X-ray Raman scattering study of MgSiO3glass at high
pressure: Implication for triclustered MgSiO3melt in
Sung Keun Leea,b, Jung-Fu Linb,c, Yong Q. Caid, Nozomu Hiraokad,e, Peter J. Engf,g, Takuo Okuchih, Ho-kwang Maob,i,j,
Yue Mengi,j, Michael Y. Hui, Paul Chowi, Jinfu Shuj, Baosheng Lik, Hiroshi Fukuil,m, Bum Han Leea, Hyun Na Kima,
and Choong-Shik Yoon
aSchool of Earth and Environmental Sciences, Seoul National University, Seoul, 151-742 Korea;cLawrence Livermore National Laboratory, Livermore, CA
94588;dNational Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan;fConsortium for Advanced Radiation Sources andgJames Franck Institute,
University of Chicago, Chicago, IL 60637;hInstitute for Advanced Research, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8601, Japan;iHigh Pressure
Collaborative Access Team, Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439;jGeophysical Laboratory, Carnegie Institution of
Washington, Washington, DC, 20015;kMineral Physics Institute, Stony Brook University, Stony Brook, NY 11794;lInstitute for Study of the Earth’s Interior,
Okayama University, Yamada 827, Misasa, Tottori 682-0193, Japan; andnDepartment of Chemistry and Institute for Shock Physics, Washington State
University, Pullman, WA 99164
Contributed by Ho-kwang Mao, March 18, 2008 (sent for review January 14, 2008)
Silicate melts at the top of the transition zone and the core-mantle
boundary have significant influences on the dynamics and prop-
primary components of the magma ocean and thus played essen-
tial roles in the chemical differentiation of the early Earth. Diverse
macroscopic properties of silicate melts in Earth’s interior, such as
density, viscosity, and crystal-melt partitioning, depend on their
electronic and short-range local structures at high pressures and
temperatures. Despite essential roles of silicate melts in many
geophysical and geodynamic problems, little is known about their
nature under the conditions of Earth’s interior, including the
densification mechanisms and the atomistic origins of the macro-
scopic properties at high pressures. Here, we have probed local
electronic structures of MgSiO3glass (as a precursor to Mg-silicate
melts), using high-pressure x-ray Raman spectroscopy up to 39
GPa, in which high-pressure oxygen K-edge features suggest the
formation of tricluster oxygens (oxygen coordinated with three Si
frameworks;O) between 12 and 20 GPa. Our results indicate that
the densification in MgSiO3melt is thus likely to be accompanied
with the formation of triculster, in addition to a reduction in
nonbridging oxygens. The pressure-induced increase in the frac-
tion of oxygen triclusters >20 GPa would result in enhanced
density, viscosity, and crystal-melt partitioning, and reduced ele-
ment diffusivity in the MgSiO3 melt toward deeper part of the
Earth’s lower mantle.
silicate melts at high pressure ? tricluster oxygen
probably dominated the differentiation of Earth in the Hadean
magma ocean where significant fractions of the Earth were melts
melts, primarily in MgSiO3 composition, at the top of the
transition zone (4–6) and in the core-mantle boundary (7, 8)
significantly contributes to the seismic heterogeneity of the
regions. Pressure-induced structural changes in the silicate melts
play an important role in the macroscopic thermodynamic,
transport, and electronic properties at high pressure (e.g., refs.
9–13). Despite their importance and implications for global
geophysical processes in the Earth’s interior as precursors to
crystalline MgSiO3phases, including perovskite and postperovs-
kite (14, 15), the high-pressure structures of MgSiO3glass and
melt remain enigmatic because of their inherent structural
disorder and the lack of suitable experimental probes at high
pressures. In other binary alkali and ternary aluminosilicate
glasses, the densification mechanism is mostly associated with an
increase, either gradual or abrupt, in the coordination number
he nature of silicate melts at high pressure and temperature
governs magmatic processes in the Earth’s interior and it
of the framework cations, such as Si and Al from 4 to 5 and 6 at
the expense of the nonbridging oxygen (NBO) (11, 12, 16–20).
However, pressure dependence of the coordination transforma-
tion for fully polymerized covalent oxide glasses, including
of the triply coordinated oxygen (O), as evidenced from ab
initio molecular dynamics simulations of the SiO2melt at high
pressures up to 20 GPa (21) and oxygen K-edge x-ray Raman
studies of amorphous B2O3at high pressures (22).
Formation of tricluster is known to be one of the dominant
factors affecting the melt properties at high pressure, potentially
explaining the anomalous pressure-induced changes in viscosity
and oxygen diffusivity (9, 23, 24). The presence of the oxygen
tricluster may significantly enhance the partitioning coefficient
of an element between crystals and Mg-silicate melts in Earth’s
interior and probably account for the low solubility of noble
sixfolded silicon ([5,6]Si) in the highly depolymerized MgSiO3
glass is expected to be associated with the formation of the
with the fully polymerized silicates [see the supporting informa-
tion (SI) for details]; however, experimental evidence for its
formation in the silicate melts and glasses at high pressure is
lacking. We have recently shown that synchrotron x-ray Raman
anvil cells (DACs) provides detailed information on the pres-
sure-induced electronic bonding changes for low-z elements in
amorphous oxides, such as borates and SiO2glasses (22, 26, 27)
and other crystalline and molecular compounds (28–32). As
discussed in detail in refs. 26 and 28, oxygen K-edge x-ray Raman
scattering is currently the only available in situ experimental
N.H., P.J.E., T.O., H.-k.M., and C.-S.Y. designed research; S.K.L., J.-F.L., Y.Q.C., N.H., P.J.E.,
T.O., H.-k.M., Y.M., M.H., P.C., J.S., B.L., H.F., B.H.L., and H.N.K. performed research; S.K.L.,
J.-F.L., Y.Q.C., N.H., P.J.E., H.-k.M., J.S., B.L., and C.-S.Y. contributed new reagents/analytic
tools; S.K.L., J.-F.L., Y.Q.C., N.H., P.J.E., H.-k.M., Y.M., M.H., P.C., H.F., B.H.L., and H.N.K.
analyzed data; and S.K.L., J.-F.L., Y.Q.C., N.H., P.J.E., T.O., H.-k.M., Y.M., M.H., P.C., and H.F.
wrote the paper.
The authors declare no conflict of interest.
Freely available online through the PNAS open access option.
bTo whom correspondence should be addressed. E-mail: email@example.com, lin24@llnl.
gov, or firstname.lastname@example.org.
ePresent address: Brookhaven National Laboratory, Upton, NY 11973.
mPresent address: SPring8, Hyogo 679-5198, Japan.
This article contains supporting information online at www.pnas.org/cgi/content/full/
© 2008 by The National Academy of Sciences of the USA
www.pnas.org?cgi?doi?10.1073?pnas.0802667105 PNAS ?
June 10, 2008 ?
vol. 105 ?
no. 23 ?
technique to directly reveal the pressure-induced electronic
bonding changes around oxygen atoms in oxide glasses. Here, we
have studied the oxygen K-edge x-ray Raman spectra of MgSiO3
glass at pressures up to 39 GPa where the formation of the
triclustered oxygen bonds at high pressures is suggested.
Results and Discussion
The oxygen K-edge spectra of MgSiO3 glass with a dominant
between 1 atm and ?12 GPa. Above 20 GPa, the spectra show a
distinct feature at ?544–545 eV, wherein the spectral features
gradually shift to higher energies with increasing pressure (Fig. 1 A
and B). The occurrence of the spectral feature near 545 eV at high
pressures may stem from a variety of pressure-induced structural
changes in the MgSiO3glass, such as the formation of theO
formation of oxygen linkingSi and highly coordinated silicon,
such asSi–O–[5,6]Si andSi–O–Si.
To explain the spectral feature observed at 544–545 eV in the
x-ray Raman spectra of the MgSiO3glass at high pressure, we
explored the oxygen K-edge x-ray Raman spectra for model
amorphous and crystalline MgSiO3and SiO2phases at ambient
conditions with known short-range local structures [e.g., bridg-
ing oxygen (BO), NBO, and edge-sharing oxygen] (Fig. 2). This
approach is particularly helpful because the connection between
x-ray Raman features and the local oxygen electronic bonding
environment has not been fully understood in these materials.
The x-ray Raman spectra for amorphous SiO2glass at 1 atm and
MgSiO3glass are similar and consist of a broad peak at ?538 eV,
consistent with previous studies of the oxygen K-edge spectra of
SiO2glasses at 1 atm (27, 33, 34) (Fig. 2A). This indicates that
both NBO (Mg–O–Si) and BO (Si–O–Si) have similar
oxygen K-edge features, arising from the insignificant effect of
Mg to the oxygen K-edge feature (Fig. 2B). At ?538 eV, the
oxygen K-edge spectrum for quartz shows a distinct feature
because ofSi–O–Si (Ref (27, 33, 34).). The spectrum for
ilmenite-type MgSiO3shows distinctive features at 537 and 541
eV. This peculiar feature for the edge-sharing oxygen configu-
ration is similar to that of stishovite (27), indicating close
proximity of oxygen around oxygen with its second nearest
neighbors. The oxygen K-edge spectrum of pyroxene shows a
angles in pyroxene (138.3° and 131.4°) compared with that in
quartz (143.6°) (Fig. 2C). The oxygen K-edge spectrum of
perovskite with all corner-sharingSi–O–Si shows an increase
Si coordination transformation (see SI for more discussion).
The abovementioned analyses of the model systems provide a
basis for interpreting the local electronic structures of the
MgSiO3glass at high pressures (Fig. 1). Most of the features
relevant to the local oxygen configurations observed for the
crystalline phases at high pressure, including both the specific
bond angles, length effect of crystals, edge-sharing oxygen (537
and 541 eV), andSi–O–Si (543 eV), cannot explain the
significant changes observed for the glasses at high pressures. In
particular, pressure-induced changes in the Mg–O distance or
the formation ofSi–O–[5,6]Si because of the reduction in NBO
can only partially contribute to the occurrence of the spectral
features at 545 eV (Fig. 1C). Previous oxygen K-edge studies of
B2O3and SiO2glasses, using x-ray Raman spectroscopy, have
shown that the edge feature is sensitive to the change in the
coordination environment of oxygen: Similar features (increase
in intensity with pressure in high energy region) were observed
for oxygen K-edge x-ray Raman spectra for B2O3glass, which has
been attributed to the formation of triply coordinated oxygen
with highly coordinated borons,
K-edge feature at 543 eV for SiO2glass has been interpreted as
the formation of stishovite-like
B (22). A similar oxygen
Si in SiO2 glass at high
pressures (27). We note that the coordination transformation of
Si in the SiO2glass is uniquely accompanied by the formation of
oxygen atoms that are triply coordinated by
pressures [plotted as energy loss (incident energy ? elastic energy) vs. nor-
malized scattered intensity]. (A) Total x-ray raman spectra. Gray area repre-
sents energy range from 539 to 549 eV. (B) Comparison of the Oxygen K-edge
spectra for amorphous MgSiO3at 1 atm and 39 GPa. (C) Oxygen K-edge x-ray
Raman spectra for MgSiO3glasses at 1 atm and 39 GPa and perovskite. Points
refer to the step size of the energy scan of the experiments (see Materials and
Oxygen K-edge x-ray Raman spectra for MgSiO3 glasses at high
www.pnas.org?cgi?doi?10.1073?pnas.0802667105 Lee et al.
theoretical calculations of the local structure of SiO2glass have
shown that a significant increase in the fraction of highly
coordinated Si at ?20 GPa (35), consistent with the changes in
the oxygen K-edge x-ray Raman spectra with pressure (27).
Molecular dynamics simulations have also reported that the
0% at 1 atm to the 30% at 30 GPa (24). Therefore, the oxygen
K-edge feature at 545 eV in the MgSiO3glass at pressures ?20
GPa can be attributed to the formation of the triply coordinated
oxygen and changes in the short- to medium-range structures
that are associated with the formation of the triply coordinated
oxygen at pressures ?20 GPa (i.e., formation of a three-member
ring, an increase in bond length, and a decrease in bond angle).
The shift in the oxygen K-edge energy of MgSiO3 glass
corresponds to the topological changes with pressure, such as the
bond length and angle, as reported in refs. 22 and 31. The
pressure-induced energy shift in the oxygen K-edge observed
here can thus be assigned to ring-statistics associated with a
reduction in the bond angle and the formation of smaller
member rings. The shift in the edge energy is apparently
irreversible when decompressed from ?30 GPa. The energy
difference of 1–2 eV between the x-ray Raman spectra of the
initial and decompressed MgSiO3glass from 30 GPa to 1 atm
cannot be simply explained by the experimental uncertainty in
the spectra, because the uncertainty in the energy calibration at
the synchrotron beam lines used in this study is ?0.4 eV (see SI
for details). This irreversible change in edge energy is consistent
with the permanent densification of MgSiO3glass likely due to
the changes in the ring-statistics.
To qualitatively understand the fraction of the oxygen
tricluster in MgSiO3glass at high pressures, the ratio of the
area under the 545 eV peak to the total area was derived (Fig.
3). Because the feature in the x-ray Raman spectrum can arise
from complex multielectronic transitions spanning from pre-
edge to the extended region, the Gaussian de-convolution of
x-ray Raman spectrum may not properly describe the changes
in bonding. The purpose of the current analysis is, therefore,
not designed to provide the quantitative information about the
fraction of tricluster (the full x-ray Raman profile would be
necessary to achieve this feat). Rather, the current fitting
approach is used to provide the qualitative trend in the
fractions of the oxygen K-edge features that are relevant to the
formation of oxygen tricluster at high pressure. A dramatic
increase in the fraction of the spectral feature at 545 eV
between 12 GPa and 20 GPa in the MgSiO3glass reveals an
electronic bonding transition, which is referred to be due to
oxygen coordination transformation. Ab initio MD simulations
2 was fitted to a model consisting of three Gaussian peaks (one for the 545 eV
peak and two for the other regions to obtain relative fraction) and the fraction
area from ?532 eV to 551 eV. As mentioned in the text, the fitting approach is
used to provide the qualitative trend in the fractions of the oxygen K-edge
features at high pressures.
SiO2and MgSiO3at 1 atm and their high-pressure phases. (A) Total oxygen
K-edge x-ray Raman spectra. (B) Oxygen K-edge spectra for amorphous SiO2
and MgSiO3at 1 atm. All of the oxygens in the crystalline quartz and amor-
MgSiO3glass at 1 atm, approximately one-third of the oxygens are BOs, and
the rest are nonbridging oxygens (NBOs) (Mg–O–Si) (40). The spectral fea-
ture in the lower energy region for MgSiO3glass is slightly broader than that
of SiO2glass. This can be explained by the existence of the NBO in the former,
in which the oxygen 2p projected partial density of states has unoccupied
states at slightly lower energy than BO (41). The features at 537 and 541 eV in
the spectrum for ilmenite-type MgSiO3are associated with oxygen p-p hy-
Oxygen K-edge spectra for pyroxene and quartz. The low energy feature for
pyroxene is probably associated with Mg–O–Si (41). The 538 eV feature in
quartz arises from the transition of the core electrons from 1s states to
nearbySi and that at ?546 eV apparently originates from its long-range
periodicity as it becomes prevalent for the larger clusters with several coor-
dination shells in full multiple-scattering simulations (33). (D) Oxygen K-edge
spectra for amorphous MgSiO3at 1 atm and perovskite. The significantly
different oxygen coordination environments for perovskite and glass at 1atm
at 1 atm) do not lead to significant changes in the x-ray Raman features.
Oxygen K-edge x-ray Raman spectra for crystalline and amorphous
Lee et al.
June 10, 2008 ?
vol. 105 ?
no. 23 ?
(12, 13) of MgSiO3liquid at the pressures of the core-mantle
boundary indicate that the reduced fraction of the NBO is a
major densification mechanism in silicate melts. Together with
the formation of the oxygen triclusters, these trends suggest
multiple densification mechanisms in the MgSiO3melt at high
pressure. The schematic atomic configuration for one of the
oxygen triclusters coordinated with threeSi atoms respon-
sible for the high-energy edge feature (Fig. 4A) is shown along
with a possible mechanism of its formation (Fig. 4B). Although
the mechanism is based on the transformation of the BO to
O with pressure, it is likely that the oxygen triclusters in
MgSiO3 glasses can also be formed from the NBO
(NBO 3O). The formation of the triclusters in Mg-silicates
may initiate the formation of smaller member rings (including
three-member rings), in which the larger member ring (e.g., a
five-member ring) acts as a strain energy reservoir in the
formation of the highly coordinated Si and tricluster (Fig. 4B)
(36). The broadening of the oxygen K-edge feature in MgSiO3
glass also suggests an increase in the topological disorder and
significant changes in the medium-range order with increasing
pressures (37), although the detailed atomic configurations
and effects on the x-ray Raman features require further
Our study of MgSiO3 glass—a precursor for the MgSiO3
melt at high pressures—indicates the formation of the oxygen
triclusters and associated changes in the atomic configuration
in short-to-medium range, with a reduction in the NBO, above
?20 GPa. These changes would affect the thermodynamic
(e.g., density, molar volume, and crystal-melt partitioning) and
transport properties of silicate melts (e.g., viscosity and dif-
fusivity) toward the deeper part of the Earth’s lower mantle.
Although the effect of temperature needs further exploration
to obtain insights into melts, the formation of oxygen triclus-
ters can be an efficient densification mechanism in the MgSiO3
melt in Earth’s mantle and may explain the atomistic origin of
the high-density Mg-silicate melts at the core-mantle boundary
(7, 8). Changes in the local electronic structure and compo-
sition of silicate melts are believed to promote the partitioning
of elements between crystalline phases and melts in the Earth’s
mantle (10, 38, 39). The increase in the fraction of the oxygen
triclusters with smaller member rings results in a reduced free
volume needed to host elements that are more incompatible.
That is, the triclustered oxygens increase the crystal-melt
partitioning coefficient (Dcrystal-melt) of elements, such as ra-
dioactive nuclides [i.e., (dO/dP)T ? (dDcrystal-melt/dP)T],
thereby significantly affecting the process of the chemical
differentiation in the Hadean magma oceans. The continuous
increase in the fraction of the triclustered MgSiO3melt at high
pressures and temperatures thus needs to be taken into
account in future modeling to improve our understanding of
the microscopic origins of the geochemical and geophysical
processes in the Earth’s interior.
Materials and Methods
MgSiO3 glass was synthesized by melting MgCO3 and SiO2 powder at
1,650°C and quenching to ambient conditions. The samples were loaded
into the sample chamber of a Be gasket in a DAC with a few ruby spheres
as the pressure calibrant without a pressure medium. Because no pressure
medium was used to minize scattering or absorption from pressure me-
dium, the stress condition upon pressurization in DAC is uniaxial. Although
the glass structure under pressure, its effect is unlikely to be significant,
mostly because of the isotropic nature of the silicate glasses without
directional bonding changes with pressure. An experiment with hydro-
static condition (with the pressure medium) is essential to explore the
effect of stress condition on the pressure-induced changes in electroning
bonding environment in oxide glasses including borate that apparently
have directional bonding associated with boroxol ring. Diamonds with flat
culets of 150–500 ?m depending on the phases and pressure ranges of the
GPa. The thickness of the sample in the gasket was ?30–80 ?m at high
pressures. Crystalline MgSiO3pyroxene was synthesized in a gas-mixing
furnace, and ilmenite and perovskite samples were synthesized in a mul-
tianvil apparatus. It should be mentioned that the crystal structures of
these materials were confirmed by x-ray diffraction and optical Raman
spectroscopy before experiments.
The x-ray Raman spectra were collected at beam line BL12-XU of the
respectively), beam line 13ID-C of the Geo Soil Enviro Consortium for
Advanced Radiation Sources (1 bar, 26 GPa, and 39 GPa), and beam line
16ID-C of the High Pressure Collaborative Access Team (at 12, 20, 30, and
2 GPa for decompressed MgSiO3glass from 30 GPa, quartz, and SiO2glass)
of the Advanced Photon Source. X-ray Raman spectra for these crystalline
samples and SiO2glass at 1 bar were collected for the samples mounted
directly on the goniometer without DAC (and thus without cold compres-
incident beam relative to the analyzer with a fixed elastic energy (E0) of
9.886 keV at beam line BL12XU, 9.692 keV at the Geo Soil Enviro Consor-
tium for Advanced Radiation Sources, and 9.686 keV at the High Pressure
Collaborative Access Team. The x-ray Raman scattering signals were col-
lected at a scattering angle of 18° at the Geo Soil Enviro Consortium for
Advanced Radiation Sources and 30° at the High Pressure Collaborative
Access Team with a linear array of six spherical Si (660) analyzers operating
in a backscattering geometry. An array of three Si (555) analyzers with a
scattering angle of 30° were used for the experiment performed at
BL12-XU (31). Oxygen K-edge spectra were collected at pressures up to 39
GPa. A two-point smoothing was used. The x-ray beam size was ?80 ?m
horizontally and 20 ?m vertically at the Geo Soil Enviro Consortium for
Advanced Radiation Sources and High Pressure Collaborative Access Team
and was 20 ? 20 ?m at the BL12-XU. X-ray Raman spectra were collected
with ?0.5 eV steps for the data collected at the High Pressure Collaborative
Access Team and BL12-XU. At the Geo Soil Enviro Consortium for Advanced
Radiation Sources, the x-ray Raman spectrum near the main edge feature
was scanned at an energy step of ?0.7 eV from 532 to 550 eV; scan steps of
1.9 eV and 1 eV were used for data collection ?550 eV and ?532 eV,
eV, which was used as the background spectrum. Raw x-ray Raman spectra
were background-subtracted, and then most of the spectra were normal-
ized to the continuum energy tail ?555 eV. By comparing the spectra from
was identified to be ?0.4 eV (see SI for details). The pressure uncertainties
oxygen tricluster (O, blue oxygen) as one of the examples (see SI for more
details). (B) Possible mechanism for the formation of the oxygen tricluster.
Here, the blue bridging oxygen in the five member ring (Left) approaches
towardSi and formsSi and tricluster oxygen (Right) with three member
rings. A similar mechanism for the tricluster formation was proposed for
borosilicate glasses at 1 atm (36).
Structure of oxygen tricluster. (A) Schematic local structure of the
www.pnas.org?cgi?doi?10.1073?pnas.0802667105 Lee et al.
and after the x-ray measurements, which can be attributed to the relax- Download full-text
ation of the sample chamber during the experiments and the pressure
gradient across the sample.
ACKNOWLEDGMENTS. We thank I. Jarrige, H. Ishii, V. Iota-Herbei, A. Lazicki,
and N. Ito for their assistance in the high-pressure XRS experiments; J. Kung
(National Cheng-Kung University, Tainan, Taiwan) for the MgSiO3glass sam-
sample. We thank Professor E. Ohtani and Dr. C. Kao for careful and construc-
tive suggestions and advice. We also thank M. Newville for helpful discussion
on theoretical calculations of x-ray Raman spectra. Use of the Advanced
Photon Source was supported by Department of Energy Basic Energy Sciences
under Contract W-31-109-Eng-38. The Geo Soil Enviro Consortium for Ad-
vanced Radiation Sources was supported by Department of Energy-Basic
Energy Sciences-Geosciences, National Science Foundation Earth Sciences
and the State of Illinois. The High Pressure Collaborative Access Team was
supported by Department of Energy Basic Energy Sciences Materials Science,
the Department of Energy National Nuclear Security Administration, the
Carnegie/Department of Energy Alliance Center, the National Science Foun-
dation, and the W. M. Keck Foundation. Experiments performed at beam line
BL12-XU of SPring-8 were partly supported by the National Synchrotron
Radiation Research Center and National Science Council of Taiwan. This work
at Lawrence Livermore National Laboratory was performed under the aus-
pices of the U.S. Department of Energy by the University of California/
work was supported by the Korea Science and Engineering Foundation Grant
2007-000-20120-0 (to S.K.L.) through the National Research Laboratory Pro-
of Excellence program of the Institute for Study of the Earth’s Interior (H.F.).
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