Geoscience Reference
In-Depth Information
magnetism, and also spin state. Several different
spin configurations are in general acceptable for
the partially filled
d
electron shell. The tradi-
tional crystal field theory with the fundamental
Hund's rule tells that at low pressures, the total
spin moment should be maximized stabilizing the
high-spin (HS) state, while at high pressures, the
total moment decreases due to the spin paring.
This spin crossover transition of iron in MgPv,
which had been long argued (e.g., Gaffney &
Anderson, 1973), was detected by the
in situ
X-ray
emission spectroscopy measurement (Badro
et al
.,
2004). However, this spin transition of iron is still
highly unclear in Pv and PPv. Unlike experimen-
tal reports, most
ab initio
studies have predicted
stability of HS in the entire pressure range of
the Earth's lower mantle, when iron is located in
the Mg site as the ferrous (Fe
2
+
) state (Caracas &
Cohen, 2005; Tsuchiya & Tsuchiya, 2006; Stack-
house
et al
., 2006; Bengtson
et al
., 2008; Umem-
oto
et al
., 2008). Although many studies were
performed within GGA, the internally consistent
LDA
Several experiments reported that MgPv and
PPv can contain substantial amount of ferric
(Fe
3
+
) iron (e.g., McCammon, 1997; Frost
et al
.,
2004; Sinmyo
et al
., 2008). Two iron incor-
poration mechanisms are considered, vacancy
formation or coupled substitution, but the latter
is more reasonable under pressure. The spin
transition also of Fe
3
+
in Pv was investigated
experimentally (Li
et al
., 2004), by GGA-level
calculations (Li
et al
., 2005), and by internally
consistent
+
U
simulations (Hsu
et al
.,
2011; Tsuchiya & Wang, 2013). Those studies
found that Fe
3
+
located in the Si site has lower
transition pressures
LDA
∼
55 GPa at low temperature
and
2000 K (Tsuchiya & Wang,
2013). However, the Si site absorbs Al
3
+
more, if
Al coexists with Fe
3
+
(Li
et al
., 2005). In addition
to the HS and LS state, in situ M ossbauer
spectroscopy measurements suggested the sta-
bilization of intermediate spin (IS) state in the
deep lower mantle (McCammon
et al
., 2008;
Lin
et al
., 2008). Theoretical modeling however
elucidated that the signal of M ossbauer implying
the IS state can rather be interpreted as a signal
of the HS state in the crystal field of distorted
bicapped trigonal prism (Bengtson
et al
., 2009;
Hsu
et al
., 2010). As reviewed here, several
factors must be considered to understand the
spin crossover of iron in Pv and PPv, which is
more complicated than that in ferropericlase (see
the next section). The spin transition observed in
Fe-bearing Al-free MgPv at
∼
75-90 GPa at
∼
U
method was also applied in the recent
studies (Hsu
et al
., 2010; Metsue & Tsuchiya,
2011). The spin transition in both Pv and PPv
with geophysically relevant iron concentration
was predicted to occur at 160
+
200 GPa within
GGA and far beyond 200 GPa within the inter-
nally consistent LDA
∼
U
. This is crystallograph-
ically quite reasonable, because the Mg site in Pv
is substantially large and thus it has a week crys-
tal field. Although lower spin transition pressures
were found only in some particular iron config-
uration and also when applying LDA (Umemoto
et al
., 2008), such iron ordering is quite unlikely in
the actual solid solution. Some
ab initio
studies,
on the other hand, proposed that the spin transi-
tion pressure in Pv decreases with increasing the
iron concentration (Bengtson
et al
., 2008), and be-
comes quite low (9 GPa within LDA and 77 GPa
within GGA) for end-member FeSiO
3
. However,
since iron-rich perovskite is thermodynamically
unstable, it is difficult to experimentally confirm
those computed results. No LDA
+
70 GPa (Badro
et al
.,
2004; Jackson
et al
., 2005) is not fully clarified
yet, but would be related to the spin transition of
Fe
3
+
. Nonhydrostaticity, low-grade crystallinity,
and some other experimental technical issues
likely lead to complications.
As widely known, the presence of iron
decreases the acoustic wave speeds mainly
because of its heavy atomic weight. Several
studies investigated the effects of Fe and Al
on the elasticity of MgPv and PPv primarily
based on the
ab initio
method (e.g., Kiefer
et al
.,
2002; Caracas & Cohen, 2005; Li
et al
., 2005;
Stackhouse
et al
., 2005; Tsuchiya & Tsuchiya,
2006; Stackhouse & Brodholt, 2008) and showed
that iron substantially reduces the shear modulus
∼
U
investi-
gation has been performed for iron-rich com-
positions including FeSiO
3
so far, though it is
expected to provide more reliable predictions.
+
