Geoscience Reference
In-Depth Information
T
≈
1500
°
C
MgSiO
3
'GARNET'
V
p
~9 km
30
800
/
s
Liquid
Al
2
O
3
2200
2 Perovskites
'Perovskite'
V
p
~10.5
km
2Pv
+
2000
PYROXENE
V
p
=
Gt
/
s
700
8.1 km
/
s
25
Al
2
O
3
1800
b
−
+
Stishovite
V
p
~10.2
km
Spinel
Garnet
−
Majorite ss
1600
600
'Ilmenite'
V
p
=
Al
2
O
3
?
1400
/
s
10.1 km
/
s
20
+
Stishovite
V
p
=
g
−
Spinel
(9.2
−
11.2 km
/
s
1200
10.3 km
/
s
500
Clinopyroxene
160
180
200
220
240
+
Garnet
Pressure (kbar)
15
Fig. 22.4
Phase relations in MgSiO
3
. The arrows
show the direction that the phase boundaries are expected to
move when corundum or garnet, is added. The approximate
compressional velocities are shown for each phase.
[
akimotoite
]
400
10
300
Cpx
200
5
Mg--ilmenite
is isostructural with true ilmenite
and corundum. Silicon is 6-coordinated, as it is
in stishovite and perovskite, dense high-elasticity
phases. Because of similarity in ionic radii, we
expect that extensive substitution of Al for MgSi
is possible. Mg--ilmenite is a platy mineral, sug-
gesting that it may be easily oriented in the man-
tle. It is also a very anisotropic mineral, rivaling
olivine in its elastic anisotropy. The arrangement
of oxygen atoms is based on a distorted hexago-
nal closest packing having a wide range of O--O
distances.
The
ilmenite
form of MgSiO
3
,alsoknownas
akimotoite
, is stable at pressures between
c
.18
and 25 GPa and temperatures from
1 .8 .6 .4 .2 0
(Ca,Mg,Fe,Na,Al)
4
Si
4
O
12
(Mg,Fe,Ca)
3
Al
2
Si
3
O
12
Mole fraction pyroxene (percent)
Fig. 22.3
Phase relations in the cpx--gt system.
Then mineral
VIII
Mg
3
VI
[MgSi]
IV
Si
3
O
12
is known as majorite, and it exhibits a wide range
of solubility in garnet. Note that one-fourth of
the Si atoms are in 6-coordination. However, the
elastic properties of MgO plus SiO
2
−
stishovite are
similar to Al
2
O
3
, so we expect the elastic proper-
ties of majorite to be similar to garnet. Majorite
also has a density similar to that of garnet.
1100 to 1900
K in the pure MgSiO
3
system. The presence of iron
lowers the transition pressure from akimotoite
to perovskite. Aluminum plays an important role
in the akimotoite stability field in that pyrope
makes complete solid solution with the MgSiO
3
end-member majorite, enlarging the garnet field
at the expense of that of akimotoite both in pres-
sure and temperature. Depending on tempera-
ture and Al content, akimotoite may be present
in certain areas of the transition zone but absent
in others (Figure 22.4) causing significant lateral
variations in seismic velocities and density. This
complicates the interpretation of seismic obser-
vations, particularly tomographic cross-sections;
<
MgSiO
3
--ilmenite
The structural formula of 'ilmenite,' the hexago-
nal high-pressure form of enstatite, is
VI
Mg
VI
SiO
3
At low temperature the following transforma-
tions occur with pressure:
→
β
−
+
2MgSiO
3
(opx)
Mg
2
SiO
4
(
sp)
SiO
2
(st)
→
Mg
2
SiO
4
(
γ
−
sp)
+
SiO
2
(st)
→
2MgSiO
3
(ilmenite)
→
2MgSiO
3
(perovskite)
