Chemistry Reference
In-Depth Information
The transition from amphibolite facies to granulite facies is marked by
the destruction of calcic amphibole into calcic plagioclase
orthopyroxene.
There may be an intermediate stage the formation of cummingtonite (
+
cal-
cic plagioclase) and then transformation of all the amphiboles into orthopy-
roxenes. These various reactions are continuous reactions.
Plagioclase also appears at retrograde metamorphism from eclog-
ite facies into amphibolite facies: omphacite (sodium-rich pyroxene)
+
+
(pyrope-rich) garnet are transformed into kelyphite, an association of pla-
gioclase
+
hornblende. There may be an intermediate stage with plagi-
oclase
diopside.
Calcic plagioclase is an important component of impure metamorphic
carbonate rocks: impure marbles and calc-silicate-gneisses.
In a metamorphism of relatively low grade, microcline is the stable form
of potassium feldspar: it is commonly found in the orthogneisses (plutonic
or volcanic origin) and some metamorphosed clastic rocks (meta-arkose, for
example). There is an isograd microcline
+
= >
orthoclase which corresponds
to a temperature of about 600 to 650°C.
Potassium feldspar is also formed by destruction of mica:
the isograd muscovite
+
quartz
= >
sillimanite
+
potassium feld-
spar
water corresponds to the disappearance of primary muscovite;
this is an important boundary within the amphibolite facies;
+
the reaction biotite
orthopyroxene (or garnet depending on
the alumina content of the initial biotite)
+
quartz
= >
water appears,
approximately, at the beginning of the granulite facies. As ferromagne-
sian minerals are involved, it is a continuous reaction that occurs over
in a certain range of temperature (
i.e.
some extent of field).
+
orthoclase
+
Igneous rocks
The classification of igneous rocks is based on the relative proportions of
alkali feldspar/plagioclase/quartz/feldspathoids. This shows the importance
and abundance of feldspars in these rocks.
Since the work of Bowen (1915/1928) and Tuttle and Bowen (1958),
crystallization of feldspars has been the subject of numerous studies of
experimental petrology. They are widely exposed in the courses of petrol-
ogy. They also clarify the critical role played by the water pressure in the
magmatic crystallization of feldspars.
Binary systems: albite (Ab) - anorthite (An) system
Consider a magma evolving at decreasing temperature under low water pres-
sure, it reaches the liquidus in L1 and crystallizes plagioclase F1. The liquid
then evolves from L1 to L2 and plagioclase shows normal zoning F1-F2.
