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1987; Inoue
et al
., 1995). High-pressure experi-
ments in the systems MgO-SiO
2
-H
2
O (MSH) and
MgO-Al
2
O
3
-SiO
2
-H
2
O (MASH) however eluci-
dated that reactions between water and mantle
materials or direct conversions of above miner-
als produce some particular hydrous minerals,
phase D and
δ
-AlOOH, which are stable even at
the lower mantle pressures beyond
While it was suggested that the stability limit of
phase D is located from
∼
40 GPa and
∼
1800 K to
∼
800 K corresponding to the rela-
tively shallower part of the lower mantle (Shieh
et al
., 1998),
δ
-AlOOH was found to have wide sta-
bility ranges both in temperature and pressure and
remain stable even at the Earth's CMB condition,
raising the possible transportation of hydrogen
into the lowermost mantle or even to the core
(Sano
et al
., 2008; Tsuchiya & Tsuchiya, 2011).
The
ab initio
simulations are useful in identi-
fying the stable hydrogen (proton) position in the
crystal lattices, which is in general difficult by
the X-ray diffractometry. Several studies reported
a particular high-pressure behavior of these two
dense hydrous phases using partly the results of
ab initio
calculations (e.g., Tsuchiya & Tsuchiya,
2009a; Panero, 2010; Liu
et al
., 2010). These
studies showed evidence for the pressure-induced
hydrogen bond symmetrization (Tsuchiya
et al
.,
2002, 2005a, 2008a), namely the usual O-H
...
O
bond changes to O-H-O under pressure. This
H-bond symmetrization is uncommon for most of
typical hydrous minerals, but is predicted to take
place at
60 GPa and
∼
25 GPa at
relatively low temperatures and thus could be po-
tential water carriers into the deep mantle (Liu,
1987; Ohtani
et al
., 1997, 2001; Irifune
et al
.,
1998; Suzuki
et al
., 2000; Sano
et al
., 2004). In-
deed, the aluminous hydrous silicate, phase Egg
(AlSiO
3
(OH)), was found in natural diamond in-
clusions (Wirth
et al
., 2007). High-
P
,
T
stability
and elasticity of these dense hydrous minerals are
thus keys to constraining the water distribution
in the deep Earth.
Phase D has a trigonal unit cell with space
group
P
3 1
m
, and the crystal structure consists
of alternating SiO
6
and MgO
6
octahedral layers
(Ohtani
et al
., 1997; Frost & Fei 1998, 1999; Ku-
doh
et al
., 1997; Yang
et al
., 1997; Suzuki
et al
.,
2001; Tsuchiya
et al
., 2005a). Among dense hy-
drous magnesium silicate minerals, only phase
D has a structure in which every Si atom is in
octahedral coordination. This is the primary rea-
son for the high stability of phase D under high
pressure. The position of the H atom in phase D
at ambient pressure has been measured by X-ray
diffraction and neutron powder diffraction (Yang
et al
., 1997; Suzuki
et al
., 2001). According to
these experiments, the hydrogen position is disor-
dered within the MgO
6
layers. The ideal chemical
composition is MgSi
2
O
6
H
2
, although depending
on the synthesis conditions, the stoichiometry is
observed to vary between 0.55 and 0.71 Mg/Si and
between 2 and 3.4 OH molecules per formula unit.
Experiments and theory reported that the crys-
tal structure of
δ
-AlOOH has an orthorhombic
unit cell with space group of
P
2
1
nm
at relatively
lower pressures (Suzuki
et al
., 2000; Tsuchiya
et al
., 2002). This phase has an aluminum and
oxygen packing similar to that of the CaCl
2
-type
structure which can be described in terms of
a slightly distorted rutile-type structure and is
known as the post-stishovite structure of SiO
2
.
∼
30 and 40 GPa in
δ
-AlOOH and phase
D respectively, because these phases both have
the atomic geometries suitable to form the sym-
metric hydrogen bond. GGA was applied in those
studies, which is essential to describe the weak
hydrogen bond (Tsuchiya
et al
., 2005a). Also this
phenomenon is recently observed or suggested ex-
perimentally (Sano-Furukawa
et al
., 2008, 2009;
Shinmei
et al
., 2008; Hushur
et al
., 2011) but at
pressures lower by
∼
10 GPa than theoretically
predicted. This may be due to thermal, isotopic,
or quantum effects.
Elastic wave velocities and density of
δ
-AlOOH
and phase D calculated by the
ab initio
method
(Tsuchiya & Tsuchiya, 2008a, 2009b) are summa-
rized in Figure 7.8. Anomalous velocity increases
seen in
V
P
and
V
of
δ
-AlOOH around 30 GPa
are caused by the H-bond symmetrization. Sim-
ilar behaviors are also for phase D at
∼
40 GPa,
even though the magnitude is smaller. Since the
H-bond symmetrization affect mainly bulk mod-
ulus (Tsuchiya & Tsuchiya, 2008a, 2009b), no
anomaly is produced in
V
S
. If comparing these
∼
