Chemistry Reference
In-Depth Information
3.2.3.3 Stability of amphiboles
The stability field of the amphibole is limited towards the low tempera-
tures - where they are replaced by chlorite, serpentine, with or without car-
bonates - and towards the high temperatures where they are replaced by
pyroxenes.
Amphibole stability depends both on their chemical composition, and
thus the chemical composition of the rock, and on physical parameters:
like all other ferromagnesian minerals, magnesian amphiboles are stable
at higher temperatures than the amphibole containing iron;
high water pressures stabilize amphibole rather than pyroxene;
oxygen fugacity is important in that iron can be in ferric or ferrous
state.
Generally the upper limits of stability are the order of 500-800C at
0.5 kb and 550-1000°C at 2 kb. These are the conditions of plutonism and
metamorphism of medium grade.
fluid
pressure
kb
(4)
(3)
(2)
3
(6)
(5)
2
(1)
1
0
T°C
300
400
500
600
700
1000
800
900
Figure 3.20
Stability of amphiboles (after Boyd, 1959, Gilbert, 1966 and Jenkins and
Bozhilov, 2003).
1
pargasite = > aluminous diopside + forsterite + nepheline + spinel + H
2
O
2
tremolite = > diopside + enstatite + quartz + H
2
O
3
ferroactinote = > hedenbergite + fayalite + quartz + magnetite + H
2
O
f
O2
fixed by the iron = wustite buffer
4
same reaction
f
O2
fixed by the fayalite = magnetite + quartz buffer
5
ferropargasite = > hedenbergite + fayalite + magnetite + plagioclase + nepheline + H
2
O
f
O2
fixed by the iron = wustite buffer
6
same reaction
f
O2
fixed by the fayalite = magnetite + quartz buffer
