Chemistry Reference
In-Depth Information
1 atmosphere - forsterite present
T °C
1600
Fo + L
L
1500
HTEn
Di + L
Pi + L
1400
Pi
Pi + Di
1300
PEn
1200
Di
PEn
+ Pi
1100
1000
En + Di
En
900
MgSio
3
10
30
50
70
90
CaMgSi
2
o
6
Figure 3.24
Diopside-enstatite system (after Carlson, 1988, modified).
L = liquid; Fo = forsterite; PEn = protoenstatite.
HTEN = phase analogous to high temperature enstatite.
En = enstatite; Pi = pigeonite; Di = diopside (solid solution = endiopside/augite).
Igneous petrology is largely based on the phase diagrams involving
pyroxene-quartz-olivine-feldspars, to which we refer the reader.
Stability of jadeite is limited towards low pressure - and high tempera-
tures - by the reaction (Figure 3.9):
NaAlSi
3
O
8
NaAlSi
2
O
4
+
SiO
2
Albite
jadeite
+
quartz
3.2.4.3 Occurrences of the pyroxenes
Igneous rocks
Pyroxenes are typically magmatic minerals that have crystallized at high
temperature with low water pressure. Therefore these are mostly minerals
of volcanic and hypabyssal rocks.
In plutonic rocks, the pyroxenes are abundant in the initial terms (gab-
bro, norite). They are rarer in differentiated terms (diorites) where they are
replaced by amphibole. They are exceptional in granitoids. Hypersthene
granitoids (charnockites) have crystallized in the granulite facies.
