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large single-crystal elastic anisotropy because of
its sheet stacking structure of SiO
6
-octahedra.
A strong preferred orientation could develop un-
der shear flow although it is still under debate as
to which slip plane is dominant for the MgSiO
3
post-perovskite phase (Iitaka
et al
., 2004; Oganov
& Ono, 2004; Wookey
et al
., 2005; Yamazaki
et al
., 2006; Merkel
et al
., 2006, 2007; Miya-
jima
et al
., 2006; Niwa
et al
., 2007; Miyagi
et al
.,
2008; Walte
et al
., 2009; Miyajima & Walte, 2009;
Okada
et al
., 2009). If we assume the presence of
horizontal shear flow, which is believed to exist at
some of the CMB, then a reasonably large seismic
velocity contrast (up to
prominent elastic properties on MgO extracted
from those previous studies is that the MgO
exhibits a substantial elastic anisotropy at ambi-
ent pressure and temperature although MgO with
cubic symmetry of NaCl-type (B1) structure is
optically isotropic. High-pressure sound velocity
experiments further suggested that the anisotropy
decreases with increasing pressure at ambient
temperature at least up to
20 GPa. The ultra-
sonic experiments of (Chen
et al
., 1998) on single-
crystal MgO to 8 GPa and 1600 K then demon-
strated that the anisotropy of MgO increases as
temperature is increased at high pressure. Recent
technical advancement in ultrasonic interferom-
etry combined with synchrotron X-ray diffraction
measurement further enabled to determine the
elastic velocities of MgO up to 24 GPa and 1650
K, almost reaching the
P
-
T
condition at the bot-
tom of the mantle transition zone (Kono
et al
.,
2010). Available experimental data for MgO from
the direct shear wave velocity measurements un-
der lower mantle pressure regime have, however,
been limited to a maximum pressure of 55 GPa
on single-crystal by Brillouin spectroscopy (Zha
et al
., 2000). Owing to the lack of the shear wave
velocity data on MgO under whole pressure range
of lower mantle, the pressure dependence of elas-
tic velocities or moduli has so far been poorly
constrained. In particular, the pressure deriva-
tive of the shear modulus (
G
0
) reported in the
previous experiments exhibited considerable vari-
ations ranging from 2.21 to 2.85, which prevents
us from modeling the detailed lower mantle min-
eralogy. The large variation of
G
0
values of MgO
also makes it difficult to evaluate the effect of
iron on
G
0
of (Mg,Fe)O ferropericlase (Kung
et al
.,
2002; Jackson
et al
., 2006; Crowhurst
et al
., 2008;
Marquardt
et al
., 2009, 2009).
Murakami
et al
. (2009) have determined shear
wave velocity measurements of polycrystalline
MgO collected by Brillouin spectroscopy in a
DAC in conjunction with synchrotron X-ray
diffraction technique under the entire lower
mantle pressure regime approaching 130 GPa.
A representative Brillouin spectrum and X-ray
diffraction image at high pressure are shown in
Figure 6.7, and the pressure dependence of the
∼
7%) may appear together
with a signature of shear wave anisotropy (Iitaka
et al
., 2004). If this were the case, the velocity
change as the D
discontinuity could be explained
by the shear flow pattern and a degree of preferred
orientation of the post-perovskite phase. A com-
bination of post-perovskite phase transition with
the LPO of post-perovskite phase is a possible
model to explain the characteristics of the D
discontinuity.
∼
6.3.3 MgO, (Mg,Fe)O
Periclase (MgO) is important not only as a geo-
physically important minerals as the end-member
of major lower mantle phase of (Mg,Fe)O, ferroper-
iclase, but also as a standard material for checking
the performance of the experimental techniques
for elasticity measurements (Jackson & Niesler,
1982; Isaak
et al
., 1989; Yoneda, 1990) and for
theoretical modeling of thermoelastic properties
(Karki
et al
., 1997). Its remarkably wide pressure
and temperature stability field (Karki
et al
., 1997;
Duffy & Ahrens, 1995) provides the optimal con-
ditions for exploring the pressure dependence of
elastic properties that is critical to the geophys-
ical applications of mineral physics data. Elastic
properties of MgO have thus been studied by
a number of methods. Large number of stud-
ies under low-pressure condition has consistently
yielded precise elastic constants of MgO at zero
pressure (Spetzler, 1970; Jackson & Niesler, 1982;
Yoneda, 1990; Chopelas, 1992; Duffy & Ahrens,
1995; Sinogeikin
et al
., 1998, 2000). One of the
