Chemistry Reference
In-Depth Information
series, pigeonite is replaced by the enstatite/bronzite, because magnesian
pigeonite crystallizes at higher temperatures (above 1300°C) than such mag-
mas (which are about 1100 to 1200°C).
In
tholeiitic plutonic complexes
(anorthosite massifs, stratiform mafic-
ultramafic complex like the Bushveld and the Skaergaard (Figure 3.26),
pigeonite and augite of primary crystallization, formed at high temperature,
have undergone changes during subsolvus progressive cooling producing
exsolutions lamellae of orthopyroxene in augite and transforming the pri-
mary pigeonite into inverted pigeonite (hypersthene with exsolutions lamel-
lae of augite). These stratiform complexes commonly contain accumulative
pyroxenites (cumulates), like bronzitites and diopsidite.
The two pyroxenes of the
calcalkaline series
, volcanic or plutonic com-
plexes (Sudbury, Giant Mascott) (Figure 3.26) are augite and hypersthene.
In the plutonic complexes, augite, and to a lesser extent, hypersthene show
the same type of exsolution as in tholeiitic plutonic complexes.
The diopside-enstatite phase diagram (Figure 3.24) shows that the tho-
leiitic series (with pigeonite) crystallizes at higher temperatures and lower
water fugacities than the calc-alkaline series (with hypersthene).
Pyroxenes are, with olivine, the major constituents of ultramafic rocks:
lherzolites (olivine
+
Opx
+
Cpx), harzburgite (olivine
+
Opx), wherlite (oli-
vine
+
Cpx), websterite (Cpx
+
Opx) and pyroxenites.
wollastonite Casio
3
hedebergite
CaFeSi
2
O
4
diopside
CaMgSi
2
O
4
salites
augite
subcalcic augite
unknown
in the nature
unknown
in the nature
pigeonite
bronzite
enstatite
MgSiO
3
ferrosilite
FeSiO
3
hyperstène
pyroxenes of the mafic - ultramafic complexes
Sudbury
Giant Mascot
Calcic and Fe-Mg pyroxenes
Bushveld
Skaergaard
Stillwater
Figure 3.26
Pyroxenes of the mafic-ultramafic complexes (data compiled by M. Besson
and unpublished data by the same author).
