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instability both in V P and V S , (Kawai & Tsuchiya,
2012c) this transition of the K-hollandite I phase
could also be related to the seismic scatter-
ers observed in the uppermost lower mantle
(Kaneshima & Helffrich, 1999) similarly to the
dissociation of NaAlSi 2 O 6 jadeite.
16
KAISi 3 O 8
V P
12
V Φ
7.4.5 NAL phase
The aluminous phase synthesized in the basaltic
composition, called ''Al-rich phase'', has a crys-
tal structure similar to the calcium ferrite (CF)
structure (Irifune & Ringwood, 1993). Later, this
Al-rich phase was proposed to be a new hexag-
onal aluminous (NAL) phase (Miyajima et al .,
2001; Akaogi et al ., 1999; Sanehira et al ., 2005) or
a CF-type phase (Kesson et al ., 1998; Hirose et al .,
1999; Ono et al ., 2001). The NAL phase has a
hexagonal crystal structure with the space group
P 6 3 /m with a chemical formula of XY 2 Z 6 O 12 ,
where X represents large monovalent or divalent
cations such as Ca 2 + ,Na + ,andK + , Y middle-
sized ones such as Mg 2 + ,and Z small Al 3 + or
Si 4 + . Z O 6 octahedra form edge-sharing double
chains (Miura et al ., 2000; Gasparik et al . 2000),
and Y atoms are located in a large tunnel made up
by the Z O 6 octahedral chains. The X cations are
randomly distributed in the ninefold coordination
sites with half-occupancy.
Studies on the phase relations in the natural
MORB reported that both NAL and CF coexist up
to about 50 GPa but NAL disappears above 50 GPa
(Perrillat et al ., 2006; Ricolleau et al ., 2008, 2010).
It was also observed that as the volumetric frac-
tion of NAL decreased with increasing pressure,
the fraction of MgPv instead increased. Although
it was unclear whether the disappearance of NAL
phase is due to the high-pressure stability relation
between the NAL and CF phases or absorbance
ofNALtoMgPv(Ono et al ., 2009; Ricolleau
et al ., 2010), more recent studies on the phase
relations in the NaAlSiO 4 -MgAl 2 O 4 join inves-
tigated using the LH-DAC (Imada et al ., 2011)
and for the NaMg 2 Al 5 SiO 12 composition by an
ab initio computation (Kawai & Tsuchiya, 2012b)
both demonstrated that the CF phase should be
recognized as the high-pressure phase of the NAL
8
V S
ρ
4
0
50
100
150
P (GPa)
Fig. 7.6 Static elastic wave velocities and density
calculated for KAlSi 3 O 8 as a function of pressure.
Nonlinear behaviors in a shaded area indicate the
ferroelastic anomaly associated with the K-hollandite
I-to-II transition.
the other suggested instability at about 22
and 23 GPa (Caracas & Boffa Ballaran, 2010).
The Born's elastic stability criteria (Born &
Huang, 1954) however clearly indicate that the
mechanical instability starts from
16 GPa with
softening of shear modulus, which causes a fer-
roelastic tetragonal instability of the tetragonal
K-hollandite I phase and its second-order phase
transition to the monoclinic K-hollandite II phase
(Kawai & Tsuchiya, 2012c). Associated with this,
velocity anomalies appear primarily in V P and V
(Figure 7.6). Then the K-hollandite II phase was
found mechanically stable up to 150 GPa (i.e., en-
tirely in the Earth's lower mantle). If considering
a positive Clapeyron slope of 7 MPa/K between
K-hollandite I and II measured experimentally
(Nishiyama et al ., 2005), the transition pressure
corresponds to the depth of 692 km and 771 km at
1300 K (cold slab geotehrm) and 1800 K (normal
mantle geotherm), respectively. Since substantial
velocity reductions are expected across the shear
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