103
MgSiO
3
is a representative chemical composition
of the Earth’s mantle. The upper/lower mantle
boundary is marked by the formation of orthorhombic
MgSiO
3
-rich perovskite (space group: Pbnm). The
MgSiO
3
perovskite was first synthesized at 30 GPa in
1975 [1]. Later, the conservation of perovskite
structure was confirmed up to 127 GPa [2]. It has
been believed that the perovskite-type MgSiO
3
-rich
phase is a predominant mineral in the lower mantle
and is stable to 135 GPa a condition corresponding to
the bottom of the mantle. However, phase transition
of MgSiO
3
perovskite was once suggested from the
seismic anomalies near the base of the mantle around
2700-km depth [3].
In order to investigate the stability and possible
phase transition of MgSiO
3
perovskite, we performed
in situ X-ray observation of pure MgSiO
3
composition
at high pressure and temperature up to 134 GPa and
2600 K corresponding to the conditions of the
lowermost mantle [4]. Angle-dispersive X-ray
diffraction spectra were collected at beamline
BL10XU. High pressure and temperature conditions
were generated in a laser-heated diamond anvil cell
(LHDAC). Temperature was measured by the
spectroradiometric method. Pressure was determined
from the unit-cell volume of platinum mixed with the
sample using P-V-T equation of state.
Results demonstrate that the Pbnm perovskite
structure is stable at least to 114 GPa and 2300 K
(Fig. 1). Above 127 GPa and 2500 K, fifteen new
peaks were observed in the diffraction pattern. These
new peaks from a new MgSiO
3
polymorph (post-
perovskite phase) can be indexed by an orthorhombic
cell with lattice parameters of a = 2.456(0)
Å
, b =
8.042(1)
Å
, and c = 6.093(0)
Å
. In order to determine
the crystal structure that possesses these lattice
parameters, molecular dynamics (MD)-aided crystal
structure design was performed. The appropriate
number of atoms (8 Mg + 8 Si + 24 O; Z=8) were
positioned randomly in a double unit cell because of
the small a-parameter. Classical MD calculations
were carried out with the (NTV) ensemble of this
system at high temperature (5000 K), and then the
system was quenched to 0 K. The calculations were
repeated, each time checking and correcting the
atomic positions until the crystal structure became
consistent with the crystal chemistry and the
calculated XRD pattern matched the observed one.
The result revealed a crystal structure of a new phase
(post-perovskite phase) with a space group of Cmcm
(Fig. 2). This is isostructural with UFeS
3
or CaIrO
3
.
The post-perovskite phase is denser than perovskite
by 1.0-1.2 % at 121 GPa and 300 K.
The post-perovskite phase has six-fold Si and
Post-perovskite Phase Transition in MgSiO
3
Fig. 1. Phase diagram of MgSiO
3
. Solid squares and open circles
indicate the stabilities of Pbnm perovskite and post-perovskite
phase, respectively. A broken line shows the transition boundary.
60 70 80 90 100 110 120 130 140
Temperature (K)
Pressure (GPa)
1600
1800
2000
2200
2400
2600
2800
Perovskite
Post-perovskite
104
eight-fold Mg coordination, and the SiO
6
-
octahedra share the edges to make an
octahedral chain like rutile-type structure.
These chains run along a-axis and are
interconnected each other by apical
oxygen atoms in the direction of c-axis to
form edge and apex shared octahedral
sheets. The octahedral sheets are stacked
along b-axis with interlayer Mg
2+
ions. This
crystal structure was further optimized by
the first-principles calculations [5]. The
calculations also indicated that the post-
perovskite phase stabilizes relative to
perovskite above 98 GPa and 0 K.
These results show that the MgSiO
3
post-perovskite is a predominant mineral in
the Earth’s lowermost mantle (D” region) at
2700 to 2900-km depth. Phase transition
can cause large seismic heterogeneities.
The D” seismic discontinuity is observed in
many regions around the world about 200-
300-km above the core-mantle boundary
(119-125 GPa). The post-perovskite phase
transition occurs at depths matching those
of the D” discontinuity (Fig. 1) and is most
likely responsible for the cause of seismic
velocity increase up to 3%. A large S-wave
polarization anisotropy (V
SH
> V
SV
) is also
observed below the D” discontinuity. It can
be explained by a strong preferred
orientation of the post-perovskite phase
under the strong horizontal shear flow [5].
The D” region has long been the most
enigmatic region in the Earth’s interior. The
newly discovered MgSiO
3
post-perovskite
phase provides a consistent way to explain
a number of seismic anomalies observed in
this region.
Kei Hirose
Department of Earth and Planetary Sciences,
Tokyo Institute of Technology
E-mail: kei@geo.titech.ac.jp
References
[1] L. Liu: Geophys. Res. Lett. 2 (1975) 417.
[2] E. Knittle and R. Jeanloz: Science 235 (1987) 668.
[3] I. Sidorin et al.: Science 286 (1999) 1326.
[4] M. Murakami, K. Hirose, K. Kawamura, N. Sata, Y.
Ohishi: Science
304 (2004) 855.
[5] T. Iitaka, K. Hirose, K. Kawamura and M. Murakami:
Nature 430 (2004) 442.
Fig. 2. Crystal structure of the post-perovskite
phase projected along [001], [100], and [010]
directions, and a stereoscopic view showing the
layer stacking structure. Coordination polyhedra of
oxygen atoms around Si atoms are shown as
octahedra, and the Mg
2+
ions are shown as balls.
Bold line indicates the unit cell.
b
c
a
c
a
c
b
b
a
c
b
a
![Fig. 2. Crystal structure of the post-perovskite phase projected along [001], [100], and [010] directions, and a stereoscopic view showing the layer stacking structure. Coordination polyhedra of oxygen atoms around Si atoms are shown as octahedra, and the Mg2+ ions are shown as balls. Bold line indicates the unit cell.](/figures/fig-2-crystal-structure-of-the-post-perovskite-phase-dimno9kz.webp)
