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and other incompatible elements with large ionic
radii (e.g., Hofmann, 1997). Since the subduction
of MORB at plate convergence zones makes the
mantle chemically heterogeneous, it is important
to study the fate of the basaltic materials for un-
derstanding the chemical evolution of the Earth
(Christensen & Hofmann 1994). MORB is more
felsic than mantle peridotite and has a quite differ-
ent mineralogy at lower mantle conditions with
large amounts (
16
SiO
2
V
P
V
Φ
12
V
S
20 vol% for each) of free SiO
2
silica (stishovite (St) and the CaCl
2
phase), cal-
cium silicate perovskite (CaPv), and an additional
aluminous compound with an approximate com-
position of MgAl
2
O
4
(Irifune & Ringwood, 1993;
Ono
et al
., 2001; Hirose & Fei, 2002).
Ab initio
simulations revealed that at the
mantle temperatures, stishovite first under-
goes a second-order phase transition to the
orthorhombic CaCl
2
-type structure (space group
Pnnm
)at
∼
8
ρ
4
0
50
100
150
200
P (GPa)
70 GPa and 2000 K, and then to the
α
-PbO
2
-type structure (
Pbna
)at
∼
Fig. 7.4
Static elastic wave velocities and density
calculated for the silica phases, stishovite (CaCl
2
)
(solid lines) and
α
-PbO
2
(dashed lines), as a function of
pressure. A vertical line represents the CaCl
2
-
α
-PbO
2
transition pressure. Nonlinear behaviors in a shaded
area indicate the ferroelastic anomaly associated with
the St-to-CaCl
2
120 GPa and
2500 K, with steep Clapeyron boundaries for both
cases (Tsuchiya
et al
., 2004c). The study tightly
constrained vague phase boundaries reported by
LH-DAC experiments (Dubrovinsky
et al
., 2001;
Murakami
et al
., 2003). The
α
-PbO
2
-type phase is
therefore stable only near above the CMB depth.
Also, there were experimental uncertainties
related to the symmetry of the
α
-PbO
2
-type phase
(Dubrovinsky
et al
., 1997, 2001). However, the
structural optimizations using different proposed
symmetries show that, in the ground state, the
Pbca
polymorph is more stable than the ones
with
Pnc
2,
P
2
1
2
1
2and
P
2 symmetry (Tsuchiya
et al
., 2004c).
The mineral physical interest in the post-
stishovite transition is the ferroelastic instability
expected to occur associated with the second-
order stishovite-to-CaCl
2
transition (Cohen,
1992; Karki
et al
., 1997). A zone-center acoustic
phonon softens accompanied by a spontaneous
development of the orthorhombic strain across
the continuous structure change from tetragonal
stishovite to orthorhombic CaCl
2
. This causes
an anomalous decrease in the shear modulus,
thus in
V
P
and
V
S
. Wave velocities in the high-
pressure
∼
transition.
Tsuchiya, 2011) are shown in Figure 7.4. Due to
applying LDA at the static condition, the post-
stishovite transitions are located at relatively
lower pressures than the above-mentioned high-
temperature transition pressures of pure SiO
2
.
However, high-pressure experiments showed
that stishovite in the basaltic composite accom-
modates several wt% of Al along with H via
the coupled substitution (Pawley
et al
., 1993)
and those impurities have substantial effects to
lower the stishovite-CaCl
2
transition pressure
(Lakshtanov
et al
., 2007).
SiO
2
shows anomalous nonlinearities both in
V
P
and
V
S
around 45 GPa (Figure 7.4) associated
with the ferroelastic post-stishovite transition
to the CaCl
2
-type phase (Karki
et al
., 1997b;
Tsuchiya
et al
., 2004c). Both the PPv transition in
MgPv and the CaCl
2
-
α
-PbO
2
transition in SiO
2
occur at 98 GPa in the static LDA calculations
(Karki
et al
., 1997c; Tsuchiya
et al
., 2004b). The
silica
phases
(Karki
et al
.,
1997a;
