Geoscience Reference
In-Depth Information
CaMgSi
2
O
6
clinopyroxene to ilmenite, if it occurs,
is probably a higher-pressure transition. It may
transform to the perovskite structure without an
intervening field of ilmenite:
VIII
(Ca
0
.
5
)
3
VI
(Ca
0
.
5
Mg
0
.
5
)
VI
Si
IV
Si
3
O
12
'majorite'
→
seismic velocity of mw is low and there cannot be
too much of it in the lower mantle if seismic data
is to be satisfied. This is one of the arguments for
a chemically stratified mantle.
(Mg
magnesiowustite
The post-
spinel
phases of mantle minerals are mix-
tures of magnesiowustite and perovskite. Mag-
nesiowustite [(Mg,Fe)O], a cubic mineral, is the
second most abundant mineral of Earth's lower
mantle. Mg-rich magnesiowustite may be stable
in the rock--salt structure throughout the lower
mantle. Iron-rich magnesiowustites may decom-
pose into two components, Fe-rich and Mg-rich
magnesiowustites, particularly if the low-spin
transition in FeO takes place. Magnesiowustite in
the lowermost mantle may remove FeO from the
outer core.
In the lower mantle Fe
2
+
favors (Mg,Fe)O over
perovskite
. The post-
perovskite
phase in the deepest
mantle is also probably very FeO-rich. When the
Fe
2
+
high-spin--low-spin transition occurs, some-
where deep in the lower mantle, solid solution
between Fe
2
+
and Mg
2
+
is probably no longer pos-
sible because of the disparity in ionic radii, and a
separate FeO-bearing phase is likely. At high pres-
sure this phase may dissolve extensively in any
molten iron that traverses the region on the way
to the core, or to be stripped out of any man-
tle that comes into contact with the core in the
course of mantle convection. An FeO-poor lower
mantle is therefore a distinct possibility. The
corollary is an iron--FeO core. If FeO is stripped
outofthelowermantle,oriftheFeOexistsindis-
persed phases or layers, the radiative conductiv-
ity and viscosity of the deep mantle may be quite
different than generally assumed. The seismic
properties of the lower mantle are broadly consis-
tent with (Mg,Fe)SiO
3
--
perovskite
, although other
phases are certainly present, such as (Mg,Fe)O. If
the mantle was efficiently differentiated during
accretion, with upward removal of most melts,
then the deep mantle may be deficient in Ca
and Al, as well as the LIL and heat producing
elements.
Fe)O
,
,
VIII
−
XII
(Ca
0
.
5
Mg
0
.
5
)
4
VI
Si
4
O
12
(perovskite)
The ionic radii (in angstroms) of some of the
ions involved in the above reactions are:
VI
Al
,
0
.
53
VIII
Mg
,
0
.
89
VI
Ca
,
1
.
00
XII
Ca
,
1
.
35
VI
Si
,
0
.
40
VI
[CaSi]
,
0
.
70
VI
Mg
,
0
.
72
XII
Mg
,
1
.
07
VIII
Ca
,
1
.
12
IV
Si
,
0
.
26
IV
[MgSi]
,
0
.
56
This table provides a guide as to whether substi-
tutions are possible.
MgSiO
3
−
perovskite
The structural formula of the high-pressure
phase of enstatite, or 'perovskite,' is
VIII
XII
Mg
VI
SiO
3
Abbreviated Mg-pv, it appears to be stable
throughout most of the lower mantle and is
therefore the most abundant mineral in the man-
tle. Mg--perovskite is about 3% denser than the
isochemical mixture stishovite plus periclase.
There are a variety of Mg--O and Si--O distances
in Mg--pv. The mean Si--O distance is similar to
that of stishovite. The structure is orthorhombic
and represents a distortion from ideal cubic per-
ovskite. In the ideal cubic perovskite the smaller
cation has a coordination number of 6, whereas
the larger cation is surrounded by 12 oxygens. In
Mg--pv the 12 Mg--O distances are divided into
four short distances, four fairly long distances
and four intermediate distances, giving an aver-
age distance appropriate for a mean coordination
number of eight.
A slightly denser,
post-perovskite
phase
(ppv), has recently been discovered. The
importance of this new phase lies in its appetite
for iron. Anything that strongly affects the
Fe-partitioning will affect the role of radiative
conductivity and compositional layering in the
deep mantle.
Olivine
−
Low-spin Fe
2
+
Fe and Mg have similar ionic radii at low-pressure
and substitute readily for each other in upper
compositions
also
transform
to
perovskite-bearing assemblages Mg--pv
+
mw. The
