Chemistry Reference
In-Depth Information
3.2.4.2 Stability of pyroxenes
Ferro-magnesian and calcic pyroxenes are minerals of high temperature, the
stability of which is limited towards the lower temperatures by hydrated
minerals, amphibole, talc, mica, etc. and/or carbonates. For instance:
5 MgSiO
3
+
H
2
O
Mg
2
SiO
4
+
Mg
3
Si
4
O
10
(OH)
2
enstatite
+
fluid
forsterite
+
talc
(equilibrium temperature of about 650-700°C for pressures P
=
P
fluide
1-3 kb;
Kitahara et al. 1966).
5 CaMgSi
2
O
6
+
H
2
O
+
3 CO
2
Ca
2
Mg
5
Si
8
O
22
(OH)
2
+
3 CaCO
3
+
2 SiO
2
diopside
+
fluid
tremolite
+
calcite
+
quartz
(equilibrium temperature of about 450-550°C for P
fluid
1 kb, this tempera-
ture depends on the H
2
O/CO
2
ratio; Metz, 1970, Skippen, 1974, Slaughter
et al., 1975)
Pyroxenes can be stable at lower temperatures if the pressure of fluid
(H
2
O) is low.
Ferrosilite FeSiO
3
is stable only at pressures higher than 14.5kb. At
lower pressure it is transformed into fayalite (Lindsley et al., 1964):
FeSiO
3
Fe
2
SiO
4
+
SiO
2
ferrosilite
fayalite
+
quartz
The diopside-hedenbergite system is relatively simple: it is a continuous
solid solution whose phase diagram is similar for example to the forsterite-
fayalite diagram of the olivines. It is somewhat more complicated towards
the hedenbergite end member due to the fact that herdenbergite is replaced
by iron-wollastonite at high temperature.
The MgSiO
3
-FeSiO
3
-CaMgSi
2
O
6
-CaFeSi
2
O
6
system (pyroxene trapeze)
is characterized by a miscibility gap between diopside-hedenbergite (and
augite) series and the enstatite-ferrosilite (and pigeonite) series. Towards
the magnesian end members, the solidus intersects the solvus that defines
this gap.
This diagram is established for pure magnesian terms. The diagrams
established for other ferro-magnesian and calcic pyroxenes show a simi-
lar topology. Introduction of iron in the system considerably lowers the
temperatures. However, the phase diagram is poorly known for iron-rich
terms.
At lower pressure, there is a eutectic between diopside and pigeonite;
temperatures are also significantly lowered.
