Chemistry Reference
In-Depth Information
widely accepted that the oriented pressures are not important in the equilib-
rium between minerals. They are however very important in the dynamics
of growth of the minerals. The concept of
metamorphic facies
(or mineral
facies) integrates these various physical factors.
The metamorphic facies are defined by one or several critical minera-
logical parageneses (a paragenese is a mineral association in equilibrium).
This concept was first introduced for the metamorphic rocks (Eskola, 1920,
1927). About 14 facies and sub-facies are currently recognized:
The
zeolite facies
(Coombs, 1960) includes a first sub-facies which cov-
ers the field of diagenesis and early metamorphism and is defined by the
index minerals heulandite (or its polymorph clinoptinolite, these two min-
erals are zeolites of formula (Ca, Na)
2
Al
2
Si
7
O
18
· 6H
2
O) or by the critical
association analcime
quartz.
A second sub-facies is much more metamorphic: the previous associa-
tions no longer exist; it is characterized by the presence of albite NaAlSi
3
O
8
and the association laumontite
+
+
quartz (laumontite is a zeolite of formula
CaAl
2
Si
4
O
12
· 4H
2
O).
The
prehnite
-
pumpellyite facies
is defined by the associations preh-
nite
quartz. Jadeite and glaucophane are totally
absent. Epidote may be present, and so, its appearance indicates a transition
towards the greenschist facies.
The
greenschist facies
(Eskola, 1920) is characterized by the associa-
tion of a non aluminous calcic-amphibole (tremolite - actinolite series) and
a sodic plagioclase (An
+
quartz and pumpellyite
+
quartz is common.
Biotite may appear in the higher metamorphic part of this facies.
The
blueschist facies
(or glaucophane schist facies) (Eskola, 1929) is
distinguished by the presence of a sodic amphibole, glaucophane. The pres-
ence of lawsonite and/or the association jadeite
<
20). The association epidote
+
quartz is used to define
two sub-facies. Pumpellyite may exist in the less metamorphic part of this
facies. The white micas are not muscovite but phengite. Biotite and feldspar
are excluded.
The
white schist facies
(Chopin, 1984) is characterized by the associ-
ation talc
+
kyanite (equivalent to magnesian chlorite at lower pressure).
Coesite, high pressure polymorph of silica, is known in some parageneses,
implying pressures above 25 kb.
The
albite-epidote amphibolite facies
(Becke, 1921) is characterized by
a association of a hornblende (aluminous calcic amphibole), sodic plagi-
oclase (An
+
20), epidote and quartz.
The
amphibolite facies
(Eskola, 1915) is characterized by the associa-
tion hornblende
<
+
calcic plagioclase (An
>
20). Clinopyroxene may occur but
no orthopyroxene.
The
granulite facies
(Eskola, 1929) is defined by the critical associa-
tion plagioclase (calcic)
orthopyroxene; garnet (of the almandine - pyrope
series). Biotite and primary hornblende are present in the hornblende
+
