence of Si2O7 groups with a
bridging oxygen in the spinel structure, ideally containing
only isolated SiO4 tetrahedra, would require a non-silicate
oxygen elsewhere which could act as site for protonation. It
is analogous to the non-silicate, underbonded oxygen in the
wadsleyite structure which also serves as a protonation site
[Smyth, 1987]. The present X-ray data, importantly, also
show evidence for Si-O-Si bonds and non-silicate oxygens
in the sample.
[8] Fourier difference analysis indicates three residuals:
The first is about 0.4e- in magnitude and located along the
Si-O bond at fractional coordinates (0.0657, 0.1843,
0.0657), about 0.8Å from the oxygen; the second is about
0.3e- and located at (1/4, 1/4, 0), a normally unoccupied
octahedral void; and the third is about 0.2e- and located at
(1/8, 1/8, 5/8), a normally unoccupied tetrahedral void.
These residuals are relatively small, but the first two at
least are robust in that they were observed in four additional
refinements of hydrous ringwoodites, but not in a sample of
nearly anhydrous ringwoodite. One possible interpretation
of these residuals is that the first is the result of positional
disorder of the oxygen toward a partially vacant tetrahedral
site. The second may be partial Si-occupancy of an octahedral void adjacent to a Si-tetrahedron, and the third may be
partial Si-occupancy of a vacant tetrahedral void adjacent to
a Si-tetrahedron. Occupancy of voids adjacent to a Sitetrahedron, such as either of the latter two positions, would
build Si-O-Si bonds and non-silicate oxygens that provide
sites for protonation.
[9] Despite direct evidence from infra-red absorption on
the same sample batch, we did not observe OH stretching
vibrations expected to occur in the range 3100 –3700 cm 1
and reported for the hydrous Mg-endmember composition
by Kudoh et al. [2000] and (with different mode structure)
by Liu et al. [2002]. The lack of observation in Raman is
unusual but may not be surprising: The laser power required
to avoid thermal instability may have been too low for the
observation of weak and broad OH stretching bands [e.g,
Kleppe et al., 2001]. On the basis that the OH stretching
modes are invisible, it is unlikely that the additional modes
at 709, 881 and 939 cm 1 originate from weaker OH
bending vibrations.
4. High-Pressure Evolution of the Raman Spectra
[10] Figure 3 shows representative Raman spectra of
hydrous ringwoodite as a function of increasing and
decreasing pressure. The silicate stretching modes shift
continuously up to the highest pressures; the 796 cm 1
mode increasing faster in frequency with increasing pressure than the 841 cm 1 mode (4.27 cm 1/GPa compared to
3.53 cm 1/GPa). This difference in pressure dependence
leads to a merging of the two bands under compression
resulting in a complete overlap at pressures greater than
45 GPa (Figure 4) – occurring at the same pressure in fact
as with hydrous, iron-free ringwoodite [Kleppe et al., 2002].
[11] The most significant change in the Raman spectrum
of hydrous iron-bearing ringwoodite under static compression is a strong growth in intensity of new bands above
Figure 2. Unpolarized Raman spectrum of hydrous g(Mg0.89,Fe0.11)2SiO4 at ambient conditions. Vertical bars
below the spectrum represent the Raman frequencies of
iron-free hydrous ringwoodite [Kleppe et al., 2002] for
comparison.
KLEPPE ET AL.: HIGH-PRESSURE BEHAVIOR OF RINGWOODITE
17 - 3
water incorporation into the olivine (a-Mg2SiO4) and wadsleyite (b-Mg2SiO4) structure have shown that the presence
of Fe influences the hydration mechanism of both crystal
structures significantly [e.g., Wright and Catlow, 1996].
[14] The low-frequency Raman signal (100– 250 cm 1)
might be associated with localized modes generated by
Fe2+, Fe3+ and H substitution in the ideal magnesium
ringwoodite host structure. The fact that this intensity
increases irreversibly under pressure may support such a
local mode model if it can be directly associated with
energetically-favored site distortions under compression. It
is worth noting that there is no report of similar effects in
any other silicates relevant to the Earth’s transition zone
largely because studies have concentrated on Mg-endmember cases. Additional work is required to understand this
low-frequency behavior and any possible correlation with
local disorder in the structure.
[15] Above 40 GPa we observe pressure-induced modes
emerging between 550– 680 cm 1. Williams et al. [1990]
reported similar, pressure-induced changes associated with
amorphization in high-pressure IR absorbance spectra of
fayalite crystals at 25– 30 GPa. They argued that the shift of
the dominant spectral features from high frequencies (SiO4
Figure 3. Selected unpolarized Raman spectra of hydrous
g-(Mg0.89,Fe0.11)2SiO4 as function of increasing and
decreasing pressure at 300 K. The SiO4 stretching modes
(arrows up) shift continuously and near 40 GPa three new
modes (arrows down) emerge out of the high background.
40 GPa in the range 550– 680 cm 1 (extrapolating down to
400 –600 cm 1 at 1 bar). At 60 GPa these new modes are
more intense than the SiO4 stretching vibrations (796 and
841 cm 1). The spectral changes are reversible without
hysteresis indicating that the pressure-induced structural
modification is not a result of disproportionation.
[12] The broad low-frequency features (80 – 300 cm 1)
gain in intensity with compression and remain strong on
decompression, indicating an irreversible modification in
the crystal structure. The Raman spectra excited at 488 nm
were identical across the entire frequency range with those
excited at 514 nm, precluding the existence of resonance or
fluorescence modes.
5. Discussion
[13] Our Raman and X-ray diffraction data indicate that
Fe-bearing ringwoodite may hydrate by creating Si-O-Si
bonds and non-silicate oxygens that could act as protonation
sites. Our suggested hydration mechanism for Fe-bearing
ringwoodite differs from the structural model for the incorporation of water into the Mg-endmember phase [Kudoh,
2001; Kudoh et al., 2000]. The existence of different hydration mechanisms for iron-bearing and iron-free ringwoodite
is not surprising because the presence of Fe offers new
possibilities for the incorporation of protons. For example,
redox reactions in which the reduction of ferric iron accompanies the OH group creation could be important and are
expected to influence critically the incorporation of H in the
Earth. Calculations on the energetics and mechanisms of
Figure 4. Pressure dependence of the Raman modes of
hydrous g-(Mg0.89,Fe0.11)2SiO4. Representative error bars
are given for the bands with errors outside the size of the
symbol. Filled and unfilled symbols are identical but used
for clarity of the plot. The region shown by the vertical
arrow corresponds to vibrations associated with SiO6
polyhedra in other structures.
17 - 4
KLEPPE ET AL.: HIGH-PRESSURE BEHAVIOR OF RINGWOODITE
vibrations) to lower frequencies under compression can be
associated with an increase in the Si coordination. The
pressure-induced modes we observe may also be associated
with vibrations of Si-units with a coordination number >4:
At least, the positions in the range 550– 680 cm 1 at 40 GPa
and pressure dependence (2.4– 3.4 cm 1/GPa) of the new
modes lies within the range of observed and calculated SiO6
modes and their frequency shifts in perovskite (1.7– 4.2
cm 1/GPa) [e.g., Hemley et al., 1989]. The reversibility of
our pressure-induced changes as well as the coexistence of
the SiO4 stretching frequencies with the new modes up to
60 GPa shows that bond breaking and disproportionation do
not occur. Pressure-induced, reversible changes in MgSiO3
orthoenstatite were reported by Serghiou et al. [2000], and
were also attributed to increased Si coordination based on
the appearance of an ilmenite-like Si-O stretching vibration.
[16] The degree to which the observed detailed structural
changes at 300 K are similar between all these structures is
unclear: In hydrous, iron-bearing ringwoodite additional
levels of transformation complexity must be expected with
the presence of Fe in different valence states, likely
increased site vacancies, and protonation. Unlike the previous suggestions of increased Si coordination at the
expense of SiO4 mode intensities, the intensity of the
SiO4 stretching vibrations in this study does not decrease
strongly with pressure. One further difference is that ringwoodite is already substantially denser than the phases used
by Serghiou et al. [2000] and Williams et al. [1990].
[17] In contrast to the present Raman observations of
hydrous iron-containing ringwoodite, high-pressure Raman
spectroscopic studies of the hydrous Mg-endmember phase
to 56.5 GPa at 300 K [Kleppe et al., 2002], and X-ray
diffraction experiments of anhydrous (Mg0.6,Fe0.4)2SiO4 to
50 GPa at 300 K [Zerr et al., 1993] as well as anhydrous gMg2SiO4 to 25 GPa at 700 K [Meng et al., 1994] showed no
evidence for new pressure-induced features in the spectra.
[18] If the presence of iron and the incorporation of
hydrogen into the ringwoodite structure changes the transformation energetics under pressure the present observations
could provide additional insight for post-ringwoodite phases
in hydrated, albeit cool, regions of subducted material in
advance of the breakdown to perovskite plus oxides. Negative P-T slopes are expected for transitions in general
involving coordination increase [Navrostky, 1980] and the
changes observed here at 40 GPa may then take place at
somewhat lower pressures in cool subducted material –
perhaps not far into the lower mantle. Single-crystal X-ray
diffraction data at high-pressures (>40 GPa) will be required
to evaluate the detailed structural characteristics of the
transformation proposed on the basis of the present Raman
spectroscopic observations. The possibility that coupled
protonation and iron substitution could activate new transition pathways between high density polymorphs might also
be usefully addressed through first-principles calculations.
[19] Acknowledgments. Work supported in part by NERC grant
GR3/10912 to APJ, an Oxford University Graduate Scholarship in Physical
Sciences to AKK and NSF grant EAR 0087279 to JRS. Research at the
Bayerisches Geoinstitut, Bayreuth, Germany supported by the EU ‘‘TMRLarge Scale Facilities’’ program (Contract No. ERBFMGECT980111 to D.
C. Rubie). We thank K. Refson for discussions, R. Hervig for the SIMS
analysis, and anonymous referees for comments.
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A. K. Kleppe and A. P. Jephcoat, Department of Earth Sciences,
University of Oxford, Parks Road, Oxford, OX1 3PR, UK. (Annette.
[email protected])
J. R. Smyth, Department of Geological Sciences, University of Colorado,
Boulder, CO 80309, USA.
D. J. Frost, Bayerisches Geoinstitut, Universität Bayreuth, 95440
Bayreuth, Germany.